Heat and Cold
Name: Eva M.
I would like to know what heat and cold are. I have
read that cold is the absence of heat. Is this true?
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.
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Update: June 2012