Lecture_28 - Part VIII 2nd law Lecture 28 Carnot cycles The...

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Part VIII — 2nd law Lecture 28 — Carnot cycles The main results of classical thermodynamics were developed in the beginning of the 19th century, during the second industrial revolution. The “second” industrial revolution refers to the develop- ment of the steam-driven engine, as a power source (the “first” relied on water power). That is, the motivation for thermodynamics was to understand how to turn heat into work. Figure 1: Steam plant. Modern power plants share the same basic design (see Fig. 1): a heat source (a fire, or a nuclear reactor) that heats a working fluid (water, in a “boiler”), which expands to do work (in a turbine), and is cooled in a condenser where the “waste heat” is extracted by a heat sink (a river, or a cooling tower), whereupon the working fluid is sent back to the boiler. Key features of this design include: a heat source, and a heating step; an expansion step, in which work is done; a heat sink, and a cooling step; a cyclic process. Immediately the question arises, how e ffi ciently can we turn the heat from the heat source into useful mechanical work? From the experiments of Joule, which lead to the first law of thermodynamics 63
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(energy conservation), we know that heat and work are both ways of changing the energy of a system. Work can be turned into heat with perfect e ffi ciency. What about the reverse process?
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