Thermal Equilibrium - Distinctions Between Thermal and Thermodynamic Equilibria

Distinctions Between Thermal and Thermodynamic Equilibria

In the seventh German edition of his Treatise on Thermodynamics, Planck mostly writes of 'thermal equilibrium' (for example, on page 2), and only sparingly of 'thermodynamical equilibrium' (for example, on page 121). Though Planck does not explicitly say so, it seems from the context of his text that he may mean that thermal equilibrium is a good concept for cases in which chemical changes are not allowed, while thermodynamic equilibrium is a good concept when chemical changes are allowed. In his The Theory of Heat Radiation, Planck freely writes of 'thermodynamic equilibrium', for example on page 22, but not at all of 'thermal equilibrium'; in this case, radiative transfer of heat is being considered, while that is not mentioned in the Treatise on Thermodynamics.

An explicit distinction between 'thermal equilibrium' and 'thermodynamic equilibrium' is made by C.J. Adkins. He allows that two systems might be allowed to exchange heat but be constrained from exchanging work; they will naturally exchange heat till they have equal temperatures, and reach thermal equilibrium, but in general will not be in thermodynamic equilibrium. They can reach thermodynamic equilibrium when they are allowed also to exchange work.

Another explicit distinction between 'thermal equilibrium' and 'thermodynamic equilibrium' is made by B. C. Eu. He considers two systems in thermal contact, one a thermometer, the other a system in which several irreversible processes are occurring. He considers the case in which, over the time scale of interest, it happens that both the thermometer reading and the irreversible processes are steady. Then there is thermal equilibrium without thermodynamic equilibrium. Eu proposes consequently that the zeroth law of thermodynamics can be considered to apply even when thermodynamic equilibrium is not present; also he proposes that if changes are occurring so fast that a steady temperature cannot be defined, then "it is no longer possible to describe the process by means of a thermodynamic formalism. In other words, thermodynamics has no meaning for such a process."