T5 - Phy sics 7B WS T5 ( rev. 3.0) Page 1 T-5. Engines and...

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Physics 7B WS T5 (rev. 3.0) Page 1 T-5. Engines and Efficiency Questions for discussion 1 . In common-sense language, efficiency is "what you want" "what you have to put in" . Let’s see how this common- sense notion of efficiency applies to heat engines and refrigerators. a) A heat engine, such as a steam engine, takes advantage of the everyday fact that “heat wants to flow from hot to cold.” The engine “siphons off” some of this flowing heat energy in the form of useful work. This is shown in the schematic diagram at right. Keeping in mind the common-sense meaning of efficiency, how would you define the efficiency of a heat engine? Your definition of e heat engine should involve the quantities W net , Q H , and/or Q L . b) Use the First Law of Thermodynamics, together with the fact that the engine runs through a cycle , to show that your definition is equivalent to e heat engine = 1 - Q L /Q H .
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Physics 7B WS T5 (rev. 3.0) Page 2 c) Your refrigerator forces heat energy to flow “against the grain,” from the cold icebox to the warm kitchen. Naturally this requires an input of work, as shown in the schematic diagram (below). Keeping in mind the common-sense definition
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This note was uploaded on 11/19/2010 for the course LECTURE 1 taught by Professor Yildiz during the Fall '10 term at University of California, Berkeley.

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T5 - Phy sics 7B WS T5 ( rev. 3.0) Page 1 T-5. Engines and...

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