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le in one direction and under the needle in the opposite In this way the magnetic direction. effect of the current is magnified and the compass-needle will be deflected. By Left-Hand applying (Sec. V:48), the direction of current may be determined. By noting the amount of deflection of the needle a rough com- Rule the In experiment 34, chapter 31, a conductor AB was suspended between the poles of a strong permanent magnet as shown in Fig. 27:15. When a heavy current was sent through the conductor from a storage battery or similar source, the conductor was pushed aside. When the current was reversed, the conductor was thrust in the opposite direction. Thus we see that a conductor carrying current in a magnetic field is acted upon by forces which cause the conductor to move. The magnetic field between the poles of the permanent magnet may be represented as shown in Fig. 27: 16(a). The current in the conductor sets up a magnetic field as in Fig. 27: 16(b). A crosssection of the conductor is shown in Fig. 27: 16(c). When the conductor is in the permanent magnetic field, Fig. as in 309 Chap. 27 MAGNETISM AND ELECTRICITY 27: 16(d), the lines of force from both the conductor and the permanent magnet are in the same direction on the force are like stretched elastic bands (Sec. V:4), they try to straighten themselves and consequently, push the conductor to the left. Similarly, if the electron flow in the conductor is reversed, the motion will be in the opposite direction. If now conductor AB (Fig. 27:15), is replaced by a single loop ABCD (Fig. 27:17), section AB will be moved toward the left by the combined forces of the permanent magnetic field and the electromagnetic field, while section CD will be thrust toward the right. Thus, the loop will set itself so that the plane of the coil will become perpendicular to the direction of the lines of force of the permanent magnet. This behaviour may be more simply explained if we consider of a Conductor Fig. 27:15 Carrying Electric Current in a Mag- Action netic Field. right and in opposite directions on the left. Thus the lines tend to be crowded on the right side, while on the left they cancel each other. Since lines of magnetic Fig. 27:17 Action of a Loop CarryElectric Current in a Magnetic ing Field. the loop to be a form of helix. The Helix Rule
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(Sec. V:49), indicates that the loop, when carrying current, will have an N-pole and an S-pole. Following the Law of Magnetism (Sec. V:2), the N-pole of the loop will be attracted to the S-pole of the permanent magnet while the S-pole of the loop will face the N-pole of the magnet. V:53 THE D'ARSONVAL GALVANOMETER Fig. 27:16 The Motor Principle. The D’Arsonval Galvanometer is a 310 MAGNETIC EFFECTS OF ELECTRIC CURRENT Sec. V;54 sensitive instrument designed to detect the presence of small direct currents of It also serves to determine electricity. Fig. 27:18 A Simple Galvanometer. the direction and compare the strengths of such currents. The simplest form of this type of gal- vanometer is shown in Fig. 27:18. It consists of a permanent horseshoe magnet and a coil of wire which is free to swing between the poles of the magnet. A stationary soft iron core is mounted inside the coil to concentrate the lines of force. Springs attached to the coil serve to conduct the electrons into and out of the coil and also to bring the coil to rest in a position where the plane of the coil faces away from the poles of the magnet, as shown in the diagram. As current passes through the coil it will turn, so that the N-pole of the coil is toward the S-pole of the magnet, and is toward the the S-pole of the N-pole of the magnet (Sec. V:52). This movement is restricted by the springs. The amount of turning is indicated by the attached pointer moving across a calibrated scale. The stronger the electric current through the coil, the greater the magnetic field produced, and so the coil higher the reading on the scale. This moving-coil type of meter is the basis of most standard electrical measuring instruments in common use. V : 54 AMMETERS AND VOLTMETERS Ammeters and voltmeters are instruments designed to measure electric current and potential difference respectively. They are essentially galvanometers of the moving-coil type (Sec. V:53), modified by the addition of suitable resisTheir scales are calibrated to tances. read directly in amperes or volts. (a) Ammeters The wire winding on the moving coil of an ammeter must be very fine and light to allow the necessary freedom of movement
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. Such a conductor cannot carry a large current without undue (Sec. V:71), and consequent heating melting of the wire. The movable coil of such an instrument is rarely allowed to carry more than 0.05 amperes. If the ammeter is to be used to mea- Fig. 27:19 Ammeter Connected in Series. 311 ) Chap. 27 MAGNETISM AND ELECTRICITY sure higher currents than this the current must be divided so that no more than 0.05 amperes will flow through the moving coil, and the rest of the current will be carried around the coil through a shunt connected in parallel with the instrument (Fig. 27:19). Such a shunt must have a very low resistance and be accurate over a wide range of temperatures. To measure the current in a circuit the ammeter must be connected in series. As a result, all the current, or a known fraction of the total current, will pass through the instrument coil. In order to avoid affecting the total current in the circuit, the resistance of the ammeter must be very low. If the ammeter were accidentally connected across the circuit, a high current would flow through it and the instrument would be “burned out”. To avoid this a fuse (Sec. V:72) should be connected in series with the ammeter. Example A galvanometer has a resistance of 5 ohms and gives a full-scale deflection with a current of 0.05 amperes. What value of shunt resistance must be used to convert it to an ammeter reading up to 5 amperes? Current to be carried by ammeter = 5 amp. Current to be carried by galvanometer = 0.05 amp. Current to be carried by shunt = 5 — 0.05 — 4.95 amp. Resistance of galvanometer = 5 ohms. Potential difference across galvanometer — 0.05 X 5 = 0.25 volts {V = IR) Potential difference across shunt = 0.25 volts (parallel connection) Resistance of shunt to be used = ^ 4.95 0.05 ohms {R — — I Fig. 27:20 Voltmeter Connected in Parallel. 312 MAGNETIC EFFECTS OF ELECTRIC CURRENT Sec. V:55 (b) Voltmeters potential Since a voltmeter is used to measure difference between two the points in a circuit, it must be connected circuit between the in these points. Therefore,
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a voltmeter must have a very high resistance to avoid parallel with drawing a large current. As a result the total current in the circuit will not be affected significantly. A galvanometer may be converted to a voltmeter by the addition of a high resistance in series with the moving coil (Fig. 27:20). Example A galvanometer has a resistance of 5 ohms and gives a full-scale deflection with a current of 0.05 amperes. What value of series resistance must be used to convert it to a voltmeter reading up to 5 volts? Potential difference across voltmeter = 5 volts Current through voltmeter = 0.05 amp Resistance of voltmeter = 5 =100 ohms (R 0.05 Resistance of galvanometer = 5 ohms Series resistance to be added = 100— 5 = 95 ohms. V:55 THE ELECTRIC MOTOR The electric motor is studied in experiment 35, chapter 31, using a St. Louis motor (Fig. 27:21). In order to understand its consider an electromagnet mounted on an axis so that it is free to rotate between the poles of a magnet (Fig. 27:22). If a current is passed through the electromagnet so as to produce an S-pole at the top, as operation shown, the S-pole will be attracted by the N-pole of the stationary field. Similarly the N-pole of the electromagnet will be attracted by the S-pole of the These forces cause stationary magnet. the electromagnet, which serves as the motor armature, to rotate. The momentum of this armature causes it to rotate so that the S-pole of the armature passes slightly beyond the N-pole of the magnet. Commutator Brushes Commutator Plates Armature — Field Magnet Fig. 27:21 St. Louis Motor. Central Scientific Co. of Canada Ltd. 313 Chap. 27 MAGNETISM AND ELECTRICITY ments of the commutator also rotate. Connection to the commutator segments is made by a pair of brushes, one leading the electrons in ( — ) and the other out ( + ). At exactly the correct instant, segment R makes contact with brush Bi causing the electrons to flow through the armature. One-half turn later, segment R makes contact with brush B 2 as segment S contacts brush Bi, etc. Thus the commutator serves to reverse the direction of electron flow with every 180° rotation of the armature. The magnetic field in which the
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armature turns is usually provided by an Fig. 27:24 Electric Motor. The Commutator Reverses the Direction of Electron Flow Every 180°. electromagnet, since such a magnet may be made more powerful than a permanIts windings are called the ent type. field coils to difTerentiate them from the armature coil. The field coils may be in series or in parallel with the armature coil, depending on the motor characteristics desired. A more advanced text should be consulted if greater detail is required. The motor described here operates on direct current only, and thus is seldom used. However, it serves to illustrate the principle electric operating on of operation of including motors, those all alternating current. S-poIe Electric Motor, The ArFig. 27:22 mature is Free to Rotate Between the Poles of the Magnet. If the current in the armature windings could be reversed at this instant, the polarity of the armature would change and repulsion and attraction would carry the armature through another 180° (Fig. Successive reversals of current 27:23). Electric Motor. The Poles Fig. 27:23 Reverse in this Position so Armature Continues to Rotate. and consequent changes of armature polarity cause the rotation to continue. Such a reversal of current can be caused of a commutator (Fig. by the 27:24). The armature wires are attached to two segments of good conducting material which are insulated from each other. As the armature rotates the seg- use 314 MAGNETIC EFFECTS OF ELECTRIC CURRENT Sec. V:56 V : 56 QUESTIONS A (b) State the Left-Hand Rule. 1. 2. (a) Draw diagrams showing the lines of magnetic force for (i) a straight wire (ii) a single turn of wire when carrying an electric current. Indicate clearly on each diagram the direction flow and of the electron of the magnetic field. (a) On a diagram of a solenoid indicate electron direction flow and the magnetic field rounding it. the sur- of the the (c) Indicate current in wires (1), (2) and (3) in which the direction of the lines of direction of force is shown. (d) Indicate the direction of the lines (5) and of force around wires (4), (6) carrying a current. (b) State the Helix Rule. (c) On the following diagrams mark the
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direction of the current and the polarity of the solenoids. 315 Chap. 27 MAGNETISM AND ELECTRICITY 3. (a) Why is soft iron, rather than steel, used as the core in most electromagnets? (b) How may the strength electromagnet be increased? of an 8. (d) How may a galvanometer be (i) an modified to convert it into ammeter (ii) a voltmeter? Make a labelled diagram of a simple D.C. motor and describe its opera- 4. (a) Make a fully labelled diagram of an electric bell connected in a tion. B circuit. (b) With the aid of this diagram describe the operation of an elec- tric bell. 5. 6. (a) What is the purpose of an automobile generator cut-out relay? (b) Describe its operation. (a) What is the purpose of a galvanoscope? (b) Describe and explain the operation of a galvanoscope. 7. (a) State the motor principle. (b) Make a labelled diagram of a simple moving-coil galvanometer and explain how its operation employs the motor principle. (c) What precaution should be obobserved when a galvanometer is connected in a circuit? 1. A galvanometer has a resistance of 1 0 ohms and gives a full-scale deflection with a current of 0.04 ampere. What value of shunt resistance must be used to convert it to an ammeter reading up to (a) 5 amperes (b) 50 amperes (c) 500 amperes. 2. A galvanometer has a resistance of 2 ohms. It gives a full-scale deflection with a current of 0.01 ampere. What value of shunt resistance must be used to convert it to an ammeter reading to 10 amperes? 3. What value of series resistance must be added to convert the galvanometer of question 1 to a voltmeter reading up to (c) 500 volts? (a) 5 volts 4. What value of resistance must be connected in series with the galvanometer (b) 50 volts of question 2 to convert it to a voltmeter reading up to 100 volts? 316 CHAPTER 28 ELECTROMAGNETIC INDUCTION V:57 THE STORY OF FARADAY Michael Faraday was one the greatest experimental scientists the world has ever known. He was born in England in 1791 and, after a
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very ordinary education, became an apprentice to a book- of pressed the great chemist. Sir Humphrey Davy, so much that at the age of twentyone he became his assistant at the Royal Institution. Within twelve years he was made a director of the Institution. From the time that Oersted demonstrated the magnetic effect of an electric current Faraday dreamed of the converse effect, the production of electricity from magnetism. After many unsuccessful experiments he finally produced a momentary current in a coil of wire wound on an iron ring by starting and stopping a current in another coil wound on the same ring (Fig. 28:1). With this small beginning Faraday began a series of experiments on electro- University of Toronto Michael Faraday binder. He read every scientific book which passed through his hands and quickly became well educated. He im- Fig. 28:1 Electricity from Magnetism. to magnetism that eventually led his invention of the dynamo. The services of Faraday were in great demand by firms but he refused great industrial wealth in order to devote his talents to scientific discovery. Honours of all kinds were showered upon him but he remained the humble scientist and experimenter to the end, devoting his time and 317 Chap. 28 MAGNETISM AND ELECTRICITY energy to discovery. When all else is forgotten, he will be remembered as the father of the electric current that serves our homes, offices, factories and communities. Some of his other discoveries, such as the methods of liquefying gases, making optical glass, of benzine, and the like, are not remembered so well even though of great importance. the isolation Although he was never one to seek publicity his name has become immortal because of his contributions to humanity through physics and chemistry. V:58 CAUSE OF AN INDUCED CURRENT It is a simple matter to demonstrate, as Faraday did, that, just as electrons in motion can set up a magnetic field, a magnetic field in motion can cause a Three such experiflow of electrons. ments are described in chapter 31, experiment 36. is i.e., no coil, (Fig. current If a galvanometer is connected to the terminals of a solenoid and a bar magnet or an electromagnet is held above the solenoid 28:2), obtained. When the magnet is thrust into the centre of the the strength of the magnetic field is increased, the galvanometer needle is momentarily deflected from its centre position. The galvanometer needle returns to the zero position when
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the magnet remains stationary in the solenoid, i.e., while the strength of the magnetic field remains constant. When the magnet is withdrawn quickly from the solenoid, the strength of the magnetic field decreases, and the galvanometer needle is again deflected, but this time in the opposite direction. It is apparent, then, that a flow of electrons originated each time the magnet was moved. This is called induced current, and the effect caused by the changing magnetic field around a conductor is called electromagnetic induction. 318 For an induced current to flow in the circuit there must, of course, be an electromotive in (Sec. V:26). This induced E.M.F. is caused by the changing magnetic field about the conductor. circuit force the Similarly, in the circuit used by Faraday (Fig. 28:1), when the switch in circuit containing the battery and the primary coil (C) was opened, the electron flow was stopped. Thus, the magnetic field about the primary coil deThis changing creased rapidly to zero. magnetic field the the secondary coil in the circuit and an E.M.F. was induced. When the switch was closed, current flowed in the primary circuit and as the magnetic field increased from zero to a maximum, an E.M.F. was induced again in the secondary circuit, causing a current to flow in the turns cut (5') of opposite direction. From these experiments we may conclude that an induced E.M.F. is caused by the changing magnetic field about the conductor. V:59 MAGNITUDE OF INDUCED E.M.F. In experiment 37, chapter 31, it will be observed that, if the magnet is slowly plunged into and withdrawn from the solenoid, there will be slight deflection of the galvanometer needle. If the action is repeated more and more rapidly, it will be seen that the amount of deflection is directly proportional to the speed of movement of the magnet. Thus, we may conclude that the strength of the induced current is proportional to the speed at which the magnet is moved or the E.M.F. induced in a circuit is proportional to the rate of change of the magnetic field cutting the conductor. If a magnet is thrust first into a coil of few turns and then, at the same speed, into a coil of many turns, and the deflection of a galvanometer in the ELECTROMAGNETIC INDUCTION Sec. V: 60 circuit is noted, it will be
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found that a stronger induced current flows in the coil having the larger number of turns. Therefore, it is evident that the E.M.F. induced in a circuit is proportional to the number of turns of the conductor cut by the varying magnetic field. N-pole at the end of the solenoid where the magnet enters. This N-pole repels the N-pole of the magnet and opposes the motion of the magnet. As the N-pole of When a powerful electromagnet with an iron core is substituted for the bar magnet in the experiment described above it will be observed that, with the rate of movement and the number of turns in the coil remaining constant, the deflection of the galvanometer needle is greatly increased. It is apparent, then, that the E.M.F. induced in a circuit is the proportional the changing magnetic field. strength of to V:60 DIRECTION OF AN INDUCED CURRENT The direction of an induced current is studied in experiment 38, chapter 31. Here, the direction of the current is noted when (a) the N-pole of a magnet is thrust into the coil; (b) the N-pole is Induced Current as the Bar Fig. 28:3 Magnet is Withdrawn from the Solenoid. the magnet is withdrawn from the coil (Fig. 28:3) the induced current is in such a direction as to produce an S-pole at the same end of the solenoid. The attraction of this S-pole of the solenoid for the N-pole of the magnet tends to oppose the withdrawal of the magnet. Similar results may be found when the electromagnet is used. These eflfects are summarized in Lenz’s Law which states: The direction of an induced current is such that the magnetic field which it produces opposes the motion or change that induces the current. Induced Current as the Bar Fig. 28:2 Magnet is Plunged into the Solenoid. withdrawn; (c) the S-pole is inserted; (d) the S-pole is withdrawn. As the Npole enters the solenoid (Fig. 28:2) a current is induced which produces an Because of their importance, primary and secondary currents require special It was noted (Sec. V:58), treatment. that secondary currents are in opposite directions when the primary is opened compared to when it is and demonstrable secondary currents are in the same direction when the primary circuit is opened (at the �
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�break”) and in the opposite direction when it the “make”). The idea of opposing mag- primary is closed. closed that the (at It is 319 Chap. 28 MAGNETISM AND ELECTRICITY As it nears the position at right angles to the first, much cutting of the lines of force netic fields is carried out here in that the induced current, while flowing in the same direction as the primary, attempts to maintain the magnetic field that is Similarly, at the completion collapsing. of the primary circuit, the induced current is attempting to oppose a magnetic field which is being built up. It is further to be noted that the induced E.M.F. is greater at the “break” because the induced current tends to strengthen the field of force of the primary. V:61 THE EARTH INDUCTOR The earth inductor is a coil with a diameter of about 18 in., made of several hundred turns of fine insulated copper wire. When the two end leads of the coil are connected to a galvanometer and the rotated through 360°, the galvanometer needle will be seen to swing first to one side and then to the other (Chap. 31, Exp. 39A). This action continues as long as the inductor is rotated. inductor rapidly is In Fig. 28:4, as the coil rotates clockwise, A downward and B upward, little cutting of the lines of force occurs and a small E.M.F. will be developed at first. occurs, and the E.M.F. produced will be larger. Throughout this 90° of rotation the induced E.M.F. has been built up from zero. It will be obvious that the E.M.F. will gradually decrease to zero in the next 90°. Throughout this rotation of 180°, the E.M.F. has gone from zero to a maximum and back to zero again (Fig. 28:5). The direction of the current has been constant. All this time DIRECTION OF CURRENT 320 ELECTROMAGNETIC INDUCTION Sec. V: 62 Fig. 28:6 Simple Alternating Current (A.C.) Generator. the magnetic field of the induced current has been opposing the motion causing it (Lenz’s Law). Rotation through the next 180° in which A will move up and B down will result in the same cycle of events as before except that the direction of the current will be reversed. The energy required
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to overcome the opposition of the fields of force is transformed into electrical energy. While the earth inductor itself is of no practical value it does permit a preliminary study of the generation of electricity by induction. This simple knowledge has made possible the development of all the many forms of electric generators which supply power for modern living. V:62 THE ALTERNATING-CURRENT GENERATOR As indicated in section V:61, mechanical energy may be used to produce electrical energy. A device used for this purpose is called a generator or dynamo. A simple generator (Fig. 28:6) consists of a coil of wire called an armature which is made to rotate between the poles of a magnet (Chap. 31, Exp. 39B). To facilitate connections to the rotating armature, slip rings {Ai, A 2 ) are connected to each end of the coil and rotate with it. Contact is made by metal or carbon brushes {Bi, B2 ) which connect the coil with the external circuit containing the lamp (L). As the coil rotates in the direction shown in the diagram, it cuts the magnetic field between the pole-pieces {S, N) of the magnet and a current is induced In position (1) the in the conductor. sides ab and cd of the conductor are momentarily moving parallel to the lines of magnetic force so there is no cutting and no induced E.M.F. Therefore no electrons will flow. In position (2) the coil is cutting an increasing number of lines of force so the induced E.M.F. is Electrons flow in the direcincreasing. tion abed so as to produce an N-pole and an S-pole in the coil which will oppose its motion (Lenz’s Law). Electrons are now leaving the slip ring B, passing through the lamp, and returning to A. In position (3) the conductor is cutting the maximum number of lines of force and the induced E.M.F. and induced electron flow are at a maximum. As the 321 Chap. 28 MAGNETISM AND ELECTRICITY (4) coil reaches position the electron flow is decreasing and, in position (5) as the motion is parallel to the lines of force once more, it ceases entirely. In the electron flow builds up again (6) but this time in the direction dcba so that electrons leave slip ring A, pass through the lamp, and return through B. The current again reaches a maximum (8), and falls to in zero
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in position (9). Thus, during one complete rotation of the coil, the induced current starts at zero, increases to a maximum, falls to zero, increases to a maximum in the opposite direction, and again falls These (7), decreases in zero. to changes are summarized in Fig. 28:7. A current with such characteristics is said to be an alternating current (A.C.) and one cycle has been described above. Alternating current is provided by all hydro-electric installations. Those who have lived in, or visited, 25-cycle power areas will have noticed the rapid flickering of the lights. This is caused by the reversal of current flow occurring 25 times per second. When the number of reversals is increased to 60 times per second as in 60-cycle power, this flickering is not noticeable. Very few areas 25-cycle of power. the world use this still 322 ELECTROMAGNETIC INDUCTION Sec. V: 64 V:63 THE DIRECT-CURRENT GENERATOR When electrons flow continuously in one direction it is said to be a direct current. Such current is required for special installations such as electroplatIn order to obtain direct current ing. from an A.C. generator, a commutator must be used in place of the slip rings (Chap. 31, Exp. 39B). For a single-coil generator as described in section V:62, the commutator consists of a collectingring made of two segments or com- Brush trons are about to reverse direction. Thus the commutator bars change brushes just as the electron flow reverses so that the current flowing through the external circuit is always in the same direction (Fig. 28:9). The current from a single turn of wire is clearly a pulsating current. To eliminate this the armature of a commercial generator consists of many such coils each with its commutator arranged around the shaft (Fig. 28:10). Such an arrangement will give a more continuous flow of current, since some of the coils will always be cutting the magnetic lines of force. Such generators are used in autoremote mobiles, places, direct-current generating systems in schools, hospitals, etc., and for electroplating and electro-purification of metals. electrical systems in Segments Fig. 28:10 Armature of a Commer- cial D.C. Generator mutator bars insulated from each other. Each bar is connected to a terminal of the coil (Fig. 28:8). The brushes
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are so placed that they rest on the insulating material between the bars at the instant when the elec- V : 64 TRANSFORMERS Transformers are used to change the voltage in an A.C. circuit as required. A transformer consists of two separate coils of insulated wire, the primary and secondary coils, wound on the same soft iron core (Fig. 28:11). Alternating currents from a generator flow through the windings of the input or primary coil. As the current changes in intensity and direction, the magnetic field surrounding the coil changes as well. Thus lines 323 — Chap. 28 MAGNETISM AND ELECTRICITY of magnetic force of changing strength and direction are made to cut the turns of the output or secondary coil. An alternating E.M.F. is induced in the secondary coil. Since the magnitude of Fig. 28:11 Step-up Transformer. the induced E.M.F. is proportional to the number of turns of the conductor cut by the changing magnetic field, the strength of the induced voltage may be varied by using a different number of the primary and secondary turns windings. For any transformer in Output Voltage _ No. of turns on secondary coil Input Voltage No. of turns on primary coil A step-up transformer has more turns on the secondary coil than on the primary coil and so serves to increase or “step up” the voltage. A step-down transformer has more turns on the primary coil than on the secondary and so serves to decrease or “step down” the voltage. In section V : 75, it will be established that the power of an electric current = the number of volts X the number of Obviously the power of the amperes. secondary current must equal the power of the primary current (except for a very small decrease owing to conversion to heat). Thus, if a step-up transformer causes a voltage rise, there must be a corresponding fall in the current. Students will be familiar with the small step-down transformers used to operate electric trains, electric bells and electric janitors around the home. Some other uses of the transformer will be described in the sections that follow. Example A transformer is to be used to provide 6 volts to operate an electric If the primary coil has 2000 turns, how door-bell on a 120 volt circuit. many turns should be on the secondary coil? Output voltage ~ 6 volts Input voltage — 120 volts Number of turns on primary = 2000 Number of turns on secondary = x Output voltage Number of turns on secondary coil Input
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voltage Number of turns on primary coil 6 X 120 2000 6.•.x=r—X 2000 =100.’. there should be 100 turns on the secondary coil. 324 ELECTROMAGNETIC INDUCTION Sec. V:65 V:65 THE TELEPHONE The first telephone was invented by Alexander Graham Bell, a Scottish emigrant to the United States, 1875, and first used between Brantford and Bell’s main interest was Paris, Ontario. he had deaf-mutes, but in many other interests and the telephone was one of the many products of his fertile mind. From his simple instru- teaching in Fig. 28:12 Telephone Transmitter. vibrate. When the vibrations are transmitted to the carbon granules, they are compressed to a vaiying degree. This, in turn, causes their electrical resistance to vary and fluctuations in the current strength are produced at the same freThis fluctuatquencies as the sounds. ing current is transmitted to the re- ceiver. into sound again. As we know so well, the purpose of the receiver is to convert these fluctuatThe ing currents receiver (Fig. 28:13) consists of a permanent magnet (M) having an electromagnet at the poles. As the fluctuating current caused by the transmitter passes through the coils {CC) of the electro- Diaphragm (D) Electromagnet (C) Star Newspaper Service Alexander Graham Bell. ment has been developed the worldthat we wide communication system know to-day. The essential parts are the transmitter and the receiver. The transmitter (Fig. 28:12) consists of a capsule containing carbon granules (C) connected the to diaphragm (D) on one side and to a carbon block (B) on the other. A current from a battery passes through the granules. metal called sheet thin a It responds to sound waves as follows: the condensations and rarefactions falling on the diaphragm cause it to Fig. 28:13 Telephone Receiver. 325 Chap. 28 MAGNETISM AND ELECTRICITY Transmitter Ground Fig. 28:14 Simple Telephone Circuit. magnet, the magnetic field is caused to change at the same frequency as the Thus the steel original sound waves. diaphragm {D) is caused to vibrate at this frequency and sound waves almost identical to those at the transmitter are reproduced at the receiver. If the transmitter and receiver circuits are directly linked,
