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electromagnetism | Magnetic force on a magnetic dipole | https://physics.stackexchange.com/questions/66203/magnetic-force-on-a-magnetic-dipole | <p>Can someone please help me understand and answer the question of "why are the forces on a magnetic dipole different in a uniform magnetic field and non-uniform magnetic field?" I know in a uniform magnetic field the magnetic dipole material will tend to align with the field lines of the magnetic field it is in but w... | <p>In uniform field, the net force is zero
$$F=\oint ids\times B=(\oint ids)\times B=0\times B=0 $$
but the torque is not zero,$\tau\neq0$, until it aligns with field lines,</p>
<p>in non-uniform field, both are nonzero $F \neq 0$ , $\tau \neq 0$.</p>
| 300 |
electromagnetism | Direction of magnetic force when magnetic field and velocity are not in same plane | https://physics.stackexchange.com/questions/69413/direction-of-magnetic-force-when-magnetic-field-and-velocity-are-not-in-same-pla | <p>We know from Flaming's Right Hand Rule how to calculate the direction of the magnetic force given the magnetic field and the velocity are in the same plane. Now suppose they are not in the same plane. As an example, consider a uniform magnetic field directed perpendicularly onto the screen and a charged particle mov... | <p>Two vectors (starting or shifted/imagined to start at the same point) always belong to the same plane, usually one plane. If the vectors are $A\to B$ and $A\to C$, just imagine that you connect $B,C$ by a straight line, thus completing a triangle, $ABC$. This triangle is already a clear "seed" of a plane, isn't it? ... | 301 |
electromagnetism | Is it correct to say "like poles attract, unlike poles repel" while two magnets are placed such that one is inside another? | https://physics.stackexchange.com/questions/73953/is-it-correct-to-say-like-poles-attract-unlike-poles-repel-while-two-magnets | <p>As we know a solenoid is considered as a electromagnet(magnet) if there's a current flowing through it. if a soft iron core is placed inside the solenoid, the former get magnetised. Consider the solenoid as a hollow bar magnet, and the magnetised soft iron core inside as a bar magnet. The poles of the magnets next t... | <p>A solenoid <em>induces</em> a magnetic field in the iron bar kept inside it. This is not the same as inserting a permanent bar magnet inside the solenoid. </p>
<p>The induced magnetic field is in the same direction as the original magnetic field. One way of thinking about this is that the <a href="https://en.wikipe... | 302 |
electromagnetism | Particle inside a charged shell | https://physics.stackexchange.com/questions/76598/particle-inside-a-charged-shell | <p>Imagine that I have a particle of charge $q$ at the center of a spherical insulating shell of charge $Q$ and radius $R$.</p>
<p>Both the particle and shell are initially at rest.</p>
<p>Now I apply a force $\mathbf{F}$ to the particle which causes it to have an acceleration $d\mathbf{v}/dt$.</p>
<p>The electric f... | 303 | |
electromagnetism | How DC and AC relays works? | https://physics.stackexchange.com/questions/79438/how-dc-and-ac-relays-works | <p>I was told long time ago that DC relay had a coil. There was a switch (2 wires, one is stable, the other one is flexible) inside the coil. The switch was parallel to the axial direction of the coil. </p>
<p>Today, I am thinking how AC relay works. I go back to think about DC relay. If what I was told is right, the ... | <p>This question is not about solid state relays.</p>
<p>In a DC solenoid type relay, a permanent magnet (sliding center core) moves and actuates switch contact(s) when energized. These are typically used in automotive applications, to provide current to a starter motor through fusible links.</p>
<p>Although AC sole... | 304 |
electromagnetism | How do I find the electric field above the center of a square plate (rather than circular)? | https://physics.stackexchange.com/questions/79858/how-do-i-find-the-electric-field-above-the-center-of-a-square-plate-rather-than | <p>I tried to integrate E due to a line of charge sweeping across the plate, but got bogged down. Any suggestions?</p>
| <p>The trick to this one is knowing how to do the integral. I am going to assume you got an integral that looks like $$\int_{-L/2}^{L/2} \frac{a \lambda dx}{(x^2+d^2)^{3/2}}.$$
The first thing to do to make this easier is non-dimensionalize the integral. Let's start by switching to a non-dimensional integration variabl... | 305 |
electromagnetism | The curl is not zero? | https://physics.stackexchange.com/questions/81063/the-curl-is-not-zero | <p>In Maxwell's equations, the curl of the electric field for a steady state processes (ie. No changing electric or magnetic field) zero. However, if we take a curl along the wire for some distance and perpendicularly out from the wire and back to its original position (perpendicularly entering the wire), it is not ze... | 306 | |
electromagnetism | Electron motion in a wire | https://physics.stackexchange.com/questions/81638/electron-motion-in-a-wire | <p>For my introductory course to electromagnetism (I'm an undergraduate student, so ELI5), I'm trying to get the right conceptual model of electron movement in a thin wire (with constant but non-zero cross section, like a cylinder) due to a constant current. It seems to me, that there are several contributing factors, ... | <p>1 and 3 are the major players here, and chaotic/brownian/random movement with lots and lots of collisions is what the electrons do even when they are being pushed from one place to another as a current. </p>
<p>The electrostatic repulsions and attractions tend to both bring them close and separate them at the same ... | 307 |
electromagnetism | What is the magnetic effect on either of the charges moving parallel | https://physics.stackexchange.com/questions/83089/what-is-the-magnetic-effect-on-either-of-the-charges-moving-parallel | <p>Consider two electrons moving parallel to each other in the same direction with same constant velocity. Will they experience any force due to either of them?</p>
| <p>Yes, they will feel both electric and magnetic force. If you apply a Lorentz boost and get into the frame where they are at rest, they will simply feel the electric field of each other.</p>
<p>However if we stay in the lab frame we will notice that the force they experience goes down with $1/\beta \gamma^2$ (relati... | 308 |
electromagnetism | Is mass of a particle changed when it is charged? | https://physics.stackexchange.com/questions/83382/is-mass-of-a-particle-changed-when-it-is-charged | <p>If a particle of mass $M$ is given an electric charge $Q$, will its mass change?</p>
| <p>What do you mean with "particle"? How would you add a charge?</p>
<p>If you add an electron to an atom you will increase the mass by the electron mass (of course) minus a tiny contribution coming from $m=E/c^2$ where E is the work done taking the electron from where it was to its actual position in the Coulomb pote... | 309 |
electromagnetism | Why is th $\hat{r}$ component zero in this integral? | https://physics.stackexchange.com/questions/86084/why-is-th-hatr-component-zero-in-this-integral | <p>I'm trying to evaluate the magnetic field by calculating the Coloumb integral $\overrightarrow{A}$, and later I will take:
$$\overrightarrow{B}=\nabla \times \overrightarrow{A}$$</p>
<p>However, in the middle of everything, I get to (cylindrical coordinates):</p>
<p>$$\overrightarrow{A}=\frac{\mu_oI}{4\pi} \oint_... | <p>Hint: substitute $u=\cos{(\phi - \phi')}$ and integrate.</p>
| 310 |
electromagnetism | Why don't stationary electric charges possess a magnetic field? | https://physics.stackexchange.com/questions/87818/why-dont-stationary-electric-charges-possess-a-magnetic-field | <p>Why don't stationary electric charges posses magnetic field, while moving charges do?</p>
| 311 | |
electromagnetism | Calculating $dB/dt$ from velocity | https://physics.stackexchange.com/questions/88989/calculating-db-dt-from-velocity | <p>I have recently carried out an experiment to verify Faradays law for a falling magnet. My starting point was to keep both the area of the coil and the number of turns constant whilst changing the velocity (the different velocities were obtained by dropping from different heights).What would be good is if a graph of ... | <p>The problem is that the magnetic field around a bar magnet aren't uniform, so different parts of your coil will experience different values of $B$. If the magnetic is small enough, you can approximate it's field by a <a href="http://en.wikipedia.org/wiki/Magnetic_dipole" rel="nofollow">dipole</a>,</p>
<p>$\mathbf{B... | 312 |
electromagnetism | Help Understanding Equation for Characteristic Time of Induced Magnetic Field | https://physics.stackexchange.com/questions/90371/help-understanding-equation-for-characteristic-time-of-induced-magnetic-field | <p>I am reading <a href="http://books.google.co.uk/books?id=Quv8awSN-RYC&printsec=frontcover&dq=magnetic%20techniques%20for%20the%20treatment%20of%20materials%20pdf&hl=en&sa=X&ei=ePKuUu2lH8yThQfs64D4CA&ved=0CDIQ6AEwAA#v=onepage&q&f=false" rel="nofollow">this</a> book, the part in particu... | 313 | |
electromagnetism | Speed of Magnetic Signal over Large Distance | https://physics.stackexchange.com/questions/89701/speed-of-magnetic-signal-over-large-distance | <p>If I had a very strong magnet on Earth and a very sensitive compass on Mars (just using planets to illustrate large distance), how long would the compass take to notice if I turned the magnet 180deg? I assume it can't beat the speed of light. Do the outer reaches of the magnetic field move as a rigid body with the... | <p>The core of your question (whether the magnetic field moves as a rigid body at long distances) has a fairly straight-forward answer: no, it does not. Changes in a magnetic field propagate at the speed of light. This is a fundamental consequence of <a href="http://en.wikipedia.org/wiki/Maxwell%27s_equations#Vacuum_eq... | 314 |
electromagnetism | Lenz's Law and Eddy Currents | https://physics.stackexchange.com/questions/90739/lenzs-law-and-eddy-currents | <p>You can determine the direction of eddy currents according to Lenz's law.
