What is a reversible process in thermodynamics?

A reversible process in thermodynamics is defined as an idealized process that can be reversed without leaving any changes in the system or its surroundings. This means that throughout the process, the system remains in thermal, mechanical, and chemical equilibrium, allowing it to return to its initial state seamlessly.

In such processes, all changes are infinitesimally small, and they can theoretically occur infinitely slowly, ensuring that the system passes through a series of equilibrium states. Because of this characteristic, the work done by or on the system, as well as heat exchanges with the surroundings, can be perfectly undone simply by reversing the process.

This concept is critical in thermodynamics as it helps establish maximum efficiencies for processes and cycles, such as in engines and refrigerators, as the efficiency calculations often rely on idealized reversible cycles.

Reversible processes are essentially a theoretical benchmark against which real processes can be compared, as true reversibility is never achieved in practice due to factors like friction, turbulence, and non-equilibrium conditions present in actual systems.