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only weak signals This is because of the will be heard. very small changes in resistance in the carbon granules of the transmitter as compared to the total resistance of the circuit. The slight variations in current do not cause enough variation in the receiver electromagnet to give satisfactory reception. To overcome this, the transmitter is connected to the primary winding of a step-up transformer. The secondary coil is connected in the line circuit through which magnified effects are carried to both receivers as shown in Fig. 28:14. Refinements such as the bell signal, automatic dialing, relays, ultra high-frequency beam wireless, sheathed cables, coaxial cables, and others are employed in the modern system. However, the above still remains the basis for all telephone communication. V:66 SELF-INDUCTANCE It has been observed that when an is broken, as when a circuit electric in a house-lighting switch turned off, a spark or “electric arc” freThis is caused by selfquently occurs. inductance which is explained below. circuit is r RINGER CO IL j DIAL MECHANISM Bell Telephone Co. Canada The Dial Telephone. 326 When a coil of wire of many turns, wound about an iron core, is connected to a battery, the current that begins to flow causes a magnetic field about the wire. As an increasing number of lines of force cut the conductor, an E.M.F. ELECTROMAGNETIC INDUCTION Sec. V:67 will be induced in the coil itself causing a current to flow in a direction opposite to the flow of current in the coil (Lenz’s Law). Thus the current will build uo more slowly than might be expected in such a circuit (Chap. 31, Exp. 40). When the connection to the battery is broken, the decreasing current causes a decreasing magnetic field about the wires. An E.M.F. causing a current that flows in the same direction as the main current will result. Since this is of relatively high voltage, a definite spark (arc) will be observed. In a circuit, an induced current that opposes any change in the flow of current through it caused by self-inductance is called a self-induced current. It is so similar to the inertia of matter that it has been termed electrois an important magnetic inertia. consideration in all alternating-current equipment design. It V : 67 THE INDUCTION COIL The induction coil is very like the transformer in construction
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, as both consist of primary and secondary coils of different numbers of turns of wire wound about a common soft iron core. Whereas the transformer can utilize the changing magnetic field caused by alternating current to step up the voltage, the same operation is impossible with a direct current for lack of changing magnetic field. However, it is possible to obtain high voltage from a low voltage direct current by the use of an induction coil. is accomplished by continually This interrupting the primary current, with the aid of a circuit breaker similar to (Sec. V:50). that in the electric bell This results in a changing magnetic field around the primary which induces a high voltage current in the secondary coil. This device was first invented by Page in the United States in 1836 and later reinvented by Ruhmkorff in Europe in 1851. The of coil (Fig. thick induction 28:15) consists of a primary coil T of a few insulated copper wire turns wound about a soft iron core B which is not solid but built up from a bundle of soft iron wires to prevent self-induction effects. Around the primary coil is a secondary coil C consisting of many turns of very fine copper wire wound in sections with parafifined-paper insulation between the layers of each section. The primary circuit is completed through Secondary Terminals a make-and-break device D. This consists of a stiff piece of spring steel with a soft iron armature on one side and a platinum (or tungsten) contact on the other. It is normally in contact with the point on the platinum (or tungsten) As the end of the adjusting screw. primary current flows, the core becomes magnetized and attracts the armature, thereby breaking the circuit. The magnetism of the core now collapses and the armature thus released is drawn back by the spring to make contact with the The primary circuit again screw. complete for the action to be repeated. As the magnetic field of the primary circuit builds up and collapses at each make and break, an alternating induced is 327 Chap. 28 MAGNETISM AND ELECTRICITY E.M.F. is set up in the secondary coil. This E.M.F. may be many tens of thousands of volts and can cause a spark several inches long to pass between the terminals of the secondary coil. Oneof each alternation has a much half greater E.M.F. than the other (Sec. V:60). As a result this current usually is viewed as a pulsating direct current. In
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actual practice the weaker of the two is practically eliminated by the use of a condenser. Induction coils can be used for operalthough modern ating X-ray tubes, tubes are mainly transformer operated. They are also employed in some forms of laboratory research where moderately high voltages are required, and in the ignition systems of automobiles. Note: Because of the very large selfinductance of the primary winding, the upbuilding and decay of the primary current are both delayed, and the slower changes in the magnetic field mean correspondingly reduced voltages the secondary coil. The self-induced forward E.M.F. in the primary circuit at the break causes arcing across the contact points which become considerably worn as a result. To prevent this a condenser E, formed from alternate sheets of tinfoil and paraffined-paper, is placed across the make and break, and the self-induced current at break surges into the con- in denser instead of arcing across the points, thereby producing a very rapid break. The condenser plates, however, are connected through the primary winding, and no sooner is the condenser charged than it discharges through the primary coil, producing a current in it in a direction opposite to the now upbuilding current from the battery. As was noted before (Sec. V:60), the induced current is weaker at the make than at the break. The condenser weakens it still further. Accordingly, the coil produces practically 328 a pulsating, E.M.F. than that of the primary. direct current of higher V:68 THE AUTOMOBILE IGNITION SYSTEM the into fitted plugs The flammable mixture of gasoline vapour and air used in internal combustion engines is ignited by a series of sparks passing across the electrodes of spark cylinder heads. These sparks, which are correctly timed for each of the cylinders, are produced by a type of induction coil, the primary of which is connected to the battery through a circuit breaker, while the secondary is connected through a “distributor” to the electrodes of the spark plugs. The scheme of the ignition system, is shown in Fig. 28:16. Spark Plugs Fig. 28:16 Simple Automobile Ignition System. The circuit breaker is operated by a cam revolving at half speed and driven from the engine crankshaft. When the platinum points are separated, the resulting change of magnetic field of force produces an induced voltage in the secondary coil. A condenser is connected to prevent
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across contact points the time. 3. 4. ELECTROMAGNETIC INDUCTION Sec. V: 69 arcing (and consequent damage to the points) and to ensure a rapid break as In described in the previous section. order to withstand the high voltages (of the order of 10,000 volts), the leads from the secondary coil to the distributor and spark plugs are covered with thick in- sulation. V : 69 QUESTIONS A 1. Read a more complete story of Faraday in a good encyclopedia and write a brief account of his accomplish- 5. ments. 2. Describe briefly Faraday's method of producing induced current for the first (a) What is the cause of an induced current? (b) What factors affect the magnitude of the E.M.F. induced in a circuit? (c) State Lenz's Law. (d) On the following series of diagrams label of the current and indicate the polarity of the upper end of the solenoid. direction the 6. the inductor with regard to direction and magnitude as the coil is revolved through 360° in the earth's field. (a) Make a diagram of a simple generator with a single coil of wire rotating between the permanent regard to current produced with magnitude and direction as the coil rotates through 360°. (b) How is the current taken from the rotating armature and fed into the poles of a magnet. Describe the external circuit? (c) Describe how a generator differs direct-current from an alter- nating-current generator. (a) Describe, using simple diagrams, an (ii) a current produced alternating-current generator by the (i) direct-current generator. (b) What is meant by (i) a single (ii) 60-cycle current (iii) pulcycle sating direct-current? 7. Explain: (i) the structure (ii) the action of 10. 8. (a) a step-up transformer (b) a step-down transformer. (a) Describe (i) a simple telephone (ii) a simple telephone transmitter receiver with regard to purpose, (a) Describe the structure of an earth inductor. (b) Describe the current produced in structure, action. (b) Describe how transmitter and receiver circuits are linked in order to keep up the strength of signals. 9. What is the effect of self-inductance when a circuit containing a coil of wire is
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(a) completed (b) broken? Describe a simple induction coil as to (a) purpose (b) structure (c) action (d) uses. 329 Chap. 28 MAGNETISM AND ELECTRICITY B 1. A transformer is required to provide 6 volts to operate an electric door-bell on a 120 volt circuit. If the primary coil has 1 600 turns, how many turns should be on the secondary? 2. A step-down transformer has 2000 turns on the primary and 200 turns on the secondary. If the primary voltage Is 22,000 volts, what is the secondary voltage? 3. An electric door-bell transformer has 960 turns on the primary and 80 turns on the secondary. When it is connected to the 110 volt circuit, what voltage is provided for the bell? 4. A generator produces at 2200 volts. This current alternating is supplied through a step-up transformer with a voltage ratio of 1:100 to the transmission lines. This is fed to a step-down transformer of voltage ratio 1000:1. (a) What is the final voltage? (b) Draw a simple diagram to show these stages. 5. A generator with an E.M.F. of 550 volts supplies energy to a 2200-volt line. has The transformer primary winding. How many turns must there be on its secondary? 1 00 turns on its 6. A transformer, used to operate a toy electric train on a 1 20-volt line, has taps on the secondary to give 1 2, 8, 6 and 4 volts. If the primary winding has 960 turns, how many turns must there be on the secondary at each tap? 330 CHAPTER 29 ELECTRICAL ENERGY V:70 THE PRODUCTION OF ELECTRICITY towards industrial Not many years ago Canada was known throughout the world as an agricultural country. A survey of our exports at the beginning of the twentieth century shows that farm products formed the basis of our economy. In the years preceding the Second World War a gradual swing greater exFrom those pansion could be noted. years to the present time the change has been almost phenomenal. Canada has become an industrial nation. Our exports now, while still including a large part of the world wheat supply and other agricultural commodities, have been expanded to take in countless manufactured farm articles machinery to paper, clothing, electrical devices, chemicals and products of mining and smelting. ranging heavy from Closely connected with these changes has been the marvellous development of Canada’s hydro-electric power
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resources. The energy of falling water has been turn the huge turbines harnessed which rotate the armatures of large genSteam-generating erators plants have become common in which (Fig. 29:1). to the energy from coal is utilized. In recent years heat from an atomic pile is These being used in the same way. latter methods provide the alternating current that is so vital to our modern way of life. Millions of horsepower have been made available to turn the wheels of industry across the country and to provide the luxuries of cheap electricity in our homes. Men of foresight have planned for new and greater feats in engineering as they divert the waterways, tunnel through mountains and improve the generators to provide Canada with power for the future. V:71 THE TRANSMISSION OF ELECTRICITY Every high-school student knows that electrical energy is readily transformed into heat energy. A current of electricity consists of a shunting of free electrons in the circuit. The energy of these electrons is converted into heat as they encounter opposition to their motion in the conIt is apparent that the greater ductor. the number of electrons moving and the greater the resistance of the conductor, the greater will be the amount of heat produced. In 1841, James Joule proved experimentally that the heat produced was proportional to the square of the current, to the resistance, and to the time the current flowed {HaPRt). It is evident then that the amount of current in a conductor is the most important single factor in heat losses dur- 331 Aerial View of Hydro-Electric Plant, Niagara Falls. Ontario Hydro 332 ELECTRICAL ENERGY Sec. V: 72 Ing the transmission of electricity. The power of an electric current is equal to the voltage times the current strength, i.e., P— VI (Sec. V;75). This is constant for a given current. Thus, if the voltage is made higher, the current becomes smaller and heat losses will be reduced. For this reason, alternating current produced at a potential difference of 2,200 to 12,000 volts is stepped up to between 50,000 and 220,000 volts for transmission. the the voltage is stepped down by transformers so that the pole voltage is usually about 2,200 volts. The familiar pole transformers step this down further to 220 and 110 volts for household circuits. This current is used to operate electric ranges, heaters, lights and various other appliances. sub-station At V:72 FUSES ^ In designing any electrical circuit care must be taken to ensure
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A. Edison of Canada Ltd. Fig. 29:3 The Incandescent Lamp. 334 ELECTRICAL ENERGY Sec. V:74 \ \ \'resistance wire. This is an alloy of nickel (80%) and copper (20%) which can be heated to a high temperature without melting, and oxidizing very slowly even when red hot. In addition, it has a resistance of about sixty times that of copper and this reduces the costs. / f { \ /1 1 I- fp [1 ^ lp ^ 1 Canadian General Electric Fig. 29:5 A Modern Tungsten Filament Bulb. bulb (Fig. 29:5) bears little resemblance to the original. Modern filaments are made of tungsten, giving greater service and whiter light. Instead of evacuated glass bulbs, we now have bulbs filled with an inactive mixture of nitrogen and argon which allows the filament to be heated to higher temperatures. Platinum lead-in wires passing through the glass have been replaced by inexpensive nickeliron alloys. Costs have gone down while efficiency has been raised to heights not dreamed of a few decades ago. V:74 OTHER APPLICATIONS OF THE HEATING EFFECT (a) Domestic Heating Appliances Nowadays, in many homes, space heaters, hot-water heaters, ranges, etc., use electricity. The appliances designed for these various purposes contain heating-elements usually made of “nichrome” the heaters, In electric heating-elements are generally supported on fireclay forms, and in some of the smaller heaters concave polished metal reflectors are used to concentrate the radiant heat into a beam. The electric range has nichrome heating-elements embedded in some suitable insulating material for protection. The elements often consist of two parts. When the switch is set at “high” these are in parallel; when at WVVWWVW •-VvWVVVWWS^ fo) (b) (c) Fig. 29:6 Regulation of Heat in an Electric Stove Element. “medium” one element is cut out; when at “low” both elements are in series (Fig. 29:6). The walls and door of the oven are filled with heat-insulating material such as fibre glass to reduce the heat loss, the oven temperature being regulated by some form of thermostat. (Fig. 29:7) With electric irons the heating-elements are
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electric arc is also applied in electric ships’ plates, Boiler welding. plates, etc., can be welded by connecting the plates to the negative of a D.C. supply, applying the positive side to the weldingrod where the weld is to be made. V:75 BUYING ELECTRICAL ENERGY Since electricity is able to do work, it is a form of energy. The faster work is done, the faster energy is utilized. The rate of doing work, i.e., the rate of utilizing energy, is the power. Electrical power is measured in watts. One watt is the power provided when a current of one ampere flows with a potential difference of one volt. Thus: Power (watts) = P.D. (volts) X Current (amps. or P = VI watts P ={IR)ir.'V = IR,Ohm'sLaw) or P = PR watts If the power of one watt is provided for one hour, the consumer must pay for 1 watt-hour of energy. Similarly, if one kilowatt (1000 watts) is used for one hour, the consumer must pay for 1 kilowatt-hour (k.w.h.) of energy. Thus electrical energy (k.w.h.) == power (k.w.) X time (hours). Note: Since 746 watts = 1 horsepower (h.p.) it follows that P or P = = VI 746 PR 746 h.p. h.p. Example Find the cost of operating an electric toaster for two hours if it draws 8 amperes on a 110 volt circuit. The electrical energy costs 4 cents per kilowatt-hour. I =z 8 amp. no volts / P = VI P=\10 X 8 = 880 watts 880= —r—- =.880 k.w. 1000 Electric energy used =.88 X 2 = 1.76 k.w.h, 1 k.w.h. costs 4 cents 1.76 k.w.h. cost 4 X 1.76 = 7.04 cents. The cost is 7 cents. 337 0 5 (i) Chap. 29 V : 76 MAGNETISM AND ELECTRICITY QUESTIONS A 1. (a) Describe what happens to alternating current from the time it produced at the generator until it is used to operate the electric toaster in your home. is Explain why the (b) changed before electricity is transmitted across the country
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. voltage is 2. (a) What is the function of a fuse in an electric circuit? (b) Describe the structure and action of a simple fuse. 3. (a) Make a labelled diagram of an electric-light bulb. (b) Explain the function of each part labelled in (a). 4. List three appliances found in each of the following, in which the heating effect of your employed, current electric (a) is house (b) your school (c) industry. 5. Define: power, watt, kilowatt, kilo- watt-hour. B 1. At the power-house, electricity at a potential of 1 2,000 volts is generated. It is prepared for long-distance transmission by being applied to a step-up transformer in which the number of turns of wire in the primary and secondary are in the ratio of 1 to 2000. (a) What will be the potential difference in the secondary? (b) How will the current strength be altered? (c) How will this affect the loss of energy through heating of the con- ductor during transmission? 2. (a) A pole transformer steps down the line potential from 5500 volts to 1 1 0 volts. What is the ratio of the number of turns of wire in the primary and secondary of the transformer? 338 (b) What change will occur in the current strength (ii) the heating effect? 3. Determine the power of an electric kettle which amperes when plugged into a 110 volt requires a current of 1 electric outlet. 4. What current is drawn by a 550 watt toaster when it is plugged into the 1 1 volt line? 5. What voltage is applied to a 2970 watt heating-element that draws a current of 1 3.5 amperes? 6. What is the power (a) in horsepower (b) in watts (c) in kilowatts, of a motor which draws a current of 3.38 amperes on the 1 1 0 volt line? 7. What is the maximum power that can be used in a circuit containing a 15 ampere fuse in the 1 1 0 volt line? 8. If an electric toaster of 550 watts and an electric kettle of 1000 watts were plugged into the same wall outlet in your home, would the 15 ampere fuse in the circuit melt? Explain your answer. 9. Find the cost of operating an electric toaster for 3 hours if it draws 5 amperes on a
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110 volt circuit. The electricity costs 3.5 cents per kilowatt-hour. 10. What is the cost to a storekeeper of leaving a 40 watt light bulb burning near his safe for 36 hours if electricity costs 3 cents per kilowatt-hour? 11. An electric stove element draws 5 amperes on the 220 volt circuit. is turned on for 4 hours, what is the cost of operating the stove at 2.2 cents per kilo- If it watt-hour? 12. A Vi h.p. electric motor In an oil furnace comes on for a period of 5 minutes 48 times each day on the average. What is the cost of the electricity for 30 days If it costs 2.5 cents per kilowatt-hour? 13 A boy on returning home from school. ELECTRICAL ENERGY Sec. V:76 at 4:30 p.m. turns on lights in the kitchen, basement, hall and bedroom. The kitchen has one 100 watt bulb, the basement two 60 watt bulbs, the hall one 40 watt bulb and the bedroom one 40 watt bulb and one 60 watt bulb. If these lights are left burning 1 0:00 p.m. what is the cost when electricity costs 3 cents per kilowatt-hour? until 339 CHAPTER 30 ELECTRONICS points of which are to the secondary of an induction coil, the discharge close together. The induction coil is turned on while the tube is being evacuated (Chap. 31, Exp, 42). At first a discharge discharge occurs between the V : 77 INTRODUCTION (a) When a lighted match, or the flame of a gas burner, is brought near the knob of a positively charged electroscope, the leaves will be seen to fall (Chap. 31, Exp. 41). The flame liberates electrons from the atoms of the gases causing positively charged ions. The electrons so set free are attracted to the electroscope and neutralize its charge. Had the charge on the instrument been negative, charge would have been lost positively charged gaseous ions. become them the to to its (b) An electrical discharge tube with an open side-arm (Fig. 30:1) is attached Fig. 30:1 Apparatus to Show Dis- charge of Electricity at Low Pressures. 340 points but, as the tube is partially evacuthe current ceases between the ated, points and begins between the tertube even though minals the they are farther apart. This discharge, though varying
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in appearance with the even degree of though the tube is evacuated as com- evacuation, continues inside pletely as possible. The current flows through the gas at reduced pressure because the gas becomes ionized. The electrons expelled from the atoms of gas are attracted to the anode where they re-enter the metallic part of the circuit. The positively charged gaseous ions are attracted to the cathode where they remove electrons from it. Where the electricity is passing through a vacuum, electrons are expelled from the cathode and proceed in straight lines directly to the anode. The behaviour of electricity as it passes through a gas or a vacuum is known as electronics. The vacuum-tube and the low pressure gas-filled tube have opened the way for a seemingly endless number of new The most applications familiar application is the radio, but this is only one of the many being used in our modern way of living. Electronics electricity. of ELECTRONICS Sec. V:78 ticularly near the anode end, shine with a bright green fluorescent light. The phenomena occurring at this stage of the discharge proved to be most important. If an object is placed some distance in front of the cathode a sharp shadow of the object is cast on the end of the tube Experiments suggest near the anode. is used to guide our ships and aircraft, to perforin calculations in minutes that would take a mathematician days, to time delicate processes, to operate radio, television, sound movies, and to perform amazing feats of mechanical control. Its future is likely to be as remarkable as its past—a challenging field for keen young scientists. V:78 CATHODE RAYS The main characteristics of the discharge of electricity through gases may be shown by the same method as was used in section V: 77(b). At normal pressures the insulating properties of the is of the the gas are too great for a spark to occur, but as the pressure is reduced a current passed and irregular streamers of light traverse the tube between the electrodes each of which is surrounded by a luminous glow. With continued lowdischarge ering pressure broadens out to a steady luminous column which extends from the anode to This is almost as far as the cathode. known as the positive column and is separated from the cathode glow by a the Faraday dark dark space called space. The pressure in the gas at this stage is about 5 mm. of mercury. Further reduction in the pressure of the gas to about positive column to shrink and to break up into alternate
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light and dark patches known as striations. The Faraday dark space increases in size at this low pressure, and the cathode glow moves away from the cathode, leaving a dark space between it and the cathode called the Crookes’ dark space (Fig. 30:2). When the pressure is about 0.1 mm. the positive column disappears altogether and negative glow becomes considerthe ably extended. At still lower pressures the 1 mm. causes negative disappears, glow the the Crookes’ dark space filling tube, and the glass walls of the tube, par- the Gases, (a) At Reduced Pressures— P, Positive Column; F, Faraday Dark Space; N, Cathode Glow; C, Crookes' Dark Space, (b) At Very Low Pressures— Cathode Rays in Crookes' Tube. that the fluorescence of the glass is due to something being emitted from the in Germany, cathode, and Goldstein, gave the name of cathode rays to this emanation. Crookes, in England, showed that the rays were shot out at right angles from the cathode, caused a rise of temperature of bodies interposed in their path, and that the rays were also capable of producing a mechanical force. 341. Chap. 30 MAGNETISM AND ELECTRICITY He hazarded a guess that the rays consisted of a stream of particles, and it was later shown that the rays could be deflected by a magnetic field and that they carried a negative charge (Chap. 31, Exp. 43) It was Sir J. J. Thomson, in England, of however, who showed in a series masterly experiments, carried out at the close of the nineteenth century, that the rays consisted of tiny, identical, negatively electrified particles which we now call electrons. In one of his experiments, in which the rays were deflected by both magnetic and electric fields, he was able to calculate their velocity and also the ratio of the charge to the mass of the the particles. The magnitude of electronic charge had been found by other means, and so he was able to obtain a value for the mass of the electron which he showed to be about 1/1840 part of the mass of the hydrogen atom. Various forms of the cathode ray tube are in use. One of these, the X-ray tube will be described in the next section. The other, the television picture-tube will be referred to in the next chapter. V : 79 X-
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RAYS While experimenting with discharge tubes at very low pressure, Roentgen, in Germany, in a fluorescent screen some distance away glowed brightly even when the discharge tube was covered with black paper, and 1895, noted that A Chest X-Ray Machine. General Electric 342 ELECTRONICS Sec. V:79 objects placed between the tube and screen produced shadows. Roentgen attributed these effects to a new form of tube, radiation its unknown which, of nature, was called X-rays. emanating account from the on cyanide, which is the substance frequently used on X-ray viewing-screens. They affect photographic plates, produce chemical changes, and are capable of discharging electrified bodies since they air. the are X-rays ionize electromagnetic waves of the same nature as visible or ultra-violet rays but having a very much shorter wave-length. One of the most characteristic properties of the rays is their penetrating power. They through many substances can opaque to ordinary light waves, the degree of penetration varying inversely as the density of the substance. Roentgen himself noticed this power of penetration and observed that human flesh was more transparent to the rays than bone. The pass It X-rays, is now known that or Roentgen rays, are produced whenever cathode rays impinge on a solid object Fig. 30:3 shows (Chap. 31, Exp. 44). a modern form of X-ray tube in which the electrons, constituting the cathode stream, originate from a heated filament (Sec. V:80) and are focused by a concave cathode on to a tungsten or molybdenum target set in a block of copper called the anti-cathode. The face of the anti-cathode is set at an angle of 45° to the axis of the tube, the X-rays coming off in the direction indicated. Most of the energy of the cathode stream is dissipated as heat, only about 1 per cent of the energy is converted into X-rays. X-Ray Photograph of a Human Hand. St. Michael’s Hospital X-rays travel in straight lines, are not deflected by either magnetic or electric fields and produce fluorescence in suitable materials such as platinum-barium medical uses of X-rays in locating fractures and foreign bodies in the human system are their most important application. Another use of X-rays is the detec- 343 Chap. 30 MAGNETISM AND ELECTRICITY tion of flaws in
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metal castings, and research scientists find them a useful aid in many of their investigations. V : 80 THE ELECTRONIC TUBE Just as a tap acts as a valve to control the flow of water through the pipes of a house, so the electronic tube acts as a valve to control the flow of electrons in an electric circuit (Chap. 31, Exp. Electronic tubes are able to 45, 46). increase or decrease the amount of current flow, or even to start or stop it, and the action is immediate. They use praccircuit. No tically no power in attempt will be made in this text to do any more than to outline the structure and action of the simplest forms of these the tubes. (a) Structure Any electronic tube has four basic parts (Fig. 30:4) : 1. A glass metal envelope enclosing the tube serves to sustain the vacuum or low pressure within the tube; 2. A cathode, which is heated either directly by an electric current or indirectly by a heater element which carries the current, serves to give ofT the electrons when heated; 3. An anode serves to receive the electrons from the cathode; 4. A base which has four or eight terminals or prongs for connecting the tube into its circuit. Such a tube is called a diode. A triode has an additional part called the grid (part d). (b) Action When the tube is connected into a circuit, the circuit is actually broken. When the cathode is heated electrons Hot Filament Fig. 30:5 Action of a Diode Tube. If a positive charge is leave its surface. applied to the anode, these electrons will be attracted across the intervening space and thus a flow of electrons will occur (Fig. 30:5). The anode is frequently referred to as the ‘‘plate” and the current 344 ELECTRONICS Sec. V:80 passing through the plate is called the “plate current”. (c) Rectification If the anode is connected to an alternating-current circuit (Fig. 30:6) it will be alternately positiv^ely and negatively While the anode is positive charged. electrons will flow from the cathode to the anode and through the plate circuit. During those half cycles when the anode the negative, electrons about the is Fig. 30:6 Electrons are Attracted Only When Plate Rectifier Circuit. is Positive. (a) (b) 345 Chap.
