A process in a closed system is reversible if the entropy change is $dS = \frac{dQ}{T}$.
A process is quasistatic if a process is infinitely slowly.
Now, if a process is reversible, this means that we are always in equilibrium and this can only be the case, if we do this process very slowly. Thus, any reversible process is quasi-static.
Although I think I understand the basic definition, I don't see why we need the concept of a quasistatic process? Which equations or concepts in thermodynamics actually rely on this kind of process?
Wikipedia lists: isobaric, isochoric, isothermal processes. So is any such process necessarily quasistatic? What about adiabatic processes?
So far I did not get why it is important in thermodynamics to look at such processes and which equations hold only for such processes?
Best Answer
In classical thermodynamics the equations are valid only for thermodynamic equilibrium. That means that your system must have his variables well defined all the time. For instance, if you have a gas the temperature must be well defined and the same across the gas. In order for this to be valid when you have a process, the process must be slow enough so that different parts of the gas keep the same temperature. That is why any reversible process is necessarily a quasistatic one (but, some quasistatic processes are irreversible).
In short, when you assume a reversible process you are assuming a quasistatic one, thus each time you use the thermodynamic equations for whatever reversible process you are implicitly assuming "quasistaticity". In particular the ones you asked, such as isobaric, isochoric, isothermal processes (if they are reversible).