E.g. If a metal sheet is losing flux into the page, it will experience induced eddy currents in a clockwise direction to replace it. </p>
<p>However, this doesn't make sense to me logically. :S Consider a straight current-carrying wire. It ha... | 315 | |
electromagnetism | Why doesn't a particle's velocity effect the strength exerted on it by an electric field? | https://physics.stackexchange.com/questions/92170/why-doesnt-a-particles-velocity-effect-the-strength-exerted-on-it-by-an-electr | <p>Here is what I know:</p>
<p>$F = E q = m a$ </p>
<p>so $a = \frac{E q}{ m}$</p>
<p>and we can increase the acceleration ($a$) of a particle in an electric field ($E$) by either decreasing its mass ($m$) or increasing its charge ($q$).</p>
<p>Here is where I am confused:</p>
<p>$a = \frac{v}{ t}$, thus $E = \... | <p>Recall that in this scenario the electric field is some given value - probably a constant - established as a parameter of the problem. If you've been given $E$, you can solve for $a$ (and therefore $v$). Or, having been given $a$, you can compute the necessary $E$ to account for such an acceleration. However, they a... | 316 |
electromagnetism | AC Electromagnets | https://physics.stackexchange.com/questions/93356/ac-electromagnets | <p>Could someone help explain the uses of AC <a href="http://en.wikipedia.org/wiki/Electromagnet" rel="nofollow">electromagnets</a>. Wherever I look it says that DC electromagnets create stronger magnetic fields. I understand why AC electromagnets could be used in transformers but why use them in motors for example? Po... | <p>In motors AC electromagnets can be useful as after half the rotation the magnetic field could be reversed and a permanent magnet placed on rotor could then continue to make a complete turn which would have stopped if the magnetic field was constant. But most of the motors rely on a constant magnetic field in which a... | 317 |
electromagnetism | A simple way to calculate the potential electrical output of a magnet. | https://physics.stackexchange.com/questions/93557/a-simple-way-to-calculate-the-potential-electrical-output-of-a-magnet | <p>I am a software engineer. This I understand very well. I am attempting to build a prototype with arduino circuits. This I know a little less about. Part of my project requires a electro-magnetic generator. This I know nothing about. </p>
<p>I looked up Gauss's law and embarrassingly the math is a bit above my pay g... | <p>It is not actually <a href="http://en.wikipedia.org/wiki/Gauss%27s_law" rel="nofollow">Guass' Law</a> (which concerns itself with electric fields) you want, but <a href="http://en.wikipedia.org/wiki/Faraday%27s_law_of_induction" rel="nofollow">Faraday's Law of Induction</a>.</p>
<p>Even then, the <em>only</em> way ... | 318 |
electromagnetism | The force exerted on a magnetic dipole $m$ in magnetic field $B$ | https://physics.stackexchange.com/questions/94605/the-force-exerted-on-a-magnetic-dipole-m-in-magnetic-field-b | <p>How can one prove that:</p>
<p>The force exerted on a magnetic dipole $m$ in magnetic field $B$, in addition to $F=\nabla(m\cdot B)$, can be expressed by </p>
<p>$$F=(m\times \nabla)\times B.$$</p>
| <p>In view of a known identity concerning the cross product:
$$(m\times \nabla)\times B = \nabla (m\cdot B)- m (\nabla \cdot B) = \nabla (m\cdot B) = F$$
because $\nabla \cdot B=0$.</p>
| 319 |
electromagnetism | Why aren't all conductors always charged? | https://physics.stackexchange.com/questions/96169/why-arent-all-conductors-always-charged | <p>If you place a conductor beside an insulator, the insulator will become negatively charged and the conductor will become positively charged. Air is an insulator. So why don't all conductors placed in air automatically become positively charged and the air around it become negatively charged?</p>
<p>See, for example... | <p>The triboelectric effect isn't a result of placing two materials next to each other, it comes as a result of rubbing them together. This is important because the two materials do not naturally want to become charged, you have to add energy, typically in the form of the friction that comes from rubbing. This overcome... | 320 |
electromagnetism | Determining the minimum pull force of a magnet required to hold it in place | https://physics.stackexchange.com/questions/96579/determining-the-minimum-pull-force-of-a-magnet-required-to-hold-it-in-place | <p>If I place a magnet on the underside of a metal object so that it is held there by its attraction to the metal, how can I determine the minimum pull force the magnet needs to have in order to keep it attracted to the metal and not fall off? Would the required pull force be equivalent to the magnet's weight?</p>
| <p>What makes a magnet attract to a surface? What makes a magnet <I>strongly</I> attract to a surface at a given field energy? Look at the pole structure of a refrigerator magnet, then "divergence" of a field. A simple Fe-Nd-B N45 magnet will pull hundreds of times its weight. But wait!</p>
<p><a href="http://imag... | 321 |
electromagnetism | induced emf when a wire or coil travel through a magnetic field | https://physics.stackexchange.com/questions/98463/induced-emf-when-a-wire-or-coil-travel-through-a-magnetic-field | <p>I've recently learned that If you move a conductor through a magnetic field, an emf is induced across the ends of the conductor: </p>
<p>$E = BLv $ </p>
<p>I've also been told that the same is true for a coil but that the equation is: </p>
<p>$E = BLvN$</p>
<p>And that for a coil, there is only an emf induced wh... | <p>You're almost right. The induced electromotive force due to external magnetic field is proportional to rate of change of magnetic flux through the circuit. In case of the coil moving in a uniform field, the flux through it (proportional to the number of turns) does not change in time. In case of the wire, the flux c... | 322 |
electromagnetism | Question about laws of conservation in electrodynamics | https://physics.stackexchange.com/questions/32335/question-about-laws-of-conservation-in-electrodynamics | <p>One way of deriving laws of energy, impulse and angular momentum of electromagnetic field conservation is following:</p>
<ol>
<li><p>Introduce two values below:
$$
\mathbf P = \frac{c}{4 \pi}[\mathbf E \times \mathbf B], \quad W = \frac{1}{8 \pi}(\mathbf E^{2} + \mathbf B^{2}).
$$</p></li>
<li><p>Using Maxwell's eq... | 323 | |
electromagnetism | How does one prove:$\nabla(\vec{\mu_m}\cdot\vec{B})\cdot\vec{dr}=0$? | https://physics.stackexchange.com/questions/34056/how-does-one-prove-nabla-vec-mu-m-cdot-vecb-cdot-vecdr-0 | <p>Work done by a magnetic force(even over an infinitesimally short displacement)=0</p>
<p><em>Net Force</em> on a current loop in an external magnetic field is given by: $$\vec{F}=\nabla(\vec{\mu_m } \cdot \vec{B})$$</p>
<p>How does one prove: $$dW=\nabla(\vec{\mu_m} \cdot \vec{B})\cdot\vec{dr}=0$$</p>
<p>$\vec{\mu... | <p>I had the same problem a few weeks ago. It is clear that an electromagnet does produce work, to solve the paradox you need to take into account the generator that runs the current and balance the energy ( Griffiths p211, introduction to electrodynamics).
