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Tt is a little dificult to begin at once with the present view, so we shall first
see how things looked in about 1920 and then take a few things out of that
picture. Before 1920, our world picture was something like this: The “stage” on
which the universe øgoes is the three-dimensional spøce of geometry, as described
by Euclid, and things change in a medium called #ne. The elements on the
stage are øarf/cles, for example the atoms, which have some properiies. Eirst, the
property of inertia: If a particle is moving it keeps on going in the same direction
unless ƒorces act upon it. The second element, then, is ƒorces, which were then
thought to be of two varieties: First, an enormously complicated, detailed kind
of interaction force which held the various atoms in diferent combinations in a
complicated way, which determined whether salt would dissolve faster or sÌlower
when we raise the temperature. The other force that was known was a long-range
interaction—a smooth and quiet attraction—which varied inversely as the square
of the distance, and was called graiation. 'Phis law was known and was very
simple. Whyụ things remain in motion when they are moving, or h# there is a
law of gravitation was, of course, not known.
A description of nature is what we are concerned with here. EFrom this point
of view, then, a gas, and indeed all matter, is a myriad of moving particles. Thus
many of the things we saw while standing at the seashore can immediately be
connected. Eirst the pressure: this comes from the collisions of the atoms with the
walls or whatever; the drift of the atoms, if they are all moving in one direction
on the average, is wind; the random internal motions are the heøf. Thhere are
wawves of excess density, where too many particles have collected, and so as they
rush of they push up piles of particles farther out, and so on. This wave of excess
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density is sound. It is a tremendous achievement to be able to understand so
much. Some of these things were described in the previous chapter.
What kznds of particles are there? "There were considered to be 92 at that
time: 92 different kinds of atoms were ultimately discovered. They had difÑferent
names associated with their chemical properties.
The next part of the problem was, 0haf are the short-range ƒorces? Why does
carbon attract one oxygen or perhaps wo oxygens, but not three oxygens? What
1s the machinery of interaction bebtween atoms? Is it gravitation? 'Phe answer
is no. Gravity is entirely too weak. But imagine a force analogous to gravity,
varying inversely with the square of the distance, but enormousÌly more powerful
and having one difference. In gravity everything attracts everything else, but
now imagine that there are £o kinds of “things,” and that this new force (which
is the electrical force, oŸ course) has the property that likes repel but unlikes
a#trac¿. The “thing” that carries this strong interaction is called charge.
'Then what do we have? Suppose that we have two unlikes that attract each
other, a plus and a minus, and that they stick very close together. Suppose we
have another charge some distance away. Would it feel any attraction? It would
feel pracficall none, because 1f the first two are equal in size, the attraction for
the one and the repulsion for the other balance out. Therefore there is very little
force at any appreciable distance. Ôn the other hand, if we get 0erw close with
the extra charge, œftraction arises, because the repulsion of likes and attraction
of unlikes will tend to bring unlikes closer together and push likes farther apart.
Then the repulsion will be /ess than the attraction. Thịs is the reason why the
atoms, which are constituted out of plus and minus electric charges, feel very
little force when they are separated by appreciable distance (aside from gravity).
'When they come close together, they can “see inside” each other and rearrange
their charges, with the result that they have a very strong interaction. 'Phe
ultimate basis of an interaction between the atoms is elecfrical. Since this force
1s so enormous, all the plusses and all minuses will normally come together in
as intimate a combination as they can. All things, even ourselves, are made of
ñne-grained, enormously strongly interacting plus and minus parts, all neatly
balanced out. Ônce in a while, by accident, we may rub of a few minuses or a
few plusses (usually it is easier to rub of minuses), and in those circumstances
we fñnd the force of electricity nbalanced, and we can then see the efects of these
electrical attractions.
To give an idea of how much stronger electricity is than gravitation, consider
two grains of sand, a millimeter across, thirty meters apart. If the force between
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them were not balanced, if everything attracted everything else instead of likes
repelling, so that there were no cancellation, how much force would there be?
'There would be a force of three rmillion tons between the twol You see, there is
very, 0er little excess or deficit of the number of negative or positive charges
necessary to produce appreciable electrical efects. 'This is, of course, the reason
why you cannot see the diference between an electrically charged or uncharged
thing—so few particles are involved that they hardly make a diference in the
weight or size of an object.
With this picture the atoms were easier to understand. 'They were thought
to have a “nucleus” at the center, which is positively electrically charged and
very massive, and the nucleus is surrounded by a certain number of “electrons”
which are very light and negatively charged. Now we go a little ahead in our
story to remark that in the nucleus itself there were found two kinds of particles,
protons and neutrons, almost of the same weight and very heavy. 'Phe protons
are electrically charged and the neutrons are neutral. If we have an atom with six
protons inside its nucleus, and this is surrounded by six electrons (the negative
particles in the ordinary world of matter are all electrons, and these are very
light compared with the protons and neutrons which make nuclei), this would be
atom number six in the chemical table, and i% is called carbon. Atom number
eight ¡is called oxygen, etc., because the chemical properties depend upon the
electrons on the ow#s¿đe, and in fact only upon hoa rmamy electrons there are. 5o
the chemzcal properties of a substance depend only on a number, the number
of electrons. (The whole list of elements of the chemists really could have been
called 1, 2, 3, 4, 5, etc. Instead of saying “carbon,” we could say “element six,”
meaning six electrons, but of course, when the elements were first discovered, it
was not known that they could be numbered that way, and secondly, it would
make everything look rather complicated. It is better to have names and symbols
for these things, rather than to call everything by number.)
More was discovered about the electrical force. The natural interpretation of
electrical interaction is that two objects simply attract each other: plus against
minus. However, this was discovered to be an inadequate idea to represent ït.
A more adequate representation of the situation is to say that the existence of
the positive charge, in some sense, distorts, or creates a “condition” in space,
so that when we put the negative charge in, it feels a force. 'Phis potentiality
for producing a force is called an electric ficld. When we put an electron in an
electric field, we say it is “pulled” We then have two rules: (a) charges make a
fñeld, and (b) charges in fields have forces on them and move. "The reason for