Second Law of Thermodynamics: Heat moves towards cold

by Joshua Hwang on November 19, 2008

I know we’ve already had a podcast about entropy (Podcast number 26). But this is an important topic, and I thought we could take a different approach to the Second Law of Thermodynamics.

You leave your ice cream out, and it melts, it always melts. You leave your coffee on the table, and your mug gets hot and the coffee gets colder.

So what?

The second law of thermodynamics states that “Heat does not spontaneously flow from colder objects to warmer objects.” Now there doesn’t seem to be anything unexpected about this statement. Duh, of course heat moves from hot things to cold things. But think about it, why should this happen? What is it about the world that makes heat flow from hot things to cold things?

If you heard the entropy podcast, it’s all about spreading out and energy. In the same way that water tends to level itself out, it wants its energy to be spread out. The energy in the coffee tends to spread out as well: the energy from the hot particles transfers to colder particles, thereby cooling the coffee down.

If you think about something like car engines, there are controlled hot explosions happening repeatedly inside a colder engine. This means that not all of the energy from the engine can go to producing work, as a lot of this energy actually goes into heating up the engine. Because of the second law of thermodynamics we have a long way to go to keeping our coffee hot and our engines efficient.

[tags]thermodynamics, physics, heat, entropy, culture[/tags]

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