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To calculate the change in entropy for a finite process, we must recognize that T is generally not constant. If dQ r is the energy transferred by heat when the system follows an arbitrary reversible process between the same initial and final states as the irre- (22.9) As with an infinitesimal process, the change in entropy %S of a system going from one state to another has the same value for all paths connecting the two states. % S ! % f i dS ! % f i
dQ
r T Let us consider the changes in entropy that occur in a Carnot heat engine that operates between the temperatures T c and T h . In one cycle, the engine takes in energy Q h from the hot reservoir and expels energy Q c to the cold reservoir. These energy transfers occur only during the isothermal portions of the Carnot cycle; thus, the con- where the negative sign represents the fact that !Q c ! is positive, but this term must rep- resent energy leaving the engine. In Example 22.3 we showed that, for a Carnot engine, Using this result in the previous expression for %S, we find that the total change in Now consider a system taken through an arbitrary (non-Carnot) reversible cycle. Because entropy is a state variable—and hence depends only on the properties of a (22.10) where the symbol indicates that the integration is over a closed path. & &
dQ
r T ! 0 % S ! 0 ! Q
c ! ! Q
h ! ! T c
T h % S ! ! Q
h ! T h " ! Q
c ! T c S E C T I O N 2 2 . 6 • Entropy 685 Quick Quiz 22.7 Which of the following is true for the entropy change of a system that undergoes a reversible, adiabatic process? (a) %S $ 0 (b) %S ! 0 (c) %S # 0 Quick Quiz 22.8 An ideal gas is taken from an initial temperature T i to a higher final temperature T f along two different reversible paths: Path A is at constant pressure; Path B is at constant volume. The relation between the entropy changes of A # % S B (b) %S A ! % S B (c) % S A $ % S B . Change in entropy for a finite process |