Entropie

According to the second law of thermodynamics, work is converted into heat, and completely reversing it is impossible even for an ideal gas undergoing reversible processes. Thus, natural processes are directional.

In the Carnot engine, which consists of reversible processes, the ratio of the heat exchanged and the temperature of the heat reservoirs is constant. This ratio is defined as the change of a new physical quantity, the entropy, represented by the symbol S.

When a reversible process is not isothermal, it can be considered as many infinitesimal isothermal processes at different temperatures. Then, the entropy change is the sum of the ratio delta-Q by T in each step. As the limit of delta-Q approaches zero, the entropy change is given by an integral.

At higher temperatures, constituents of any substance are in higher disorder. When a cold substance absorbs heat, its constituents get more disordered. For a hot substance, the change in the randomness of its constituents is minor. Thus, entropy change quantifies the increase in disorder of a system.