文档介绍：: The Second Law of Thermodynamics
[IAW 42-50; VN Chapter 5; VWB&S-, , Chapter 7]
Concept and Statements of the Second Law (Why do we need a second law?)
The unrestrained expansion, or the temperature equilibration of the two bricks, are familiar
processes. Suppose you are asked whether you have ever seen the reverse of these processes take
place? Do two bricks at a medium temperature ever go to a state where one is hot and one is cold?
Will the gas in the unrestrained expansion ever spontaneously return to occupying only the left side
of the volume? Experience hints that the answer is no. However, both these processes, unfamiliar
though they may be, patible with the first law. In other words the first law does not prohibit
their occurrence. There thus must be some other “great principle” that describes the direction of
natural processes, that tells us which first patible processes will not be observed. This is
contained in the second law. Like the first law, it is a generalization from an enormous amount of
observation.
There are several ways in which the second law of thermodynamics can be stated. Listed
below are three that are often encountered. As described in class (and as derived in almost every
thermodynamics textbook), although the three may not appear to have much connection with each
other, they are equivalent.
1) No process is possible whose sole result is the absorption of heat from a reservoir and the
conversion of this heat into work. [Kelvin-Planck statement of the second law]
Q
T2
System
W This is not possible
T1
2) No process is possible whose sole result is the transfer of heat from a cooler to a hotter body.
[Clausius statement of the second law]
Q
T2
For T1 < T2 , this is not possible
T1
Q
1B-1
3) There exists a property called entropy, S, which is a thermodynamic property of a system. For a
reversible process, changes in this property are given by