In other words, if the magnetic moment is created by a current... | 324 |
electromagnetism | What law of electro-magnetics explains this? | https://physics.stackexchange.com/questions/34265/what-law-of-electro-magnetics-explains-this | <p>I took my son to a science museum where they had a solenoid oriented vertically with a plastic cylinder passing through the solenoid. An employee dropped an aluminum ring over the top of the cylinder when there was no current going through the solenoid. Then they turned on the current going through the solenoid and... | <p>I'll start this with <strong><a href="http://en.wikipedia.org/wiki/Right_hand_grip_rule#Direction_associated_with_a_rotation" rel="nofollow">Right Hand Grip rule</a></strong> for solenoids...</p>
<p>"The coil (solenoid) is held in the right hand so that the fingers point the direction of current through the winding... | 325 |
electromagnetism | Effects of EMP in superconductor-based devices/equipment? | https://physics.stackexchange.com/questions/36145/effects-of-emp-in-superconductor-based-devices-equipment | <p>What are possible effects of electromagnetic pulse / EMP on superconductor-based devices/equimpent/transportation?</p>
<p>Are they resilient or more sensitive to EMP?</p>
| 326 | |
electromagnetism | Do magnets work in outer space? | https://physics.stackexchange.com/questions/37920/do-magnets-work-in-outer-space | <p>Is there any media where magnet lose its property?</p>
| <p>Magnets work perfectly in the vacuum – and in the absence of a gravitational field. They don't depend on any "environment" or "medium". And the electromagnetic force is independent of gravity, too.</p>
<p>In analogy with sound, some 19th century physicists thought that electromagnetic phenomena required a medium ca... | 327 |
electromagnetism | Is it possible to see domains in a metal/magnet under microscope | https://physics.stackexchange.com/questions/39024/is-it-possible-to-see-domains-in-a-metal-magnet-under-microscope | <p>I was just curious to know that is it possible to see domains of a magent/metal in motion on being magnetized, under microscope. If there is someone who has access to microscopes, can help me out on this. I was thinking of buying one.
Question is lil stupid but I am putting it here because I know no better place !!<... | <p>You actually have a couple of options without have to resort to a microscope. One of the most interesting is <a href="http://en.wikipedia.org/wiki/Magnetic_field_viewing_film" rel="nofollow">Magnetic Field Viewing Film</a> which should allow one to view magnetic fields easily over large domains. The film is <a hre... | 328 |
electromagnetism | Magnetic field lines can be entirely confined within the core of a toroid, but not within a straight solenoid. Why? | https://physics.stackexchange.com/questions/38067/magnetic-field-lines-can-be-entirely-confined-within-the-core-of-a-toroid-but-n | <p>I need a full explantation for this concept.</p>
<p>Magnetic field lines can be entirely confined within the core of a toroid, but not within a straight solenoid.</p>
| <p>This is a solenoid and its magnetic field lines.</p>
<p><img src="https://i.sstatic.net/R11ij.png" alt="solenoid"></p>
<p>This is a <a href="http://en.wikipedia.org/wiki/Toroidal_inductors_and_transformers" rel="nofollow noreferrer">toroid</a> and its magnetic field lines </p>
<p><img src="https://i.sstatic.net/Y... | 329 |
electromagnetism | Bend or concentrate magnetic field? | https://physics.stackexchange.com/questions/41011/bend-or-concentrate-magnetic-field | <p>What are the ways to modify the form of magnetic field from the permanent magnet? For example I have a permanent neodymium magnet. Its magnetic field is distributed at large volume around the magnet, with decreasing strength at larger distances from the magnet. I'd like to make it concentrated in very small distance... | <p>To modify the distribution of magnetic field (irrespectively of the source - PMs or coils), you need some material with non-unit magnetic permeability, such as steel. These materials 'concentrate' the field lines, pulling them in away from the surrounding air (which has a permeability of very nearly 1). So to shield... | 330 |
electromagnetism | Proving the existence of the magnetic potential | https://physics.stackexchange.com/questions/41210/proving-the-existence-of-the-magnetic-potential | <p>Suppose $\vec{B}$ is a differentiable vector field defined everywhere such that $\nabla\cdot \vec{B}=0$. Define $\vec{A}$ by the integral
$$A_1=\int_0^1 \lambda(xB_2(\lambda x,\lambda y,\lambda z)- yB_3(\lambda x,\lambda y,\lambda z)) d\lambda$$
Together with its two cyclic permutations for $A_2,A_3$</p>
<p>I'm try... | <p>You are right in both your specific questions: your $\lambda$ derivative is right and the partial derivatives can go inside the integral.</p>
<p>You have, however, one crucial mistake in your original formula, which should read
$$A_1=\int_0^1 \lambda(\quad z\quad B_2(\lambda x,\lambda y,\lambda z)- yB_3(\lambda x,\... | 331 |
electromagnetism | how do you destroy magnetic field - demagnetize? | https://physics.stackexchange.com/questions/43550/how-do-you-destroy-magnetic-field-demagnetize | <p>And what happens with the magnetic field of a star that goes supernova? The magnetic radiation is scattered through the cosmos? Each particle will go away with its own magnetic radiation?</p>
| <p>In a star the magnetic field isn't a "thing" in it's own right, it's the result of motion of charged particles in the star. When a supernova goes bang the matter ejected will have some peculiar motion, i.e. motion relative to the overall outward flow, and as a result there will be magnetic fields in the ejected matt... | 332 |
electromagnetism | What happens when a ferromagnetic object encounters a field too strong for it? | https://physics.stackexchange.com/questions/43584/what-happens-when-a-ferromagnetic-object-encounters-a-field-too-strong-for-it | <p>Just thinking aloud ... It is possible for a star to grow so large it collapses under it's own gravity. Along a parallel path (so to speak) when a conductor carries a current too large for it to sustain, it burns out.</p>
<p>Can a ferromagnetic object be imparted a field too strong for it? What happens then? </p>
... | <p>In a ferromagnet the field is the result of the aligned elecron spins. Once the spins are all aligned the field cannot be made any stronger. You could put a piece of iron in an external field more powerful than the iron could generate on it's own, but it would no especially surprising effect, and when you withdrew t... | 333 |
electromagnetism | Lorentz model and energy exchange | https://physics.stackexchange.com/questions/43878/lorentz-model-and-energy-exchange | <p>The Lorentz model, describing the electron of the atom as an harmonic oscillator forced by an oscillating electric field $\vec{E}$, shows that the dipole moment $\vec{D}$ obeys the following equation in the stationary regime:
$$\vec{D}=\frac{q^2}{2m\omega_0}\frac{\vec{E}}{\omega_0-\omega-i\gamma_d}$$
where $\omega_0... | <p>In the driven oscillator approximation you gave above the external field is so strong that the electron motion "follows" $\vec{E}(t)$ (i.e. it moves with the same phase $+\pi$ = the opposite phase). It is possible when the initial (before switching $\vec{E}(t)$ on) oscillations are "absent" or their contribution is ... | 334 |
electromagnetism | Will a magnet loose too much strength it cut with a hacksaw? | https://physics.stackexchange.com/questions/47574/will-a-magnet-loose-too-much-strength-it-cut-with-a-hacksaw | <p>Can a magnet be cut with a hacksaw, by hand, without losing its essential characteristic of being magnetic? I know i would have to be very careful with heat but I don't know how careful</p>
| <p>This would likely mess up the polarity of the magnet near the area where you are cutting. It would depend on several factors, namely how big the magnet is compared to the hacksaw blade (the bigger the magnet the more likely you are to preserve it) and what the magnet is made out of, for instance if the magnet is ste... | 335 |
electromagnetism | How is the polarity of a magnet decided (before its creation)? | https://physics.stackexchange.com/questions/48130/how-is-the-polarity-of-a-magnet-decided-before-its-creation | <p>This is not how to determine a magnet after it's created, but rather before it is created. How is the polarity of a magnet created (why does one side go this way and the other go the opposite)? Is it randomly decided based on forces acting on the soon-to-be magnet, or is it aligned with the Earth’s magnetic fields... | <p>The alignment is basically random, but it can be tipped in favor of one direction or another based on the ambient magnetic field of the Earth. In fact, when molten ferromagnetic material upwells from deep within the Earth, at, say, the Mid-Atlantic Ridge (see <a href="http://en.wikipedia.org/wiki/Seafloor_spreading#... | 336 |
electromagnetism | Does the shielding around a solenoid actuator affect the force on the plunger? | https://physics.stackexchange.com/questions/48427/does-the-shielding-around-a-solenoid-actuator-affect-the-force-on-the-plunger | <p>I'm looking to minimize the weight of some solenoid actuators I want to build, so I'm trying to better understand the effects of the shielding / frame commonly found on solenoid actuators.</p>
<p>My understanding is the force exerted on the plunger is essentially only dependent on the cross sectional area of the pl... | <p>I spent some time learning <a href="http://femm.info/" rel="nofollow noreferrer">FEMM</a> to figure this out.</p>
<p>The answer is yes, the case does make a notable difference in the pull on your plunger.</p>
<p>Without a case the pull on my sample plunger was 0.036 N</p>
<p><img src="https://i.sstatic.net/Jy9i3.... | 337 |
electromagnetism | 4-velocity and electromagnetic fields | https://physics.stackexchange.com/questions/49736/4-velocity-and-electromagnetic-fields | <p>Can anyone see a reason for $$\left(1+{U_\rho U^\rho\over c^2}\right)\left(U_\nu{d^2 U^\nu\over d\tau^2}\right)=0$$?</p>
<p>Here $U^\rho$ is the 4-velocity for a particle and $\tau$ the proper time. The context is for a particle moving in an electromagnetic field.</p>
<p>I believe it may be useful to introduce the... | <p>The left parentheses are equal to zero due to $U_{\rho}U^{\rho}=-c^2$. This is true for timelike vectors in the (-1,1,1,1) signature.</p>
| 338 |
electromagnetism | How to count magnetic repulsion | https://physics.stackexchange.com/questions/47612/how-to-count-magnetic-repulsion | <p>I have two equal flat round magnets. I know amount of force $F$ which attracts iron objects to one of them and geometric characteristics of magnets. I want to fix first of magnet and some additional mass in the air by second magnet. To do so, I am going to orient magnets so that second magnet repulse first and addit... | <p>To a good approximation, normal magnets can be treated as dipole magnets, in which case the force between them can be found in <a href="http://en.wikipedia.org/wiki/Magnetic_dipole#Forces_between_two_magnetic_dipoles" rel="nofollow">this wikipedia article</a>. To avoid link-only answers, here it is:
$$\mathbf{F} = \... | 339 |
electromagnetism | Relationship between current through a motor and it's load | https://physics.stackexchange.com/questions/51332/relationship-between-current-through-a-motor-and-its-load | <p>When a motor, connected to a battery that has a constant voltage, spins without a load it's speed is higher than with load. I'm told that because of back emfs the current is very small when there's no load because of the higher speed. And so when there is a load the back emf is less as the motor spins slower, and so... | <p>In a DC motor, when the armature rotates its coils cut the magnetic and induce a voltage in the coils. This voltage is of opposite polarity of the voltage that is powering the motor (the battery) and is called the back emf. It is modeled as a voltage source that is proportional to the speed of the motor times a con... | 340 |
electromagnetism | Current induced when dropping a magnet through a coil | https://physics.stackexchange.com/questions/51341/current-induced-when-dropping-a-magnet-through-a-coil | <p>When graphing the induced current in a coil while a magnet is dropped through it why is the total area equal to 0?
