# Reversible and Irreversible Processes in a Cyclic Model with a Piston

I have a question that pertains to how we can recover the initial or previous state in a reversible process and how that becomes impossible with an irreversible process.

In the quasit-static model with a piston there is a classic example where weights are removed in an infinitesimal manner so that the expanding gas never deviates far from equilibrium as we continue the process along an isotherm. In the idealized, reversible process we assume there is no friction or dissipative forces. You can always recover the previous state of the gas just by adding the weights back.

My question is about what happens in an irreversible process if you permit friction. The gas will not push the piston up as far because some of the energy is no longer used to do work in expansion. The energy is lost to friction as heat, so the expansion is less. But if you add the same weight back which was removed, would the piston actually not compress by the same (lessened) amount that it expanded? The friction should be the same whether you're going up or down, but you can't actually recover your previous state since this is not a reversible process.

My thinking is that since the temperature and pressure would change in the irreversible process with friction, that adding the weight back would not give you a compression that equally offsets the expansion when you removed the weight. Is this correct? Or does the gas actually compress to the previous volume but the temperature and pressure have still changed?

***Assume in 1 case that heat remains in gas and in another case that heat is permitted to enter the surroundings

• Where does the heat go, back into the gas, or is heat transfer allowed with the surroundings? Oct 7, 2019 at 14:19
• How about we explore both of those scenarios. When I was originally thinking about it I assumed if heat went to the surroundings then the gas could be re-compressed to the original volume by adding the weight back. But then I started wondering that if it lost the heat to the surroundings, temperature and pressure would be lower, so by adding the weight back the compression would be MORE than the expansion. I feel like I am confusing myself. Oct 7, 2019 at 14:34
• Check out this thread in another forum that addresses questions of piston friction: physicsforums.com/threads/…. If I remember correctly, it considers the specific case of an adiabatic system. Oct 7, 2019 at 14:45
• @ChetMiller Hi Chet. I want to say, for the quasi-static adiabatic expansion and compression with friction, since the only way to transfer the entropy generated in the system to the surroundings is by heat transfer, the adiabatic system cannot be returned to its original state (original entropy). But I've made some missteps lately that my confidence has taken a hit. Oct 7, 2019 at 16:10
• while this thread at physicsforums is rather informative and interesting, I am having trouble parsing it to find the answer to my particular question posed above. Oct 7, 2019 at 16:23