The Second Law of Thermodynamics

The second law of thermodynamics describes the direction of processes towards stable equilibrium states. In this chapter, the second law is motivated and entropy is introduced as the relevant thermodynamic property: the second law is a balance law for entropy, which is not conserved, but can only be produced, not destroyed. Combination of first and second law for reversible processes yields the Gibbs equation, which relates entropy to measurable properties. Property relations for entropy are found for incompressible liquids and solids, and for ideal gases. Evaluation of heat transfer, stirring and expansion processes shows that the second law as introduced—with positive entropy generation rate—leads to equilibration of temperature, and decay of fluid motion. Thermodynamic temperature must be positive to ensure mechanical stability of a system. The chapter closes with brief discussions of the microscopic interpretation of entropy, and its relation to disorder.