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30 MAGNETISM AND ELECTRICITY flow. current Thus the cathode will be repelled and no current passes will through the tube in one direction only, from cathode to anode, and a direct current flows in the plate circuit. The tube has been used to obtain direct current from an alternating-current supply. Such a process is called rectification. The tube used for the purpose is designated a rectifier tube. Half-wave rectification by a diode is illustrated in Fig. 30:7. (d) Amplification A triode tube has a fifth part called the grid (Fig. 30:8) which controls the current passing through the tube. The grid is a special wire or mesh between the cathode and the anode. Any change in grid voltage causes a proportional change in the amount of current flowing through the tube. Thus, if we have a very weak alternating current, too weak to be used directly, it would be applied to the grid as a signal voltage. As the grid voltage varies it causes an identical amplified variation in the plate current. The grid is given a negative charge just large enough that it never actually becomes positive when the signal is applied. Thus, no electrons are attracted to the grid itself so the tube operates at maximum efficiency. Fig. 30:9 illustrates this amplification action as a radio signal is impressed on the grid of the triode. 346 Triode Tube as an Amplifier. ELECTRONICS Sec. V:82 Sound Waves Microphone Aerial - Aerial Vacuum Tube Amplifiers Vacuum'\ Tube Generator of High Frequency Waves'Modulated Carrier Waves Tuner / Detector / 00000(5 ^ // 7 ‘ ( Tube Used as an Amplifier Vacuum Tube Used as a Modulator (a) TRANSMISSION Audio-frequency Amplifier Radio Frequency Amplifier (b) RECEPTION Fig. 30:10 Radio Transmission and Reception. V:81 RADIO TRANSMISSION AND RECEPTION The triode tube can be used as an oscillator which is merely a type of alternating-current generator capable of providing frequencies in the order of 150,000 to 1,500,000 cycles or more per As these variations in current second. are impressed on a transmitting antenna, waves, electromagnetic carrier called In the waves, are sent out into space. transmitting-studio a microphone transforms sound waves into a fluctuating This current is used to direct current. modulate, or cause variations in, the amplitude of the carrier waves from the oscillator. differences
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set free by light. Students Positively Charged Anode Serves as Plate to Attract Emitted Electrons Light-Sensitive Cathode Gives Off Electrons when Light Hits It Electrons Escape from Surface Electric Bell Fig, 30:12 Photoelectric Cell. burglar alarms, control dangerous machinery, reproduce sound for movies, count articles on production lines, and for hundreds of other purposes contributing to the efficiency and safety of industrial processes (Fig. 30:13). The photronic cell is used almost universally to-day for all the above pro- 348 Fig. 30:13 Photoelectric to Operate an Electric Bell. Cell Used ELECTRONICS Sec. V:83 The Component Parts of a Light-Meter. Canadian General Electric interested in photography will recognize the light-meter as a simple application of the photronic cell. In these devices light energy is con- verted into electrical energy. This, in part, provides the answer to the scientists’ dream of eventually tapping the huge resources of the sun itself. V : 83 QUESTIONS 1. (a) What is the meaning of the term electronics? (b) State two facts fundamental to electronics. thermionic (c) Distinguish emission and photo-emission of elec- between trons. 2. (a) What are cathode rays? (d) Describe cathode rays. (c) How are cathode rays produced? properties some of 3. (a) Explain how X-rays are produced. (b) Describe some properties of Xrays. (c) How are these properties associated with the uses of X-rays? 4. (a) Make a labelled diagram of a simple diode tube. State the purpose of each part. (b) How does a triode tube differ from a diode tube? (a) What is meant by rectification? (b) Explain why a diode tube may be used as a rectifier. (a) What is meant by amplification? (b) Explain why a triode tube may be used as an amplifier. 5. 6. 7. Describe briefly how radio signals are sent from one station to another. 8. (a) Describe the structure of a simple photoelectric cell (ii) (i) a photronic cell. (b) What transformation of energy takes place in each of the above cells? 349 CHAPTER 31 EXPERIMENTS ON MAGNETISM AND ELECTRICITY EXPERIMENT 1 To distinguish between magnetic and
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non-magnetic substances. (Ref. Sec. V:2) Apparatus Bar magnet, assortment of small articles made from iron, paper, glass, wood, copper, nickel, rubber, tin, silver, plastic, etc. Method Approach each of the articles in turn with the bar magnet. Tabulate your results. Observations State which objects were attracted and which were not attracted by the magnet. Conclusions 1. State which were magnetic substances and which were non-magnetic substances. 2. Define (a) magnetic substances, (b) non-magnetic substances. EXPERIMENT 2 To determine if the magnetic strength is equal in all parts of the bar magnet. (Ref. Sec. V;2) Apparatus Bar magnet, iron-filings. Method Sprinkle iron-filings liberally on a piece of paper. Roll the bar magnet in the filings and observe. Clean the magnet carefully with a cloth at the end of the experiment and return the filings to their container. Observations 1. Are the filings attracted equally to all parts of the bar magnet? 2. Where do most of the filings collect? 350 EXPERIMENTS ON MAGNETISM AND ELECTRICITY 3. Where does the least number of filings collect? 4. Compare the number of filings collected at each end of the magnet. Conclusions 1. Where is the magnetic strength of a bar magnet concentrated? What are the these areas called? 2. Where is the least magnetic strength in a bar magnet? EXPERIMENT 3 To determine the difference between the poles of a bar magnet, (Ref. Sec. V:2) Apparatus Bar magnet, chalk, non-magnetic stand, brass support. Method 1. Mark one end of the magnet with chalk so that it may be readily identified. 2. Suspend the magnet horizontally from the non-magnetic stand by means of thread supporting a brass support. 3. Allow the magnet to swing freely until it comes to rest. Note the direction in which the marked end is pointing. 4. Reverse the ends of the magnet in the support and again let it swing freely until it comes to rest. Observations 1. In what direction did the marked end of the magnet point when it came to rest the first time? 2. In what direction did the marked end of the magnet point when it came to rest the second time? Conclusions 1. Along what directional line will a freely suspended bar magnet always come to rest? 351 Chap. 31
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2. MAGNETISM AND ELECTRICITY Why do we call one end of a bar magnet the north-seeking pole and the other end the south-seeking pole? What abbreviations for these are in common use? EXPERIMENT 4 To investigate the effect of bringing like and unlike magnetic poles together, (Ref. Sec. V:2) Apparatus Two bar magnets of known polarity, non-magnetic stand, brass support (Fig. 22:3). Method 1. Suspend one bar magnet from the non-magnetic stand by a thread and a brass support so that it is free to move in a horizontal plane, or balance a bar magnet horizontally on the convex surface of a watch glass. 2. Approach the N-pole of the suspended magnet with the N-pole of the other magnet. 3. Repeat, using the N-pole of the suspended magnet with the S-pole of of the other. 4. Now approach the S-pole of the suspended magnet with each end of the other. 5. Tabulate your results. Observations Pole of Suspended Magnet Pole of Other Magnet Observation N-pole N-pole S-pole S-pole N-pole S-pole N-pole S-pole Conclusions 1. What is the effect of like magnetic poles on each other? 2. How do unlike magnetic poles affect each other? 3. The above two conclusions combined into one statement give us the Law of Magnetism. State this important law. Questions 1. If the polarity of the magnets to be used is not known, how could you determine this information? 2. Why is it correct to say that the north magnetic pole of the earth contains S-pole magnetism? 352 EXPERIMENTS ON MAGNETISM AND ELECTRICITY EXPERIMENT 5 To study the magnetic field about a bar magnet, (Ref. Sec. V:4) Apparatus Two bar magnets, iron-filings, sheet of paper. Method 1. Lay a bar magnet on the desk and place a book of approximately the same thickness as the magnet on either side of it. Place the paper over the magnet so that the edges of the paper are supported by the books (a magnet board may be used to support the magnet Sprinkle iron-filings evenly over and paper instead of the books). the surface of the paper while tapping it gently. 2. Repeat the above procedure using the two bar magnets placed with their N-poles about 2 in. apart. 3
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. Repeat part 2 with an N-pole and an S-pole about 2 in. apart. Observations 1. Where are the lines of filings most concentrated? 2. Where are the lines least concentrated? 3. What is the general shape of the lines formed? 4. Do any of the lines appear to cross each other? 5. Where do the lines seem to begin and end in the case of the single bar magnet? 6. Describe the lines formed when like poles are adjacent to each other. 7. Describe the lines formed when unlike poles are adjacent to each other. Conclusions 1. What causes the iron-filings to take up their positions about the bar magnet? 2. What evidence have you found that supports the theory that lines of force repel each other? 3. What property of lines of force explains the attraction of unlike poles? 4. What evidence have you found that would indicate that the field is strongest close to the poles of the magnet? Questions 1. Make a diagram of each of the patterns formed by: (a) A single bar magnet. (b) Two bar magnets with like poles adjacent. (c) Two bar magnets with unlike poles adjacent. 2. Indicate by means of a diagram what you would expect the field of force about a horseshoe magnet to look like. Verify your prediction by experiment. 353 Chap. 31 MAGNETISM AND ELECTRICITY EXPERIMENT 6 To study induced magnetism. (Ref. Sec. V:6) Apparatus Bar magnet, long iron nail, iron tacks, magnetic compass. Fig. 31:2 Method 1, Place one end of the nail near the tacks to test for magnetic power. Now hold one end of the bar magnet near one end of the nail. Allow the other end of the nail to come in contact with the tacks. Lift the magnet and the nail together. Then move the magnet away from the nail. 2. With the head of the nail near the N-pole of the magnet, approach the N-pole of the compass-needle with the point of the nail. Repeat this with the head of the nail near the S-pole of the magnet. Observations 1. Did the nail by itself possess magnetic powers? 2. When a pole of the magnet was close to the nail, what was the effect on the tacks? 3. What occurred when the magnet was moved away from the nail? 4. With the N-pole of the magnet near the head of
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the nail, how was the N-pole of the compass-needle affected? 5. What occurred when the S-pole of the magnet was used? Conclusions 1. Why did the nail act as a magnet when under the influence of the bar magnet? 2. Why did it not remain a magnet when the bar magnet was removed? 3. What polarity was induced in the end of the nail (a) nearer to (b) farther from the pole of the magnet used? Questions 1. Why do we specify an iron nail and iron tacks? 2. Why should a long nail be used? 354 EXPERIMENTS ON MAGNETISM AND ELECTRICITY EXPERIMENT 7 To determine if the earth's magnetic field can cause induced magnetism. (Ref. Sec. V:6) Apparatus Dipping-needle (Fig. 22:8), soft iron bar about 12 in. long, hammer, magnetic compass. Method 1. Determine the direction of the magnetic lines of force at your location on the earth’s surface by means of the dipping-needle (Sec. V:5). 2. Test the iron bar for magnetic polarity by approaching the N-pole of the compass-needle with each end in turn. 3. Holding the iron bar at the angle indicated by the dipping-needle, strike the bar sharply several times with the hammer. 4. Again test the bar for magnetic polarity using the magnetic compass. Observations 1. What is the magnitude of the angle of inclination? 2. What was the result of testing each end of the iron bar with the compass in the first case? 3. After striking the bar what changes were observed when it was tested with the compass? Conclusions 1. Why was the test different in the second case than in the first? 2. From your observations what magnetic polarity must the north mag- netic pole of the earth possess? EXPERIMENT 8 To study magnetic shielding. (Ref. Sec. V:6) Apparatus Demonstration magnetic compass-needle, bar magnet, thin sheets of iron, glass, copper, wood. Method 1. Hold the bar magnet to one side of the compass-needle so that it will be attracted towards the magnet. 2. Move the point of the compass-needle to one side and release it, allowing it to begin swinging from side to side. Note the number of vibrations in 10 seconds and the time in which it finally comes to
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rest. 3. Insert the iron sheet between the compass-needle and the magnet and repeat the above procedures, taking care that all other conditions remain the same as before. 4. Repeat part 3 using other materials. 355 Chap. 31 MAGNETISM AND ELECTRICITY Observations 1. Number of v.p.s. without shield 2. Time to come to full stop without shield 3. Number of v.p.s. with iron shield 4. Time to come to full stop with iron shield. 5. Repeat the above observations when other materials are inserted. Conclusions What is magnetic shielding? Questions 1. Why is it necessary to shield a magnetic compass on board a ship? 2. How can such magnetic shielding be accomplished? EXPERIMENT 9 To magnetize a steel wire, (Ref. Sec. V:7) Apparatus Bar magnet, steel wire or thin knitting-needle, magnetic compass. Method 1. Test the ends of the wire for magnetism, using the compass. 2. Using the N-pole of the bar magnet stroke the wire from one end to the other several times, always in the same direction. Lift the magnet well away from the wire as you return it to the starting point for each stroke. Test each end of the wire for polarity, using the compass. 3. Repeat the above steps using the S-pole of the bar magnet to stroke the wire. Observations State observations for each part described above. 356 EXPERIMENTS ON MAGNETISM AND ELECTRICITY Conclusions 1. What indication did you have that the wire was not magnetized before it was stroked with the magnet? 2. What makes you believe that the wire has become a bar magnet during the experiment? 3. What \vas the polarity of the end of the wire last touched by the pole of the bar magnet? Questions 1. Why were both ends of the wire tested for magnetic polarity? 2. Why was the bar magnet lifted away from the wire as it was returned at the end of each stroke? 3. Explain why the wire became magnetized. EXPERIMENT 10 To investigate some experimental evidence for the theory of magnetism. (Ref. Sec. V:7) Apparatus A magnetized steel wire, wire cutters, magnetic compass. Method 1. Determine the polarity of the steel wire. 2. Cut the wire in half and determine the polarity of each half. 3. Cut the pieces into smaller and smaller sections and determine
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the polarity of each piece. Observations By means of diagrams illustrate your observations. Conclusion From these observations what inference can be made about the basic structure of a magnet? EXPERIMENT 11 To investigate further experimental evidence for the theory of magnetism. (Ref. Sec. V:7) Apparatus Test-tube, iron-filings, stopper, bar magnet, magnetic compass. Method 1. Fill the test-tube with iron-filings and insert the stopper. Test the tube for magnetic polarity. 2. Stroke the test-tube from end to end several times using one pole of the bar magnet as was done in experiment 9. Test the tube again for magnetic polarity. 3. Shake the test-tube vigorously and again test it for polarity. 357 Chap. 31 MAGNETISM AND ELECTRICITY Observations 1. Did the tube of filings act as a magnet when first tested? 2. Was any movement of the filings noticed as the test-tube was stroked? 3. What was observed in the second polarity test? 4. How did the shaking afifect the magnetic properties? Conclusion How does this experiment support the theory of magnetism? Question Would there be any limit to the magnetic strength produced? Explain. EXPERIMENT 12 To produce static electricity by friction, (Ref. Sec. V:10) Apparatus Hard rubber or ebonite rod, glass rod, cat’s fur, piece of silk, small scraps of papers, sawdust and other light objects. Method 1. Rub the ebonite rod with the cat’s fur. Bring the rod near the objects. 2. Repeat the above procedure, rubbing the glass rod with the silk. Note Lucite rubbed with a sheet of rubber or polystyrene (the material used in transparent vegetable containers) is an excellent substitute for glass and silk. Observations Describe the behaviour of the objects as the rod approaches in each case. Conclusion What is the efTect of rubbing an ebonite rod with cat’s fur or a glass rod with silk? 358 EXPERIMENTS ON MAGNETISM AND ELECTRICITY Questions 1. What transformation of energy is involved in producing static elec- tricity by friction? 2. What evidence is there that this new power of attraction is not magnetism? 3. Describe instances encountered static electricity. in your everyday life in which you have EXPERIMENT 13 To show that there are different kinds of
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electrical charges and to establish the law of electrical charges. (Ref. Sec. V:11) Apparatus Two ebonite rods, two glass cat’s fur, silk. rods, insulated stand, thread, stirrup, Method 1. Charge an ebonite rod by rubbing it with cat’s fur. Suspend the charged rod in the stirrup attached by a thread to the insulated stand. Approach the charged end of the ebonite rod with the other charged ebonite rod. 2. Approach the charged end of the ebonite rod with the charged end of a glass rod which has been rubbed with silk. 3. Replace the suspended ebonite rod by a charged glass rod and repeat the procedures outlined above. Observations What is observed in each of the above parts? Conclusions 1. Are there different kinds of electrical charges? Name each kind. 2. What simple law of electrical charges may now be stated? Question What standard method have we of producing each kind of charge in the laboratory? EXPERIMENT 14 To study the use of the pith-ball electroscope. (Ref. Sec. V : 13) Apparatus Pith-ball electroscope, ebonite rod, cat’s fur, glass rod, silk. Method 1. Charge the ebonite rod and slowly bring it near the pith balls. Allow them to touch the rod, and observe. 359 Chap. 31 MAGNETISM AND ELECTRICITY 2. After charging the pith balls approach them slowly with (a) a negatively charged rod and (b) a positively charged rod. 3. Repeat the above procedures using a charged glass rod in part 1. Observations Describe what is observed in each part above. Conclusions 1. Explain each observation. 2. How may the pith-ball electroscope be used to (a) detect (b) identify an electrical charge? Questions 1. What is meant by “charging by contact”? Explain this process. 2. Why is repulsion the only sure evidence that an object is charged? EXPERIMENT 15 To study the use of the gold-leaf electroscope. (Ref. Sec. V:13) Apparatus Gold-leaf electroscope, ebonite rod, cat’s fur, glass rod, silk, stick of sealing-wax, piece of flannel. Method 1. Charge the ebonite rod and slowly bring it near the knob of the electroscope until
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it touches. Withdraw the charged ebonite rod. 2. Approach but do not touch the knob of the charged electroscope with (a) another negatively charged rod (b) a positively charged glass rod (c) a stick of sealing-wax that has been rubbed with flannel. 3. Remove the charge from the electroscope by touching the knob with a finger. 4. Repeat parts 1 and 2, touching the knob with a charged glass rod. Observations Describe the observations for each part above. Conclusions 1. What charge is given to the electroscope when it is “charged by contact” with (a) a positively (b) a negatively charged rod? 2. What procedure can be used to identify an unknown charge on an object? EXPERIMENT 16 To distinguish between conductors and insulators. (Ref. Sec. V:14) 360 7 EXPERIMENTS ON MAGNETISM AND ELECTRICITY Apparatus Gold-leaf electroscope, ebonite rod, cat’s room. fur, various objects in the Method 1. Place a negative charge on the electroscope by touching the knob with the charged ebonite rod. 2. Approach the knob of the charged electroscope with the charged ebonite rod. Note what happens. 3. Rub the charged rod on a water pipe and again approach the knob. Note what happens. 4. Recharge the rod after each test and repeat step 3, rubbing the charged rod on rubber, slate, wood, your hand, and dipping it in water. Observations Note the effect on the electroscope leaves in each case. Conclusions 1. List those materials which allowed the charge to escape. What are such materials called? 2. List those that did not permit the charge to escape. What are they called? EXPERIMENT 1 To produce induced electrical charges. (Ref. Sec. V:15) Apparatus Two metal spheres on insulated stands, proof-plane, ebonite rod, cat’s fur, glass rod, silk, gold-leaf electroscope. Method 1. Charge the electroscope negatively by the contact method. 2. Test each sphere for the presence of a charge by bringing it near the knob of the charged electroscope. 3. Place the two spheres A and B in contact as shown in Fig. 23:6. Approach, but do not touch, sphere A with the end of a negatively charged ebonite rod. Remove B from A and then remove
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the rod. Again test each sphere as in part 2. 4. Touch the spheres to each other and test each for a charge as in part 2. 5. Repeat parts 3 and 4, using a positively charged rod. Observations State what was observed. Explanation Explain your observations in this experiment with reference to the electron theory. 361 9 Chap. 31 MAGNETISM AND ELECTRICITY Conclusions 1. In each case what charge was induced on (a) the sphere near the inducing charge (sphere.4), (b) the sphere distant from the inducing charge (sphere B)? 2. What do you conclude about the magnitude of these two induced charges? EXPERIMENT 18 To charge a gold-leaf electroscope by induction. (Ref. Sec. V:16) Apparatus Gold-leaf electroscope, ebonite rod, cat’s fur, glass rod, silk. Method 1. Charge an ebonite rod negatively and approach, but do not touch, the knob of the electroscope with it. Keeping the charged rod near, touch the knob with a finger, i.e., “ground” the knob. Remove your finger. Then remove the charged rod. 2. Repeat the procedure, using a positively charged glass rod. Observations Record the behaviour of the leaves in each step of the method by means of a series of diagrams. Explanation Using the electron theory, indicate on the diagrams what occurred in each of the above steps. Conclusions What charge can be induced on the electroscope with (a) a negatively charged rod, (b) a positively charged rod? Question Describe how to charge an electrophorus (Sec. V:17) by induction. Explain fully. EXPERIMENT 1 To investigate the distribution of electric charges on a charged object. (Sec. V;19) Apparatus Electrophorus, proof-plane, gold-leaf electroscope, Biot’s spheres (Fig. 23:11), several insulated conductors of different shapes. Method A. Using Biot's Spheres 1. Charge the disc of the electrophorus and transfer the charge by con- tact to the insulated metal sphere. 362 EXPERIMENTS ON MAGNETISM AND ELECTRICITY 2. Test the insulated metal hemispheres for electric charge by means of a proof-plane and the electroscope. 3. Then place the hemispheres tightly around the charged sphere. Remove the hemispheres and using the proof
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-plane and electroscope again test them for charge. 4. Test the sphere for the presence of electric charge. Observations Describe clearly what occurred in the above steps. Explain fully. Conclusion Where do electric charges reside on a charged object? B. Using Conductors of Different Shapes 1. Using the disc of the electrophorus, charge the conductors by contact. 2. Touch various parts of each conductor with the proof-plane and bring it to a given distance from a charged electroscope each time. Note the amount of deflection in each case and compare the density of charge on the various parts of each conductor. (a) Fig. 31:5 Observations Describe clearly what occurred. Conclusion Where is there the greatest density of charge on the surface of an irregularly-shaped object? EXPERIMENT 20 To study the action of pointed conductors. (Ref. Sec. V:20) Apparatus Wimshurst machine, metal point, candle. Method Attach the metal point to one terminal of the Wimshurst machine. Turn the handle rapidly to generate an electric charge. Hold the lighted candle in front of the point when it has become highly charged. Observation What happens to the flame of the candle? 363 Chap. 31 MAGNETISM AND ELECTRICITY Explanation What causes this phenomenon? Conclusion What effect do points have on the charge residing on a conductor? Application Explain the action of lightning-rods. EXPERIMENT 2 1 To study the action of a voltaic cell. (Ref. Sec. V:23, 24) Apparatus Glass vessel, strips of copper and zinc metal, flash-light bulb, socket, water, potassium dichromate. Method 1. Slowly add 10 ml. of concentrated sulphuric acid to 200 ml. of water, with continuous stirring. Pour this solution (the electrolyte) into the glass vessel. Immerse the two plates in the electrolyte parallel to each other but not touching. Note what occurs. 2. Connect the plates by wires joined to the flash-light bulb. Note what happens at both plates and to the flash-light bulb. 3. Add 14 teaspoonful of potassium dichromate and stir until it dis- solves. Observations 1. Is there any change noted at both plates in part 1? 2. (a) What changes occur at the plates in part 2? (b) What happens to the light bulb in part 2? 3. What is the result of adding potassium
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dichromate? 4. What eventually happens to the electrodes as the action continues? Explanations 1. What is the cause of the changes at the zinc plate? 2. What must have been produced to make the light bulb glow? 3. Account for the change in the glowing that is observed. 4. What is the action of the potassium dichromate? 5. Why will the action of the cell eventually stop? Conclusions 1. How is electric current produced in a voltaic cell? 2. What is (a) local action (b) polarization? How may they be prevented? Questions Why is the voltaic cell as used in this experiment not in common use to-day? 364 EXPERIMENTS ON MAGNETISM AND ELECTRICITY 2. What name, describing its action, can be given to the potassium dichromate in this experiment? EXPERIMENT 22 To determine the relationship between potential difference fV)^ current strength (I), and resistance (R) of an electric circuit (Ohm's Law). (Ref. Sec. V;33) Apparatus Resistance coil*, a number of dry cells, galvanometer (Sec. V:53), knife switch. VVWWW^ ( G I I.X I Method Fig. 31:6 1. Connect one dry cell, the resistance coil, the galvanometer, and the knife switch in a series circuit as indicated in the diagram. Close the switch and note the deflection of the galvanometer. Open the switch. 2. Repeat the above procedure using two, three, four and then five dry cells in series. Record all your results in the table below. Observations Number of Cells Galvanometer Deflection (Divisions) No. OF Cells Galvanometer Deflection 1 2 3 4 5 Explanations 1. What is represented by number of cells? How could it be repre- sented using an electrical symbol? 2. What is represented by the galvanometer deflection? How could it be represented using an electrical symbol? 3. How do the ratios obtained by dividing the number of cells by the *The instructor should ensure that the resistance used is of a correct value to give good results with the galvanometer and potentials employed. 365 Chap. 31 MAGNETISM AND ELECTRICITY galvanometer deflections compare? sistance R. This ratio is called the re- Conclusions 1. State Ohm’s Law. 2. Express it, using symbols. Questions 1.
