GITAM, Department of Engineering Physics

# Reversible and Irreversible Process

A reversible process from the thermo dynamical point of view is one which can be retraced in the opposite direction by slightly changing the external conditions, so that the working substance passes through exactly the same conditions as it does in the direct process.

If heat is produced by the working substance in the direct process, the same quantity will be absorbed by it in the reverse process. Again, if work is done by the substance in the direct process, an equal amount of work will be done on the substance in the reversible process. Thus, it is clear that there is no wastage of energy in the reversible process. In a reversible process, the system passes through a series of equilibrium states. Eg. The slow isothermal and slow adiabatic process.

The process which cannot be retraced in opposite order by reversing the controlling factors is called an Irreversible process. Eg.

1.      The conduction of heat from a hot to a cold body.

2.      Production of heat by friction or by the passage of current through an electrical resistance.

3.      Transfer of heat by radiation.

Most of the process in nature are irreversible. During an irreversible change the system is not in equilibrium at all instances of time. Irreversible process consists of non-equilibrium states which cannot be represented on a P-V diagram.

# Conditions for Reversibility

1. The substance undergoing a reversible change must at all instances be in thermodynamic equilibrium with its surroundings. It means the pressure and temperatures of the working substance must never differ appreciably from its surroundings at any stage of the cycle of operation.
2. All the processes taking place in the cycle of operation must be infinitely slow.
3. There should be complete absence of frictional forces.
4.  There should not be any loss of energy due to conduction, convection or radiation during the cycle of operation.

These conditions are never strictly realized in practice because no mechanical process is frictionless and no insulator or conductor is perfect. Thus reversibility is an ideal case. However, the conditions necessary for reversibility can be fulfilled approximately and such process may be regarded as reversible within the limits of experimental errors.