# Quasistatic vs Reversible processes

Would it be fair to state the difference between quasistatic and reversible processes as follows?:

1. A process is quasistatic if at every point in the process the system is in equilibrium with itself.

2. A process is reversible if it is quasistatic and at every point in the process the system is in equilibrium with its environment.

Quasistatic precess: quasistatic process is the process that evolves so slow that every point can be viewed as equilibrium during the process.

As for the reversibility of the process, literally reversible process is quasistatic process in which the increasing in entropy is extremely small,i.e.,the entropy is constant throughout the process. To summary, quasistatic process is the process in which every instantaneous states is equilibrium;reversible process is the quasistatic process in which the entropy does not increase,but the quasistatic is not necessarily a reversible, that depents on the entropy—increasing or not.

In addition, I think that whether a process is quasistaic has nothing to do with if the system is isolated or not.If the system is isolated it is in equilibrium itself.If the "system" is in contact with something else, they can composite a big system whose entropy determines the reversibility of the process.

Quasistatic means that, over the entire process path, the rate of transfer of heat at the system interface with the surroundings and the rate at which work is done at the system interface with the surroundings takes place extremely slowly.

Internally reversible means that, over the entire process path from the initial state of the system to its final state, the system is never more than slightly removed from being at thermodynamic equilibrium.

Fully reversible means that both the system and the surroundings (as a separate complementary system) experience internally reversible process paths.

• Do you have by any chance some examples in mind? – Rodrigo Fontana Jan 11 '18 at 5:48
• This definition you give is similar o one found in Reichl, A modern Course in Statistical Physics, "a reversible change change is one for which tha system remains infinitesimally close to the thermodynamic equilibrium - that is, is performed quasi-statically." In some sense, it seems to me he uses both concepts as synonyms (although they may not be...). – Rodrigo Fontana Jan 11 '18 at 6:58
• @RodrigoFontana I think we can think of cases in which a system experiences a non-quasi-static change, and yet is close to thermodynamic equilibrium throughout. – Chet Miller Jan 11 '18 at 12:44

If you search Wikipedia, you can get the followings. 1. a quasi-static process is a thermodynamic process that happens slowly enough for the system to remain in internal equilibrium.

So your statement is good but misses "happens slowly"

1. In thermodynamics, a reversible process is a process whose direction can be "reversed" by inducing infinitesimal changes to some property of the system via its surroundings, while not increasing entropy.

So your statement is a bit off. A process that is quasistatic and is in equilibrium with its surrounding might not be reversible.

• Although your comment (2) makes sense to me, I always have a conceptual doubt related to the sentence of "not changing the entropy". If Im not mistaken, defined in thermodynamics textbooks, we can read a concept of entropy as the (small) heat exchange over temperature in a $reversible process$. I think this can also be found in Clausius essays. Please, correct me if Im wrong, but isn't it in contradiction with your second statement? (I think some examples are in order but cant think any). – Rodrigo Fontana Jan 11 '18 at 5:17