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1 Cp(T) dT
1 (7–38) where s° is the value of s° at T2 and s° is the value at T1. Thus,
. s °(T), kJ/kg K
(Table A-17) FIGURE 7–33
The entropy of an ideal gas depends
on both T and P. The function s°
represents only the temperaturedependent part of entropy. s1 s°
1 R ln P2
P1 (kJ/kg · K) (7–39) (kJ/kmol · K) (7–40) It can also be expressed on a unit-mole basis as
1 Ru ln P2
P1 Note that unlike internal energy and enthalpy, the entropy of an ideal gas
varies with specific volume or pressure as well as the temperature. Therefore,
entropy cannot be tabulated as a function of temperature alone. The s° values
in the tables account for the temperature dependence of entropy (Fig. 7–33).
The variation of entropy with pressure is accounted for by the last term in
Eq. 7–39. Another relation for entropy change can be developed based on
Eq. 7–31, but this would require the definition of another function and tabulation of its values, which is not practical.
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This document was uploaded on 11/28/2012.
- Spring '09