Basic conditions of equilibrium thermodynamics
Since the equilibrium process is always a process that is infinitely slow, all practically realizable processes can only approximate the equilibrium ones to some extent. This circumstance imposes certain restrictions on the difference between the concept of ‘thermodynamic’ characteristics and ‘thermophysical characteristics’, since the degree of closeness of the states of the system to the intermediate equilibrium states requires control, and, consequently, a criterion for proximity to a given equilibrium. The latter can not be done without analyzing the relaxation stages of approximation to local equilibrium: analyzing the spectrum of the relaxing characteristics (variables) and the characteristic relaxation time for each variable. We recall that in thermodynamics we consider the longest relaxation time during which an equilibrium is established for all the parameters of the given system. In this case, naturally, the question arises of the self-consistency of the description of the relaxation stages and the proper equilibrium state, which is practically never discussed in the framework of the non-equilibrium thermodynamics.