In one of my books, a process is said to be isentropic from which they conclude that it is adiabatic thus reversible. I don't think "isentropic" is a sufficient condition for these conclusions. Can someone enlighten me ?
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Taken from wikipedia:
In thermodynamics, an isentropic process is an idealized thermodynamic process that is adiabatic and in which the work transfers of the system are frictionless; there is no transfer of matter and the process is reversible.
An isentropic process is, by definition, adiabatic and reversible.
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$\begingroup$ I disagree. What about an irreversible cycle in which the entropy of the working substance (the system) returns to its original value at the end of the cycle? Of course, in such a situation, the change in entropy of the surroundings is not zero. $\endgroup$ Commented Jul 5, 2017 at 20:01
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$\begingroup$ @ChesterMiller "The word 'isentropic' is occasionally, though not customarily, interpreted in another way, reading it as if its meaning were deducible from its etymology. This is contrary to its original and customarily used definition." That's from the second paragraph. Typically it is not used to mean "no change in entropy" although that is what the name implies. $\endgroup$– JMacCommented Jul 5, 2017 at 20:37
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$\begingroup$ Thanks @JMac. I'm just wondering what the OP had in mind when he asked this. $\endgroup$ Commented Jul 5, 2017 at 22:11
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$\begingroup$ @ChesterMiller Oh, I totally got where it was coming from. Once you get used to the naming convention it just seems reasonable that isentropic should mean what you would think it means; but I kept my answer simple because I think it spoke for itself well given the specific question asked. $\endgroup$– JMacCommented Jul 5, 2017 at 22:18
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$\begingroup$ @JMac seems odd to have the term "isentropic" mean something different from its mathematical definition of just $\Delta S_\text{system} = 0$. The same way how isothermal means $\Delta T_\text{system} = 0$, isobaric means $\Delta P_\text{system} = 0$, isochoric means $\Delta V_\text{system} = 0$, etc... So it seems Wikipedia is likely confused here --- it's actually the term, "reversible" that means $\Delta S_\text{universe} = 0$, while "isentropic" only refers to the system's entropy change. Thus it's possible for an irreversible process with $\Delta S_\text{system} = 0$ to be isentropic $\endgroup$– ManRowCommented Dec 9, 2019 at 6:30
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Let me clarify all of your doubts. Suppose a system is going through a process from state 1 to state 2. There are two possibilities, the process may be reversible and may be irreversible. Let's talk in general, $$dS=(\Delta Q/T)+ dS_{gen}$$ so, first doubt whether isentropic process is always reversible, suppose the system is going from 1 to 2 by irreversible process in the case taken above, so there will be entropy change due to heat exchange and some entropy will be generated. But think about the case when heat is given to the system. therefore, $\Delta Q/T$ will be negative where as $dS_{gen}$ is positive. So, if $dS_{gen}$ cancels out with $(\Delta Q/T)$, $dS$ will be zero but process is irreversible. so always we can't say that isentropic process is reversible process. Continuing the discussion, let suppose the above process is reversible, so $dS_{gen}=0$, also if the process is adiabatic, therefore, there is no heat exchange $(\Delta Q/T=0)$.\ This means from our equation dS=0, also Therefore, reversible adiabatic process is always isentropic process. talking about the reverse, the first case we discussed, the process was isentropic but not an adiabatic one. so, an isentropic process can not always be declared as an adiabatic process. Hope you have got it.
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$\begingroup$ can a irreversible process of a subsystem be reversed by increasing the total entropy of the system? $\endgroup$ Commented Jul 10 at 12:21