[Encyclios]

Reversible and irreversible transformations

A thermodynamic transformation is defined reversible if it, starting from a state of thermodynamic equilibrium, takes place in such a way that the system and the environment can always be brought back to their respective initial states, going through the same transformation without any trace of it remaining. This definition has two fundamental consequences

• the first is that a reversible transformation goes through a succession of equilibrium states, which means that it is a quasi-static transformation;
• the second is that a reversible transformation can, during the inverse transformation, make the system and the environment pass through the same states encountered in the direct transformation, by means of equal and opposite operations; the mechanical and thermal energy exchanges of the direct transformation are equal and opposite to those of the inverse transformation.

Ultimately, a reversible transformation, once traveled in the two directions, does not result in any change in the system or the environment. To conclude, we emphasize that a reversible transformation is not absolutely feasible in reality, so it is pure abstraction. It is useful, for example, to know the maximum value of the work obtainable in a transformation characterized by positive work or the minimum value for the work to be spent in a transformation with negative work.