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Manfred comments on Heat vs. Motion - Less Wrong
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Consider two identical flywheels made of iron:
a) starts in room temperature, you apply force and make it spin. b) stands still but you heat it uniformily with several flames.
Suppose that in both cases the same amount of energy has been put into the flywheels.
In both cases the atoms are moving in high speed. Now if you look at the flywheels with an infrared camera would they look the same? This is not a rethorical question.
Huh, nobody's answered this.
The torched iron would emit more infrared than the spinning iron.
The reason is because thermal motion isn't just any ol' motion - it's motion that has had time to come to equilibrium between alllll the different ways the atoms in the solid can move. For example, the first atom could move left, and the second atom move right, and the third atom move left, and so on. All told there are as many ways for the atoms to move as there are atoms in the solid, which is more than 10^23, which is way more than the measly 1 way of moving that is "all atoms go around the center." In order to emit infrared light you need the atoms oscillating against their neighbors at high frequency, which is a big chunk of those 10^23 ways the atoms can move, but doesn't have anything to do with "all atoms go around the center."