Equilibrium states are states of zero velocity and uniform temperature, and, as a consequence, of zero production of entropy. This distinguishes them from the more general steady states, in which the production of entropy is nonzero due to gradients of temperature and velocity. This fundamental difference leads to many implied differences between equilibrium states and steady states. Most importantly, there are universal variational principles for determining equilibrium states — the extremum principles of thermodynamics — which do not have any counterpart for the steady states (despite many claims in the literature). The extremum principles are related to the dynamical stability of equilibrium states. Closely connected to the stability are the static constitutive inequalities — additional properties of the equilibrium response functions which do not follow (at least not directly) from the second law of thermodynamics. Basic assertions about the equilibrium states of coexistent phases and the proofs of the existence of equilibrium states are also based on extremum principles.
KeywordsPotential Energy Regularity Assumption Load Environment Surface Traction Singular Surface
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