228 MILNE art. f 



But 



AQ = Ac -f AW. 



If now the change is reversible, AQ = tAt], and so in this case 



Af = - (AW - pAv). 



Thus the decrease in f is equal to the excess of external work 

 performed over the work of expansion against the external pres- 

 sure. Hence the name "free energy" function. 



If any process occurs at constant pressure and constant 

 temperature, and if A'Q is the heat absorbed and A'TF the ex- 

 ternal work performed, 



whence 



also 



Hence 



or 



t 



Ae + pAv - Af ^ A'Q; 



A'Q = Ae + A'W. 



Ae + pAv - Af ^ Ae + A'W, 



{A'W - pAv) ^ -Ar. 



Thus the excess of external work performed over that of mere 

 expansion cannot exceed — Af . 



Now suppose that the system is enclosed in an environment of 

 constant pressure and constant temperature. Then if any 

 process occurs of itself, the only external work is that of expan- 

 sion, and so 



A'W = pAv. 



Therefore 



Af ^ 0. 



