## Isentropic Process and Reversible Process

Hi all,

Will like to check if my below thoughts are right:

1) For a totally reversible process, it has to be both internally and externally reversible. And the definition of externally reversible means heat transfer between a system and reservoir where the temperature are the same.

2) For an isentropic process, it has to be internally reversible and adiabatic.

From the above 2 definition, can i say that
A) For a totally reversible process, it will not be isentropic since it involve heat transfer, albeit the temperature difference is infinitisimal small.(based on definition of externally reversible)

B) An isentropic process is not a totally reversible process but only an internally reversible process

For another question on the same topic, in an adiabatic reversible expansion of a piston system( i.e isentropic) , can I visualise the reason this process is seen as isentropic is because although the volume increases, meaning the molecules are more random(increase entropy), the counter effect is brought about by the reduction in pressure and temperature, hence slowing down the molecule speed, resulting in a zero entropy change??
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 Quote by noobie_doobie Hi all, Will like to check if my below thoughts are right: 1) For a totally reversible process, it has to be both internally and externally reversible. And the definition of externally reversible means heat transfer between a system and reservoir where the temperature are the same. 2) For an isentropic process, it has to be internally reversible and adiabatic. From the above 2 definition, can i say that A) For a totally reversible process, it will not be isentropic since it involve heat transfer, albeit the temperature difference is infinitisimal small.(based on definition of externally reversible) B) An isentropic process is not a totally reversible process but only an internally reversible process For another question on the same topic, in an adiabatic reversible expansion of a piston system( i.e isentropic) , can I visualise the reason this process is seen as isentropic is because although the volume increases, meaning the molecules are more random(increase entropy), the counter effect is brought about by the reduction in pressure and temperature, hence slowing down the molecule speed, resulting in a zero entropy change??
I don't quite understand how you conclude that isoentropic process is not reversible

Yes to the "another question"
 I think maybe it is good to define that the only kind of reversible processes are a)internally reversible, b)externally reversible and c)totally reversible. We should avoid using the term general term 'reversible process' as in many websites since there are subtle differences between each reversible process which have to be considered. Going back to my conclusion in point B) that isentropic process is not totally reversible, this is because the process might have internal irreversibility but is negated through heat transfer. Hence, with internal irreversibility, the process is no more totally reversible. One question, although the process is isentropic for this case, does it implies that the surrounding has be to isentropic too?? As for my statement A), maybe I will rephrase to say that i) for a totally reversible single path process, it might not be isentropic since it involve heat transfer, albeit the temperature difference is infinitisimal small.(based on definition of externally reversible). This can be seen from the isothermal expansion in Carnot cycle, which is a totally reversible process, yet not isentropic. ii) for a totally reversible single path process, only an adiabatic expansion is isentropic, as seen from adiabatic expansion in Carnot Cycle. iii) only a cyclic process, consisting of totally reversible single path processes, will result in a zero entropy change to the system and environment. A cyclic process, consisting a mixture of totally reversible single path and isentropic(not necessary reversible) single path, might cause the system to have zero entropy change but not the environment.

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