ABSTRACT
Now, the second law being stated as a postulate, it is interesting to introduce it in another way. It consists in using the entropy function which must be, firstly, introduced and secondly studied. It is interesting to begin with the general notion of balance-sheet. It is directly related to the non-conservative property of entropy. The concept of entropy being intimately related to the notions of reversibility and irreversibility, it is judicious to give here a further criterion of reversibility. A process is reversible if one can envisage another experiment which can, at the same time, bring back both the system and its surroundings to their initial state. For every closed system, there exists a state function called entropy. Like other state functions, the entropy change accompanying a transformation depends only on its initial and final states.
