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Name: Eva M.
Status: educator
Age: 50s
Location: N/A
Country: N/A
Date: 7/13/2003


Question:
I would like to know what heat and cold are. I have read that cold is the absence of heat. Is this true?


Replies:
Heat is energy. Usually we think of heat as the fraction of the total energy possessed by a particle that can easily be given to another particle in a collision. Thus, if some molecule is whizzing across the room and it whacks into another molecule that is moving more slowly, the slow molecule will generally move faster after the collision, while the fast molecule will generally move slower. Energy will have been transferred from the fast molecule to the slow molecule, and that is the sort of energy that is useful to think of as heat.

In another example, suppose there is a molecule that behaves like two masses connected by a spring, and suppose that the molecule is not going anywhere, but that its masses are spinning around and oscillating in and out. Clearly, the molecule has energy. Now if some less energetic molecule drifts by and gets whacked by one of the masses, the whackee generally will gain some energy at the expense of the whacker. This is also heat transfer.

Now that was a microscopic view of heat energy. How about two huge collections of molecules: one whose molecules are, on average, zipping around rapidly, spinning, and oscillating; and one whole molecules are, on average, just slowly drifting around. We would say that the zipping-around collection is hot, and the slowly-drifting collection is cold. What would happen if we put the zipping-around collection right next to the slowly-drifting collection? Let us let them collide, but not intermingle. When zippers collide with drifters, as we have seen, zippers generally lose energy while drifters generally gain it. Sooner or later, both collections will end up with the same average energy -- the same temperature. In this macroscopic view, heat is what gets transferred from a hot thing to a cold thing, when the two things touch.

Tim Mooney



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