During a transformation thestate variablesof the systemvary, to reach another state of equilibrium. The transition from the equilibrium state (1) to the equilibrium state (2) generally takes placeout of balance.
We then distinguish between (see Fig. 2.4):
transformationsreversible(or ideal): these are infinitely slow transformations formed from a succession of equilibrium states
transformationsirreversible: these are rapid and brutal transformations out of equilibrium
Reversibilityof a transformation requires that the system passes through an infinity of different intermediate states few equilibrium states (quasi-static states). The transformationsnaturalspontaneous are irreversible: they can only evolve in one direction (e.g. the expansion of a gas from HP to LP, the flow of heat from HT to LV, etc.).
