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SECOND LAW OF THERMODYNAMICS AND ENTROPY 257

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The entropy attains its maximum value when the system reaches a stable equilibrium
state from a non-equilibrium state. This is the state of maximum disorder and is one of maximum
thermodynamic probability.

5.15. TEMPERATURE-ENTROPY DIAGRAM

If entropy is plotted-horizontally and absolute temperature vertically the diagram so obtained
is called temperature-entropy (T-s) diagram. Such a diagram is shown in Fig. 5.24. If working
fluid receives a small amount of heat dQ in an elementary portion ab of an operation AB when
temperature is T, and if dQ is represented by the shaded area of which T is the mean ordinate, the

width of the figure must bedQ
T

. This is called ‘increment of entropy’ and is denoted by dS. The
total heat received by the operation will be given by the area under the curve AB and (SB – SA) will
be corresponding increase of entropy.

SA dS SB

S(Entropy)

A

a

b

B

T (Temp.)

Fig. 5.24. Temperature-entropy diagram.
From above we conclude that :
Entropy change, dS Q
T
= Heat change ( )
Absolute temperature ( )
“Entropy may also be defined as the thermal property of a substance which remains
constant when substance is expanded or compressed adiabatically in a cylinder”.
Note. ‘s’ stands for specific entropy whereas ‘S’ means total entropy (i.e., S = ms).

5.16. Characteristics of Entropy

The characteristics of entropy in a summarised form are given below :

1. It increases when heat is supplied irrespective of the fact whether temperature changes
   or not.


2. It decrease when heat is removed whether temperature changes or not.