Thermodynamics
SECOND LAW OF THERMODYNAMICS
DS
DQ
Tabs
(For a reversible process)
where
S
=
the change in entropy of a system during some process (Btu/°R)
D
Q
=
the amount of heat added to the system during the process (Btu)
D
Tabs
=
the absolute temperature at which the heat was transferred (°R)
The second law can also be expressed as DSO for a closed cycle. In other words, entropy must
increase or stay the same for a cyclic system; it can never decrease.
Entropy is a property of a system. It is an extensive property that, like the total internal energy
or total enthalpy, may be calculated from specific entropies based on a unit mass quantity of the
system, so that S = ms. For pure substances, values of the specific entropy may be tabulated
along with specific enthalpy, specific volume, and other thermodynamic properties of interest.
One place to find this tabulated information is in the steam tables described in a previous chapter
(refer back to Figure 19).
Specific entropy, because it is a property, is advantageously used as one of the coordinates when
representing a reversible process graphically. The area under a reversible process curve on the
T-s diagram represents the quantity of heat transferred during the process.
Thermodynamic problems, processes, and cycles are often investigated by substitution of
reversible processes for the actual irreversible process to aid the student in a second law analysis.
This substitution is especially helpful because only reversible processes can be depicted on the
diagrams (h-s and T-s, for example) used for the analysis. Actual or irreversible processes cannot
be drawn since they are not a succession of equilibrium conditions. Only the initial and final
conditions of irreversible processes are known; however, some thermodynamics texts represent
an irreversible process by dotted lines on the diagrams.
Rev. 0
Page 71
HT-01
