Physics - Feynman Lectures on Physics, Atoms in Motion P2

 Hi, I'm posting a kind of resume of the things that I learned in the second part of the first chapter of this link: https://www.feynmanlectures.caltech.edu/I_01.html

Matter is made of atoms

What scientific statement would contain the most information in the fewest words?

R: The atomic hypothesis, that all things are made of atoms, little particles that move around in perpetual motion, attracting each other when they are a little distance apart, but repelling upon being squeezed into one another.

In this sentence, there's an enormous amount of information.

We can only start to see atoms if we magnify it for at least a billion times. The atoms are 1 or 2x10⁻⁸ cm in radius, 10⁻⁸ is called an angstrom, so we say they are 1 or 2 angstroms(Å) in radius.

If an apple is magnified to the size of the earth, then the atoms in the apple are approximately the size of the original apple.

A group of atoms is called a molecule.

Atoms are continually jiggling and bouncing, turning and twisting around one another.

The particles are "stuck together", they attract each other.

The whole group is "glued together".

Water can flow, but it doesn't disappear, things don't just fly apart, because of the molecular attraction.

The jiggling motion is what we represent as heat. The more heat, the more motion you get, and when the motion decreases, the heat also decreases.

With enough heat, it's possible for you to get so much motion that the pull between them isn't enough to keep them together.

Just like you would do if you were in a crowded and hot place.

But with this, they start to fly apart, water becomes steam, and so on.

Also remember, they are flying apart, and not disappearing.

Now let's see some properties of steam or any other gas.

Imagine a room with around a hundred tennis balls, bouncing in perpetual motion.

When they bombard the wall, this pushes the wall away.

This means that the gas exerts a jittery force, in order to confine a gas we must apply pressure on it. (pressure times the are is the force.).

Clearly, the force is proportional to the area, if we increase the area but keep the number of molecules per cubic centimeter the same, we increase the number of collisions with the wall in the same proportion as the area was increased.

Now, if we put twice as many tennis balls(molecules) in this room(or more specifically a gas container/tank), doubling the density and let them have the same speed, i.e, the same temperature. Then in a close approximation, the number of collisions is going to be doubled, and since each molecule has the same speed as before, the pressure is proportional to the density.

If we consider the true nature of the forces between the atoms we would expect a slight decrease in pressure because of the attraction between the atoms, and a slight increase because of the finite volume they occupy. But, to an approximation, if the density is low enough that there are not many atoms, the pressure is proportional to the density.

There's also another thing about the atomic theory that is if we increase the temperature(motion) without changing the density, the pressure also increases because the "balls" are hitting often and harder than before.

Now let's consider another situation, imagine that this room with the balls, now has a piston in the wall, that slowly compress the balls into a smaller room.

What is going to happen to the atoms?

Well the speed(temperature) of the atoms are going to increase, because they hit earlier in the wall picking up speed, so we can say that the atoms increase in temperature when being compressed and under expansion the temperature decreases.

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Now, this is the end of part two, if you want to read the original material, you can access this link