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Name the units used for measuring potential difference, current strength, and resistance. 2. Define “ohm”. EXPERIMENT 23 To determine the resistance of an unknown resistance by the voltmeter-ammeter method. (Ref. Sec. V:38) Apparatus Voltmeter, ammeter, rheostat, dry cells, unknown resistance. Method 1. Connect the voltmeter, ammeter, rheostat and dry cells in a circuit with the unknown resistance X as shown in the diagram. 2. Adjust the rheostat, R, until any suitable value of current flows. 3. Determine the value of the current in amperes by reading the Determine the potential difference in volts by reading ammeter. the voltmeter. 4. Calculate the value of X in ohms. Observations V=^ I = Calculations Determine the value of X using Ohm’s Law. Conclusion What is the value of the unknown resistance? 366 EXPERIMENTS ON MAGNETISM AND ELECTRICITY Questions 1. Why may the setting of the rheostat be varied without affecting the final result? 2. Why is the rheostat used in this circuit? 3. Why is the voltmeter connected in resistance? parallel with the unknown EXPERIMENT 24 To determine the resistance of an unknown resistance by the substitution method. (Ref. Sec. V:38) Apparatus Ammeter, resistance box, rheostat, dry cell, unknown resistance. Method 1. Connect the ammeter, rheostat and dry cell in series with the unknown resistance. Adjust the rheostat to get a large deflection of the ammeter. 2. Remove the unknown resistance and substitute the resistance box with all plugs removed. Replace the plugs singly and in groups of two or more until you get the same ammeter reading as before. Observation What is the total resistance indicated by the unreplaced plugs? Conclusion What is the value of the unknown resistance? EXPERIMENT 25 To study electrolysis of water. (Ref. Sec. V.41) Apparatus Simple electrolytic cell, pair of platinum electrodes, electrolyte consisting of one part sulphuric acid slowly added to approximately ten parts of water with constant stirring, source of direct current, test-tubes, wood splints. Fig. 31:8 367. Chap. 31 MAGNETISM AND ELECTRICITY Method 1. Fill the electrolytic
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cell almost to the top with electrolyte. Completely fill the two test-tubes with the same liquid and invert one over each electrode as shown in the diagram. Connect the electrodes to the current source noting which is the cathode ( — ), and which is the anode ( + ) 2. Allow the current to flow until one test-tube is almost filled with gas. Measure and compare the lengths of the gas column in the two test-tubes. 3. Remove the test-tubes and immediately lower a burning splint into each in turn. Note the results. Observations 1. What is observed at the anode and at the cathode as the current flows? 2. Compare the volume of gas produced at the anode with that at the cathode in a given time. 3. What was observed when the splint was lowered into each of the gases in part 3? Conclusions 1. What gas is produced at (a) the anode (b) the cathode? 2. What are the relative volumes of the two gases? 3. Define cathode, anode, electrolyte, electrolysis. Explanation Write a brief explanation of electrolysis of water (Sec. V:41). Include in your explanation a description of what the current consists of in (a) the external circuit (b) the electrolyte. Questions 1. Why was sulphuric acid added to the water in making the electrolyte? 2. Why were the electrodes made of platinum? 3. What would be the eflfect on the quantities of gases produced of (a) increasing the current strength, (b) increasing the length of time that it flows. The Hoffman water voltameter (Fig. 26:2) may be used in place of the above apparatus. EXPERIMENT 26 To study electrolysis of copper sulphate solution. (Ref. Sec. V:42) Apparatus Electrolytic cell, pair of carbon electrodes, solution of copper sulphate, source of direct current, test-tube, wood splints. 368 EXPERIMENTS ON MAGNETISM AND ELECTRICITY Method 1. Fill the electrolytic cell almost to the top with the copper sulphate solution. Connect the electrodes to the current source. Allow the current to flow for several minutes. Note what occurs at both electrodes. Note any change in the colour of the electrolyte. 2. Collect a test-tube full of the gas liberated at the anode as described in the previous experiment
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and test it with a glowing splint. examine the deposit at the cathode. Also, Observations Describe all observations. Conclusion What is obtained at (a) the anode (b) the cathode? Explanation Write a brief e.xplanation of the electrolysis of copper sulphate solution. EXPERIMENT 27 To discover the factors that affect the amounts of materials liberated at the electrodes during electrolysis. (Ref. Sec. V:43) Apparatus As in experiment 25, with ammeter and rheostat in series with the electrodes. Fig. 31:9 Method 1. (a) Set up the electrolytic cell as in experiment 25, and allow a current of known strength to flow for one minute. Measure the length of the oxygen column collected over the anode. (b) Repeat part 1 (a) allowing the current to flow for two minutes, three minutes, etc. 3G9 — Chap. 31 2. MAGNETISM AND ELECTRICITY Repeat part 1 the strength etc., of that previously used. (a) using a current of one-half the strength, twice Observations 1. Current Strength Constant Time Current Flows (min.) Length of Oxygen Column (cm.) 2. Time Constant Current Strength (amp.) Length of Oxygen Column (cm.) Conclusion What factors affect the amounts of materials liberated at the electrodes during electrolysis? EXPERIMENT 28 To determine the strength of an electric current using a copper voltameter. (Ref. Sec. V:44) Apparatus Copper voltameter, copper sulphate solution, ammeter, rheostat, switch, source of direct current, stop-watch, balance, alcohol, emery-paper. Method 1. Clean the cathode with the emery-paper. Wash it in water. Dip 4. it in alcohol and let it dry by evaporation. Weigh it accurately. 2. Arrange apparatus as in Fig. 26:3. Simultaneously close the switch and start the stop-watch. Take ammeter readings every minute and let the current flow for exactly 15 minutes. 3. Remove the cathode, being careful not to dislodge any of the deposit from it. Rinse in water, dip in alcohol, and allow it to dry as before. Weigh it accurately. Observations Ammeter Readings 1. Initial weight of cathode 1 = 2. Final weight of cathode Weight of copper deposited = = 3. Time of
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current flow gm. gm. gm. sec. Current strength (average of ammeter readings) = amp. Calculations 1. Determine the weight of copper deposited in 1 second. 370 EXPERIMENTS ON MAGNETISM AND ELECTRICITY 2. Knowing that the electrochemical equivalent of copper is 0.000329, calculate the current flowing in the circuit. Conclusions 1. What was the calculated current? 2. How docs it compare with the ammeter reading? 3. Define ampere, coulomb. Questions 1. What possible sources of error were there in this experiment? 2. W'hy would a silver voltameter be more accurate than a copper voltameter? EXPERIMENT 29 To study electroplating. (Ref. Sec. V:45) Apparatus Electrolytic cell, copper sulphate solution, copper anode, object to be plated (a key) as cathode, source of direct current, rheostat, emerypaper, carbon tetrachloride. Method Carefully clean the object with emery-paper, wash in carbon tetrachloride and wash in water. Arrange apparatus as in the diagram. Close the circuit. Adjust the rheostat so that only a small current flows. After several minutes, remove the electrodes and examine them. Observations What is observed? Conclusion What is meant by electroplating? 371 Chap. 31 MAGNETISM AND ELECTRICITY Explanation Write a brief explanation for copper plating. Questions 1. In order to electroplate with any metal, what must (a) the cathode (b) the anode (c) the electrolyte consist of? 2. (a) Why should the cathode be cleaned? (b) Why should a small current be used? 3. Why was there no change in the colour of the copper sulphate solution in the above experiment? EXPERIMENT 30 To illustrate the principle of the lead-acid storage cell, (Ref. Sec. V:46) Apparatus Two strips of lead metal, emery-paper, two dry cells, glass tumbler, solution of sulphuric acid (one part concentrated sulphuric acid added to ten parts water), flash-light bulb, socket, galvanometer with fuse. Sulphuric Acid Solution Method 1. Thoroughly clean the lead strips, using the emery-paper. Fill the tumbler about three-quarters
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full of sulphuric acid solution. Immerse the lead strips in the solution so that they do not touch each other, connect them in series with the dry cells and let the current flow for about 5 minutes. Momentarily insert the galvanometer into the circuit to determine the direction of the current. Note any changes observed at each electrode. 372 2. EXPERIMENTS ON MAGNETISM AND ELECTRICITY Remove the diy cells and insert a flash-light bulb in their place. Note what happens. Momentarily insert the galvanometer to determine the direction of the current again. Let the current continue to flow until the flash-light bulb goes out and note any changes in the plates. Observations Describe all observations. Explanation 1. (a) What type of cell do you have in part 1? (b) What energy transformation takes place in it? 2. (a) What type of cell do you have in part 2? (b) What energy transformation takes place in it? 3. Describe briefly the chemical changes involved during the above energy transformations. Conclusions 1. What does a lead-acid storage cell consist of? 2. Describe what takes place during discharging and recharging. Questions 1. How may a hydrometer be used to determine the condition of a storage battery? 2. Why must water be added to a storage battery periodically? Why should it be distilled water? EXPERIMENT 31 To investigate the magnetic field surrounding a conductor of electricity. (Ref. Sec. VMS) Apparatus Several dry cells, conductor, iron-filings, small magnetic compass, piece of cardboard, switch. Method 1. Pass the conductor vertically through the piece of cardboard and connect it with the dry cell and switch so that the electrons will flow upward in the conductor when the switch is closed. Sprinkle ironfilings on the cardboard and gently tap it. Note the results. 2. Explore the magnetic field around the conductor with the compass- needle and plot the direction of the needle on a diagram. 3. Grasp the wire with your left hand so that the Angers point in the direction in which the N-poles are deflected. Compare the direction of the electron flow and that in which your thumb points. 4. Reverse the direction of electron flow and repeat the procedure in 3 and 4. 373 Chap. 31 MAGNETISM AND ELECTRICITY Observations Describe all observations made above. Conclusions 1. Describe the magnetic field around a wire carrying electric
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current. 2. State the Left-Hand Rule. Question Describe the magnetic field obtained if the above conductor were coiled into a single loop. EXPERIMENT 32 To investigate the magnetic field surrounding a helix carrying electric current, (Ref. Sec. V:49) Apparatus Dry cell, helix, iron-filings, magnetic compass, piece of cardboard. Method 1. Set the helix into a slit in the cardboard and support it horizontally. Join it to the dry cell and switch. Close the switch and sprinkle ironfilings on the cardboard, tapping it gently as you do so. Describe what occurs. 2. Determine the polarity of the magnetic field about the helix, using the N-pole of the compass. 3. Grasp the helix with your left hand so that the fingers point in the direction of the electron flow. Toward which pole does your thumb point? 4. Reverse the direction of electron flow and repeat parts 2 and 3 above. 374 EXPERIMENTS ON MAGNETISM AND ELECTRICITY Observations Describe all the observations made above. Conclusions 1. Compare the magnetic field about a helix carrying a current, with that about a bar magnet. 2. State the Helix Rule. EXPERIMENT 33 To discover the factors affecting the strength of an electromagnet, (Ref. Sec. V:50) Apparatus Two di-y cells, about 5 ft. of copper wire, cylindrical piece of soft iron about 4 in. long and in. in diameter, compass-needle, small iron weights (tacks, etc.) switch. Method 1. Wind about 10 turns of wire into a loose coil around the piece of soft “iron”. Remove the iron core and join the coil to a dry cell and switch. Close the switch and test the coil for magnetism. Determine the number of iron weights that can be lifted. 2. Insert the soft iron core, close the switch and test as before, noting any difference in the strength of the magnetism. 3. Using twenty turns of magnet wire around the iron core repeat the observations made in parts 1 and 2. 4. Using the two dry cells in series, repeat part 3. Observations State the number of iron weights lifted in each of the above cases. Conclusion What factors affect the strength of an electromagnet? Explanation State why each of these factors increases the strength of an electromagnet. Questions The galvanoscope (
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Fig. 27:14) consists of several coils of wire with different numbers of turns, e.g., 1, 25, 100, in each coil. 1. Describe how this instrument may be used (a) to determine the to compare the strengths of different direction of a current (b) currents. 2. What is the purpose of the coils with different numbers of turns? 375 Chap. 31 MAGNETISM AND ELECTRICITY EXPERIMENT 34 To determine the effect of a magnetic field upon a conductor carrying electric current, (Ref. Sec. V:52) Apparatus Battery of dry cells, strong horseshoe magnet, switch. Method 1. Assemble the apparatus as in the diagram with the conductor suspended between the poles of the magnet at right angles to the Close the switch and note what direction of the lines of force. happens. 2. Reverse the direction of the current and repeat part 1. Observations Describe what is observed. Conclusion What is the effect of a magnetic field upon a conductor carrying an electric current? This is the motor principle. Explanation What causes the conductor to move in each case? 376 EXPERIMENTS ON MAGNETISM AND ELECTRICITY Questions 1. If the above conductor were coiled into a helix and freely suspended in a magnetic field, what would be the effect (a) When a small current is passed through it? (b) When a larger current is passed through it? (c) When the current is reversed? (d) When the field is reversed? Make labelled sketches to indicate polarities and direction of movement. 2. What energy transformation occurs? Application Describe the construction of the D’Arsonval galvanometer. EXPERIMENT 35 To investigate the principle of operation of a simple directcurrent motor, (Ref. Sec. V:55) Apparatus St. Louis motor (Fig. 27:21), dry cells, rheostat, magnetic compass, switch. Method 1. Examine the motor carefully. Identify the field magnets, armature, Connect the brushes in series with the brushes and commutator. battery, rheostat and switch. Close the switch and give the armature a gentle push to start it rotating. 2. Remove the magnets. Close the switch and adjust the rheostat so that a small current passes through the armature winding. Determine the polarity of the armature. Note the position of the commutator segments and brushes. Slowly rotate the
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armature through 180°. Note the changes in the position of the commutator segments and brushes as you do so. Again determine the polarity of the armature. Replace the magnets. 3. (a) Vary the current by changing the rheostat setting. (b) Vary the magnetic field by changing the distance of the poles from the ends of the armature. What effect do each of these have on the speed of rotation of the armature? 4. (a) Reverse the direction of the current. (b) Reverse the direction of the magnetic field. What effect do each of these have on the direction of rotation of the armature? Observations Describe all the observations. 377 Chap. 31 MAGNETISM AND ELECTRICITY Conclusions 1. Why does the armature of a direct current motor rotate? What energy transformation takes place? 2. What governs the speed of rotation of the armature? 3. What governs the direction of rotation of the armature? Explanation 1. What is the purpose of the commutator in a direct-current motor? Explain its action. 2. Explain why the factors mentioned in conclusions 2 and 3 affect the rotation of the armature. Question How does this motor differ from commercial D.G. motors? EXPERIMENT 36 To determine the cause of an induced current, (Ref. Sec. V:58) Apparatus Galvanometer, two solenoids wound on hollow spools so that one with few turns of wire will slip entirely inside one with many turns, bar magnet, dry cell, switch. 1 mm T (a) Fig. 31:14 Method Connect the larger solenoid to the galvanometer. 1. Thrust the magnet into the centre of the solenoid. Keep it stationary for a few moments and then withdraw it quickly. 2. Connect the terminals of the smaller solenoid through a switch to a dry cell. Close the circuit. Thrust smaller solenoid into the centre 378 EXPERIMENTS ON MAGNETISM AND ELECTRICITY of the larger. withdraw it. Leave it stationary for a few moments and then 3. Place the smaller solenoid (the primary coil) inside the larger one After a few (the secondary coil), and close the primary circuit. moments open the circuit again. Observations Observe the galvanometer needle at each stage of the experiment. Conclusion State the cause of an induced current. EXPERIMENT 37 To determine what factors affect the magnitude of an
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induced electromotive force (E.M.FJ. (Ref. Sec. V:59) Apparatus Same as for experiment 36, iron core to fit in hollow core of primary coil. Method 1. With the larger solenoid attached to the galvanometer, insert the bar magnet into the hollow core first slowly and then rapidly. Compare the strengths of the induced E.M.F.’s. 2. With the larger solenoid attached to the galvanometer plunge the bar magnet into its hollow core. Then attach the galvanometer to the solenoid with fewer turns of wire and plunge the bar magnet into it at the same speed as before. 3. Attach the larger solenoid to the galvanometer. Connect the smaller to the dry cell to make an electromagnet. solenoid (primary coil) Plunge the electromagnet into the hollow core of the secondary coil. Now insert the iron core into the centre of the primary coil and plunge the primary into the secondary. Observations State what is observed in each part above. Conclusion What factors affect the magnitude of an induced E.M.F.? Question Why were the induced currents produced at the “make” and “break” of the primary circuit in experiment 36 much greater than those produced by other means? EXPERIMENT 38 To investigate the direction of an induced E.M.F. (Ref. Sec. V:61) (Lenz's Law), 379 Chap. 31 MAGNETISM AND ELECTRICITY Apparatus Solenoid with many turns, bar magnet, galvanometer, dry cell, high resistance. Method 1. Connect the dry cell through the high resistance to the galvanometer. Note in what direction the galvanometer needle is deflected when a current of known direction is passing through it. 2. Connect the solenoid to the galvanometer and note the direction of the needle deflection and thus determine the direction of the current when: (a) The N-pole of the magnet is thrust into the coil. (b) The N-pole is withdrawn. (c) The S-pole is inserted. (d) The S-pole is withdrawn. 3. Using the Helix Law, determine the polarity of the upper end of the solenoid for each part of 2. Observations Direction of Current Across Front of Solenoid Polarity of Upper End of Solenoid Operation N-pole inserted N-pole withdrawn S-pole
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inserted S-pole withdrawn Conclusions 1. What effect does the magnetic field produced have on the motion that is inducing the current? 2. State Lenz’s Law. Questions 1. Make a series of diagrams to illustrate the above observations. Show the direction of motion of the magnet and the polarity produced. 2. Use Lenz’s Law to explain the production of an induced E.M.F. at the make and break of a primary circuit (Experiment 36, part 3). EXPERIMENT 39 To demonstrate the production of an alternating current and to study the principle of the A.C» and D.C. generator. (Ref. Sec. V:61 to 63) Apparatus Earth inductor (Fig. 28:4), galvanometer, connecting wires, St. Louis motor with both A.C. and D.C. armatures ( Fig. 27:21). 380 EXPERIMENTS ON MAGNETISM AND ELECTRICITY A. Using the Earth Inductor Method Connect the two leads of the earth inductor to the terminals of the galvanometer. Rotate the coil rapidly through 360° and note the action of the galvanometer needle. Observations State what is observed. Explanation 1. What is the cause of the induced E.M.F.? 2. Why does it vary in direction throughout the 360° rotation? Illus- trate your answer by a series of diagrams. Conclusion What is produced by each complete rotation of a coil in a magnetic held? B. Using the St. Louis Motor Model Method 1. Connect the leads from the A.C. armature to the galvanometer. Spin the armature with the huger and note the action of the galvanometer needle. 2. Replace the A.C. armature with the D.C. armature. Spin the armature as before and again note the action of the galvanometer needle. Observations State what occurred. Explanation 1. Answer the questions found in the explanation in part A. 2. How is the induced alternating current carried to an external circuit? 3. How may this induced alternating current be changed to direct current? Name and describe the device used to do this. Conclusions 1. What is a generator? 2. What energy transformation occurs in the operation of a generator? 3. What is the difference between an A.C. and a D.C. generator? Questions 1. How could you produce 60 cycle A.C. with the model used
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in this experiment? 2. What modihcation in the structure of the apparatus would permit a reduction in the speed of rotation of the armature in question 1? 381 Chap. 31 MAGNETISM AND ELECTRICITY EXPERIMENT 40 To study self-inductance. (Ref. Sec. V:67) Apparatus Coil with many turns wound around a soft iron core, three dry cells, switch, 6-volt lamp, neon lamp and socket. Fig. 31:15 Method 1. Connect the coil in series with the dry cells and switch as shown in the diagram. Join the lamp in parallel with the coil. 2. Close the switch. Note the effect on the lamp. 3. Open the switch. Note the effect on the lamp. Observations Make note of the observations. Explanation With the aid of Lenz’s Law account for the phenomena noted above. Conclusion What is meant by self-inductance? Question What causes the arc when a switch is opened? EXPERIMENT 41 To demonstrate the conductivity of a gas (An introduction to electronics). (Ref. Sec. V:77) Apparatus Gold-leaf electroscope, ebonite rod, cat’s fur, glass rod and silk, Bunsen burner. Method 1. Charge the electroscope positively. Hold a flame near the knob of the electroscope. Note what happens. 2. Charge the electroscope negatively and repeat step 1. 382 EXPERIMENTS ON MAGNETISM AND ELECTRICITY Observations Describe what is observed in each part above. Explanation What effect did the flame have on the particles of gas causing it to become a conductor of electricity? Why were both the positively and negatively charged electroscopes discharged? Conclusion What is electronics? EXPERIMENT 42 To study the changes in the conductivity of a gas as its pressure is reduced. (Ref. Sec. V:77) Apparatus Electrical discharge tube with open side-arm (Fig. 30; 1), vacuum-pump, induction coil, battery. Method Attach the electrodes of the discharge tube to the terminals of the secondaiy of the induction coil. The discharge points of the induction coil should be close together. Turn on the induction coil and evacuate the tube as completely as possible with the vacuum-pump. Note what happens. Observations 1. Where did the electrical discharge occur before and after evacuation of the tube? 2. Describe the discharge in the tube at various
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stages of evacuation. Explanation Why does the discharge occur in the evacuated tube rather than between the discharge points? Conclusion What effect does reducing the pressure of a gas have on its electrical conductivity? Question. According to the electron theory, what constitutes the current of electricity (a) through a gas at ordinary or reduced pressures (b) through a vacuum? EXPERIMENT 43 To study some properties of cathode rays. (Ref. Sec. V:78) Apparatus Crookes’ tube (Fig. 30:2b), induction coil, dry cells, bar magnet. 383. Chap. 31 MAGNETISM AND ELECTRICITY Method 1. Set up apparatus as in diagram. Darken the room. Complete the circuit and observe the appearance of the end of the tube remote from the cathode. 2. Erect the target or metal cross in the path of the rays and again observe as in part 1 3. Approach the sides of the tube with first the N-pole and then the S-pole of the bar magnet and observe as above. Observations Describe what is observed. Conclusion List three properties of cathode rays. Question What evidence is provided in part 3 above to prove that cathode rays are streams of electrons? EXPERIMENT 44 To study the production and nature of X-rays. (Ref. Sec. V:79) Apparatus X-ray tube (Fig. 30:3), induction coil, gold-leaf electroscope. Method 1. Join the secondary terminals of the induction coil to the anode and cathode of the X-ray tube. Charge the electroscope and place it with its knob near the target of the tube. Start the induction coil and observe the leaf of the electroscope. 2. Repeat the above procedure placing (a) a piece of cardboard (b) a piece of wood (c) a piece of lead plate between the knob of the charged electroscope and the X-ray tube. Observations State what is observed in each case above. Conclusions 1. How are X-rays produced? 2. Account for the above observations. EXPERIMENT 45 To study thermionic emission of electrons in a vacuum-tube, (Ref. Sec. V:80) Apparatus Diode tube (B), two switches (Kj and Kg), “A” battery, “5” battery, galvanometer (G). 384 — EXPERIMENTS ON MAGNETISM AND ELECTRICITY Method 1.
