The Thermal Bottleneck If the first law of thermodynamics says you can't win, then the Second Law of Thermodynamics says you can't even break even. The First Law is essentially a statement of conservation of energy and asserts that you can't get more energy out of a heat engine than you put in. But the Second Law says that no heat engine can use all the heat produced by a fuel to do work. The Carnot cycle sets the ideal efficiency which can be obtained if there is no friction, mechanical losses, leakage, etc., but real machine efficiencies are much less.

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HyperPhysics***** Thermodynamics

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Qualitative Statements: Second Law of Thermodynamics The second law of thermodynamics is a profound principle of nature which affects the way energy can be used. There are several approaches to stating this principle qualitatively. Here are some approaches to giving the basic sense of the principle. 1. Heat will not flow spontaneously from a cold object to a hot object. Further discussion 2. Any system which is free of external influences becomes more disordered with time. This disorder can be expressed in terms of the quantity called entropy. Further discussion 3. You cannot create a heat engine which extracts heat and converts it all to useful work. Further discussion 4. There is a thermal bottleneck which contrains devices which convert stored energy to heat and then use the heat to accomplish work. For a given mechanical efficiency of the devices, a machine which includes the conversion to heat as one of the steps will be inherently less efficient than one which is purely mechanical. Further discussion Alternative statements: Second Law of Thermodynamics

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