I was studying some basic thermodynamics concepts and I noticed that in reversible processes in my book always the adiabatic word is used.

  1. So are all reversible processes adiabatic? (ques 1)

  2. Or are at least all reversible expansion adiabatic? (ques 2),

  3. And are all irreversible processes adiabatic? (ques 3)

  4. or at least all irreversible expansion are adiabatic? (ques 4)

  5. If all of these statements are wrong then when are they true?

  6. This adiabatic word is troubling me very much as it is almost used everwhere. Why?

I know adiabatic means $Q=0$ (that is the system does not exchange heat energy with the surrounding) so no need to explain adiabatic.


1 Answer 1


To answer your question, at first we have to know what is reversible process. A reversible process is a process where the effects of following a thermodynamic path can be undone be exactly reversing the path. An easier definition is a process that is always at equilibrium even when undergoing a change. i.e. in reversible process we can draw the process backward by changing conditions.

So it is not needed that all the reversible processes must be adiabatic. Isothermal, isochoreic, and isobaric processes can also be reversible. As example, in Carnot's Engine, the working substance first undergoes a reversible isothermal expansion, then reversible adiabatic expansion. After that the working substance compressed reversibly and isothermaly and then reversibly and adiabaticaly. So here we get almost all types of reversible process.

Similarly we can't say all irreversible process are adiabatic but we can say most of them occurs adiabatically.

In practical all the changes in thermodynamics state supposed to occurs in quasistatic process.

  • $\begingroup$ Can you please explain this quasi-static a bit more in this context?? $\endgroup$
    – Freelancer
    Commented Oct 14, 2015 at 16:05
  • $\begingroup$ All right, a process is called quasistatic if all the parameters of the system$(p,V,T,etc)$ vary physically indefinitely slowly so that the system is found all the time in an equilibrium state. This means that at any stage of the process, there will be enough time for the parameters to equalize over the entire system and such a process will represent a continuous succession of equilibrium states infinitely close to each other. They are practically quasi-equilibrium or quasi-static. $\endgroup$ Commented Oct 14, 2015 at 16:19

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