Reversible and Irreversible Processes

A reversible process is an idealised process carried out infinitely slowly, in such small steps that the system passes through an unbroken sequence of states, each one only infinitesimally displaced from true thermodynamic equilibrium -- this is called a quasi-static process. A reversible process is, in addition, required to have absolutely no dissipative effects present anywhere along the way -- no friction between moving parts, no viscous drag within a fluid, no electrical resistance in a circuit, and so on -- since any such dissipation converts organised mechanical or electrical energy irreversibly into disorganised heat.

Because a reversible process proceeds through a sequence of near-equilibrium states with nothing dissipated along the way, it can, at least in principle, be run backward through exactly that same sequence of intermediate states, restoring both the system and its surroundings to precisely their original condition, with absolutely no leftover trace anywhere that the process ever took place. This is the defining property that gives the process its name.

An irreversible process, by contrast, is any process that cannot be undone in this way without leaving some permanent change behind, somewhere in the system or its surroundings (or both). A great many familiar processes are irreversible for one clear reason or another:

• A block sliding across a rough surface and coming to rest under friction: the block's ordered kinetic energy is converted into disorganised heat in the block and the surface; that heat never spontaneously reconverts itself back into ordered kinetic energy that sends the block moving again.
• The free (unrestrained) expansion of a gas into an evacuated space: the gas rushes in on its own, without any external force resisting it and without passing through any well-defined sequence of intermediate equilibrium pressures, so it cannot be treated as quasi-static at any stage.
• Heat conduction across a finite temperature difference: heat flows from the hotter region to the colder one at every intermediate stage of the process, so running the process "backward" would require heat to flow spontaneously from cold to hot -- precisely what the Clausius statement of the second law forbids.
• The mixing of two different gases, or of hot and cold portions of the same fluid: once mixed, the gases (or fluid portions) do not spontaneously separate themselves back out again.

In the real physical world, every process that occurs at a finite, non-zero rate is, strictly, irreversible to some degree, however small -- some friction, some finite temperature gradient driving the process, or some other small dissipative effect is always present. A perfectly reversible process is therefore only a mathematical idealisation and a limiting case, never something that is achieved exactly. …