Onsager’s reciprocal relations in irreversible processes
V. Zlatić, R. Monnier · Oxford University Press eBooks · 2014
This chapter describes irreversible processes in the context of Onsager’s proposal that the currents flowing in response to applied thermal or electrical forces are the same as those responsible for the regression of thermodynamic fluctuations toward thermal equilibrium. Expressing the resulting entropy change in terms of the first moment of the fluctuating quantity, the continuity equation shows that the time derivative of the first moment is equal to the corresponding uniform current density. The resulting rate of change of entropy density is equal to the scalar product of that current density with a thermodynamic force defined as the derivative of the entropy density with respect to the moment. In a stationary state, the entropy thus generated is transported out of the system by an entropy current through its surface. When several quantities fluctuate simultaneously, the entropy change contains interference terms. The statistical theory of fluctuations and the principle of microscopic reversibility then lead to Onsager’s reciprocal relations for the corresponding transport coefficients.