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How does heat energy get from one place to another?
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The answer is, all five methods (radiation, transport, mixing, molecular conduction, and transformation) are important ways of moving heat energy from one place to another.
We've already considered the first one, when we dealt with radiation and how it drives the diurnal temperature cycle. We'll deal with the second one in the module on air masses and weather patterns. Number five, the transformation into another form of energy, will be discussed under sounding diagrams and convection. Methods three and four, the direct contact and mixing, are pertinent to this module.
Both techniques are familiar from daily life. Consider the problem of heating up a pot of chili on the stove. (Non-Texans may substitute stew for chili if you can relate to it better, but it just won't taste the same.) You stick the pot on the stove, and turn on the burner. The burner, whether it's gas or electric, heats up the bottom of the pot.
How does the heat get from the bottom outside of the pot to the bottom inside of the pot? It has to pass through the pot bottom somehow. This is done by direct molecule-to-molecule conduction. When a substance heats up, that means its molecules vibrate, spin, and jump around faster. When one molecule vibrates faster, especially in a solid or liquid where all the molecules are tightly or loosely tied together, the one next to it gets kicked by the motions and starts vibrating faster too.
For some substances, this is a very efficient way of conducting energy. This is especially true for many metals. Indeed, certain metals are selected for the bottoms of pots precisely because they are very efficient at conducting heat from one place to another. Unfortunately, most liquids (including chili) and gases are not nearly so efficient, because the individual molecules are not tightly connected to each other. If you're not careful with your chili, the bottom part will become very hot (as hot as the bottom of the pot), while the rest of the chili will hardly warm up at all. The result? One scorched pot of chili.
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