ABSTRACT
One way to introduce the second law of thermodynamics is to begin by defining the entropy function. Entropy is a very puzzling function for different reasons. Perhaps is it because it is a non-conservative quantity? Perhaps is it also because it may appear as being only a “simple” mathematical function?. Finally, it is a highly puzzling concept because it seems to be related with the very troublesome one of “time’s arrow”. In any case, for the purpose, it is a very important function because it is a component of the functions of Helmholtz and Gibbs energies. “It is impossible to construct a machine that is able to convey heat by a cyclical process from one reservoir (at a lower temperature) to another at a higher temperature unless work is done on the machine by some outside agency”. According to Clausius’ statement, the second law is a postulate. It is accepted because deductions from it correspond to the experience.
