Entropy Change for a Reversible, Isothermal Process

In general, it is not possible to evaluate ΔS directly by integrating dQ/T.
However, in the special case of an

internally reversible



process it is possible and easy too!
Because temperature is constant, T can be pulled out of the intergral:
Internally Reversible, Isothermal Processes
PV diagram showing a process patch from state 1 to state 2.
This is particularly useful for determining the change in entropy of a

thermal reservoir

or for the two reversible, isothermal steps in the

Carnot Cycle

Constant T
T1 = T2
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Ch 7, Lesson B, Page 4 - Entropy Change for a Reversible, Isothermal Process