Chapter 2 - Irreversible Thermodynamics

Abstract

Irreversible thermodynamics is a phenomenological extension to classical reversible thermodynamics that seeks to capture the nonequilibrium nature of irreversible processes. The degree of disequilibrium of a system is characterized by its thermodynamic affinity, which provides a driving force for equilibration. A nonzero affinity produces a nonzero flux of the relevant thermodynamic quantity as the system is driven toward equilibrium. Irreversible processes generate a positive entropy, where the rate of entropy production is equal to the product of the associated affinity and flux. Since the entropy of the universe increases as the result of an irreversible process, the original state of the universe can never be recovered, in accordance with the Second Law. Irreversible thermodynamics typically involves three simplifying assumptions: (a) purely resistive system, where the fluxes at a given instant depend only on the value of the affinities at that instant; (b) linearity, where quadratic and higher-order terms of the flux equations are neglected; (c) Onsager reciprosity, which defines the inherent symmetry of the kinetic coefficient matrix. Onsager reciprosity is based on a microscopic reversibility of the system near equilibrium and is applicable to a wide range of phenomena with coupling between different types of driving forces and fluxes. Irreversible thermodynamics is broadly applicable to variety of materials processes which cannot be described using standard reversible thermodynamics. Several examples include: (a) thermophoresis (i.e., the Soret effect), where a thermal gradient in a material gives rise to a chemical concentration gradient; (b) thermoelectricity (i.e., the Seebeck effect), where a thermal gradient gives rise to an electrical current; (c) electromigration, where an electrical current gives rise to a concentration gradient; and (d) piezoelectricity, in which mechanical work on the system gives rise to an electrical current.