ABSTRACT
A system is out-of-equilibrium when at least one thermodynamic force is not null. Thermodynamic forces originate thermodynamic flows. In out-of-equilibrium thermodynamics, there are two regimes: the linear and nonlinear regime. The distinction depends on the relationships between flows and forces. When the relationships between flows and forces are linear (for example, in heat conduction or diffusion), after fixing the contour conditions, a system evolves to a steady state wherein the forces become a linear function of the spatial coordinates, and the flows become constant. In such a steady state, the entropy production of the system is minimized. In the linear regime, the symmetry principle holds: the asymmetry of the forces is always larger or equal to that of the flows. A different behavior is observed in the nonlinear regime. An example of the nonlinear relationship is that between the chemical force and flow. After fixing the contour conditions, a chemical system can evolve to either a stable or an unstable or an oscillatory state.
