Page 245 - Standard Handbook Of Petroleum & Natural Gas Engineering

218 General Engineering and Science

From the second law, for a reversible process,

SQ
dS = - or SQ = TdS (2- 129)
T
Combining Equations 2-128 and 2-129 gives
dU = TdS - PdV (reversible, P-V work only) (2-130)

Reversible Isobaric Processes
The second law is written as

SQ = TdS
but, since the heat capacity C = SQ/dT, the second law becomes
CdT = TdS (2-13 1)

Thus, for constant pressure processes, the entropy increase is written as

dT
dS = C, - C,d(enT) (reversible isobaric process) (2-132)
=
T
Reversible Constant Volume Processes
A constant-volume process is called zsochoric and, for such processes, the entropy
increase is written as

dT
dS = C, - = C,d(tn T) (reversible isochoric process) (2-133)
T
Reversible Isothermal Changes: Maximum Work
The variation of entropy with volume and pressure under conditions of constant
temperature is determined by using Equation 2-130:

(2-134)

or rearranging:

(2-135)

The variation of entropy with pressure is likewise written as

(2-136)