Consider an isolated system. We know that it will reach equilibrium after enough time. According to what I have read so far, the system should undergo irreversible processes to reach equilibrium, but is there any counterexample to this statement? i.e a system and a nonequilibrium initial condition such that processes used to reach equilibrium are all reversible?

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    $\begingroup$ Not if the system is isolated. $\endgroup$ – Chet Miller Mar 13 at 14:03
  • $\begingroup$ @ChetMiller Why? and what is an example if it is not isolated? $\endgroup$ – onurcanbektas Mar 13 at 14:20
  • $\begingroup$ Well, if it is not isolated, then you can make the process reversible by exchanging heat and work reversibly with the surroundings. $\endgroup$ – Chet Miller Mar 13 at 15:03

Assume an isolated system. Start from an equilibrium state $0$ and remove some internal constraint so that the system spontaneously moves to another equilibrium state $1$. The difference in entropy $\Delta S = S_1 - S_0$ between the entropies of the final $S_1$ and initial $S_0$ is never negative (2nd law), and it is positive for an irreversible process the amount being characteristic and a measure of irreversibility.

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  • $\begingroup$ I think the OP is saying the isolated system starts out in disequilibrium. $\endgroup$ – Bob D Mar 13 at 17:24
  • $\begingroup$ @Bob_D That is OK, just measure the entropy variation at any instant after the system is allowed to equilibrate and has left state $0$. $\endgroup$ – hyportnex Mar 13 at 17:34

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