Onsager Symmetry Relation

Abstract

In nature, every process is irreversible. The restriction on reversible processes, as done in classical thermodynamics, gives a limiting case only. That is, a process is not reversible unless it proceeds infinitesimally slowly and therefore is independent of time. In nature, however, all processes proceed with finite speed and therefore are time dependent. The second law of classical thermodynamics can be stated as follows: The entropy increases if an irreversible process is occurring. One can state this sentence in the opposite way as well: The irreversible process occurs because it is connected with an increase of entropy. Here the increase of entropy is looked upon as force which generates the thermodynamic process. With this picture it is possible to get a relation between an increase of entropy and the rate of the thermodynamic process [1, 2].