Non-relativistic Thermodynamics IV (Sign Questions and Onsager Symmetry Relations in Phenomenological Theory)

The signs of thermodynamical state functions are defined in terms of the sign of absolute temperature T. 1) The 2nd Law's principle of evolution determines the signs of the (symmetrised) phenomenological coefficients L(α β). 2) The 2nd Law's principle of (cinetic) equilibrium determines the signs of the remaining state functions (mass densities of the chemical components A in a C(> 1)-component fluid, its elastic modulus, and other state functions appearing in the coupled linearised equations of motion). These sign conditions show that the solutions of these equations are damped waves existing only in the future. It is remarkable, that the phenomenological coefficients satisfy Onsager's symmetry relation LA B = LB A for the diffusion of substances AB... = 12 ... C, if both principles are applied.