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Connect the circuit as shown in the diagram using the plate and filament voltages recommended for the tube available. 2. With Ki open (a) close Kz making the plate positive and watch the galvanometer for any deflection. (b) reverse the terminals of the ‘‘5” battery so the plate is negative and repeat part (a). 3. With Kr closed— (a) close K 2 making the plate positive and note any galvanometer deflection. (b) reverse the terminals of the “B” battery so the plate is negative and repeat part (a). Observations Switch Plate Charge Galvanometer Deflection 2(a) 2(b) 3(a) 3(b) Explanation. 1. Explain the effect on the filament of closing switch Ki. 2. Account for the above observations. Conclusions What two conditions must be fulfilled so that electrons will flow through a vacuum-tube? 385 Chap. 31 MAGNETISM AND ELECTRICITY EXPERIMENT 46 To demonstrate rectification by a diode tube. (Ref. Sec. V:80) Apparatus Diode tube, two 45-volt “B” batteries, “A” battery, neon bulb. Method 1. Connect the two “B” batteries in series and join these to the neon bulb. Reverse the battery connections. 2. Connect the neon bulb to the 110-volt alternating-current outlet. 3. In the circuit shown in experiment 45 replace the “B” battery by the 110- volt alternating-current source, and the galvanometer by the neon bulb. Observations Describe the way in which the neon bulb glows in each step above. Explanation Account for each observation. Conclusions 1. What is meant by rectification? 2. Explain how rectification occurs in the diode tube. 3. Make a diagram to illustrate the direct current produced in the plate circuit on the diode tube in step 3. EXPERIMENT 47 To demonstrate photo-emission of electrons. (Ref. Sec. V;82) Apparatus Zinc plate, gold-leaf electroscope, arc-lamp. Method 1. Attach the knob of the electroscope to the zinc plate by a fine wire. Charge the zinc plate negatively. Allow the beam of light from the arc-lamp to fall upon the charged zinc plate. 2. Repeat part 1 with the zinc plate positively charged. Observations What is observed in each
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step above? Explanation Account for the observations. Conclusions What is meant by photo-emission of electrons? 386 MODERN DEVELOPMENTS IN PHYSICS This Electron Microscope Permits Viewing of Particles Smaller Than One 10-Millionth of an Inch in Any Diameter. It Provides Magnification 50 Times Greater Than Heretofore Possible— So Great That a Human Hair Would Take on the Dimensions of a Subway Tunnel. R.C.A. Victor Company, Ltd. CHAPTER 32 MODERN DEVELOPMENTS IN PHYSICS gets of research have become ordinary articles of commerce. This century has seen the “horseless carriage”, originally a luxury, become a necessity for business and pleasure. The airplane in one generation has undergone an amazing development for peace and war. Electronics has given birth in turn to radio, radar and television. In this chapter, a few modern developThese are ments are to be described. THIS TWENTIETH CENTURY This has been called the age of Physics —a time when apparent luxuries have become commonplace necessities or gad- 389 Chap. 32 MAGNETISM AND ELECTRICITY selected because the preceding chapters serve as a preparation for them and because they are topics that will be of considerable interest to many students. Cathode-Ray Oscilloscope Fig. The principle of the cathode-ray tube (Sec. V;78) is shown in 32:1. Electrons emitted by the hot cathode travel at high speed toward the (c) (a). Many pass through the anode opening in the anode and form the electron beam (b) which passes between the (di d 2 ) two sets of deflecting plates and impinge on the fluorescent screen (i). A spot of light appears on the electron beam the screen wherever strikes. The spot of light is made to travel by applying voltages to the deflecting plates, thus causing the electron beam to sweep from side to side, move up and down, or undergo both motions simultaneously. Each point on the screen struck by the beam continues to fluoresce for a short time after the beam has gone by and does not quite die out before the next passage of the beam revives it. This fact combined with persistence of vision RCA Victor Company, Ltd. Canadian Marconi Co. Fig. 32:2 Commercial Cathode Ray Oscilloscope and Tube. Fig. 32:3 The Image Orthicon. 390 MODERN DEVELOPMENTS IN PH
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YSICS of the human eye produces the illusion of a continuous line of light on the screen. Thus, it is easy to see that if one voltage is used to make the beam sweep from side to side at a known uniform rate, another voltage applied to the ver- Illus. Courtesy of Canadian Marconi Co. A Picture Tube deal deflection plates will make the spot write the autograph of the latter voltage on the screen, revealing in graphic form frequency, wave form and other its characteristics. The commercial cathode-ray oscilloscope and tube is pictured in Fig. 32:2. It combines in one circuit a cathode-ray tube, a sweep circuit, amplifiers, a power supply, all properly synchronized. This is probably the most widely used instrument in electronics, particularly in the testing and repair field. All kinds of sound—speech, noise, music—can be ana- lyzed by the use of this instrument. Television In radio, electromagnetic waves modulated by the original sound-effects are transmitted through space. In television, a picture controlled by the light from the scene is transmitted at the same time as the sound. The feeble energy from such waves controls electric currents and magnetic fields in such a way as to reproduce the original sound and scene. In the modern television camera, the essential component is an image orthicon. Fig. 32:3, not much bigger than a rolled-up magazine, but very complicated, sensitive and costly. It receives an optical image, /, of the scene on a thin metal coating, M, on the inside are surface tube. ejected from this screen in direct propor- Electrons the of Fluorescent Coating 391 Chap. 32 MAGNETISM AND ELECTRICITY Scene in Television Studio During a Broadcast. Canadian Broadcasting Corp. tion to the brightness of the light that falls on it. These collect on the target screen, T, nearby and produce an elec- Fig. 32:5 The Effect of the Earth's Cur- vature on Television Waves. tron image of the scene. This electron image is scanned by an electron beam, B, from the electron gun, G, in much 392 the same manner as you read a page in this book but much faster (covering 525 lines in 1/3 sec.). The beam is controlled as in the cathode-ray tube. The speed of the electrons in the beam is so regulated that some of the electrons are repelled in a return beam whose intensity varies with the concentration of electrons on the target screen
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and, consequently, with the brightness of light from the scene. This return beam constitutes a weak current which is amplified, sent to a transmitter where it modulates ultra high frequency carrier waves that are broadcast much as are radio waves. The main part of the receiving aptube, paratus called a kinescope (Fig. 32:4), that is another cathode-ray is MODERN DEVELOPMENTS IN PHYSICS essentially the same as in the cathoderay oscilloscope. The signal received by the antenna is amplified and then is Bell Telephone Company of Canada Fig. 32:6 A Relay Station. applied to the tube in a manner that controls the intensity electron beam shot out by its electron gun. The the of motion of this beam is synchronized with the image orthicon. the scanning of of the beam and Since the intensity hence the brightness of the spot on the screen varies with the signal received, the beam produces a reproduction of the original scene 30 times per second. The illusion of a continuous picture is explained by the rapidity of scanning and our persistence of vision. Television waves travel in a straight path. Each transmitter’s coverage is an area with a radius of about 130 miles on account of the earth’s curvature (Fig. 32:5). Obviously, this presents a problem in telecasting events of national coast. To importance from coast overcome the difficulty a series of relay stations (Fig. 32:6) has been developed which is used both for television and telephone conversations using short waves called microwaves to carry the energy. Each frequency carries several messages. Waves of this kind can be focused and aimed at the next station so that little energy is lost. to The sound system of television is of the type called frequency modulation (F.M.) unlike that of ordinary radio which is amplitude modulation (A.M.). In the latter, the signal modulates (alters (i) Carrier Frequency (ii) Sound Pattern (iii) Amplitude Modulated Carrier Fig. 32:7 (a) Amplitude Modulation. (b) Frequency Modulation. 393 Chap. 32 MAGNETISM AND ELECTRICITY the form of) the amplitude of the carrier wave as in Fig. 32:7a. Such a wave is affected by all kinds of outside elecIn tromagnetic disturbances the former, the signal modulates the frequency of in Fig. 32 : 7b. Such a wave is free from carrier wave as (static). the static. In one method for colour television
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pass and where they become deflected. The arrangement of lenses is similar to that in the optical microscope but the object to be studied is very much thinner (1/100,- 395 ). Chap. 32 MAGNETISM AND ELECTRICITY ELECTRON MICROSCOPE OPTICAL MICROSCOPE ^SOURCE OF ILLUMINATION (Electrons) (Light) CONDENSER LENS (Magnetic) (Glass) SPECIMEN STAGE OBJEaiVE LENS (Magnetic) (Glass) PROJECTOR LENS (Magnetic) (Glass) IMAGE (Viewing Screen) (Eye Piece) — Fig. 32:9 A Comparison of Electron and Optical Microscopes. R.C.A. Victor Company Ltd. 000 cm. thick). The instrument must be evacuated to prevent deflection of the electron stream as it collides with air molecules. The varying penetrations of the electrons through the object produce a sort of shadow picture on the fluorescent viewing screen or photographic film. Magnifications of up to 100,000 diameters have been obtained. The electron microscope has proved to be a very valuable tool in the hands of the research worker. It has revealed new facts about particle shapes and sizes which affect such processes as the covering power of paint pigments, the wear- ing quality of rubber tires and the operation of chemical catalysts. It has Comparison of Some Magnifiers Object Diameter (Microns Magnification (Diameters) Fine Machine Work 25-100 Pond Life Fungi 10-25 Bacteria Structure of Bacteria Large Virus Colloidal Particles Small Viruses 1-2 0.25 0.10 0.05 0.01 Instrument Hand lens Low power optical Medium power optical 8 20 200 1000. High power optical 2000. Special optical microscope or electron microscope 4000. Electron microscope 20,000 Electron microscope Large Molecules 0.002 100,000 Electron microscope 396 MODERN DEVELOPMENTS IN PHYSICS salts themselves. erated by the This phenomenon was called radioactivity and was extensively investigated by Pierre and Marie Curie. Using pitch-blende, a natural ore rich in uranium compounds, they succeeded, after a long series of repeated crystallizations, in isolating two new elements, polonium and radium, which than radioactive more were y- Rays provided new information in such fields as metallurgy, chemistiy, ceramics, crystallography, plastics, textiles, biology and medicine. NUCLEAR
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. The number of protons in the nucleus of the atom is the atomic number or nuclear charge. Since the atom is electrically neutral, the Fig. 32:12 Structure of Some Atoms. (MN = Mass Number; AN = Atomic Number) 398 MODERN DEVELOPMENTS IN PHYSICS Fig. 32:13 Isotopes of Hydrogen. Tritium MN = 3 AN = 1 number of electrons revolving about the nucleus must equal the number of protons in the nucleus, and hence must also equal the atomic number. of the This Rutherford’s atom served until the discovery by Sir James Chadwick in 1932 neutron. particle (which was earlier predicted by Rutherford) has the same mass as the proton It is now bebut possesses no charge. lieved that atomic nuclei are built up of protons and neutrons. The sum of the protons plus the neutrons make up what is called the mass number of the atom. In general the number of neutrons the mass in number minus the nuclear charge or atomic number. For example, the helium nucleus which has an atomic number of 2 and a mass number of 4 must contain 2 protons and 2 neutrons. the nucleus is equal to Finally, to complete the picture of the atom, Niels Bohr postulated that the electrons revolved in various orbits, or energy levels, about the nucleus. The Bohr Concept for a number of typical atoms is shown in Fig. 32:12. Isotopes About this time, it was discovered by Rutherford and Aston in England that although all atoms of an element have the same chemical properties some differed from others in mass. Such forms of any element are called isotopes. They postulated that all the isotopes of any one element must have the same number of protons and electrons and hence the same atomic number, but must differ in the number of neutrons in the nucleus and hence must differ in mass num- (Fig. 32:13). ber. Most elements consist of a number For example, hydrogen is of isotopes. thought to consist of a mixture of three Protium is the isotopes commonest isotope of hydrogen. Deuterium is the isotope of hydrogen that is found in heavy water. The nucleus of called is much used in experimental work in nuclear physics. Tritium is a very rare isotope of hydrogen that is of importance in the hydrogen bomb. deuterium, deuteron, a Artificial Transmutation The above interpretation of natural radioactivity suggested to
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Tn of nuclear transformation, Nuclear Fission, in which the uranium nucleus was split into two nearly equal parts with the release of a large amount of energy (Fig. 32:15). Another interesting feature of the fission of uranium is that during the process further neutrons are emitted in relatively larger numbers than those absorbed. Thus from each uranium nucleus undergoing fission two or more neutrons are released. The neutrons so released can then cause fission in other uranium nuclei and so on causIf allowed to ing a chain reaction. proceed in this way, provided that there is a suitable amount of fissionable matter present, the whole of the material is transformed in a very short time, with the release of vast quantities of energy. This is the principle of the atomic bomb. It has been proven that the combined mass of the products is a little less than the mass of original material. Dr. Albert Einstein as early as 1905 using his famous equation, E =; MC^, predicted what we now know—that the mass lost becomes transformed into the energy of the explosion. E represents the energy, M the loss in mass and C, a constant quantity equal to the speed of light (Sec. IV: 6). Thus we can see that a small amount of mass is transformed into a tremendous amount of energy. Nuclear Reactors If the neutrons from the fission of U 235 are absorbed or slowed down as they are produced, the chain reaction can be controlled and the energy released put to useful purposes. Graphite and certain other light substances which possess the property of absorbing neutrons, are used to control the rate of neutron emission in atomic fission and are called moderators. The assembly of uranium (or other fissile material) and graphite (in the form of blocks or rods) is known as an atomic pile or nuclear reactor (page 402), the rate of energy release being controlled by adjusting the length of the graphite rods inserted in the pile. The energy released is transfomied into heat which is used for various purposes. Energy is variously measured. The engineer may express it as B.T.U. per pound (Sec. 111:20). The nuclear physicist may express it in electron volts per atom. One B.T.U. equals 665 X 10^^ million electron volts. estimated 13,600 B.T.U. per pound, this is 4 electron volts per atom of carbon. By comparison one atom of uranium produces 200 million This means that one electron atom of uranium produces 50
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million times as much energy as one atom of carbon or if you have equal masses of coal and uranium the amount of energy released from the uranium is 3 million times as great. There is little wonder, therefore, that the search for uranium goes on at a feverish pace. gives volts. that, coal It is if Atomic Fusion to In 1951 when man was congratulating himself on his conquest of the atom through turning fission his advana new and still more exciting tage, process was evolved by Dr. Edward Teller in the United States. You will recall Hans Bethe’s theory of the origin of the sun’s continuous heat by the union of four atoms of hydrogen to form an atom of helium accompanied by the conversion of a small amount of mass into energy (Sec. Dr. Teller employed this process using the isotopes of hydrogen—protium, deuterium and tritium—resulting in the production of a source of energy far greater than ever The hydrogen bomb realized was created which was fantastically devastating. A source of energy was born that was so amazing that it may well be the source of energy of the future. Ill: 3). before. 401 Chap. 32 MAGNETISM AND ELECTRICITY Atomic Knergy of Canada Ltd. Model of N.R.X. Reactor, Chalk River, Ontario. star Mewspaper Service U.S.S. Nautilus. The First Atomic Powered Submarine. 402 MODERN DEVELOPMENTS IN PHYSICS COOLING WATER TRAVELS Space does not permit dealing at any length with the many and varied uses of atomic energy. Suffice it to say that biological, medical, agricultural and industrial research are greatly benefiting from the developments made in this field as are some users of industrial and domestic power. are as yet beyond man's fondest dreams. The possibilities 403 ANSWERS Unit I—Mechanics A Chapter 2, Section I : 4—Page 16 4 (a) 1000 mm., 100 c.m., 10 dm., 1000 m. 10.000 dg., 1000 gm. 1, 3. 11.46 (ii) 8.3. 9 (b) 5, 5, 5, 5, 5 (b) 1,000,000 mg., 100,000 eg., 10 (b) 37., 34., 37. 11 (b) (i) B (a) A A B A 1520, 3.8, 605.4 cm.; (
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b) 1 2.3, 6,000,050.465 c.c. (c) 100 m. (i) 28.1 ft. (hi) 64.4 km. per hr. (ii) 22.5 cu. dm. (hi) 22.5 litres 225.000 dg., 22.5 kg.; (b) (i) 28.4 gm. (ii) 909. kg. (hi) 5.4 pints 102.70; (b) (ii) 0.35 (hi) 27 X 101; (j) (i) 14 x 102 (h) 0.50 (hi) 9.0 Chapter 3, Section I : 9—Page 22 30,000, 2.36, 60,005.44 sq. cm.; (c) 2,500,000, 2 (a) 37.4, 15.7, 39.4 in.; (b) (i) 30.5 cm. (ii) 1.61 km.; (ii) 856 cm.; (d) (i) 58.7 R. per sec. (ii) 17.9 m. per sec. 3 (a) (i) 1500 sq. cm. (ii) 15. sq. dm.; (b) (i) 22,500 c.c. 4 (a) 22,500 gm., 22,500,000 mg., 2,250,000 eg., 5 (i) 3.1 litres (ii) 3080 ml. 36. (i) 66.56 (ii) 71.25 (hi) 67.17; (c) 6 (a) (i) 1 (b) 2.70 gm. per c.c. 3 (b) 8.0. B 1 5 gm. per c.c. lb. 6 20.3 cu. ft. cu. ft. 16 72.8 gm. or 6. gm. per c.c. 24 1.18 gm. per c.c. 2 52.5 or 53. gm. 3 268 c.c. 4 62.5 or 63. lb. per cu. ft. 7 4.87 12 1781.3 or 18 X lO^ lb. 17 8.5 gm. per c.c. 8 54.1 c.c. 9 800 or 8 X 10^ gm. 13 7.64 gm
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. per c.c. 18 (a) 19.5% (b) 13.4% 5 64. 10 0.068 11 0.59 15 0.80 14 0.8722 20 5.9 19 0.42 c.c. 23 0.9 gm. per c.c. 21 9.9 gm. per c.c. 22 8.7 gm. per c.c. Chapter 4, Section I : 15—Page 32 1 (b) (i) 100 gm. (ii) 790 gm. 3 (c) (i) 2.6 gm. (ii) 4.3 c.c B 5 (i) 7.14 (ii) 70 c.c 2 (a) 68 gm.; (b) 625.0 lb. 4 10.5 1 (a) 30 gm.; (b) 162.5 lb. cu. ft. per cu. ft. 10 8.90, 0.840 or 1.6 X 102 lb. 20 18.8 or 19. cm. 26 14. cm. 25 1/3 11 32.0 gm. 7 (i) 105.9 gm. (ii) 8.30 gm. per c.c. 12 No 13 120 gm. 17 11.7 c.c. 22 100 gm. 16 36.6 or 37. gm. 21 20.4 or 20 cm. 3 (a) 25 ml. (b) 0.40 6 (i) 2.5 (ii) 0.096 cu. ft. (hi) 156 lb. 9 (i) 5.36 (ii) 0.73 15 156. 19 1.3 24 0.21 8 1.30 14 0.9 gm. per c.c. 18 0.90 gm. per c.c. 23 187.5 or 188. c.c. Unit II—Sound A Chapter 6, Section II : 9—Page 61 5(b) 1 100 ft. per sec. 9 1 150 ft. per sec. 10(a) 1 100 ft. per sec. 1 (a) 3.75 cm.; (b) 15 cm. (b) 288 m. per sec. 6 (a) 1099, 1055, 1135 ft. per sec.; (b) 335, 321.8, 345.8 m. per sec. (b) 6
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.7°C. 12 (a) 56.5 ft. 2.67,.67 sec. 4 (a) 320 v.p.s.; (b) 667 v.p.s. 3 (a) 1170 or 11.7 X 10^ ft. per sec.; 5 (a) 1.50 ft.; (b) 56.0 cm. 7 (a) 15°C.; 11 (a) 357 m.persec.; (b) 42°C. 14 750 m.p.h.’ 10 259 v.p.s. 13 22.6 sec. 8 5595 ft. 9 1.33 m. Chapter 7, Section II : 18—Page 71 6 (b) (i) 750 v.p.s., (ii) 100 v.p.s.; (c) (i) 208.3 or 208 v.p.s. (ii) 50 v.p.s. 404 ANSWERS B (a) 1 4 150 teeth 8 25, 80, 35 cm. v.p.s. 13 160 v.p.s. 960, 1920, 7680 v.p.s.; (b) 240, 60, 15 v.p.s. 5 256, 320, 384, 512 v.p.s. 9 1080, 270 v.p.s. 2 240 v.p.s. 6 900, 225, 675 v.p.s. 10 312 v.p.s. 11 400, 2500 gm. 3 320 r.p.m. 7 587 v.p.s. 12 384 A Ch.\pter 8, Section II : 25— Page 79 2 (a) (i) 48 in. (ii) 1 200 ft. per sec.; (b) 24 in. B 1 (a).302 m. (i) 5 ft. (ii) 5 15.5°C. A 7 1781 ft. Unit III—Heat A 4 4 X 10-3 lb. ft.; (b) 1100 ft. per sec. 1 6 33.3°C. 7 18.8 cm. 2 1:2 8 4 beats per sec. 3 320 v.p.s. 4 11.9 in., 9 486 or 474 v.p.s. Chapter 9, Section II : 34—Page 93 Chapter 11, Section III : 4—Page 114 Chapter 12, Section III : 11
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— Page 124 2 37.8, 176.7, —140, — 45.6°C. 3 (a) —40°; (b) 4 (a) (i) 330, 250° K. (ii) 25, -36° C.; (b) (i) 309.7, 255.2, °K. (ii) 212, B 1 59, 392, —76, — 459.4°F. 320° — 459.4°F. Chapter 14, Section III : 32—Page 160 2 1600, 8400 cal. (i) A (b) B 350. cal., (ii) 5328 B.T.U., (iii) 179.4 cal. 4 (b) 0.087. 7 4000, 9 (a) 27,000 cal; (b) 8100 cal.; (c) 21,700 cal. 1 100,000 or 10 X 104 4 6000 or 60 X 10“ B.T.U. cal. 8 40°C. gm. 9 0.104 14 204.3 or 204 gm. 18 187.5 or 188 gm. 24 27 gm. 28 534 cal. 1.2 X 102 25 46.6 or 47°C. 29 40°C. 30 53°C. 10 0.0933. 15 12.8 or 13°C. 19 208.3 gm. 2 39,000 or 39 X lO^ B.T.U. 5 1320 or 13.2 X 10^ cal. 11 0.21 12 0.031 16 0.44 20 80 cal. 26 530 or 5.3 X 10^ cal. 31 3640 cal. 6 101 cal. 3 91,000 or 91 X lO^ 7 56.3 13 292.6 or 293 gm, 17 1250 or 12.5 X 10^ gm. 22 82 cal. 23 0.50 27 541 or 5.4 X 10^ cal. 33 121 or 32 0.112 or 0.11 21 78 cal. Unit IV—Light A Chapter 16, Section IV : 8—Page 182 5 (d) 2 in. 7 (b) 8.3 min. B 1 30 ft. 2 0.8 in. 3 148 or 1.5 X 10^ m. 4 1.3 sec. 5 5.87 X lO^^ miles, 9.46 X 1042 km.
