Whenever I hear people talk about heat, they often explain that its, like, “particle vibration”, which I think I understand. Stuff doesn’t just change direction on its own though; it needs a force to interact with, like other particles or fields.
Does that mean that when you only have one atom, it doesn’t meaningfully have a temperature, and instead just a mass and velocity, and uninteracted with it would just keep going in one direction? And “heating it up” is just the same as speeding it up? Or is the thermal “internal kinetic energy” also a subatomic thing?
Comments
Correct – temperature is property of bulk material. While in some specialized cases we will talk about the temperature of a small collection of particles (a couple dozen), generally we don’t talk about the temperature of a system until you have enough particles that statistical mechanics dominates (so, several million particles on the low end).
The main reason is that while it is handy to think about temperature as “how fast particles are moving” it’s really defined using entropy – energy flows in such a way that the entropy of the system increases. So, when an object feels “hot” that is because if you touch it, energy is flowing from that object to you. You can only really guarantee that situation when there are many, many particles.
And when looking at a single particle, it’s even worse. The idea of temperature being related to the speed of the particles – while this is still a simplification you should say “it’s the speed of the particles about the center of mass of the system.” That is, you have air in your car, and it’s some temperature. You start driving, and thus the air molecules now all have a higher velocity, but that doesn’t mean the air temperature has gone up, because their velocity about their center of mass (aka – measured from the center of the car) hasn’t increased at all. And for a single particle, no matter how fast it is moving, the center of mass of that particle is wherever the particle is. So, it never has a velocity about its own center of mass, so you can’t even try to define a temperature for it.