The area represents the charge in the coil but why must the resultant flow of charge be 0?</p>
| <p>The EMF induced in the coil is given <a href="http://en.wikipedia.org/wiki/Faraday%27s_law_of_induction" rel="nofollow">by</a>:</p>
<p>$$
\varepsilon=-N\frac{d\Phi_B}{dt}
$$</p>
<p>where $d\Phi_B$ is the magnetic flux through the coil and $N$ is the number of windings. The current through the coil is given by Ohm'... | 341 |
electromagnetism | How do transformers work? | https://physics.stackexchange.com/questions/51936/how-do-transformers-work | <p>A <a href="http://en.wikipedia.org/wiki/Transformer" rel="nofollow">transformer</a> is basically a primary inductor connected to a voltage $U_P$ which you want to transform. You also have an iron rod and a secondary inductor. So when there is a current $I_P$ the iron rods becomes magnetic. When you connect the prim... | <p>It's always handy to have some background information:</p>
<p>In Europe the mains voltage is 230 Volts, which is too much for a lamp for example, so it needs to be lowered to for example 12 Volts. This is done by using a transformer.</p>
<p>The primary winding is connected to the mains voltage of 230 Volts. The AC... | 342 |
electromagnetism | How to interpret Faraday's Law? | https://physics.stackexchange.com/questions/52749/how-to-interpret-faradays-law | <p>Faraday's law is given by:</p>
<p>$\nabla \times E = -\frac{\partial{B}}{\partial{t}}$</p>
<p>On the right hand side of the equation, we have a quantity representing how $B$ changes over time. On the left hand side of the equation we have a quantity representing the spatial variation of $E$ (the curl of $E$).</p>... | <p>I think your last question stated in the following way might clear up your confusion. If I setup an electric field $\mathbf E(t, \mathbf x)$ such that $\nabla\times\mathbf E(t_0,\mathbf x_0)\neq 0$ at some spacetime point $(t_0, \mathbf x_0)$, then Faraday's law tells me that whatever $\mathbf B$-field there is aro... | 343 |
electromagnetism | How to calculate how weak does a magnet get when you get an other magnet closer to it? | https://physics.stackexchange.com/questions/55087/how-to-calculate-how-weak-does-a-magnet-get-when-you-get-an-other-magnet-closer | <p>I heard that when you take two magnets and get them closer together so they reject each other (north pole to north pole or south pole to south pole) they weakens. Does anybody knows how to calculate how much it weakens depending on their distance? </p>
<p>And does it work the other way around? If you take two magne... | 344 | |
electromagnetism | Does a photon have a north and south pole? | https://physics.stackexchange.com/questions/55554/does-a-photon-have-a-north-and-south-pole | <p>A photon has an oscillating magnetic and electric field.</p>
<p>Is the magnetic field a dipole?</p>
| <p>A photon is not the source of an oscillating magnetic and electric field, and it does not have poles. Your confusion stems from the fact that photons can be seen as field quanta for electromagnetic fields. In a sense they are the field, but they do not create it. </p>
| 345 |
electromagnetism | Magnetic properties of matter | https://physics.stackexchange.com/questions/55991/magnetic-properties-of-matter | <p>When a dielectric is placed in an electric field,it gets polarized. The electric field in a polarized material is less than the applied field. Now my query is, when a paramagnetic substance is kept in a magnetic field, the field in the substance is more than the applied fiekd. What is the reason for this opposite b... | <p>The external field causes the <a href="http://en.wikipedia.org/wiki/Magnetic_dipole" rel="nofollow">magnetic dipole</a> moments $\mathbf m$ of the atoms in the material to align with the applied field $\mathbf B$. If one now imagines summing up the fields due to all of the tiny little dipole moments that are now al... | 346 |
electromagnetism | Conservation, Maxwell tensor | https://physics.stackexchange.com/questions/56332/conservation-maxwell-tensor | <p>Can someone please explain to me how the conservation of the energy momentum tensor $$\nabla_\beta T^{\alpha \beta}=0$$</p>
<p>imply the conservation of the Maxwell tensor $$\nabla_\beta F^{\alpha \beta}=0$$?</p>
<p>Additional info: </p>
<p>Note that the $\nabla_\beta$ denotes the covariant derivative.</p>
<p>En... | <p>The claim isn't correct and even the way how the second equation is called, "conservation of Maxwell tensor", is deeply misleading. It makes no sense to call it a conservation law because the integral $\int d^3 x\,F^{0\mu}$ isn't any natural conserved quantity.</p>
<p>The second equation is called one of the Maxwe... | 347 |
electromagnetism | Direction of the Area Vector (with regards to magnetic dipole) | https://physics.stackexchange.com/questions/59498/direction-of-the-area-vector-with-regards-to-magnetic-dipole | <p>I'm learning about torque on a conductive coil in a magnetic field. I have been taught that $\vec\tau = \vec\mu \times \vec{B}$, where $\vec\mu$ is the magnetic dipole moment. Also, $\mu = I\vec{A}$, where $\vec A$ is the area vector of the loop.</p>
<p>To find the direction of the area vector, I am told to use t... | <p>As far as I know, the area vector is a purely mathematical object whose definition is related to the orientability of the surface (in this case, a disk). This is a property of surfaces embedded in an Euclidean space that allows to choose surface normal vector to the surface at every point. For an oriented surface, t... | 348 |
electromagnetism | Electromagnet, ideal turns depending on ohm | https://physics.stackexchange.com/questions/60945/electromagnet-ideal-turns-depending-on-ohm | <p>More turns -> stronger field<br>
more turns -> longer copper wire<br>
longer copper wire -> more resistance(ohm)<br>
<br>
at what turn does the resistance make the electromagnet weaker? - I want to make an ideal electromagnet.
