1.3.3 Reversible Processes

A quasi-static process goes from the initial state to the final state through a series of equilibrium states. A modification of the quasi-static process gives what is called a reversible process.

A quasi-static process is one that is sufficiently slow. A reversible process is slow too, but it is more than that. In a reversible process a very slight change in the external conditions can change the direction of the process. For example, return to the illustration of a gas trapped inside a cylinder by a movable piston. If the inside pressure is greater than the outside pressure, the piston will move outward. This is true even if the outside pressure is just a microscopic amount less than the inside pressure. Of course, then even a very small expansion of the volume inside the cylinder will cause the pressure inside to drop to equality with the outside pressure and the expansion will stop. But never mind. Think of what would happen if we even slightly increased the outside pressure. It would now exceed the inside pressure and the expansion would reverse and become a compression.

If a process takes place such that it is, at every step along the way, reversible in this sense, the entire process is said to be reversible.

This doesn't just apply to expansions and compressions. One can heat a system reversibly too. Just think of the entire system being immersed in a constant temperature bath with a temperature control. If the system and the bath are in temperature equilibrium the slightest twitch of the temperature control will result in heating becoming cooling or vice versa.

Many processes can be done, at least conceptually, in a reversible manner. Some cannot. For example the free expansion of a gas into a vacuum can't be done reversibly.

What is the difference between a quasi-static process and a reversible one? Both take place via equilibrium states. But only the reversible process requires that the internal and external conditions be "balanced". A quasi-static process must be slow, but it could be slow because it is slowed artificially. All the while it could be unbalanced with respect to the external conditions.

Reversible processes are important in thermodynamics. Indeed, they are among the most commonly studied processes. Of course they are not "real". We can't actually carry out infinitely slow processes. They are idealizations of real processes and as such are very useful.