CHAPTER 4 THE SECOND LAW
4.6 APPLICATIONS 128
To evaluate the entropy change, we need a reversible path from the initial to the final
state. The net quantity of heat transferred to phaseíduring an infinitesimal time interval
is∂qíD
P
ì§í∂qíì. The entropy change of phaseíis the same as it would be for the
reversible transfer of this heat from a heat reservoir of temperatureTí: dSíD∂qí=Tí.
The entropy change of the entire system along the reversible path is found by summing over
all phases:
dSD
X
í
dSíD
X
í
∂qí
Tí
D
X
í
X
ì§í
∂qíì
Tí
D
X
í
X
ì>í
∂qíì
Tí
C
∂qìí
Tì
(4.6.7)
There is also the condition of quantitative energy transfer,∂qìíD ∂qíì, which we use
to rewrite Eq.4.6.7in the form
dSD
X
í
X
ì>í
1
Tí