AND OF THE SECOND LAW 69 



PART III 



PHYSICAL INTERPRETATIONS 

 SECTION A 



OF THE SIMPLE REVERSIBLE OPERATIONS IN THERMODYNAMICS 



Change under Constant Volume 



WE found above that the entropy of a state was precisely 

 denned in a physical way by the number of complexions of that 

 state. Now let us see what happens to this number of com- 

 plexions when an ideal gas experiences some of the simpler 

 changes, of a reversible (non-cyclical) character. We will begin 

 with the case in which the volume of the gas remains constant 

 while its temperature rises, the final state of the gas having a 

 higher temperature than its initial state. 



We see from Eq. (7), p. 51, that c grows and from Eq. (4), 

 p. 50, that C diminishes. MAXWELL'S Law, given by Eq. (5), 



p. 50, shows for a given velocity that the number -j- of molecules 



c dp 



possessing the given velocity is less in the final state than it was 

 in the initial state, and as the total number n of molecules in the 

 gas is unchanged, there will be a greater variety of velocities in 

 the final state. This makes the number of possible permutations 

 greater in the final state, thus increasing the number of com- 

 plexions; consequently, as entropy varies with the logarithm of 

 the number of complexions, we see that the entropy of the final 

 state is greater than in the initial state and this agrees with 

 experience. 



