What is Heat?

First, you need to know what heat is. Heat is the rattling, wiggling motion of all the atoms that make up a substance. It's a form of motion energy, but special because the solid as a whole isn't going anywhere. How is that possible? Imagine a large crowd of people standing around, in line, perhaps, for concert tickets. They could be standing quietly or shoving and milling about, even though the crowd isn't going anywhere. It's like that with solid objects, which are really vast "crowds" of individual atoms, that may be quiet

(cold) or may be milling around excitedly (hot).

A hot (jumpy) solid contains more energy per unit volume than a cold

(quiet) solid. If we put a hot solid next to a cold, then some of the energy in the hot solid will flow into the cold solid. This is called the flow of heat.

OK, now we need to think about how heat flows from something hot

(your hot chocolate inside your thermos) to something less hot (the atmosphere). There are three ways:

(1) CONDUCTION. If the hot solid is touching the cold solid, then jumpy (hot) atoms can bump against their quiet (cold) neighbor atoms, and if they do the hot guys get quieter and the cold guys heat up.

Then the newly-hot-used-to-be-cold guys start heating up *their* neighbors, in turn, and so forth until the heat flows deep into the cold solid.

(2) CONVECTION. If the hot solid is not touching the cold solid, there will usually be some third material between the two. If the third material is also a solid, we just have conduction from hot to third material, and from third material to cold solid, category (1), already dealt with, phooey.

But if the third material is a liquid or gas, something new can happen. Suppose for the sake of argument the hot solid is on the bottom, the cold on top, and a liquid in between. This describes things like a saucepan on the stove or the inside of the Earth. A hot atom will jostle a neighbor cold atom in the liquid, heating it up.

When a layer of liquid atoms near the hot solid are all jumping around, they take up more room, as you'd not be surprised to hear.

That is, the liquid expands. When it expands, it weighs less per fluid ounce, quart or gallon than the cold liquid above it. Now, you know how light fluids (like oil) float to the top of heavier fluids

(like water)? And hot gas (in a hot-air balloon or from a smokestack) rises up through colder gas?