The Second Law of Thermodynamics
The concept of entropy will be introduced from two different starting points. This chapter explains how the entropy will be seen as a quantification of the degree of irreversibility of a process. As a result of an examination of the properties of reversibly operated heat engines, there naturally develops a quantity which has all the properties of a thermodynamic state function. This state function is the entropy. These findings lead to a statement of the Second Law of Thermodynamics, which, together with the other laws of thermodynamics lay the foundation for the thermodynamic method of describing the behavior of matter. The chapter compares two processes: the isothermal reversible expansion of an ideal gas and the free expansion of an ideal gas. It suggests that all reversible Carnot cycles operating between the same upper and lower temperatures must have the same efficiency—namely, the maximum possible.