Thermodynamic notion of entropy

Franjo Sokolić

Formulation of the principles of thermodynamics is one of the major achievements of physics. They introduce two functions of state, internal energy and entropy, but while energy is conserved, entropy increases for irreversible processes. Even for ideal, reversible processes, the efficiency of the heat engines is under hundred percent, which is related to the fact that entropy cannot decrease. The main question is the following one: Is it necessary to have some irreversible step at the level of each elementary process for the increase of entropy?