11. Equilibrium (reversible) and non-equilibrium (irreversible) processes

We got to know different processes: isothermal (temperature is constant), isobaric (pressure is constant) and isochoric (volume is constant). At the same time, one of the most important circumstances was not sufficiently emphasized. It was assumed that all these processes proceeded very slowly. For example, gas compression in the cylinder runs so smoothly that at any time a new state of equilibrium with new values of pressure and volume can be established. Such slow processes are called equilibrium processes.

If, after slow compression, the gas is allowed to expand slowly, it passes through the same sequence of equilibrium states as in compression. For this reason, equilibrium processes are also called reversible.

The states of heat equilibrium and reversible processes are primarily the subject of study in the theory of heat phenomena. In this case, the trends are the simplest.

The process of state change, followed by imbalance in the system, is called non-equilibrium. Let the piston compress the gas in the cylinder very quickly. Then the equilibrium between the individual parts of the gas will be disturbed, and only after some time the gas will move to a new equilibrium state. This process is irreversible: with rapid compression, the pressure under the piston is higher at first than in the rest of the cylinder, while with rapid expansion it is lower, i.e. the gas states at compression and expansion are different.

Real processes are not infinitely slow, and in fact they are all irreversible. The idea of a reversible process is the same idealization of real processes as, for example, the movement of an absolutely solid body or the flow of an ideal fluid in mechanics. If real processes are slow enough, they can be considered reversible with a good approximation.