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6 52.8 X 1042 7 4 3 yg^rs. Chapter 17, Section IV : 20—Page 194 A. 5 (a) 7. 405 A 2 (c) dioptres. B I 1.20 B (i) 10 o’clock (ii) 5.45 o’clock (v) — 60 cm. 5 — 9.23 in. 11 — 4.5 in. 12— 8.6 cm. 1 oc 13 20 in. 14 2.2 in. Chapter 18, Section IV ; 34— Page 211 ANSWERS 4 (a) 6 3 ft. (i) 50 cm. (ii) 60 cm. (iii) 90 cm. (iv) 7 4 in. 10 36 cm. 8 4.3 ft. 9 3 in. 124,000 miles per sec.; (d) 2.47 5 (b) glass 7 (b) water 9 (d) 6.7 2 479 X 102 3 (i) 35° (ii) 28° (iii) 20° per sec. ZD = 37° 5 One with critical angle of 30° 7 — 9.2 in. (v) — 60 cm. (iii) 90 cm. (iv) II 32 in. 13 6 in. (i) 0.9 in. (ii) 11 X 18 (a) 5.5 in.; (b) 36 in. x 60 in. cc 12 24 cm. 6 (a) 8 5 cm. 14 24 cm. 4 Zi = 48.5° (i) 50 cm. (ii) 60 cm. 10 80 X 17 9 9 metres 15 —20 cm. 16 140, 12.7 cm. Chapter 19, Section IV : 43— Page 225 1 (c) 7.3 X 1014 ^ p 3 (a) 3.1 in.; (b) 1.5 in. 7 (b) 160 X 9 (d) 192 cm. Unit V—Magnetism and Electricity Chapter 20, Section IV : 50—Page 235 6 (c) (i) 8.3 X (ii) 6.7 X (d) (i) 25 mm. (ii) 20 mm. Chapter 25, Section V : 39— Page 293 A (b) 22 volts 4 (c) 1 ohms (ii) 5.5, 2.8 amp. (i) 90 ohms (ii) 6 (b) 14 ohms. 1.
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2 amp. (iii) 60, 48 volts; (d) (i) 13 B 2 212.5 or 21 X 10 volts 1 12 ohms 4 (a) 0.12 amp.; (b) 11.9 or 12 volts and 0.06 volts 5 (a) 22 ohms; (b) 5.5 ohms 6 (a) 41.3 ohms; (b) 0.333 amp.; (c) 2.66 amps. 7 (b) 6 volts; (c) 0.9 ohm 8 16.7 or 17 ohms. 9 (a) 3.4 ohms; (b) 3.3 amp.; (c) Chapter 26, Section V : 47—-Page 302 11 (a) 2.4, 1.6 amp.; (b) 18 volts. 1.7 amp. 10 0.2 amp. 3 3.3 amp. B (a) 3.55 gm.; (b) 12.1 gm. 4 0.003 amp. 8 0.002 cm. 5 1.26 amp. 9 3.95, 0.37 gm. Chapter 27, Section V : 56—^Page 315 6 0.02 amp. 10(a) 2 amp.; (b) 15 hr. 2 (a) 9.5 gm.; (b) 32 gm. 3 (a) 2.4 gm. (b) 0.30 7 (a) 0.18 amp.; (b) 0.0028 1 gm. gm B 1 (a) 0.08 ohm; (b) 0.008 ohm; (c) 0.0008 ohm, 20.002 ohms 3 (a) 115 ohms; (b) 1240 ohms; (c) 12490 ohms 4 9998 ohms. Chapter 28, Section V : 69—Page 329 B 4 220 volts 5 400 turns 6 (i) 96 turns 1 80 turns 2 2200 volts (ii) 64 turns (iii) 48 turns (iv) 32 turns. 3 9.2 volts B Chapter 29, Section V ; 76—Page 338 1 (a) 24 X 10® volts; (b) lZ2000th; (c) greatly diminished 50 times (ii) 2500 times (ii) 372 watts (iii) h.p. 4 5 amp. 7 1650 watts 3 1650 watts 0.372 kw. 8 No. cents
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11 9.7 cents 12 $1.12 13 6 cents, 406 5 220 volts 2 (a) 50:1; (b) (i) 6 (i) 0.498 10 4.3 9 6 cents INDEX Aberration, chromatic, 210, 224; spherical, 193 Abnormal expansion of water, 124 Absolute, temperature, 123; zero, 123, 132 Absorption of radiant energy, 133, 142 {Exp. 168) _ Absorption spectra, 219 Accommodation, lens, 229 Accuracy, degree of, 11; limits of, 13 Achromatic lens, 224, 233 Acid, 295, 299 Acoustics of buildings, 87-89 Action of points, 276 {Exp. 363) Addition, 14 Additive theory of colour, 222 Air, lens, 205, 206; liquefaction of, 155; pres- sure effect on the boiling-point, 148 Airships, 31 Alcohol, 295 Alcoholometer, 29 Alkali, base, 299; metals, 347 Alloys, 291; densities of, 21 Alnico magnet, 266 Alpha rays, 398 Alternating current, 322; 25-cycle, 322; 60- cycle, 322 Alternating current generator, 321 {Exp. 380) Amalgamate with mercury, 281 Amber, 268 Ammeter, 311, 312 Ammonia gas, 152, 154 Amperes, 281, 288, 298 Ampere-hours, 302 Amplihcation, 346 Amplifier, 346 Amplitude, modulation, 393; of vibration, 50, 64, 76, 78; of wave, 52 Angle, critical, 200; of declination, 262; of deviation, 200; of incidence, 184; of inclination, 263; of reflection, 184; of refraction, 196, 198; visual, 231 Angstrom, 2l4 Anions, 295 Annular eclipse, 179 Anode, 295 Antenna, 285, 347, 393 Anthracene, 218 Anti-cathode, 343 Anti-freeze hydrometer, 29 Aperture, 227 Approximate numbers, 13; rules for using, 14 Aquastat, 119 Aqueous humour, 228 Arc, electric, 326, 336, 382; furnace, 337; lamp, 336, 386; welding, 336, 337 Archimedes, 1, 3, 24, 25 Archimedes’ principle, 25 {Exp. 39, 40); ap- plications of, 29-31
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Area, measurement of, 10; of conductor and resistance, 288 Argon, 335 Aristotle, 1, 3, 49, 175 Armature, 321, 323, 377 Arrangement of cells, 283 Arrhenius, Svante, 295 Artificial, magnets, 260; transmutation of ele- ments, 399 Astigmatism, 230, 231 Astronomical telescope, 233, 234 {Exp. 254) Atmosphere, pressure of, 120, 148, 150 Atmospheric refraction, 202 Atom, 112, 132, 397; structure of, 269, 270, 398 ; Bohr concept, 399 Atomic, bomb, 401; energy, 3, 401; fusion, 401; number, 398; pile, 112, 331, 401 Atropine, 228 Attraction, electrical, 268 {Exp. 358) ; mag- netic, 260 {Exp. 352) Audibility, limits of, 65 Audio-frequency, 347 Auditoria, acoustics in, 59, 77, 88-89 Auditory nerve, 49, 82 Automobile, engine, 159; cooling system, 145; generator cut out relay, 307, 309; ignition system, 328 Axis, principal, 189, 204; secondary, 189 Bacon, Roger, 3, 226 Balance, 12, 34, 35 Balance wheel of watch, 116 Ball and ring apparatus, 115 {Exp. 164) Balloons, 30 Bar magnet, 260 {Exp. 350, 351) Base (Alkali), 295 Battery, 29, 279, 284; hydrometer, 29; rat- ing, 302 Beam, of light, 176; wireless, 326 Beats, 78, 79 {Exp. 106) Becquerel, Henri, 397 Bel, 64 Bell, Alexander Graham, 325 Bell-in-vacuo, 51 {Exp. 96) Bell-jar, 51 Bellows, camera, 226 Beryllium, 400 Beta ray, 398 Bethe, Dr. Hans, 113 Bifocal lenses, 231 Bimetallic strip, 115 Binary colours, 221 Binoculars, 201 Biot’s spheres, 276 {Exp. 362) Black, 221 Blind spot, 229 Blip, radar, 394 407 1 ; INDEX Bohr, Niels, 399; concept of the atom, 399 Boiling-point, 148; effect of air pressure, 148; table of, 152 Bomb calorimeter, 144 Break and make, 379 Breaking the sound barrier, 60, 61 Breezes, land and
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sea, 130, 131 Bridges, expansion in, 116 Brilliancy, 202 British system of measurement, 9 British thermal unit, 139, 401 Brushes, electric, 314, 321, 323 Bucket and cylinder, 39 Bunsen burner, 163, 164 Buoyancy, 24 Buoyant force, 24 Burette, 12 Burton, Dr. F., 395 Caesium, 219, 347 Calculations with approximate numbers, 13, 14, 15 Calibrating a thermometer, 120, 124 Calipers, 12 Caloric theory, 109 Calorie, 138; food, 139 Calorihc values of fuels, 144 Calorimeter, 142; bomb, 142, 144 Camera, lens, 226, 227; pin-hole, 176-178 Camouflage, 223 Can, overflow, 40, 43 Canadian National Exhibition Band Shell, 58 Carbon dioxide, 156 Carbon resistors, 291; rods, 297; filament, 334 Carrier waves, 347, 393 Catch bucket, 40, 43 Cathode, 295 Cathode rays, 341, 397 {Exp. 383) Cathode ray oscilloscope, 390; tube, 342, 389- 393 Cations, 295 Cat’s fur, 268 Cell, arrangement of, 283; dry, 279; electrolytic, 295, 296, 367, 369; primary, 300; secondary, 300; storage, 300, 372; voltaic, 279, 364 Celsius, 122; scale, 121 Centigrade scale, 121 Centigram, 10; centimetre, 9, 10 Centre of curvature, mirrors, 189; lens, 204 C. G. S. system of measurement, 9 Chadwick, Sir James, 399 Chain reaction, 401 Changes of state, 146 Characteristics of musical sounds, 63 Charges, electric, kinds of, 268, 269; distri- bution of, 276 Charging, by induction, 273 {Exp. 362) ; by contact, 271 {Exp. 359, 360) Churchill, Sir Winston, 394 Ciliary muscles, 229 Circuit, 281 ; breaker, 284, 327 Climate, 146, 150 Clock, 50, 117 Closed tube, 73, 74 {Exp. 102) Clouds, screening effect of, 135 Coal, 112-113, 156, 401 Coaxial cable, 326 Cobalt, 259 Coefficient of expansion, of gases, 123; of liquids, 119; of solids, 116 Cohesion, force of, 146 Coil, 305 Coil spring, 54 Cold frames
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, 135 Cold light, 176, 218 Collimator tube, 219 Colour, blindness, 223; chart, 221; disc, 215; filters, 221; pigments, 221, 223 {Exp. 253); importance of, 213, 223; nature of, 220222; printing, 223, 224; television, 394; theories of, 220, 222; uses of, 223; vision, 223 Coloured lights, 222 Commutator, 314, 322, 323, 377 Compass, magnetic, 261 Compensating for expansion, 116 Complementary colours, 221 Compound bar, 115, 165 Compound microscope, 232, 396 Compression of gases, ll2 Concave lens, 203; images in, 208 {Exp. 250) focal length, 205 {Exp. 248) Concave mirror, 188; images in, 190 {Exp. 241, 242) Concave reflector, 58, 235 Condensation, 54, 74, 75, 79, 325 Condenser, 328 Condensing lens, 233, 235 Conductance, 289 Conduction of heat, 126, 156; in gases, 128; in liquids, 128; in solids, 126, 134 {Exp. 166) Conductometer, 126, 166 Conductors, electric, 272, 288 {Exp. 361) ; dif- ferent shapes, 276 {Exp. 363) Conductors, of electricity, 272; of heat, 127 Cones, for vision, 229 Conservation of energy, 1 1 Constantan, 291 Continuous spectra, 219 Convection currents, 128-131 ; in liquids, 128130 {Exp. 166); in gases, 130, 135, 142 {Exp. 167) Convention of signs, 192, 205, 208 Converging (Convex) lens, 203; images in, 207 {Exp. 250) ; focal length, 205 {Exp. 248) Chemical, change, 112; compounds, 295; effects of electric current, 295; energy, 300 Converging pencil, 176 Convex mirror, 188; images in, 191 {Exp. Chemistry and colour, 220 Chlorine, 154 Choroid coat, 228 Chromatic, aberration, 210, 224; scale, 84 Chromium plating, 299 243) Cooling system of automobile, 145 Copper, 288; ions, 297; plate, 279; plating, 299 Copper sulphate, electrolysis, 297 {Exp. 370) 408 INDEX Copper voltameter, 298 {Exp. 370) Cornea
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, 228 Corpuscular theory- of light, 181 Corrosion, 299 Cosmic rays, 132 Coulombs, 281 Crest, 52 Critical, angle, 200; pressure, 155; tempera- ture, 155 Crookes, Sir William, 341 Crookes’ tube, 383; dark space, 341 Crystalline lens, 228 Cubic measure, 10 Curie, Pierre and Marie, 397 Current, see electric alternating, direct, convection, Curved mirrors, 188 Cut glass, 202 Cut-out relay, automobile generator, 307-309 Cycle, 50; alternating current, 322 Cyclotron, 400 Cylinder, volume of, 36; automobile, 328 D’Arsonval galvanometer, 310, 377 Davy, Sir Humphrey, 110, 156, 295, 317, 336 Decibel, 64, 92 Decimetre, 9 Declination, angle of, 262 Decomposition of water, 295, 296 Defects, of vision, 230 Degrees, 120; absolute (Kelvin), 123; centi- grade (Celsius), 121; Fahrenheit, 121 Demagnetization, 265 Density, 15; definition, 18 {Exp. 35, 36, 37); effect of expansion on, 124; in relation to convection, 129; maximum density of water, 19, 20; table of, 21 Depolarizing agents, 280 Depth, apparent change in {Exp. 246) Detector circuit, 347 Deuterium, 399 Deuterons, 399 Deviation, angle of, 200 {Exp. 247) Dewar flask, 156 Dial, telephone, 326; thermometer, 117 Diamonds, 199, 202 Diaphragm, camera, 226, 227; telephone, 325 Diatonic scale, 82, 83 Diesel engine, 156, 159, 160 Difference of potential, 280-282 Differential thermometer, 167 Diffusion of light, 184, 185 Dilatometer, 19, 20 Diode tube, 344, 345; as a rectifier, 345 {Exp. 386) ; thermionic emission, 344 {Exp. 384) Dioptre, 205 Dip, magnetic, 263 Dipping needle, 263 Direct current, 346; generator 323 380); motor, 313, 314 {Exp. 377) {Exp. Direction of electron flow, 374 Discharge, electrical, 341 ; through gases, 340, 341; tubes, 219 Discontinuous spectra, 219, 220 Discord, 84 Dispersion, of light, 213 {
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Exp. 251) Displacement, 30 Dissociation, 295, 296 Distance formula, lens, 208; mirror, 192 Distilled water, 295, 373 Distinct vision, least distance of, 229, 231 Distribution of charges, 276 {Exp. 362) Distributor, 328 Diverging lens, 203 Diverging pencil, 176 Division, 14, 15 Doppler’s principle, 66 Drums, 87 Dry, cell, 279-281; dock, 30; ice, 156; steam, 151 Dunlap Observatory, 234 Dynamo, alternating current, 317, 321 Ear, 81, 82; frequency limits of hearing, 65 Earth inductor, 320 {Exp. 380) Earth’s magnetic field, 260 Ebonite rod, 268 Echoes, 58, 59, 394 Eclipses, of moon, 179; of sun, 178, 179 Edison, 334 Efficiencies, of heat engines, 156; of electric heater, 137 Einstein, Dr. Albert, 2, 4, 132, 401 Elasticity, 54 Electric, arc, 326, 336; bell, 306, 307; conductors, 361; eye, 348; furnace, 337; generator, A.C., 321, D.C., 323; heating appliances, 335; insulators, 361; light bulb, 334; organ, 88; power, 337; symbols, 285; welding, 337 Electric charges, 268; induced, 272, 273, 277 {Exp. {Exp. distribution 362) ; kinds of, 269; law of, 269 {Exp. 359) 361); 276 of, Electric circuit, 281; types of, 283 Electric current, 279, 281; and electron flow, 282; alternating, 346, 347; direct, 346; chemical effects of, 295 {Exp. 367-373); energy, 331; induced, 318 {Exp. 378-380); magnetic effects of, 304 {Exp. 373, 374) ; measurement of, 281, 298 {Exp. 370) ; rectified, 345 {Exp. 386) Electric motor, 309, 310; St. Louis, 313, 377; {Exp. 376) Electrical discharge tube, 340 {Exp. 382, 383) Electrical energy, 300, 301, 337 Electricity, 1; heat from, 331, 333; production See Static of, 331; transmission of, 331. Electricity, Electric Current Electrochemical equivalent, 298 Electrodes, 295 Electrolysis, 295, 296 ; uses, 297 ; laws of
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, 298 {Exp. 369) ; of water, 295, 296 {Exp. 367) ; of copper sulphate solution, 297 {Exp. 368) Electrolyte, 279, 295 Electrolytic cell, 295, 296, 367 Electromagnet, 52, 263, 304, 306 {Exp. 375) Electromagnetic, induction, 317, 318; inertia, 327 Electromagnetic, waves, 132, 181, 219, 347; spectrum, 217 Electromagnetism, 317 Electromotive force, 282 {Exp. 379, 380) 409 1 INDEX Electron, flow, 282, 296, 297 ; microscope, 388, 395; theory, 269; volt, 401 Electrons, 269, 342, 397 ; thermionic emission of, 344 [Exp. 384) ; photo-emission of, 347 {Exp. 386) Electronics, 283, 340, 389 {Exp. 382) Electronic tube, 344 ; as amplifier, 346 ; as rectifier, 345 Electrophorus, 273, 274 {Exp. 362) Electroplating, 297, 299 {Exp. 371) Electrorefining, 297 Electroscopes, 270. See gold-leaf and pith-ball Electrostatic, induction, 272, 273 {Exp. 361) ; machine, 275 Electrostatics, 268 Electrotyping, 299 Elements, 270, 295; discovery of, 219; mean- ing of, 112; transmutation of, 399 Elementary (molecular) magnets, 265 Emergent ray, 200 Emission of radiant energy, 133 {Exp. 167) Emission theory of light, 181 E.M.F., 282; induced, 318 {Exp. 379, 380) Energy, 3, 4, 137; atomic, 401; chemical and electrical, 300, 331; conservation of. 111; equation, 401 ; from foods, 139, 144; light, 175; transformation of, 3 Energy levels (orbits), 181, 399 Engines, 156-160 Equilibrium, 147, 148 Equivalent weight, 298 Error, percentage, 13; possible, 13 Ether, 132 Evaporation, boiling and, 148 Exact numbers, 1 Expansion, 115, 119, 122-124 {Exp. 164-165); and density, 124; unusual for water, 19, 20 Eye, 227 Eyepiece, 232 Fahrenheit, D. G., 121 ; scale, 121 Faraday, Michael, 3, 262, 295, 298, 317 Faraday�
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; determination of, 64, 65; of waves, 53; of various sources, 65; ultrasonic, 65, 92 Frequency modulation, 393 Friction, 156; electricity produced by, 268, 270 {Exp. 358) Fuels, 113, 137, 144 Fundamental, 69, 75 Furnace, electric, 337 Fuses, 333 Fusion, meaning of, 146; atomic, 401; heat of, 148 {Exp. 170) Future of sound, 92 Galileo, 2, 3, 120 Galvani, L., 279, 295 Galvanometer, D’Arsonval, 310, 311 Galvanoscope, 309, 375 Gamma ray, 219, 398 Gas, 113; thermometer, 124 Gaseous ions, 340 Gases, thermal conductivity of, 128; convection in, 130 {Exp. 167) ; density of, 21; expansion of, 122-124; liquefaction of, 154; molecular motion of, 110; sound transmission in, 54; spectra of, 219 Generator, 308; A.C., 321 {Exp. 380); D.C., 323 {Exp. 380) Geographic North Pole, 260 Geometric construction of images, 186, 190, 204, 207 Gilbert, Dr. William, 2, 3, 4, 259, 262, 268 INDEX Glare, light, 185 Glass, insulator, 288; plate, 199 {Exp. 245); rod, 268 Gold-leaf electroscope. 271; charging by contact, 272; charging by induction, 273, 274 {Exp. 362) ; identifying charge, 272 {Exp. 360) Graduated cylinder, 12, 34 Gram, 10 Graphite, 299, 401 Greenhouse, 134, 135 Grid, 301, 344, 346, 348 Ground, 273, 274, 285 of, 128-131 Half-wave rectification, 346 Hammond organ, 88 Harmonic, 69 Headlights, car, 194 Hearn, R. L., Generating Station, 158 Heat, 1, 3; absorption of, 133 {Exp. 168); conductivity of, 126-128 {Exp. 166) ; con166) ; expanvection sion caused by, 122-124 {Exp. 164. ’65) ; insulators, 127, 128, 142; measurement of, 137-162; nature of, 110; of fusion, 146-150 {Exp. 170); of vaporization, 150132-136 153 {Exp. 167); sources of.
