Reversible and Irreversible Process

"A thermodynamics reverse process is one that takes place infinitesimally slowly and its direction at any point can be reversed by an infinitesimal change in the state of the system."

    In fact, a reversible process is considered to proceed from the initial state to final state through an infinite steps, infinitesimally small stages. At the initial, final and all intermediate stages, the system is an equilibrium state. This is so because an infinitesimal change in the state of the system at each intermediate step and negligible.

    "When a process goes from the initial to the final state in a single step and can not be carried in the reverse order, it is said to be an irreversible process."

    Here the system is in equilibrium state in the beginning and at the end, but not at intermediate points.

Example:

    Consider a certain quantity of a gas contained in a cylinder having weightless and frictionless piston, the expansion of the gas can be carried by two methods:

    Let the pressure applied to the piston be 'P' and this is equal to the internal pressure of gas. Since the external and internal pressures are exactly balanced, the piston remains stationary and there is no change in volume of the gas. Now suppose the pressure on the piston is decreased by an infinitesimal amount dP. Thus external pressure on the piston is P – dP since internal pressure is 'P', the piston will move up and gas will expand by infinitesimally small amount. The gas will therefore be expanded infinitely slowly i.e. by thermodynamically reversible process. At all stages in the expansion of gas, dP being negligibly small, the gas is maintained in a stage of equilibrium throughout. If at any point of the process, the pressure is increased by dP. The gas would contract reversibly.

    In the other hand the expansion is irreversibly, if the pressure on the piston is decreased suddenly. It moves upwards rapidly in single operation. The gas is in equilibrium stage in the initial and final stages only. The expansion of the gas, in this case, takes place in an irreversible manner.