Entropy and Irreversibility of Thermodynamic Processes

Abstract

This chapter is devoted to the concept of entropy and the second law of thermodynamics which for isolated systems can be expressed as the entropy maximum principle. The latter is used to derive the conditions of thermodynamic equilibrium between two subsystems of an isolated system: thermal, mechanical and with respect to the flow of matter. In particular, we obtain the relation between a reversible flow of heat and entropy, which leads to the fundamental relation between the internal energy, volume, amount of substance and entropy. As an example of the fundamental relation we derive the entropy of an ideal gas. We also give a few examples of how the entropy of a system changes in reversible and irreversible processes. Then heat devices: a heat engine, refrigerator and heat pump, are studied as examples of practical applications of the first and second laws of thermodynamics. In particular, we discuss the efficiency of the Carnot cycle and its relation to the thermodynamic temperature. We also discuss a few other thermodynamic cycles used in petrol, diesel and steam engines. Finally, we formulate and discuss briefly the third law of thermodynamics.