<br><br>
(Sorry for being vague but I'm looking for some kind of formula or an example) <... | <p>You need to combine some equations, lets list them:</p>
<p>Whe asume that you want the formula in terms of potential diferential, with Ohm's Law, $\Delta V = I R$, and the magnetic field inside one solenoid:</p>
<p>$$
B = \frac{N}{L}\mu I = \frac{N}{L}\mu \frac{\Delta V}{R} \quad\quad (1)
$$</p>
<p>where $N$ , $L... | 349 |
electromagnetism | Attraction and repulsion of Magnetic materials | https://physics.stackexchange.com/questions/62653/attraction-and-repulsion-of-magnetic-materials | <p>Why are diamagnetic materials repelled when placed in magnetic field and why are paramagnetic materials attracted when placed in magnetic field?</p>
| <blockquote>
<p>Okay... I write this as an inspiration by Feynman..!</p>
</blockquote>
<p>These "WHY?" and "How's such a thing possible?" are quite related in a way that they make the question <em>ambiguous</em>. The question author wouldn't be satisfied for sure, hearing a physicist who try to answer such <em>hatin... | 350 |
electromagnetism | Is it possible for a charged, fast-moving object to slow down and enter geo-stationary orbit? | https://physics.stackexchange.com/questions/62723/is-it-possible-for-a-charged-fast-moving-object-to-slow-down-and-enter-geo-stat | <p>I've had a wild idea which I can not discuss at length in this forum, but it comes down to the following problem:</p>
<p>A sphere of radius R=~10μm and mass m=~10-16Kgr is travelling towards the earth at v = ~10^8m/sec. The sphere carries a charge Q and intersects the earth's magnetic field perpendicularly, at a di... | <p>Your sphere is traveling at $10^8 \, \mathrm{m/s}$ (33% the speed of light) but Earth's escape velocity is $11200 \, \mathrm{m/s}$. There is absolutely no way the sphere could ever enter any sort of orbit around the Earth, regardless of the interaction between it and the Earth's magnetic field.</p>
<p>$$KE_{relati... | 351 |
electromagnetism | Small charged sphere's motion in earth's magnetosphere? | https://physics.stackexchange.com/questions/63520/small-charged-spheres-motion-in-earths-magnetosphere | <p>Suppose that a spherical metal sphere with mass $m=10^{-16}kgr$ radius $R=10μm$ charge $Q=10^{-9}C$ travels with $v=c/3$ and is trapped in the earth's magnetosphere at a distance around $r = 1000km$. <em>The exact numbers are not that important, I am providing them for order of magnitude considerations</em>. </p>
<... | <blockquote>
<p><em>Will magnetic mirroring and magnetic drift be the same as with plasma</em></p>
</blockquote>
<p><strong>No.</strong> It's behaviour will be vastly different than that for a particle. Large metal objects have free electrons/ions which will attract the charged particles in the magnetosphere. Ultima... | 352 |
electromagnetism | What really is the Magnetic Force on a wire? | https://physics.stackexchange.com/questions/65368/what-really-is-the-magnetic-force-on-a-wire | <p>I have a doubt regarding the significance of a force on a wire. Well, first of all, I know that if I have a particle and if there are several forces acting over it, then we can compute one total force $F$ that gives the same effect as the combination of the several forces, and this force is just the vector sum.</p>
... | <p>The force law you show gives us the <em>total</em> force on the wire. This force comes from the sum total of forces on all the electrons moving through the wire. So imagine that your wire is supported at either end, and the magnetic field is strictly between the two supports, so the total force on the wire is – in... | 353 |
electromagnetism | Energy in magnetic fields | https://physics.stackexchange.com/questions/7238/energy-in-magnetic-fields | <p>If I calculate the energy contained in the electric field for an electric dipole p in an electric field E, I get (ignoring the terms independent of orientation):</p>
<p>$U = - \vec{p} \cdot \vec{E}$</p>
<p>which is as expected. However, if I do the same for a magnetic dipole m in a magnetic field B, I get (again ... | <p>First of all, energy is energy. It may be converted from any form to any other form and there is absolutely no ambiguity about the sign of any form of energy. If $E$ is correctly defined so that it is conserved, the signs in front of all quantities are well-defined.</p>
<p>Your observations about $E^2$ and $B^2$ ha... | 354 |
electromagnetism | Why is there independent motion of Earth's magnetic poles? | https://physics.stackexchange.com/questions/7264/why-is-there-independent-motion-of-earths-magnetic-poles | <p>Earth's N and S magnetic poles "wander independently of each other and are not at exactly opposite positions on the globe" [from WIKI's "Earth's magnetic field"]. Can these independent motions be consistent with the supposed "dynamo effect" from electric currents of a liquid outer core? Can Earth's spheroidal shape ... | <p>Well, Earth does not have a giant bar magnet inside, the outer core is an giant mass of conductive fluid in a complex motion guided by magnetohydrodynamics -- the problem is complicated enough to be unsolvable analytically and really gives no hope of simple solution.</p>
<p>So one should rather think of a geomagnet... | 355 |
electromagnetism | Need some help interpreting a formula inspired from Coulomb's law | https://physics.stackexchange.com/questions/8856/need-some-help-interpreting-a-formula-inspired-from-coulombs-law | <p>It has been more than a decade since I did all vector related math and physics so pardon me if my question does not make sense. I am reading some article that says it was inspired from Coulomb's law and gives the following expression to calculate the partial force that one particle is exerting on another at time $n$... | <p>It's nothing you can rigorously derive from Coulomb's law, but the idea is probably the following: the rightmost factor
$$\frac{p^j - p^i}{|p^j - p^i|}$$ is just the unit vector pointing from particle $i$ to particle $j$, just as in Coulomb's law or Newton's law. Normally, for Coulomb's law, you'd have a factor of $... | 356 |
electromagnetism | How to estimate inductive coupling between distant air coils | https://physics.stackexchange.com/questions/8968/how-to-estimate-inductive-coupling-between-distant-air-coils | <p>I have two air coils (assume they are simple, circular wire loops). They both have diameter <strong>d</strong>. There is a distance <strong>D</strong> between their centres.</p>
<p><strong>D</strong> is much greater than <strong>d</strong> (more than 10x greater)</p>
<p>Both coils are at different angles, <strong>... | <p>I think it's neater to express the various directions in terms of unit vectors. We can convert to angles at the end. So say that $\hat n_1$ and $\hat n_2$ are unit vectors perpendicular to the axes of the coils, and let $\hat r$ be a unit vector pointing from the center of coil 1 to the center of coil 2. Since the c... | 357 |
electromagnetism | Electron gun - electron in cylindrical anode | https://physics.stackexchange.com/questions/11175/electron-gun-electron-in-cylindrical-anode | <p>In an electron gun, the heating filament heats the cathode, releasing electrons by thermionic emission. I've read that <em>"electrons are negatively charged particles and the positively charged cylindrical anode develops a strong electric field that exerts a force on the electrons, accelerating them along the tube"<... | <p>any electric field will accelerate electrons, according to the Lorentz force law:</p>
<p>$\vec{F} = q \vec{E} + \vec{v} \times \vec{B}$,</p>
<p>where q is the charge of the electron, and $\vec{E}$ is the electric field. I assume there is no magnetic field in this example, so the second term is zero.</p>
| 358 |
electromagnetism | One point to change electric field | https://physics.stackexchange.com/questions/13662/one-point-to-change-electric-field | <p>Can there be a charge configuration in space such that at any instant of time I can change the electric field at one and only one point?</p>
| <p>The electric field obeys Maxwell's equations and in particula the Gauss's law. This means that $\rm div E \sim \rho$. If you change the field at only one point, this will introduce an infinite divergence and consequently infinite density. In other words, you'd have to introduce a point charge. But then the field $E$... | 359 |
electromagnetism | Magnetic field in the Centre of Circular loop wire with Current? | https://physics.stackexchange.com/questions/14067/magnetic-field-in-the-centre-of-circular-loop-wire-with-current | <p>By biot-savart:</p>
<p>$$\bar{H} = \frac{I}{4\pi} \oint \frac{d\bar{l} \times \bar{r}}{r^{3}}$$</p>
<p>so</p>
<p>$$\bar{H} = \frac{I}{2a} \hat{n}$$</p>
<p>Please, explain the last implication. I cannot find such integral to match the results. The radius of the loop is $a$. The current is $I$. $d\bar{l}$ is a vec... | <p>For the path element $d\vec{l}$ around a circle with radius $a$ you can write $d\vec{l}=rd\phi\vec{e_\phi}$ with $\vec{r}=-r\vec{e_r}$ (note the minus sign, since the vector points <em>from</em> the wire to the center) you get </p>
<p>$$\vec{H}=\frac{I}{4\pi}\oint \frac{r^2 (-\vec{e_\phi}\times \vec{e_r})}{r^3}d\ph... | 360 |
electromagnetism | Mangnetic Flux summing up like Kirhoff? | https://physics.stackexchange.com/questions/14071/mangnetic-flux-summing-up-like-kirhoff | <p>You have a coil over an iron hearth. There is a current in coil which creates the flux $\phi_{1}$. The flux then distributes over the wider area in the iron (using wrong word?): $\phi_{2}$ the flux over the middle and the $\phi_{3}$ over the left.</p>
<pre><code> |-----------------|
