ABSTRACT
In this chapter, the authors define and develop the idea of entropy and provide some important examples. The name 'entropy' comes from the Greek en meaning inside and tropos meaning transformation. As a prelude to discussing entropy, there is an important theorem that applies to cyclical processes. This theorem is known as the Clausius inequality. The Clausius inequality contains the profound implication that processes can occur only if the net entropy of the universe increases or stays the same. The thermodynamic state of a system can be specified by any pair of independent state functions. In particular, a state is equally well specified by the pair entropy (S) and temperature (T) as by the pair pressure (P) and volume (V). Just as it is possible to represent a reversible process as a line joining a succession of equilibrium states on a PV diagram, the same can be done on a TS diagram.
