quality:correction
Onsager relations

The Onsager reciprocal relations have been considered the fourth law of thermodynamics. They describe the relation between thermodynamic flows and forces in non-equilibrium thermodynamics, under the assumption that thermodynamic variables can be defined locally in a condition of local equilibrium. These relations are derived from statistical mechanics under the principle of microscopic reversibility (in the absence of external magnetic fields). Given a set of extensive parameters X (energy, mass, entropy, number of particles and so on) and thermodynamic forces F (related to their related intrinsic parameters, such as temperature and pressure), the Onsager theorem states that

where i, k = 1,2,3,... index every parameter and its related force, and

are called the thermodynamic flows.