# How to conceptually identify reversible and irreversible processes?

When I studied thermodynamics for the first time I didn't really get much the conceptual understanding on reversibility, but nonetheless I've got a rough understanding and a mathematical criterion for it.

The rough understanding I got was the following: If we consider a process connecting two equilibrium states we might ask whether the inverse process could occur naturally or not. If it can the process would be reversible and otherwise it would be irreversible.

The criterion for a reversible process would be $\Delta S =0$. The whole point is that the entropy maximum postulate states that the entropy must be maximized. If when a constrant is removed a system goes from $A$ to $B$ with $\Delta S > 0$, then certainly $S_B>S_A$, hence naturally the system could never return from $B$ to $A$, because it wouldn't maximize the entropy.

On the other hand if $\Delta S = 0$, we would have $S_A = S_B$ and nothing would prevent the return.

This is a mathematical criteria and more than that, requires the idea of entropy.

My question here is: suppose we are simply given the description of a process (for example: "a piece of hot metal is thrown into cold water" or "a pendulum with a frictionless support swings back and forth"), in that case we don't know the fundamental relation, and there's no mathematics whatsoever here.

In that case, just from a conceptual description of a process how can one judge whether the process is reversible or irreversible? What is a criterion that can be used when we are discussing the process just conceptually without math involved and without any knowledge about entropy?

• The main criteria of reversibility is you can return to your initial state by reversing the operation; no information is lost. Irreversibility means there is a form of dissipative work- information is lost during the operation and hence you can't return back to your initial state even by reversing the operation. – user36790 Mar 16 '16 at 12:36
• As Feynman said: make a movie of the process, then show the film backwards. If the audience laughs, the process was irreversible. – rob Mar 16 '16 at 16:46
• My answer on this topic over on Worldbuilding.SE may be useful. – NauticalMile Mar 16 '16 at 16:53
• I think you may have a misconception. A reversible process implies there is no entropy generated as the system moves between two states ($S_{\mathrm{gen}}=0$). This does not mean that the entropy is constant. When the entropy remains constant ($\Delta S=0$), and the process is called isentropic. A general open or closed system can have interactions with the surroundings, and a reversible process does not in general mean the process is also isentropic. When the system is isolated, however, there is no interaction with the surroundings, and a reversible process must also be isentropic. – NauticalMile Mar 16 '16 at 17:53