| ... | <p>A fundamental postulate of electromagnetism is that the flux of the magnetic field through any <a href="http://en.wikipedia.org/wiki/Gaussian_surface" rel="nofollow">closed surface</a> is zero. This is essentially a statement that there are no <a href="http://en.wikipedia.org/wiki/Magnetic_monopole" rel="nofollow">... | 361 |
electromagnetism | What Is the Physical Size of a Magnet? | https://physics.stackexchange.com/questions/15343/what-is-the-physical-size-of-a-magnet | <p>With present day materials and technology what is the physical size of a one milli Tesla magnet? How much "power" it has to attract pieces of iron? Please compare it with the objects we have around. What difference in size and "power" it gets each order of magnitude that I go up? What is a weak magnet what is a powe... | <p>I would recommend you take a look at the K&J Magnetics website, particularly the magnet properties calculator: <a href="http://www.kjmagnetics.com/calculator.asp" rel="nofollow">http://www.kjmagnetics.com/calculator.asp</a> </p>
<p>Your question has too much ambiguity for a real answer, but I think that perhaps... | 362 |
electromagnetism | Magnetic Fields | https://physics.stackexchange.com/questions/15856/magnetic-fields | <p>If running a current through a magnet can reverse the magnetic field, then how strong does the current have to be and how would it best be run through the magnet.</p>
| <p>If you had solenoid or another type of electromagnet that produced its magnetic field from a current carrying like a Helmholtz coil then you could switch the direction of the current to reverse the direction of the magnetic field. Simply running a current through a permanent magnet would not reverse the magnetic fie... | 363 |
electromagnetism | Poisson's Equation | https://physics.stackexchange.com/questions/16472/poissons-equation | <p>First up, I needed to compute the potential at some external point due to charge stuffed inside the region between two concentric cylinders, the volume charge density being given.<br/> Two methods came to mind but they are apparently yielding different answers(maybe I goofed up somewhere)
<br/> The first is to solve... | <p>It does give the same result--- it would be good to give your steps, if you want to find the error. The easiest way to see this is to just use Gauss's law: if you make a cylinder of radius R, the electric field outward times the surface area is the charge inside. If the length of the Gaussian cylinder is L, and the ... | 364 |
electromagnetism | Power Generation from Axial and Transverse Emf | https://physics.stackexchange.com/questions/18915/power-generation-from-axial-and-transverse-emf | <p>We consider a flat rectangular plate moving horizontally in a vertical magnetic field,motion being in a direction perpendicular to the length of the plate. We have an emf=BLV between the tips,in the lenhgth wise direction[the axial emf]. During the formation of the axial emf a current flows along the length of the c... | <p>I'm assuming that the magnetic field you're referring to over here is the Earth's field, then we have the field strength of $6.5 * 10^{-5}$. If we assume the vehicle to be a bullet train, then we get the velocity of $300 km / h$ which is $83.33 m/s$ Assuming a scenario where a rectangular plate of $1m$ is being used... | 365 |
electromagnetism | Magnetization of coin on a railway track | https://physics.stackexchange.com/questions/19374/magnetization-of-coin-on-a-railway-track | <p>The rumor was you could make a magnet by leaving a piece of iron on a train track. The train going over it would magnetize it. </p>
<p>Is it true?</p>
| <p>This is almost a duplicate to <a href="https://physics.stackexchange.com/questions/18340/can-you-magnetize-iron-with-a-hammer/18341#comment43493_18341">can you magnetize iron with a hammer</a> . Have a look at it. </p>
<p>The only difference is that the rail lines are fixed in their north south direction for years.... | 366 |
electromagnetism | Would the north poles of two magnets repel each other if a weaker south pole was inserted between them? | https://physics.stackexchange.com/questions/19383/would-the-north-poles-of-two-magnets-repel-each-other-if-a-weaker-south-pole-was | <p>My son asked me this question and I was stumped - my intuition says that is the south pole was strong enough the attraction between the north and south poles would outweigh the repulsion between the two north poles - but how strong would the south pole need to be in this case?</p>
| <p>Assuming long skinny solenoidal magnets, so that their ends look like point magnetic sources, the force between two poles is $M1M2/r^2$, just like electrostatics, with pole density replacing charge density, where $M1$,$M2$ is the strength of the pole, and $r$ is the separation between the poles.</p>
<p>So you have ... | 367 |
electromagnetism | How are magnetic fields transmitted? | https://physics.stackexchange.com/questions/20455/how-are-magnetic-fields-transmitted | <p>A common analogy for gravity is the ball-on-a-rubber-sheet model. In this model, mass distorts spacetime and creates a 'valley' into which other mass can fall. Is this same principal valid for magnetic fields as well (proton-electron)? If so, then how is the repulsion effect modelled?</p>
<p>I ask because the under... | <p>Two things:
First, it is more intuitive to treat gravitational force as equivalent to electrostatic force, due to the existence of monopoles (and gravitational field lines do not form closed loops). There is a magnetic analogue to gravity, known as gravetomagnetism, frame-dragging, or the Lense-Thirring effect.</p>
... | 368 |
electromagnetism | Would a metal enclosure (such as a shipping container) protect its contents from the effects an electromagnetic pulse? | https://physics.stackexchange.com/questions/21180/would-a-metal-enclosure-such-as-a-shipping-container-protect-its-contents-from | <p>I was watching a program about disaster preparedness, and it was suggested that the metal enclosure of a common shipping container (of the <a href="http://en.wikipedia.org/wiki/Intermodal_container" rel="nofollow">intermodal variety</a>) would be sufficient to protect its contents from a large electromagnetic pulse ... | <p>Looks like a metal enclosure would be OK, provided its seams and joints are electromagnetically closed , see <a href="http://www.wbdg.org/ccb/FEDMIL/std188_125_1.pdf" rel="nofollow">http://www.wbdg.org/ccb/FEDMIL/std188_125_1.pdf</a> , however, I am not sure this requirement is satisfied in off-the-shelf containers... | 369 |
electromagnetism | Does inducing a current in a wire have any effect on the strength of a permanent magnet? | https://physics.stackexchange.com/questions/21062/does-inducing-a-current-in-a-wire-have-any-effect-on-the-strength-of-a-permanent | <p>Does inducing a current in a wire result in any changes in the strength of a permanent magnet? Specifically, what would the results of the following controlled experiment be? </p>
<p>You set up two alternators that each simply consist of a permanent magnet located inside a stationary loop of wire as depicted <a hre... | <p>Friction is a process that converts kinetic energy into heat. Its electrical equivalence is restistance; that converts electrical energy into heat. Materials with high magnetic hysteresis can convert magnetic energy into heat. </p>
<p>You may be confused because the usual source of magnetic energy is the physical m... | 370 |
electromagnetism | Are there any non magnetic materials that attract to each other as if they were magnetic? | https://physics.stackexchange.com/questions/24446/are-there-any-non-magnetic-materials-that-attract-to-each-other-as-if-they-were | <p>Are there any non magnetic materials that attract to each other as if they were magnetic?</p>
<p>This is an argument I am having with a friend. </p>
<p>Thanks,</p>
| 371 | |
electromagnetism | Magnetized nail lifting another nail | https://physics.stackexchange.com/questions/24616/magnetized-nail-lifting-another-nail | <p>I have two nails (made of Fe). A and B.</p>
<p>A can not lift B.
If I rub A on the magnets north pole, then it can lift B.
Then if I rub it again but on the south pole, it can not lift B. </p>
<p>why is that? Why cant it lift B anymore?</p>
<p>the title I have chosen for this question is really bad. If someone ... | <p>Iron is composed of small <a href="http://www.britannica.com/EBchecked/topic/357033/magnetic-dipole" rel="nofollow">magnetic dipoles</a> all with their north south axis pointing randomly.</p>
<p>By rubbing the nail in a consistent specific direction the small dipoles line up with the magnetic field, so when you are... | 372 |
electromagnetism | Effect of Charged Particles trapped in Magnetic Field on that Field | https://physics.stackexchange.com/questions/29744/effect-of-charged-particles-trapped-in-magnetic-field-on-that-field | <p>Given a stream of moving charged particles that encounter a uniform magnetic field such that they are trapped in a circular orbit, what effect do these particles have on the net magnetic field over time? Would the magnetic field get stronger or weaker as the number of trapped particles increase?</p>
| <p>They always reduce the field, and this is the law that magnetic fields induce currents that reduce their strength, a special case of LeChatelier's principle.</p>
| 373 |
electromagnetism | How to transfer energy from a generator to a storeage battery | https://physics.stackexchange.com/questions/29512/how-to-transfer-energy-from-a-generator-to-a-storeage-battery | <p>and thank you in advance for taking the time to read my question. To give an idea of my working level, I'm a 21 year old computer science student entering my senior year at college. It's been a few years since my Electricity/Magnetism course, and i'm a bit rusty on the Lorentz Force.
