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0308 Notes - The First Law of Thermodynamics For any...

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3/8/10 Oregon State University PH 202, Lecture 27 1 The First Law of Thermodynamics For any thermal process: ± E th.process = Q process + W process The Second Law of Thermodynamics “Heat fl ows spontaneously only from a substance at a higher temperature to a substance at a lower temperature.” (By “spontaneous,” we mean that we don’t have to do any work on the system in order to make this fl ow happen .) The fact that heat fl ow does happen spontaneously allows us to harness that fl ow to get some useful work—analogous to harnessing the spontaneous fl ow of water over a dam.
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3/8/10 Oregon State University PH 202, Lecture 27 2 Q: What’s the theoretical maximum ef fi ciency of a heat engine? A: It’s not e = 1, because that would mean Q C = 0. We have to allow some Q C , or else there’s nothing to keep the spontaneous fl ow going. The most ef fi cient engine theoretically possible is called a Carnot engine . Its ef fi ciency is directly related to the absolute temp- eratures of the hot body and the cold body whose difference is caus-
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