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111; specific heat, and 139-146 {Exp. temperature, 169) ; 119-121; thermometers, 119-122; transfer of, 126; theory of, 109, 110; and work, 156 Heat exchange, principle of, 137, 140; dur- radiation 172) ; {Exp. {Exp. 109, of, ing changes of state, 146 Heat from electric current, 331, 333 Heat from nuclear fission, 401 Heating appliances, electrical, 335 Heating systems, hot water, 145, 152; hot air, 130, 131 119, 129, 130, Heavy water, 399 Helium, 113, 124, 220, 398, 400 Helix, 305; rule, 306 {Exp. 375) Herschel, Sir W., 216 Herschel’s divided tube, 77, 78 {Exp. 105) Hoffman water voltameter, 296 Horse-power, 337 Horseshoe, magnet, 260 ; electromagnet, 306, 307 Hot-air heating, 130, 131 Hot-water, heating, 119, 129, 130, 145, 152; supply, 129-130 frequency limits of Hues, 221 Human, ear, 82; hearing, 65; eye, 227 81, Humours, aqueous, vitreous, 228 Huygens, C., 181 Hydro-electric power, 331 Hydrogen, 113, 154, 280, 295, 399; bomb, 401; ion, 296, 398; isotopes, 399, 401 Hydrometer, 27-29 {Exp. 44, 45) Hydroxyl ion, 296, 297 Hypo (sodium thiosulphate), 196 Image, in camera, 226; real, 178, 190, 192, 207; virtual, 185, 186, 191, 192, 207; in {Exp. 250), convex, lenses, concave, 207 185 207 {Exp. 239), concave, 190 {Exp. 242), con{Exp. 243), inclined, 187 {Exp. vex, 191 240), in pin-hole camera, 178 {Exp. 237) in mirrors, plane, {Exp. 250) : Image orthicon, 391 Incandescence, 175 Incandescent lamp, 334 Inch, 7, 9, 10 Incident rays, 183 Inclination, angle of, 263 Index of refraction, 198, 224 {Exp. 243) Induced charges, 272, 273, 277 {Exp. 361) Induced current (E
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.M.F.) 318; cause of, 318 {Exp. 378); direction of, 319 {Exp. 380); self, 327 magnitude of, 318 {Exp. 379) ; {Exp. 382) Induced magnetism, 263, 264 {Exp. 354, 355) Induction, 263; charging electroscope by, 273, 274 {Exp. 362) Induction coil, 327 ; uses, 328, 383 Infra-red radiations, 132, 216 In parallel, 284, 312 Input, 112 In series, 283, 312 Instruments, measuring, 12; musical, 85-88 Insulators, of electricity, 272, 288 {Exp. 361) ; of heat, 128, 142; of sound, 64, 88, 89 Intensity of sound, 63, 64, 77, 78 Interference, 54, 55; of sound waves, 76-79 Internal combustion engine, 159; ignition sys- tem, 328 International ampere, 298 Invar, 117 Invisible radiation, 216, 218 Ionization, in gases, 340; theory of, 295 Ions, 295; gaseous, 340 Ion-pairs, 295 Iris, 228; reflex, 228 Iron, 259; electric, 335, 336; filings, 259, 260; soft, 263, 264 Irregular reflection, 185 Isotopes, 399 ; of hydrogen, 399 Jupiter, moons of, 179 Kaleidoscope, 188 Keeper, magnetic, 265, 266 Kelvin, Lord, 123 Kelvin (absolute) temperature, 123 Kettle, electric, 335 Kilo, gram, 10; metre, 9; volt, 282; watt, 337 Kilogram calorie, 139 Kilowatt-hour, 113, 337 Kinescope, 391 Kinetic theory, 110 Knife switch, 284 Knowledge, seven steps to, 4 Ice, dry, 156; heat of fusion of, 150 {Exp. 170) Ignition system, 328 Lactometer, 29 Lamp, arc, 336, 386; electric 334 Land breezes, 130, 146 Larynx, 81 411 ) INDEX Laterally, displaced, 199; inverted, 186 Law of: conservation of energy 111; electrolysis, 298; electrostatics, 269 {Exp. 359) ; of sound, 64; Lenz, 319 {Exp. intensity 380); magnetism, 260, 352; reflection, of sound, 58, of light, 184; refraction of light, 197; vibrating strings, 67-69 {Exp. 99
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) Lead, 397, 398 ; spongy, 301 Lead-acid storage battery, 300 {Exp. 372) Lead peroxide, 301 Left-hand rule, 305 {Exp. 374) Length, measurement of, 7, 8, 9; of conduc- tor and resistance, 288 Lenses, accommodation, 229; action of, 203 {Exp. 250); applications of, 210; crystalline, 228; focal length of, 205 {Exp. 249); formulae, 208 ; power of, 205 Lenz’s Law, 319 {Exp. 380) Lifting electromagnet, 308 Light, 1, 175; colour of, 213-214; diffusion of, 185; dispersion of, 213; nature of, 175; reflection of, 183 {Exp. 238) ; refraction of, 196 {Exp. 243); sources of, 175; theories of, 181 ; transmission of, 96, 132, 176; velocity of, 179, 180 Light, energy, 349; meter, 349; microscope, 396; sensitive metals, 347; year, 180; waves, 394 Lightning, 276; rods, 276, 277 Limits of audibility, 65 Line spectra, 219 Lines of magnetic force, 262, 263 {Exp. 353) Linear measure, table of, 9, 10 Liquefaction, 146; of gases, 154 Liquid, buoyancy of, 24, 25, 26 ; heat conductivity of, 128, 166; convection in, 128-130 {Exp. 166); density of, 18, 21; expansion of, 19-20, 119; specific gravity of, 21 {Exp. 38, 42, 45) Liquid air, 155, 156 Litre, 10 Local action, 280 {Exp. 364) Lodestone, 259 Longitudinal vibrations, 51 {Exp. 95) Longitudinal waves, 54 {Exp. 97) Loop or coil, 305 Loops, 57, 69, 74, 86 {Exp. 99) Loudness of sound, 50, 63-64, 77, 78 Lower fixed point, 120 Lucite, 358 Luminous bodies, 175 Magnet, 260; kinds, 266 Magnetic, circuit breakers, 307; compass, 261; effects of electric current, 304, 305 {Exp. 373, 374); field, {Exp. 353); lenses, 395, 396; poles, 260 {Exp. 350-351) ; shield, 264 separator, 264; substances, 259 {Exp. 350) {Exp. 355
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) ; 262, 306 Magnetism, 1, 259; induced, 263 {Exp. 354, 355) laws of, 260, 352 {Exp. 352); terrestrial, 260-263; theory of, 264 {Exp. 357) Magnetite, 259 Magnetization {Exp. 356) Magnification formula, lens, 208; mirror, 192 Magnifying glass, 231 412 Magnifying power, 231, 232, 233, 395 Major triad, 84 Make and break, 379 Manganin, 288, 291 Mass, 10, 11, 19, 112, 114; number, 398, 399 Matter, states of, 146; and energy, 112, 114, 401 Maximum density of water, 20 Mean, position, 50; solar day, 11 Measurement, 7 ; accuracy of, 11; of heat, 137-162; of of resistance, 292; standard units of, 8; systems of, 7 of mass, length, 10; 9; Measuring devices, 7, 11, 12 Mechanics, 1 Media, for transmitting sound, 51, 52 {Exp. 96); light, 52, 96; radiant energy, 132 Megacycle, 394 Megohm, 282 Melting, definition, 146; point, 147 Meniscus, 34 Mercury, 118-120, 128, 281; switches, 284 Metals, heat conductivity of, 126 {Exp. 166) ; 21; expansion of, density of, {Exp. 165); plating of, 299 {Exp. 371); purification of, 297; specific heat of, 139, 144, 146 {Exp. 169) 115 Meteorological balloons, 31 Method, of science, 4; of mixtures, 144, 148 Metre, 9 Metric system, 9 Mho, 289 Mica, insulator, 288 Michelson, A. A., 180 Microampere, 282 Micrometer screw gauge, 12 Microscope, electron, 388, 395-397 ; optical, 232, 396 Microwaves, 393 Mile, 9 Mini, 10; litre, gram, ampere, 282; metre, 9; volt, 282 Minimum deviation, 200 Mirage, 202 Mirrors, 183; applications of, 193; curved, 184; formulae, {Exp. 240); parallel, 187; plane, 183 {Exp. 239) 192; inclined, 187 10; Mixtures, method of, 144, 148 Moderators, 401 Modes of vibration, in air columns, 74, 75; in
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strings, 69 {Exp. 100) Modulation, 347, 393 Molecules, 19, 112, 128, 132, 134, 152, 297; motion of, 110; kinetic theory of, 110 Molecular theory of magnetism, 265 {Exp. 357) Monochromatic light, 199; flame attachment, 253 Moseley, 398 Motor, principle, 309, 310 {Exp. 376, 377); St. Louis, 313; structure of, 313; direct current, 314 {Exp. 377 Movies, projector, 235; sound, 90, 91 Multiplication, 14, 15 Museum of Science and Industry, 58 Music, 49; analysis of, 391 INDEX Musical, interval, scales, 82-84; sounds. 63 82; instruments, 85-88; Nautilus (submarine), 402 Nearsightedness, 230 Negative, charge, 269-270; photographic, 227 Neon, tube, 176; lamp, 382, 386 Nerve cells, 229 Neutral body, 268, 270 Neutralization, 297 Neutron, 269, 397, 399 Newton, Sir Isaac, 2, 3, 181, 213 Newton’s disc, 215 {Exp. 252) Niagara, power, 332; region, 146 Nichrome, 288, 335 Nickel, 259, 300 Nitrogen, 124, 156, 335, 399; fixation, 337 Node, 57, 69, 74, 99 Noise, 49, 63, 391 ; noise level, 64 Non-electrolyte, 295 Non-luminous, 175 Non-magnetic substances, 259 {Exp. 350) Non-periodic vibrations, 63 Normal, 184, 189 North magnetic pole, 260 N-pole (north-seeking pole), 260 Nuclear, charge, 398; fission, 400; physics, 1, 397 ; reactors, 112, 401, 402 Nucleus, 269, 397 Numbers, exact, 11; approximate, 13 Objective, optical, 232 Octave, 65, 82 Oersted, H. C., 304, 317 Ohm, G. S., 287 Ohm, 3, 281, 288 Ohm’s Law, 287 {Exp. 365) Oil, 112, 113, 160 Oil-immersion microscope, 232 Opaque, 134, 176 Open tube, 74, 86 Optical, centre, 204; density, 197, 199; disc, 183, 184; instruments, 226-235; microscope, 396 Optics, 175 Orbit (or
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energy level), 269, 399 Organ, electric, 88; pipes, 85-87 Origin of sound, 49 Orthicon, 391 Oscillator, 347 Oscilloscope, sound tracings, 63, 70 Ounce, 10 Output, 112 Overflow can, 40, 43 Overtone, 69, 75, 86 Oxidizing agent, 280 Oxygen, 156, 295, 399 Parabola, 194 Parabolic mirrors, 194, 234 Parallax, method of, 34 Parallel, circuit, 283, 284; mirrors, 187 Partial eclipse, 178 Pencils, converging and diverging, 176 Pendulum, 50, 117 {Exp. 94); clock, 12 Penumbra, 178 Percentage error, 13 Percussion, 112 Period of vibration, 50; of wave, 53 Periodic vibrations, 63 Periscope, 201 Perm.alloy, 263 Permanent magnets, 266 Permeability (magnetic), 263 Persistence of vision, 215, 390, 393 Perspex, 201 Phase, 52, 76-78 Phonograph, 90 Phosphors, 394 Phosphorus, 176 Photoelectric cell, 347 Photoelectricity, 347 Photoemission of electrons, 347 {Exp. 386) Photography, 178, 216, 223, 227, 349 Photon, 181 Photosynthesis, 113, 114 Photronic cell, 348 Physical states of matter, 146 Physics, 1 ; definition, 3, 8 Piano, 85 Pictures, motion, 90, 91, 235 Pigments, 221, 223 Pin-hole camera, 176, 177; image in, 226 {Exp. 237) 178, Pint, 10 Pipe organ, 85-87 Pipeless furnace, 131 Pitch, 50, 63-65, 214 {Exp. 99) Pitchblende, 397 Pith-ball electroscope, 270; charging by contact, 271; identifying charge, 271 {Exp. 359) Planck, Max, 181 Plane mirrors, 185; images in, 185, 186 {Exp. 239) Plate, 344; current, 345 Platinum, 296 Plimsoll line, 29, 30 Point of incidence, 184 Points, action of, 276 {Exp. 363) Polarization, 280, 364 Poles, magnetic, 260 Polonium, 397 Polystyrene, 358 Pond, freezing of, 20 Positive charge, 269, 270 Possible error, 13 Potassium, 347; dichromate, 280, 364; nitrate, {Exp. 350-352) 130;
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permanganate, 128, 166 Potential, difference, 280-282; energy, 112 Pound, 10 Power, meaning, 337; of electric current, 324, 337; of lens, 205 Prefixes (metric), 9 Presbyopia, 231 Pressure, atmospheric, 120; cooker, 147; critical, 155; effect on boiling point, 147, 148; in water, 282 Primary, cells, 300; coil, 323, 378; currents, 318, 319, 327 ; colours, 220-222 Principal axis, of lens, 204; of mirror, 189 Principal focus, of lens, 204; of mirror, 189 Principle, {Exp. 39, 40) ; Doppler’s, 66; of flotation, 26 {Exp. 43); Archimedes’, 25 413 7 INDEX of generator, 321, 323 {Exp. 380) ; of heat exchange, 137, 140 {Exp. 169) ; of hydrometer, 28 {Exp. 44) ; of lead-acid storage direct current cell, 300 {Exp. 372) ; motor, 309, 310 {Exp. 376, 377) of Printing, colour, 223, 224 Prisms, 199, 213; reversed, 215, 224 {Exp. 247) ; total reflection, 202 Projector, slide, 235 Proof plane, 272 Protium, 399 Proton, 269, 397, 398 Protractor, 263 Psychology and colour, 220 Pulsating current, 323 Pupil of eye, 228 Purification of metals, 297 Push button switch, 284 Pyrex, 1 1 Pyrite (fool’s gold), 109 Quality of sound, 63, 70, 81 {Exp. 101) Quantity, of heat, 137-139; of electricity, 281, 298 Quanta, 132, 181 Quantum theory, 181 Quartz, prism, 218; lenses, 218 Queen Elizabeth, the, 30 Quick-freeze units, 154 Radar, 394; screen, 395 Radiant, energy, 132; heating, 136 Radiation, 142, 135, 131, 126, 156 {Exp. 167, 168) Radioactivity, 397 Radio, frequency, 347 ; transmission and recep- tion, 347; waves, 219 Radiometer, 134 Radium, 112, 397 Rainbow, 199, 216 Range, electric, 335; finder, 227 Rarefaction, 54, 77, 79, 325 Rays, alpha, 398; beta, 398; cathode, 341, 397; cosmic,
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132; gamma, 219, 398; infrared, 152, 216; light, 175, 176; ultra-violet, 132, 347 Reactors, nuclear, frontispiece, 401-403 Real image, 178, 207 Rear-vision mirror, 193 Receiver, telephone, 325 Receiving station, radio, 347 Reciprocating steam engine, 157 Recomposition of white light, 215 {Exp. 252) Recorder, sunlight, 206 ; tape, 90 Recordings, 90 Rectification, 345 {Exp. 386) Rectifier, tube, 302, 346 ; circuit, 345 Rectilinear propagation, 176, 177 Reducing heat loss, 127 Refining of metals, 297 Reflected rays, 183 Reflecting telescope, 233, 234 Reflection of light, 183, diffuse, 184; laws of, 184 {Exp. 238) ; regular, 184; total, 200 Reflection of sound, 57, 73; laws of, 57 Reflex time, 59 414 Refracting angle, 199 Refracting telescope, 233 Refraction, of light, 196; atmospheric, 202; 197, 198 {Exp. 243) ; laws of, index of, 197 {Exp. 243); through glass plate, 199 {Exp. {Exp. IM) 245) ; through prisms, 199 Refractive index, 197, 198 {Exp. 243) Refractometer, 198 Refrigerator, electric, 153; gas, 154 Regular reflection, 184 Relativity, Theory of, 132 Relay station, T.V., 393 Reproduction of sound, 90-92 Resistance, electrical, 112, 281, 288; factors affecting, 288; measurement of, 292 {Exp. 365-367) ; table of, 288; unit of, 281, 288 Resistance box, 291 Resistors, 288; in series, 288; in parallel, 289; types of, 291, 292 Resolving power, 395 Resonance, 73; and interference, 73-80; and velocity of sound, 60, 75 {Exp. 102) Retina, 229 Reverberation, 59, 88 Rheostat, 292 Rod, unit of measurement, 9 Rods, for vision, 229 Roentgen, 342 ; rays, 343 Rotor of steam turbine, 158 Rounding-off numbers, 14 Rubidium, 219 Ruhmkorff, 327 Ruler, 12, 34 Rumford, Count, 109, 156 Rutherford, 398 Salts, 295 Saturation effect (magnetism), 265 Sav
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175 South magnetic pole, 261 Spark, 326; discharge, 275; plugs, 328 Specific gravity, 20, 301 ; determination of, 27, 28 {Exp. 38, 41, 42) Specific-gravity bottle, 38 Specific heat, definition of, 139 {Exp. 169) Spectacles, 230 Spectra, 213, 214; kinds, 219 {Exp. 251) Spectroscope, 218, 219 Spectrum analysis, 219 {Exp. 253) Sphere, volume of, 36 Spherical, aberration, 193; mirrors, 188 S-pole, 260 Spongy lead, 301 Spot lights, 336 Square measure, 10 Standard pressure, 120 Standing waves, 55-57 {Exp. 98) States of matter, 146 Static, 394; electricity, 268 {Exp. 358) Steam, engine, 157; generating plants, coloured, 223 158, 331; heating, 152; turbine, 157; trap, 151 Step, -down transformer, 324; -up, 324 Steps to knowledge, 4 St. Louis Motor, 313 {Exp. 377, 380) Storage battery, 279; hydrometer, 29 Storage cell, 300; action in, 301, 302 {Exp. 372); structure, 300, 301; uses, 302 Stringed instruments, 85 Strings, vibrating, modes of, 69-70 {Exp. 100); laws of, 67-69 {Exp. 99) Sublimation, 146, 156 Submarine, 30, 402 Subtraction, 14, 15 Subtractive theory of colour, 221 Substitution method for resistance, 292, 293 {Exp. 367) Sugar, 295 Sulphate ions, 296 Sulphuric acid, 279, 301 Sun, 1 12, 175, 349 Sunlight recorder, 206 Sunset, 202 Superposition, of light waves, 181; of.sound waves, 54-57 {Exp. 101) Switches, 284 Symbols, electric, 285 Sympathetic vibrations, 76 {Exp. 103) of liquids, 119; coefficient Tables of; boiling-points, 152; calorific values of fuels, 144; coefficient of cubical expansion of linear expansion of solids, 117; critical pressures and temperatures, 155; efficiencies of heat engines, 156; electrical conductors and insulators, 272; electrical symbols, 285; electrochemical electromagnetic waves, 217; heat conductivities, 127; heats of fusion, 150; heats of vaporization, 152; indices of refraction, 199; linear measure, 9; mass
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, 10; prefixes, 9; resistances, 288; specific gravities, 21; specific heats, 139; units of electricity, 282; velocity of sound and temperature, of sound in various media, 61; volume, 10; wave-lengths of coloured lights, 214 equivalents, velocity 298; 60; Tape recorder, 90 Telephone, 325, 326 Telescopes, 233, 234 {Exp. 254) Television, 391 ; colour, 394; picture tube, 391 Temperature, meaning of, 119; absolute, 123; critical, 155; scales, 121; and quantity of heat, 137; and resistance, 288 Temporary magnets, 266 Terrestrial telescope, 234 Theories, atomic, 270; colour (additive and subtractive), 220-223; electron, 269; heat, 109; light (corpuscular, 181; magnetism, 264-265; molecular (kinetic), (Einstein), 132; wave, of 110; relativity heat, 132, of light, 181, 197, of sound, 54 Thermionic emission of electrons, 344 {Exp. 295; quantum), (Arrhenius), ionization emission, 384) Thermodynamics, 109 Thermograph, 117, 118 Thermometers, 1 19-122 ; calibrating, 120, 124; construction of, 120; dial, 117; how to use, 164 1 Thermos bottle, 134, 156 Thermoscope, 134 Thermostat, 118 Thomson, Sir J. J., 342, 397 Time, 8, Tonic, 82 Total eclipse, 178 Total reflection, 200; prisms, 201, 202 Traffic signals, 223 Transfer of heat, 126 Transformers, 323, 324 415 INDEX Translucent, 176 Transmission of, 126, 128, 132; light, 176; radiant energy, 132134; radio waves, 347; sound, 49-62 electricity, 331 ; heat, Transmitter, telephone, 325 Transmitting studio, 347 Transmutation, artificial, 399 Transparent, 134, 176 Transverse vibrations, 50, 78, 132 {Exp. 94, 95) Transverse waves, 52, 78, 132 {Exp. 97) Triad, major, 84 Triode tube, 344, 346; amplifier, 346; oscil- lator, 347 Tritium, 399 Troughs, 52 Tubes, closed and open, 73, 85-87 Tuning-fork, 55 {Exp. 96) ; interference of sound around, 76
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{Exp. l04) Tuning musical instruments, 79 Tungsten, 335 Turbines, 331; steam, 157 U, see Uranium below Ultrasonic frequencies, 65, 92 Ultra-violet, lamps, 218; radiation, 218; light, 132, 347 Umbra, 178 Unison, 79 Units, electrical, 282; heat, 138; measure- ment, 8-11; power, 337; sound, 64 Universal hydrometer, 28 Upper fixed point, 120 Uranium, U, 112; U-235, 400; disintegration series, 397; salts, 397; and coal, 401 Vacuum, 52, 132, 154; bottle, 134; evaporator, 147; tube, 132, 340 {Exp. 384, 386); tunnel (light), 180 Valves, 344 Vaporization, 146; heat of, 150 {Exp. 172) Variable resistors, 292 Velocity, definition, 53; of light, 179, 198; of radio waves, 132; of sound, 59, 75 {Exp. 102 ) Ventilation, 130 Vernier calipers, 12 Vertex, 189 Vibrating strings, laws of, 67-69 {Exp. 99) ; mode of vibrations 69 {Exp. 100) Vibration, 325; definition, 49; forced, 66; frequency for light, 214; kinds of, 50, 51; air columns, 74; of longitudinal, 51; of strings, 67-69; sympathetic, 76; transverse, 50 View finder, camera, 227 Virtual image, 185, 186, 191, 207 Vision, defects of, 230; persistence of, 215, 390, 393 Visual, angle, 231; cells, 229 Vita glass, 218 Vitreous humour, 228 Vocal cords, 81 Voice, 81 416 Volt, 282, 288 Volta, A., 3, 273 Voltage-drop, 282, 283, 288 Voltaic cell, 279 {Exp. 364) Voltameter, water, 296, 368; copper, 298 {Exp. 370) Voltmeter, 311-313 Voltmeter-ammeter method for resistance, 292 {Exp. 366) Volume, 10, 36 Watch, stop, 12; balance wheel, 116-117 Water, 295; boiling point of, 120; conductivity of, 127, 128; convection in, 129; density of, 18-20; displacement, 30; electrolysis of, 295, 296; expansion of, 124; heating
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for secondary schoo 1 39903263 CURR HIST,X- CJ STUDENT ohool BASIC PHYSICS THE MACMILLAN COMPANY OF CANADA LIMITED checks (NOTE TO SELF: Add to this table as we go along with examples from each section.) Now you don’t have to memorise this table but you should read it. The best thing to do is to refer to it every time you do a calculation. 1.9 Temperature We need to make a special mention of the units used to describe temperature. The unit of temperature listed in Table 1.1 is not the everyday unit we see and use. Normally the Celsius scale is used to describe temperature. As we all know, Celsius temperatures can be negative. This might suggest that any number is a valid temperature. In fact, the temperature of a gas is a measure of the average kinetic energy of the particles that make up the gas. As we lower the temperature so the motion of the particles is reduced until a point is reached 8 where all motion ceases. The temperature at which this occurs is called absolute zero. There is no physically possible temperature colder than this. In Celsius, absolute zero is at 273oC. Physicists have deflned a new temperature scale called the Kelvin scale. According to this scale absolute zero is at 0K and negative temperatures are not allowed. The size of one unit kelvin is exactly the same as that of one unit Celsius. This means that a change in temperature of 1 degree kelvin is equal to a change in temperature of 1 degree Celsius| the scales just start in difierent places. Think of two ladders with steps that are the same size but the bottom most step on the Celsius ladder is labelled -273, while the flrst step on the Kelvin ladder is labelled 0. There are still 100 steps between the points where water freezes and boils. ¡ water boils ---> |----| |----| |----| |----| |----| 102 Celsius 101 Celsius 100 Celsius Celsius 99 Celsius 98 |----| |----| |----| |----| |----| 375 Kelvin 374 Kelvin 373 Kelvin 372 Kelvin 371 Kelvin ice melts ---> |----| |----| |----| |----| |----| 2 1 0 -1 -2 Celsius Celsius Celsius Celsius Celsius...... |----| |----| |----| |----| |----| 275 Kelvin 274 Kelvin 273 Kelvin 272 Kelvin 271 Kelvin |----
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| |----| |----| |----| |----| -269 Celsius -270 Celsius -271 Celsius -272 Celsius -273 Celsius |----| |----| |----| |----| |----| 4 Kelvin 3 Kelvin 2 Kelvin 1 Kelvin 0 Kelvin absolute zero ---> (NOTE TO SELF: Come up with a decent picture of two ladders with the labels |water boiling and freezing|in the same place but with difierent labelling on the steps!) This makes the conversion from kelvin to Celsius and back very easy. To convert from Celsius to kelvin add 273. To convert from kelvin to Celsius subtract 273. Representing the Kelvin temperature by TK and the Celsius temperature by ToC, TK = ToC + 273: (1.1) It is because this conversion is additive that a difierence in temperature of 1 degree Celsius is equal to a difierence of 1 kelvin. The majority of conversions between units are multiplicative. For example, to convert from metres to millimetres we multiply by 1000. Therefore a change of 1m is equal to a change of 1000mm. 1.10 Scientiflc Notation, Signiflcant Figures and Rounding (NOTE TO SELF: still to be written) 9 1.11 Conclusion In this chapter we have discussed the importance of units. We have discovered that there are many difierent units to describe the same thing, although you should stick to SI units in your calculations. We have also discussed how to convert between difierent units. This is a skill you must acquire. 10 Chapter 2 Waves and Wavelike Motion Waves occur frequently in nature. The most obvious examples are waves in water, on a dam, in the ocean, or in a bucket. We are most interested in the properties that waves have. All waves have the same properties so if we study waves in water then we can transfer our knowledge to predict how other examples of waves will behave. 2.1 What are waves? Waves are disturbances which propagate (move) through a medium 1. Waves can be viewed as a transfer energy rather than the movement of a particle. Particles form the medium through which waves propagate but they are not the wave. This will become clearer later. Lets consider one case of waves: water waves. Waves in water consist of moving peaks and troughs. A peak is a place where the water rises higher than
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when the water is still and a trough is a place where the water sinks lower than when the water is still. A single peak or trough we call a pulse. A wave consists of a train of pulses. So waves have peaks and troughs. This could be our flrst property for waves. The following diagram shows the peaks and troughs on a wave. Peaks Troughs In physics we try to be as quantitative as possible. If we look very carefully we notice that the height of the peaks above the level of the still water is the same as the depth of the troughs below the level of the still water. The size of the peaks and troughs is the same. 2.1.1 Characteristics of Waves : Amplitude The characteristic height of a peak and depth of a trough is called the amplitude of the wave. The vertical distance between the bottom of the trough and the top of the peak is twice the amplitude. We use symbols agreed upon by convention to label the characteristic quantities of 1Light is a special case, it exhibits wave-like properties but does not require a medium through which to propagate. 11 the waves. Normally the letter A is used for the amplitude of a wave. The units of amplitude are metres (m). 2 x Amplitude Worked Example 1 Amplitude Amplitude Question: (NOTE TO SELF: Make this a more exciting question) If the peak of a wave measures 2m above the still water mark in the harbour what is the amplitude of the wave? Answer: The deflnition of the amplitude is the height that the water rises to above when it is still. This is exactly what we were told, so the answer is that the amplitude is 2m. 2.1.2 Characteristics of Waves : Wavelength Look a little closer at the peaks and the troughs. The distance between two adjacent (next to each other) peaks is the same no matter which two adjacent peaks you choose. So there is a flxed distance between the peaks. Looking closer you’ll notice that the distance between two adjacent troughs is the same no matter which two troughs you look at. But, more importantly, its is the same as the distance between the peaks. This distance which is a characteristic of the wave is called the wavelength. Waves have a characteristic wavelength. The symbol for the wavelength is ‚. The units are metres (m). ‚ ‚ ‚ The wavelength is the distance between any two adjacent points which
