The Feynman Lectures
by Richard Feynman
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Atoms in Motion
We are not interested with where a new idea comes from – the sole test of its validity is experiment.
Atoms make up things. Atoms are (1 angstrom) big. They move, the faster the hotter. Atoms resist being pushed too close together.
There is too much physics to learn all of it in 4 years, even though physics is about summarizing reality into laws. But the laws need mathematical tools to be understood and described. Understanding what the words even mean is hard. Hence it takes a lot of effort in explaining and understanding physics.
Physics is work in progress. It is far from done.
Even in a single drop of water there is a wondrous world, a microcosm in itself, waiting to be explored.
Water is made up of molecules. They are made up of atoms. 2 Hydrogen and 1 oxygen form one water. The particles stick together only somewhat, hence they move together as a fluid. If the temperature is increased, the particles speed increases, until they fly around without hanging together. This is steam.
Bouncy molecules (like in steam) create a force by bumping into things and transferring some of their momentum. If we were to contain the bouncy molecules, we have to push against them. Thats what pressure is.
If we increase the area we want to contain, more particles will bounce into it – increasing the amount of force we need to apply. Furthermore if the molecules move faster (are hotter) they will be harder to contain as well.
When a piston is pushed in, the collisions accelerate the balls – this increases both pressure and temperature.
Ice is water molecules locked into place because molecules attract each other to some degree. If they don't move fast enough, that attraction overcomes the motion and the particles are locked in place.
The position of the molecules is determined by the "crystalline" array. They are not "random" like in the fluid, but are determined by the other atoms, even some that are millions of atoms away.
Ice Crystals are arranged in a hexagonal grid. Hence the symmetry of snowflakes. Also ice has holes on a molecular level, hence ice shrinks when it melts and expands when it freezes. Because there is more space between the molecules when frozen.
Even in the crystal, atoms keep vibrating. They still have heat. If vibrating enough they can shake themselves free and then the crystal melts.
The surface of water: Sometimes a molecule gets knocked so hard, that it flies away. Over time, more and more water molecules get knocked out, the water evaporates.
Molecules that leave in that way, are moving more quickly than the rest of the molecules in the water – they take away some of the momentum and therefore some of the heat. Hence when water evaporates it cools down.
Blowing on water cools it because one replaces moist air close to the surface with drier air, which means that more water can evaporate, which has a cooling effect.
Water molecules are not special in that they can escape and rejoin the fluid. Air molecules, nitrogen, oxygen etc. do the same thing, dissolving into the water.
Salt are made up of ions. Ions have some extra or fewer electrons. Electrons are charged. Hence Ions are charged. Salts contain positively and negatively charged ions, which attract.
Ends of the water molecule are also charged, so they also attract more to some ions than to others. Sometimes they break out ions from the salt. This is salt dissolving in water. At the same time, different ions already in the water, might rejoin the crystal, crystallizing out of the solution. The process is dynamic, it's a back and forth.
Sometimes atoms "change partners". They change the molecules they are attached to. We call that chemical reactions. Dissolving and Crystallizing, as well as the phase transitions don't change molecules, they are just physical processes.
Nature does not care what we call it, she just keeps on doing it.
Physics tries to answer, why certain atoms want to stick with certain other atoms. Why 2 oxygen atoms and not 3 or 4?
When atoms snap together to form molecules, they accelerate. Like little magnets when they snap together. This acceleration means things heat up. Hence when a lot of them undergo chemical changes like this we have burning. When the heat is high enough, the molecules can generate light. That's why a fire has flames that emit light.
In chemistry, people try to find out the exact arrangements of atoms that give rise to certain molecules and how they behave due to that structure.
Chemical formula are a way of "drawing" pictures of the arrangements of atoms. They are often flawed, because real arrangements exist in 3D not in 2D.
Molecular names are long and complicated, because it is hard to describe the exact arrangement of where which atom goes in the molecule and how they connect to each other.
Atoms will make particles move in random ways, so by this random jiggling of particles we can deduce that there must be atoms, jiggling them around. This is known as Brownian Motion.
Everything is made of atoms. That is the key hypothesis.
When we say we are a pile of atoms, we do not mean we are merely a pile of atoms, because a pile of atoms which is not repeated from one to the other might well have the possibilities which you see before you in the mirror.