THERMODYNAMICS WITH INTERNAL VARIABLES .1. GENERAL CONCEPTS

The first part of this review replaces the concept of internal variables of state in the general context of non-equilibrium thermodynamics. This offers the slightest deviation from the classical Theory of irreversible processes (TIP) while exhibiting a flexibility and versatility that other attempts at generalizing TIP cannot sustain. Here the basic working hypotheses on which the application of internal-variable theory relies, are specified. The use of TIP, after introduction of the appropriate axiom of local equilibrium state, or of a pseudopotential of dissipation is emphasized. The role of internal state variables in describing miscrostructures and the current misunderstanding with internal degrees of freedom are clarified. However, the resemblance or identity with order parameters of phase-transition theory as well as essential differences with extended thermodynamics are acknowledged. In all, internal variables epitomize the use of systems of evolution in mathematical continuum physics.