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The function of the second law
The first law of thermodynamics is a generalization of the principle of conservation of energy to include heat. It places a restriction on the changes of a system which are energetically possible. Not all such changes occur, however, and we have already acknowledged this fact in discussing thermal equilibrium and hotness. If two bodies are placed in thermal contact it would be energetically possible for their temperatures to diverge; it would not violate the first law. However, we know that this does not happen. The temperatures converge and eventually thermal equilibrium is established. Thus there is an essential irreversibility of nature, a natural direction for change, which we need to take into account in trying to describe thermal processes. The first function of the second law is to express this irreversibility.
Secondly, although we know that work may be converted into heat by a suitable dissipative mechanism (Joule's paddle wheels, or a resistor), we have not examined the conversion of heat into work. The first law emphasized the equivalence of heat and work as forms of energy, but it tells us nothing about the conversion from one form to the other; and, in particular, it tells us nothing about the efficiency with which heat may be converted into work, a matter of enormous practical importance.
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