I wanted to create a sort of "H... | <p>First a few lines of basics. If you put a loop into the magnetic field and this loop turns within it, the magnetic flux through loop shall change according to the formula</p>
<p>$$\Phi_B = \vec{B} \cdot \vec{A} = B A \cos\phi = B A \cos\omega t,$$</p>
<p>where $\vec{B}$ is magnetic field strength, $\vec{A}$ is ar... | 374 |
electromagnetism | Find wavelength from relative permittivity and frequency | https://physics.stackexchange.com/questions/30710/find-wavelength-from-relative-permittivity-and-frequency | <p>If i have a EM wave with frequency 1MHZ and εr=9 in a perfect dielectric ,is it possible to find the wavelength λ and wave propagation speed ? It seems impossible to me..</p>
| <p>(1) $\lambda \nu = c$</p>
<p>(2) $c^2 = \frac{1}{\mu_0 \epsilon_r \epsilon_0}$</p>
| 375 |
electromagnetism | EMF in a half-ring shaped conductor around a solenoid | https://physics.stackexchange.com/questions/30715/emf-in-a-half-ring-shaped-conductor-around-a-solenoid | <p>A half-ring shaped conductor is being placed around a solenoid. This solenoid has a changing magnetic field.</p>
<p>a) There is a current and EMF (Electromagnetic force) in the half-ring shaped conductor
b) There is a current but no EMF in the half-ring shaped conductor
c) There is no current but there is an EMF in... | <p>Since there is a steadily changing magnetic flux, there are static closed loops of electric field lines circling the solenoid.</p>
<p>If the half-ring shaped conductor is placed around the solenoid, the mobile charge carriers in the conductor will redistribute in such a way that there is zero <em>net</em> E field i... | 376 |
electromagnetism | How to apply Guass Law to Voltages | https://physics.stackexchange.com/questions/29950/how-to-apply-guass-law-to-voltages | <p>So, I know $\oint E\centerdot dA = 4\pi Q_{enc}$</p>
<p>I'm trying to solve for a TEM mode with two concentric (infinite) cylindrical wave guides of radius a and b, $a<b$. I know that for TEM modes, I can solve by assuming that the outside and inside are at two different potentials, $\pm V$. </p>
<p>I'm told th... | 377 | |
electromagnetism | Farady's law and div B = 0 | https://physics.stackexchange.com/questions/30405/faradys-law-and-div-b-0 | <p>I'm reading a book on electromagnetism and I am a bit confused about some things in Maxwells equations. This is what I don't like about many physics books: they are very wordy, but at the end you don't know what is an experimental fact, what is a "theorem", what is an assumption and so on,...</p>
<p>Anyway the ques... | <ol>
<li>No.</li>
<li>No.</li>
</ol>
<p>More precisely, Biot-Savart relates the curl of $B$ to currents, which is independent of specifying the divergence.</p>
| 378 |
electromagnetism | Physical interpretation of the Maxwell stress tensor | https://physics.stackexchange.com/questions/409568/physical-interpretation-of-the-maxwell-stress-tensor | <p>In 'Introduction to Electrodynamics' by D. Griffiths, shortly after introducing the Maxwell stress tensor there is a paragraph concerning the physical interpretation of the stress tensor $\boldsymbol{T}$</p>
<blockquote>
<p>Physically, $\boldsymbol{T}$ is the force per unit area (or stress) acting on the surface.... | <p>It means a surface element whose tangent plane has a normal in the $j$th direction. For a flat surface, we can shorten that to the normal to the surface pointing in the $j$th direction. For a curved surface, each infinitesimal patch has its own tangent plane.</p>
| 379 |
electromagnetism | Do magnets wear out? | https://physics.stackexchange.com/questions/411253/do-magnets-wear-out | <p>Can a magnet ever wear out or lose strength?</p>
<p>If you break a magnet it (seemingly) gets weaker, but what about from normal use?</p>
<p>Or even very heavy use, like placing 2 magnets facing each other, so that they detract from each other, does that strain cause it to wear quicker?</p>
<hr>
<p><sub>(Note, I... | <p>Yes, a magnet, as time passes, will lose part of his strength. There are two main reasons: </p>
<ol>
<li><strong>Thermal energy</strong>: it causes the disorientation of the atomic magnetic momenta.</li>
<li>If you have a bar magnet free in space it’s easy to see (using <a href="https://en.m.wikipedia.org/wiki/Amp%... | 380 |
electromagnetism | Force on circular loop | https://physics.stackexchange.com/questions/413159/force-on-circular-loop | <p>Would the forces between two circular loops, carrying currents in the same direction, be attractive or repulsive?
Would the forces between two circular loops, carrying current in the opposite directions, be attractive or repulsive?
How to find a force acting on a circular loop?
Please explain with a picture. </p>
| <p>This picture sums up the two scenarios pretty well:</p>
<p><a href="https://i.sstatic.net/6NEvN.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/6NEvN.jpg" alt="enter image description here"></a></p>
<p>To give a little bit of context as to what is happening in each scenario:</p>
<h2>1. When the cur... | 381 |
electromagnetism | Is it correct way of saying that a moving electric field causes magnetic field? | https://physics.stackexchange.com/questions/419379/is-it-correct-way-of-saying-that-a-moving-electric-field-causes-magnetic-field | <p>$$\mathbf B=\mathbf V \times \mathbf E \mu _0 \epsilon _0$$</p>
<p>$\mathbf V$ is the velocity vector of moving electric field. Rest of the parameters follow usual notations for those.</p>
| <p>First of all, <strong>fields don't move</strong> because fields are everywhere in space. For example temperature is a field that has a value at every point in space and all the time, but when the sources moves, the field changes. </p>
<p>So, I suppose you mean by "moving electric field" that the electric field chan... | 382 |
electromagnetism | Electromagnetism Ampere's law Application to solenoid | https://physics.stackexchange.com/questions/423382/electromagnetism-amperes-law-application-to-solenoid | <p>Electromagnetism;Ampere's Law
Application for finding magnetic field strength(B) inside a current carrying solenoid</p>
<p>Question is that why we multiply the current in one loop to the number of turns(enclosed in amperian rectangular loop) ALTHOUGH the current flowing(charges flowing per unit time) is SAME throug... | <p>Answer by @Frecher is correct! I will try to give physical essence to your question.</p>
<p>Each loop of current generates its own magnetic field regardless of whether they are part of the same circuit or not. Think of two loops which are in the same circuit and have the same current flowing through them. Now separ... | 383 |
electromagnetism | How is the waveform of an electromagnetic radiation detected and generated by a resonator (like in FM Radios)? | https://physics.stackexchange.com/questions/424907/how-is-the-waveform-of-an-electromagnetic-radiation-detected-and-generated-by-a | <p>We know that a resonator consists of an inductor and capacitor. And we also know that tuning them in a specific way will get an Electromagnetic radiation detected with similar characteristics. But when the frequency of the original EM changes how do we detect the change(Like In the FM Radios, the frequency changes b... | <p>A pure sine wave RF signal does not carry any information and an ideal resonant circuit would not be able to receive information.</p>
<p>In order to pass some information with an RF carrier, it has to be modulated somehow.</p>
<p>When a carrier is modulated, regardless of the modulation method (AM, FM, etc.), it i... | 384 |
electromagnetism | How do you measure and distinguish between E and D fields, and B and H fields | https://physics.stackexchange.com/questions/429980/how-do-you-measure-and-distinguish-between-e-and-d-fields-and-b-and-h-fields | <p>I’m pretty familiar with maxwells equations, light waves, fields and materials etc, but I’m not clear on how the the various fields are actually measured. How is this typically done? Can the fields inside a material be directly measured?</p>
| 385 | |
electromagnetism | forces on moving charges in a magnetic field | https://physics.stackexchange.com/questions/430088/forces-on-moving-charges-in-a-magnetic-field | <p>When a conductor is moving relative to a magnetic field (for example a magnet falling in copper pipe, or a Eddy current brake in a train) , it is considered that the conductor moves, the conductor contains electrons, therefore the electrons are moving relative to the magnetic field, therefore EMF is generated.</p>
... | <blockquote>
<p>My question is relative to the positive charges in the atom nucleii
that are also moving relative to the magnetic field when the metal is
moving, are they too generating a force?</p>
</blockquote>
<p>The emf generated in a conductor due to its movement in a magnetic field will be acting or exerti... | 386 |
electromagnetism | What if... you had a bowl of electrons? | https://physics.stackexchange.com/questions/431751/what-if-you-had-a-bowl-of-electrons | <p>My chemistry teacher used to tell us that if you had a soup bowl with only electrons in it, the explosion could make you fly to Pluto. Was he right? Could this happen?</p>
| <p>The answer would depend how densely the electrons are packed. Let's say we have 1 kg of electrons, meaning we would have about <span class="math-container">$N = 10^{30}$</span> of them. For simplicity, let's approximate by arranging all of these electrons arranged in a spherical shell of radius <span class="math-con... | 387 |
electromagnetism | How can electromagnetic waves heat non-conducting media? | https://physics.stackexchange.com/questions/432949/how-can-electromagnetic-waves-heat-non-conducting-media | <p>According to <a href="https://ws680.nist.gov/publication/get_pdf.cfm?pub_id=906602" rel="nofollow noreferrer">this</a> source, the divergence of the Poynting vector is related to the total energy density of an electromagnetic wave, which is (locally) expressed as</p>
<p><span class="math-container">$$-\nabla\cdot S=... | <blockquote>
<p>So how is it then, that radiation heats media with no free charge carriers? Or does the radiation induce free carriers by photo-ionization first?</p>
</blockquote>
<p>This is an excellent question, very well reasoned. As other answers have described there are various microscopic processes whereby non-co... | 388 |
electromagnetism | What causes a moving positive point charge moving right in a uniform into the page magnetic field to specifically move upwards? why not downwards? | https://physics.stackexchange.com/questions/434268/what-causes-a-moving-positive-point-charge-moving-right-in-a-uniform-into-the-pa | <p>What causes a moving positive point charge moving right in a uniform into the page magnetic field to specifically move upwards? why not downwards?</p>
<p>Upwards and downwards (on the plane of the paper) are viable options. <strong>why does the charge only move upwards?</strong></p>
<p>Doesn't this violate the na... | <blockquote>
<p>The reason I ask this is because many posts imply that electromagnetism is parity invariant as we use the right hand rule twice in many cases, negating its arbitrariness, but here we use it only once.</p>
</blockquote>
<p>That last bit is incorrect. If you want a description for the dynamics which de... | 389 |
electromagnetism | What exactly is magnetic flux? | https://physics.stackexchange.com/questions/439108/what-exactly-is-magnetic-flux | <p>It is intuitive to think of an electric field, which describes the variance in the force acting on a charged particle if it were located in a certain position. However it is not so easy to understand what magnetism is and what flux is.
