276 HEAT. 



gas the lines of equal pressure and of equal volume are represented by 

 curves of the type 



where, for the one, c is the specific heat at constant pressure, and for the 

 other the specific heat at constant volume. 



The total Entropy of a system is unchanged by the perform- 

 ance of reversible cycles between its parts. When a body is 



worked between two given temperatures we have seen that , the 



Os 



entropy taken from the source, equals ^ K , the entropy given to the re- 



0R 

 frigerator ; the total quantity is therefore unchanged. 



But if the cycle is a perfectly general reversible one we still have 



Y 



2,0-0 



or the total entropy received by the working substance is zero. But in 

 order that the cycle should be reversible, all passages of heat must take 

 place between bodies differing infinitely little in temperature, so that Q 

 and 0, being the same for giver and receiver, the entropy gained by the 

 working substance in any part of the system is equal to that lost by the 

 source whence the heat comes. Hence the total entropy lost by the 

 surroundings of the working substance is also zero. 



Quantities Analogous to Entropy. The conception of entropy is some- 

 what difficult in that, as it does not directly affect the senses, there is 

 nothing physical to represent it. It may assist the reader if we point 

 out some already familiar quantities which are analogous to it. Since 



Entropy = heat energy -f temperature, 

 . . Heat energy = entropy x temperature. 



Also in the Carnot reversible cycle, 



Energy transformed = Q s x 



f) 



- 



= Entropy x fall of temperature. 

 Comparing this with gravitation energy 



Gravitation energy = Mass x level above zero, 



the level being measured by work done on unit mass raised from zero level, 

 and Energy transformed = mass x fall in level. 



Then entropy is analogous to mass and temperature to work measure 

 of level. 



Or taking the case of electrical energy, 



Electrical energy = charge x potential. 



