Onsager reciprocity

Onsager reciprocity is a principle in thermodynamics stating that certain responses in a system are symmetrical. If a small change in one property causes a response in another, then reversing these roles produces a similar effect. For example, if heating a material causes it to produce an electric current, then applying an electric current should similarly influence heat flow. This symmetry, arising from the fundamental laws of physics, helps scientists understand and predict how systems like engines, conductors, and biological processes behave under different conditions.