An isentropic process is a reversible-adiabatic process. For an adiabatic process =0, and for a reversible process, =0. From Eq. (38.26), for an isentropic process, . However, in Eq. (38.37), substitution of isentropic condition yields
or
or
(38.39)
Using , we have
(38.40)
Considering Eq. (38.38), in a similar way, yields
or
(38.41)
Using , we get
(38.42)
Using we can write
(38.43)
Combining Eq. (38.40) with Eq. (38.43), we find,
(38.44)
This is a key relation to be remembered throughout the chapter.
=0, and for a reversible process, 
=0. From Eq. (38.26), for an isentropic process, 
, we have
, we get
we can write
