Appendix A Thermodynamic interaction between two discrete systems in non-equilibrium 0 There are two different descriptions of thermodynamic systems: the field formulation and the description as a discrete (or lumped) system. The field formulation or continuum thermodynamics deals with balance equations [Wilmanski (1988);Jou, Casas-Vazquez and Lebon (2001); Muschik, Papenfuss and Ehrentraut (2001)], which model together with the constitutive relations (equations of state) and the initial and boundary conditions of the process going on in the system. After having inserted the constitutive relations into the balance equations, we obtain a system of partial differential equations whose analytical solutions can be calculated only in sufficiently simple cases. In numerous practical applications, these continuous models are replaced by approximations usually obtained by means of finite differences or finite elements. Calculations are carried out after having chosen an appropriate algorithm with respect to the problems of stability and convergence. Consequently for practical reasons it seems to be more convenient to have a direct description of the coupled thermodynamic behavior of a finite number of interacting elements or cells. This is just one of the reasons for introducing the concept of discrete systems [Muschik (1990)] because this gives a simple and effective possibility for describing thermodynamics and interactions of elements being in non-equilibrium. But here we encounter the analogous difficulties, which appears in classical thermodynamics, as noted by Truesdell and Bharatha [Truesdell and Bharatha (1977)] "the formal structure of classical thermodynamics describes the effects of changes undergone by 0 This Appendix represents results of the paper by W. Muschik and A. Berezovski (Thermodynamic interaction between two discrete systems in non-equilibrium. J. NonEquilib. Thermodyn., 29, (2004)