w p dV ex ( . ) Here, p ex at each stage is equal to ( p in + dp ) in case of compression [Fig. . (c)].
In an expansion process under similar conditions, the external pressure is always less than the pressure of the system i.e., p ex = ( p in – dp ). In general case we can write, p ex = ( p in + dp ). Such processes are called reversible processes. A process or change is said to be reversible, if a change is brought out in such a way that the process could, at any moment, be reversed by an infinitesimal change.
A reversible process proceeds infinitely slowly by a series of equilibrium states such that system and the surroundings are always in near equilibrium with each other. Fig. . (c) pV-plot when pressure is not constant and changes in infinite steps (reversible conditions) during compression from initial volume, V i to final volume, V f .
Work done on the gas is represented by the shaded area. Fig. . (b) pV-plot when pressure is not constant and changes in finite steps during compression from initial volume, V i to final volume, V f .
Work done on the gas is represented by the shaded area. Processes other than reversible processes are known as irreversible processes. In chemistry, we face problems that can be solved if we relate the work term to the internal pressure of the system. We can relate work to internal pressure of the system under reversible conditions by writing equation .
as follows: w rev ex p dV p dp dV