274 



HEAT. 



Entropy. 



Definition of Entropy. If a substance at a temperature 6 receives 



Q 



heat Q, it is said to receive entropy -g . 



Similarly if it parts with heat Q, when at a temperature 6, it is said 

 to lose entropy *? . If the gain or loss of heat affects the temperature, 



we may divide the gain or loss into elements each at practically constant 

 temperature, and then the amount of entropy gained or lost is 



The Entropy gained by a body in passing from one state 

 to another depends solely on the initial and final states. Let 



FIG 157. Entropy. Change in passing from A to B 

 the same by all paths. 



A in the indicator diagram (Fig. 157) represent some standard state of 

 the body, B some other state. Then we shall show that, however the 

 body passes from A to B, the entropy received is the same, or the 

 entropy at B depends solely on the position of B and not on the mode in 

 which the state changes from A to B. 



Let ACB, ADB, be two paths from A to B. Let the substance be 

 carried round the cycle AOBDA by a reversible process,* then 



2~ from A through C to B + 'sS- from B through D to A = 0. 



* Inasmuch as the conditions of reversibility are external conditions, any series 

 of changes of temperature, pressure, and volume in a body can be imagined to be 

 carried out reversibly by suitably adjusting the external surroundings. 



