## Reversible and Irreversible Proccesses

Jacqueline Lee
Posts: 21
Joined: Fri Sep 25, 2015 3:00 am

### Reversible and Irreversible Proccesses

Can someone explain the difference between a reversible and irreversible process?

If a system at equilibrium is a reversible process, does that mean a system not in equilibrium is irreversible process? But wouldn't systems eventually return to equilibrium?

Ronald Yang 2F
Posts: 86
Joined: Fri Sep 25, 2015 3:00 am

### Re: Reversible and Irreversible Proccesses

Yes, a system not in equilibrium would undergo an irreversible process. Whether a process is reversible or irreversible refers to the state of the system at a certain point.

A process is reversible when an infinitesimal change in the external pressure can reverse that process. For example, when a gas in a piston is at equilibrium (Pext=2 atm and Pgas=2 atm), when the gas is expanding, an infinitesimal increase in the external pressure would cause the gas to compress rather than expand (opposite processes).

A process is irreversible when an infinitesimal change in the external pressure cannot reverse that process. That's when a system is not at equilibrium. For example, when a gas in a piston is under the conditions of Pext=1 atm and Pgas=2 atm (not equilibrium) and is expanding, an infinitesimal increase in the external pressure won't result in the opposite process of compression - the gas will keep expanding.

And yes, the processes eventually return to equilibrium. It's just a matter of whether the system starts at equilibrium or not.