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Chapter_7-Second_Law - Young and Freedman Introduction Many...

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Young and Freedman Introduction Many thermodynamic processes proceed naturally in one direction but not the opposite. For example, heat always flows from a hot body to a cooler body, never the reverse. Heat flow from a cool body to a hot body would not violate the first law of thermodynamics; energy would be conserved. But it doesn’t happen in nature. Why not? It is easy to convert mechanical energy completely into heat; this happens every time we use a car’s brakes to stop it. Going in the reverse direction, there are plenty of devices that convert heat partially into mechanical energy. (An automobile engine is an example.) But even the cleverest would-be inventors have never succeeded in building a machine that converts heat completely into mechanical energy. Again, why not? The answer to both of these questions has to do with the directions of thermodynamic processes and is called the second law of thermodynamics . This law places fundamental limitations on the efficiency of an engine or a power plant. It also places limitations on the minimum energy input needed to operate a refrigerator. So the second law is directly relevant for many important practical problems. We can also state the second law in terms of the concept of entropy, a quantitative measure of the degree of disorder or randomness of a system. The idea of entropy helps explain why ink mixed with water never spontaneously unmixes and why a host of other seemingly possible processes are never observed to occur. Directions of Thermodynamic Processes Thermodynamic processes that occur in nature are all irreversible processes . These are processes that proceed spontaneously in one direction but not the other. The flow of heat from a hot body to a cooler body is irreversible, as is the free expansion of a gas discussed in Chapter 6. Sliding a book across a table converts mechanical energy into heat by friction; this process is irreversible, for no one has ever observed the reverse process (in which a book initially at rest on the table would spontaneously start moving and the table and book would cool down.) Our main topic for this chapter is the second law of thermodynamics , which determines the preferred direction for such processes. Despite this preferred direction for every natural process, we can think of a class of idealized processes that would be reversible. A system that undergoes such an idealized reversible process is always very close to being in thermodynamic equilibrium within itself and with its surroundings. Any change of state that takes place can then be reversed (made to go the other way) by making only an infinitesimal change in the conditions of the system. For example, heat flow between two bodies whose temperatures differ only 1
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infinitesimally can be reversed by making only a very small change in one temperature or the other.
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