Apparently magnetism is an effect of special relativity for moving charges, but t... | <blockquote>
<p>It is intuitive to think of an electric field, which describes the variance in the force acting on a charged particle if it were located in a certain position</p>
</blockquote>
<p>The electric field <span class="math-container">$\mathbf E$</span> is actually just a force per unit charge <span class="... | 390 |
electromagnetism | Is the amplitude of an EM wave the combination of the electric AND magnetic fields added together? | https://physics.stackexchange.com/questions/439761/is-the-amplitude-of-an-em-wave-the-combination-of-the-electric-and-magnetic-fiel | <p>For instance, to get the TOTAL energy of an EM wave(s) or intensity you square the amplitude. But do you first add or combine the strengths of the e and m fields?</p>
| <p>Suppose you have two sources of Electric field, <span class="math-container">$E_1$</span>, and <span class="math-container">$E_2$</span></p>
<p>Then <span class="math-container">$\vec{E_{tot}}=\vec{E_1}+\vec{E_2}.$</span></p>
<p>So the total intensity is
<span class="math-container">$$E^2_{tot}=(\vec{E_1}+\vec{E_2... | 391 |
electromagnetism | Can concentrating light increase the intensity of the electromagnetic field? | https://physics.stackexchange.com/questions/441765/can-concentrating-light-increase-the-intensity-of-the-electromagnetic-field | <p>(Correct where applicable)</p>
<p>Light is a wave on the electromagnetic field (or the electromagnetic field can be interpreted as a representation of the magnitude and direction of the force photons will have on another charge).</p>
<p>When we concentrate light, we are essentially changing the paths of the electr... | <p>Yes there is a bigger EM field when photons are concentrated. But the field lines of a photon are perpendicular to the line of travel. Note that the field is quantized in energy, so even though it looks like a bigger wave only part of it corresponding to the single photon energy can be absorbed at a time. Also alt... | 392 |
electromagnetism | How do virtual photons exist without violating conservation of energy? | https://physics.stackexchange.com/questions/441939/how-do-virtual-photons-exist-without-violating-conservation-of-energy | <p>If virtual particles cancel out after being created from spacetime flunctuations because they come in matter-antimatter pairs, how do virtual photons cancel out, due to a photon being its own antiparticle?</p>
| 393 | |
electromagnetism | Reason for force in Stern-Gerlach experiment | https://physics.stackexchange.com/questions/447457/reason-for-force-in-stern-gerlach-experiment | <p>I'm currently working at an assignment, and I'm having some trouble understanding how the magnetic field deflects the silver atoms passing trough it. From what I understood, the atoms are deflected up or down of a specific amount according to their magnetic moment, but I can't understand what is causing this phenome... | <p>If an atom has a magnetic <em>moment</em> (not momentum), that means it acts like a tiny dipole magnet -- like a tiny bar magnet. A uniform magnetic field will not exert a net force on a dipole magnet, because (thinking of it as a bar magnet) the magnetic field will push on one pole and pull on the other pole with ... | 394 |
electromagnetism | Why does a moving charge create electricity | https://physics.stackexchange.com/questions/449075/why-does-a-moving-charge-create-electricity | <p>Now i have been studying a chapter called current electricity and i found out that moving chages can create electricity why is this possible? Is it the holes and the electrons combining together and creeating heat and light and us pecieving it as electricity?
I haven't put much thought into it but i am also very imp... | <p>The term “electricity” doesn’t necessarily refer to any one specific thing, but rather to a whole class of phenomena related to charge, current, voltage, etc. However, it isn’t unreasonable to focus on current and say that electricity is current. So in the remainder of this answer I will talk about current. </p>
<p... | 395 |
electromagnetism | Magnetic field inside a current carrying wire | https://physics.stackexchange.com/questions/454909/magnetic-field-inside-a-current-carrying-wire | <p>Assume an infinite wire carrying DC current. According to Ampere's force law， the moving electrons inside the wire are influenced by the Lorenz force pointing to the center. As a result, the current distribution is changed and finally all current flows in the center.</p>
<p>My question is, what is wrong with the ab... | <p>You're right, the usual picture of a <em>uniform</em> current density is a mathematical idealization. Realistically, electrons will accumulate near the axis of the wire in such a way as to create an outward pointing electric field that negates this effect. The equilibrium current distribution will no longer be unifo... | 396 |
electromagnetism | Can a coil and a magnet moving together produce a voltage? | https://physics.stackexchange.com/questions/458916/can-a-coil-and-a-magnet-moving-together-produce-a-voltage | <p>In other words, if a coil is wound around a fixed magnet and the combination is rotated or otherwise moved, will the coil produce emf? Must the permanent magnet move relative the the coil to produce emf? I recall seeing a demonstration during a physics presentation back in the 70s where this was the case. The magnet... | <p>Have you heard about the <a href="https://commons.m.wikimedia.org/wiki/File:Faraday_disk_generator.jpg" rel="nofollow noreferrer">Faraday disk generator</a>?</p>
<p><a href="https://i.sstatic.net/wihRm.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/wihRm.jpg" alt="Faraday Disc Generator"></a><br>
A ... | 397 |
electromagnetism | Can a magnetic field be induced without an electric field? | https://physics.stackexchange.com/questions/463014/can-a-magnetic-field-be-induced-without-an-electric-field | <p>Can a magnetic field be induced without an electric field?
Because, as far as I know, a time varying electric field induces a magnetic field an vice versa.
But in the case of conductors carrying currennt, it doesn't seem that electric field varies with time, then how is a magnetic field induced?</p>
| <p>One of Maxwell’s four equations for electromagnetism in a vacuum shows how magnetic fields are produced:</p>
<p><span class="math-container">$$\nabla\times\mathbf{B}=\frac{1}{c}\left(4\pi\mathbf{J}+\frac{\partial\mathbf{E}}{\partial t}\right).$$</span></p>
<p>(I’ve written it in Gaussian units.)</p>
<p>From this ... | 398 |
electromagnetism | What is electric flux density? | https://physics.stackexchange.com/questions/464020/what-is-electric-flux-density | <p>We say the electric flux is the number of field "lines", thus electric flux density is the number of field "lines" per a given area. However, let's say we had a point charge <span class="math-container">$Q$</span> centered at the origin and we were to enclose this charge with a surface of radius <span class="math-co... | <p>It seems that you're likely to be using non-standard terminology. </p>
<p>If I understand your comments correctly, you seem to be associating the name "electric flux density" with the vector field <span class="math-container">$\vec D= \varepsilon \vec E$</span>. If that is the case, then this is definitely non-stan... | 399 |
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