Formulation advanced by de Groot and Mazur

de Groot and P. Mazur [GRO 69a] (formtjlas 70, p. 390) agree that the state of a composite system depends on N + 4 variables y and therefore write that 

We can just as well describe the system using the entropy or the internal energy instead of looking at the free energy. For example, with the mass internal energy e, we obtain  

If we move from one equilibrium state to another slightly different equilibrium state, the polarizations p and m respectively become p + if p and m +. The corresponding 

A polarized fluid medium is such that at equilibrium, isotropy is ensured, so that e and p are reduced to scalars, as are k and jc- Thus, as a function of the electric and magnetic susceptibilities k and p, we have  

Non-equilibrium states, for their part, are characterized by the classic thermodynamic variables, in addition to which 

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