Reversible and Irreversible Processes

Processes occur when there exists a driving force for a change of state between parts of the system or between the system and surroundings. If the driving force is finite, the process is irreversible; and if it is infinitesimal in magnitude, the process is reversible.
Examples of reversible processes: .

  • Slow expansion or compression

  • Pendulum motion without friction

  • Heat transfer through a near zero temperature difference

All spontaneous processes occurring in nature are irreversible. An irreversible process is a process that cannot be reversed (i.e. returning the system to its original state) without a net change in heat and work transfers to the surroundings. That is, for an irreversible process, it is possible to reverse the system to its original state, but the surroundings are changed as a result. In contrast, for a reversible process, after completing the forward and the reverse process, there is no net change in the system or the surroundings.
Examples of irreversible processes:

  • A gradient within a system, or between two systems (non-equilibrium effects)

  • heat transfer due to a finite temperature difference between the system and its surroundings (temperature gradient).

  • mixing of the same two fluids initially at different pressures and temperatures (pressure gradient and temperature gradient).

  • viscous flow of a fluid (velocity gradient).

  • solid-solid friction (velocity difference).

For a reversible process to happen, the difference in pressure (and temperature) between the system and surrounding required is very minimum. i.e., \(P_{\text{system}} \approx P_{\text{surrounding}}\). In other words, the reversible process happens with near zero driving force. Under this condition, we can take that \(P_{\text{surrounding}} = P_{\text{system}} = P\).

By assuming the process to be reversible one, we can find the minimum work requirement for effecting a change (or the maximum work obtainable from a change).


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