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are in phase. Two points in phase are separate by an integer (0,1,2,3,...) number of complete wave cycles. They don’t have to be peaks or trough but they must be separated by a complete number of waves. 2.1.3 Characteristics of Waves : Period Now imagine you are sitting next to a pond and you watch the waves going past you. First one peak, then a trough and then another peak. If you measure the time between two adjacent peaks you’ll flnd that it is the same. Now if you measure the time between two adjacent troughs you’ll 12 flnd that its always the same, no matter which two adjacent troughs you pick. The time you have been measuring is the time for one wavelength to pass by. We call this time the period and it is a characteristic of the wave. Waves have a characteristic time interval which we call the period of the wave and denote with the symbol T. It is the time it takes for any two adjacent points which are in phase to pass a flxed point. The units are seconds (s). 2.1.4 Characteristics of Waves : Frequency There is another way of characterising the time interval of a wave. We timed how long it takes for one wavelength to pass a flxed point to get the period. We could also turn this around and say how many waves go by in 1 second. We can easily determine this number, which we call the frequency and denote f. To determine the frequency, how many waves go by in 1s, we work out what fraction of a waves goes by in 1 second by dividing 1 second by the time it takes T. If a wave takes 1 2 a second to go by then in 1 second two waves must go by. 1 = 2. The unit of frequency is the Hz or s¡1. 1 2 Waves have a characteristic frequency. f = 1 T f T : frequency (Hz or s¡1) : period (s) 2.1.5 Characteristics of Waves : Speed Now if you are watching a wave go by you will notice that they move at a constant velocity. The speed is the distance you travel divided by the time you take to travel that distance. This is excellent because we know that the waves travel a distance ‚ in a time T. This means that we can determine the speed. v = ‚ T v ‚ T : speed (
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m:s¡1) : wavelength (m) : period (s) There are a number of relationships involving the various characteristic quantities of waves. A simple example of how this would be useful is how to determine the velocity when you have the frequency and the wavelength. We can take the above equation and substitute the relationship between frequency and period to produce an equation for speed of the form v = f ‚ v ‚ f : speed (m:s¡1) : wavelength (m) : frequency (Hz or s¡1) Is this correct? Remember a simple flrst check is to check the units! On the right hand side we have velocity which has units ms¡1. On the left hand side we have frequency which is 13 measured in s¡1 multiplied by wavelength which is measure in m. On the left hand side we have ms¡1 which is exactly what we want. 2.2 Two Types of Waves We agreed that a wave was a moving set of peaks and troughs and we used water as an example. Moving peaks and troughs, with all the characteristics we described, in any medium constitute a wave. It is possible to have waves where the peaks and troughs are perpendicular to the direction of motion, like in the case of water waves. These waves are called transverse waves. There is another type of wave. Called a longitudinal wave and it has the peaks and troughs in the same direction as the wave is moving. The question is how do we construct such a wave? An example of a longitudinal wave is a pressure wave moving through a gas. The peaks in this wave are places where the pressure reaches a peak and the troughs are places where the pressure is a minimum. In the picture below we show the random placement of the gas molecules in a tube. The piston at the end moves into the tube with a repetitive motion. Before the flrst piston stroke the pressure is the same throughout the tube. JLK When the piston moves in it compresses the gas molecules together at the end of the tube. If the piston stopped moving the gas molecules would all bang into each other and the pressure would increase in the tube but if it moves out again fast enough then pressure waves can be set up. UWV 465 When the piston moves out again before the molecules have time to bang around then the increase in pressure moves down the tube like a pulse (single peak). The piston moves out so fast that a pressure trough
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is created behind the peak. ØŁ æ Æ As this repeats we get waves of increased and decreased pressure moving down the tubes. We can describe these pulses of increased pressure (peaks in the pressure) and decreased pressure (troughs of pressure) by a sine or cosine graph. ´˜ˆ £¥⁄ ƒ¤§ ˇ— ¢¡ 14! " # $ % & ’ ( ) * +,. / Incident ray There are a number of examples of each type of wave. Not all can be seen with the naked eye but all can be detected. 2.3 Properties of Waves We have discussed some of the simple characteristics of waves that we need to know. Now we can progress onto some more interesting and, perhaps, less intuitive properties of waves. 2.3.1 Properties of Waves : Reection When waves strike a barrier they are reected. This means that waves bounce ofi things. Sound waves bounce ofi walls, light waves bounce ofi mirrors, radar waves bounce ofi planes and it can explain how bats can y at night and avoid things as small as telephone wires. The property of reection is a very important and useful one. (NOTE TO SELF: Get an essay by an air tra–c controller on radar) (NOTE TO SELF: Get an essay by on sonar usage for flshing or for submarines) When waves are reected, the process of reection has certain properties. If a wave hits an obstacle at a right angle to the surface (NOTE TO SELF: diagrams needed) then the wave is reected directly backwards. If the wave strikes the obstacle at some other angle then it is not reected directly backwards. The angle that the waves arrives at is the same as the angle that the reected waves leaves at. The angle that waves arrives at or is incident at equals the angle the waves leaves at or is reected at. Angle of incidence equals angle of reection i = r 15 (2.1) Incident ray i = r i r : angle of incidence : angle of reection i r In the optics chapter you will learn that light is a wave. This means that all the properties we have just learnt apply to light as well. Its very easy to demonstrate reection of light with a mirror. You can also easily show that angle of incidence equals angle of reection. If you look directly
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into a mirror your see yourself reected directly back but if you tilt the mirror slightly you can experiment with difierent incident angles. Phase shift of reected wave When a wave is reected from a more dense medium it undergoes a phase shift. That means that the peaks and troughs are swapped around. The easiest way to demonstrate this is to tie a piece of string to something. Stretch the string out at and then ick the string once so a pulse moves down the string. When the pulse (a single peak in a wave) hits the barrier that the string is tide to it will be reected. The reected wave will look like a trough instead of a peak. This is because the pulse had undergone a phase change. The flxed end acts like an extremely dense medium. If the end of the string was not flxed, i.e. it could move up and down then the wave would still be reected but it would not undergo a phase shift. To draw a free end we draw it as a ring around a line. This signifles that the end is free to move. 16 2.3.2 Properties of Waves : Refraction Sometimes waves move from one medium to another. The medium is the substance that is carrying the waves. In our flrst example this was the water. When the medium properties change it can afiect the wave. Let us start with the simple case of a water wave moving from one depth to another. The speed of the wave depends on the depth 2. If the wave moves directly from the one medium to the other then we should look closely at the boundary. When a peak arrives at the boundary and moves across it must remain a peak on the other side of the boundary. This means that the peaks pass by at the same time intervals on either side of the boundary. The period and frequency remain the same! But we said the speed of the wave changes, which means that the distance it travels in one time interval is difierent i.e. the wavelength has changed. Going from one medium to another the period or frequency does not change only the wave- length can change. Now if we consider a water wave moving at an angle of incidence not 90 degrees towards a change in medium then we immediately know that not the whole wavefront will arrive at once. So if a part of the wave arrives and slows down while the rest is still moving faster before it arrives
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the angle of the wavefront is going to change. This is known as refraction. When a wave bends or changes its direction when it goes from one medium to the next. If it slows down it turns towards the perpendicular. 2 17 Air Water If the wave speeds up in the new medium it turns away from the perpendicular to the medium surface. Air Water When you look at a stick that emerges from water it looks like it is bent. This is because the light from below the surface of the water bends when it leaves the water. Your eyes project the light back in a straight line and so the object looks like it is a difierent place. 18 Air Water 2.3.3 Properties of Waves : Interference If two waves meet interesting things can happen. Waves are basically collective motion of particles. So when two waves meet they both try to impose their collective motion on the particles. This can have quite difierent results. If two identical (same wavelength, amplitude and frequency) waves are both trying to form a peak then they are able to achieve the sum of their efiorts. The resulting motion will be a peak which has a height which is the sum of the heights of the two waves. If two waves are both trying to form a trough in the same place then a deeper trough is formed, the depth of which is the sum of the depths of the two waves. Now in this case the two waves have been trying to do the same thing and so add together constructively. This is called constructive interference. A=0.5 B=1 A+B=1.5 If one wave is trying to form a peak and the other is trying to form a trough then they are competing to do difierent things. In this case they can cancel out. The amplitude of the resulting 19 wave will depend on the amplitudes of the two waves that are interfering. If the depth of the trough is the same as the height of the peak nothing will happen. If the height of the peak is bigger than the depth of the trough a smaller peak will appear and if the trough is deeper then a less deep trough will appear. This is destructive interference. A=0.5 B=1 B-A=0.5 2.3.4 Properties of Waves : Standing Waves When two waves move in opposite directions, through each other, interference takes place. If the two waves have the same frequency and wavelength then a speciflc type of constructive interference
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can occur: standing waves can form. Standing waves are disturbances which don’t appear to move, they look like they stay in the same place even though the waves that from them are moving. Lets demonstrate exactly how this comes about. Imagine a long string with waves being sent down it from either end. The waves from both ends have the same amplitude, wavelength and frequency as you can see in the picture below: 1 0 -1 -5 -4 -3 -2 -1 0 1 2 3 4 5 To stop from getting confused between the two waves we’ll draw the wave from the left with a dashed line and the one from the right with a solid line. As the waves move closer together when they touch both waves have an amplitude of zero: 20 1 0 -1 -5 -4 -3 -2 -1 0 1 2 3 4 5 If we wait for a short time the ends of the two waves move past each other and the waves overlap. Now we know what happens when two waves overlap, we add them together to get the resulting wave. 1 0 -1 -5 -4 -3 -2 -1 0 1 2 3 4 5 Now we know what happens when two waves overlap, we add them together to get the resulting wave. In this picture we show the two waves as dotted lines and the sum of the two in the overlap region is shown as a solid line: 1 0 -1 -5 -4 -3 -2 -1 0 1 2 3 4 5 The important thing to note in this case is that there are some points where the two waves always destructively interfere to zero. If we let the two waves move a little further we get the picture below: 1 0 -1 -5 -4 -3 -2 -1 0 1 2 3 4 5 Again we have to add the two waves together in the overlap region to see what the sum of the waves looks like. 1 0 -1 -5 -4 -3 -2 -1 0 1 2 3 4 5 In this case the two waves have moved half a cycle past each other but because they are out of phase they cancel out completely. The point at 0 will always be zero as the two waves move past each other. 21 When the waves have moved past each other so that they are overlapping for a large region the situation looks like a wave oscillating in place. If we focus on the range -4, 4 once the waves have moved over the whole region. To make it clearer the arrows at the
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top of the picture show peaks where maximum positive constructive interference is taking place. The arrows at the bottom of the picture show places where maximum negative interference is taking place. 1 0 -1 0 As time goes by the peaks become smaller and the troughs become shallower but they do not -3 -1 -2 -4 1 3 2 4 move. 1 0 -1 For an instant the entire region will look completely at. -4 -3 -2 -1 0 1 2 3 4 1 0 -1 The various points continue their motion in the same manner. -4 -3 -2 -1 0 1 2 3 4 1 0 -1 0 Eventually the picture looks like the complete reection through the x-axis of what we started -2 -1 -4 -3 4 3 2 1 with: 1 0 -1 0 Then all the points begin to move back. Each point on the line is oscillating up and down -1 -3 -4 -2 2 1 4 3 with a difierent amplitude. 22 1 0 -1 -3 -2 -4 0 If we superimpose the two cases where the peaks were at a maximum and the case where the same waves were at a minimum we can see the lines that the points oscillate between. We call this the envelope of the standing wave as it contains all the oscillations of the individual points. A node is a place where the two waves cancel out completely as two waves destructively interfere in the same place. An anti-node is a place where the two waves constructively interfere. -1 1 4 3 2 Important: The distance between two anti-nodes is only 1 2 ‚ because it is the distance from a peak to a trough in one of the waves forming the standing wave. It is the same as the distance between two adjacent nodes. This will be important when we workout the allowed wavelengths in tubes later. We can take this further because half-way between any two anti-nodes is a node. Then the distance from the node to the anti-node is half the distance between two anti-nodes. This is half of half a wavelength which is one quarter of a wavelength, 1 4 ‚. Anti-nodes To make the concept of the envelope clearer let us draw arrows describing the motion of points along the line. Nodes Every point in the medium containing a standing wave oscillates up and down and the amplitude of the oscillations depends on the location of the point. It is convenient to draw
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the envelope for the oscillations to describe the motion. We cannot draw the up and down arrows for every single point! Reection from a flxed end If waves are reected from a flxed end, for example tieing the end of a rope to a pole and then sending waves down it. The flxed end will always be a node. Remember: Waves reected from a flxed end undergo a phase shift. The wavelength, amplitude and speed of the wave cannot afiect this, the flxed end is always a node. Reection from an open end If waves are reected from end, which is free to move, it is an anti-node. For example tieing the end of a rope to a ring, which can move up and down, around the pole. Remember: The waves sent down the string are reected but do not sufier a phase shift. 23 Wavelengths of standing waves with flxed and open ends There are many applications which make use of the properties of waves and the use of flxed and free ends. Most musical instruments rely on the basic picture that we have presented to create speciflc sounds, either through standing pressure waves or standing vibratory waves in strings. The key is to understand that a standing wave must be created in the medium that is oscillating. There are constraints as to what wavelengths can form standing waves in a medium. For example, if we consider a tube of gas it can have both ends open (Case 1) one end open and one end closed (Case 2) both ends closed (Case 3). † † † Each of these cases is slightly difierent because the open or closed end determines whether a node or anti-node will form when a standing wave is created in the tube. These are the primary constraints when we determine the wavelengths of potential standing waves. These constraints must be met. In the diagram below you can see the three cases difierent cases. It is possible to create standing wave with difierent frequencies and wavelengths as long as the end criteria are met. Case 1 L Case 2 L Case 3 L The longer the wavelength the less the number of anti-nodes in the standing waves. We cannot have a standing wave with 0 no anti-nodes because then there would be no oscillations. We use n to number to
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anti-nodes. If all of the tubes have a length L and we know the end constraints we can workout the wavelenth, ‚, for a speciflc number of anti-nodes. Lets workout the longest wavelength we can have in each tube, i.e. the case for n = 1. ‚ = 2L ‚ = 4L n = 1 Case 1: In the flrst tube both ends must be nodes so we can place one anti-node in the 2 ‚ and we also know this middle of the tube. We know the distance from one node to another is 1 distance is L. So we can equate the two and solve for the wavelength: 1 2 ‚ = L ‚ = 2L Case 2: In the second tube one ends must be a node and the other must be an anti-node. We are looking at the case with one anti-node we we are forced to have it at the end. We know the distance from one node to another is 1 2 ‚ but we only have half this distance contained in the tube. So : 1 2 ( 1 2 ‚) = L ‚ = 4L 24 NB: If you ever calculate a longer wavelength for more nodes you have made a mistake. Remember to check if your answers make sense! Case 3: Here both ends are closed and so we must have two nodes so it is impossible to construct a case with only one node. Next we determine which wavelengths could be formed if we had two nodes. Remember that we are dividing the tube up into smaller and smaller segments by having more nodes so we expect the wavelengths to get shorter. ‚ = L ‚ = 4 3 L ‚ = 2L n = 2 Case 1: Both ends are open and so they must be anti-nodes. We can have two nodes inside the tube only if we have one anti-node contained inside the tube and one on each end. This means we have 3 anti-nodes in the tube. The distance between any two anti-nodes is half a wavelength. This means there is half wavelength between the left side and the middle and another half wavelength between the middle and the right side so there must be one wavelength inside the tube. The safest thing to do is workout how many half wavelengths there are and equate this to the length of the tube L and then solve for ‚. Even though its very simple in this case we should practice our technique: 2(
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1 2 ‚) = L ‚ = L Case 2: We want to have two nodes inside the tube. The left end must be a node and the right end must be an anti-node. We can have one node inside the tube as drawn above. Again we can count the number of distances between adjacent nodes or anti-nodes. If we start from the left end we have one half wavelength between the end and the node inside the tube. The distance from the node inside the tube to the right end which is an anti-node is half of the distance to another node. So it is half of half a wavelength. Together these add up to the length of the tube ‚) = Case 3: In this case both ends have to be nodes. This means that the length of the tube is one half wavelength: So we can equate the two and solve for the wavelength: 1 2 ‚ = L ‚ = 2L To see the complete pattern for all cases we need to check what the next step for case 3 is when we have an additional node. Below is the diagram for the case where n=3 25 Case 1: Both ends are open and so they must be anti-nodes. We can have three nodes inside the tube only if we have two anti-node contained inside the tube and one on each end. This means we have 4 anti-nodes in the tube. The distance between any two anti-nodes is half a wavelength. This means there is half wavelength between every adjacent pair of anti-nodes. We count how many gaps there are between adjacent anti-nodes to determine how many half wavelengths there are and equate this to the length of the tube L and then solve for ‚. 3( 1 2 ‚) = L ‚ = 2 3 L Case 2: We want to have three nodes inside the tube. The left end must be a node and the right end must be an anti-node, so there will be two nodes between the ends of the tube. Again we can count the number of distances between adjacent nodes or anti-nodes, together these add up to the length of the tube. Remember that the distance between the node and an adjacent anti-node is only half the distance between adjacent nodes. So starting from the left end we 3 nodes so 2 half wavelength intervals and then only a node to anti-node distance: 2( 1 2 ‚) + ‚) = Case 3: In this case both ends have to
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be nodes. With one node in between there are two sets of adjacent nodes. This means that the length of the tube consists of two half wavelength sections: 2( 1 2 ‚) = L ‚ = L 2.3.5 Beats If the waves that are interfering are not identical then the waves form a modulated pattern with a changing amplitude. The peaks in amplitude are called beats. If you consider two sound waves interfering then you hear sudden beats in loudness or intensity of the sound. The simplest illustration is two draw two difierent waves and then add them together. You can do this mathematically and draw them yourself to see the pattern that occurs. Here is wave 1: 26 Now we add this to another wave, wave 2: When the two waves are added (drawn in coloured dashed lines) you can see the resulting wave pattern: To make things clearer the resulting wave without the dashed lines is drawn below. Notice that the peaks are the same distance apart but the amplitude changes. If you look at the peaks they are modulated i.e. the peak amplitudes seem to oscillate with another wave pattern. This is what we mean by modulation. 2Amax 2Amin The maximum amplitude that the new wave gets to is the sum of the two waves just like for constructive interference. Where the waves reach a maximum it is constructive interference. The smallest amplitude is just the difierence between the amplitudes of the two waves, exactly like in destructive interference. The beats have a frequency which is the difierence between the frequency of the two waves that were added. This means that the beat frequency is given by fB = f1 j f2 j ¡ (2.2) j ¡ f2 fB = f1 j : beat frequency (Hz or s¡1) : frequency of wave 1 (Hz or s¡1) : frequency of wave 2 (Hz or s¡1) fB f1 f2 27 2.3.6 Properties of Waves : Difiraction One of the most interesting, and also very useful, properties of waves is difiraction. When a wave strikes a barrier with a hole only part of the wave can move through the hole. If the hole is similar in size to the wavelength of the wave difiractions occurs. The waves that comes through the hole no longer looks like a straight wave front. It bends around the edges of the hole. If
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