How Do We Know

The kids I teach ask a ton of questions. I’ve been pretty good at fielding them…until today. Today I was stumped.

We were talking about these particles always being in motion, even in solids. I got a pretty obvious question that I’ve never considered: How do we know? How do we know the molecules are always in motion? I told them heat is caused by the particles having kinetic energy, so if the substance has heat, it must have motion. They didn’t really buy that. Their answer way, “so we don’t really know they are always in motion…”

I guess this is what I get for teaching them to be skeptical…

Dr Nebel’s Answer

That is a wonderful question, which I wish a lot more people would ask. It would help quell of a lot of erroneous thinking. But, to try and answer your question—I’m not sure that do any better than you already have.

1. We see in Brownian motion the phenomenon of atomic-molecular motion.

2. In turn, atomic-molecular motion explains the phenomenon of diffusion.

3. We see that the rate of diffusion increases or decreases with temperature. Such experimentation leads us to believe that what we are really measuring, as we measure temperature, is the degree of atomic-molecular motion, i.e., kinetic energy of atoms/molecules.

4. Extrapolating the relationship between temperature and atomic-molecular motion downward, we determine that all atomic-molecular motion will come to a stop at a temperature of -273.15˚C (-459.67˚F). This is known as absolute zero. Despite very sophisticated efforts, this temperature has never been achieved, much less anything lower. This fact supports the concept that temperature is a measure of atomic molecular motion (kinetic energy).

5. Have kids imagine themselves as molecules. Undulating their bodies and swiveling their hips can be equivalent to atomic-molecular motion (kinetic energy). Have kids note that their bodies can undergo such motions whether or not they are holding hands with other kids. To be sure, if they are not holding hands such movements will probably cause them to move about the room—representing a gas. But holding hands–representing a solid–does not preclude such movement.

6. This is to say that the attractions among atoms and molecules that lead to the substance being a solid, liquid, or gas are separate from the atomic-molecular motion. Atomic-molecular motion persists whether a substance is in a solid, liquid, or gas state, despite the fact that both are affected by temperature.

7. In conclusion, the fact that a solid block/stone gives off heat to a cooler environment is explained by a slowdown in the movement of its atoms/molecules, i.e., a loss of kinetic energy. True, we do not see this directly. However, if one chooses to disbelieve this theory, you are stuck with the task of coming up with another more logical explanation for the source/cause of energy given off by a solid block/stone as it cools. Can you do so?

Again, thank you for the question. I will be interested to know your kids respond to my answer. In the meantime, please don’t stop teaching your kids to be skeptical and please ask more questions. I encourage others to join in the discussion as well