26 SECTIONAL ADDRESSES. 



The general equation which is applicable in any particular group of 

 cases is obtained by applying the principles of energy and entropy to 

 reversible transformations. By taking both these quantities as depending 

 only upon the state of a system the following equations are obtained : — 



Group I, Case I. — Perfect gas under uniform pressure. 



Heat entry = dH = G,dT -\-pdv. Work done = dW = pdv. 

 In this case, at constant temperature, dil = dW. 



Case II. — Any fluid under uniform pressure, 



dR = C,dT+T:% dv; dW^pdv. 



oTjf 



Thus, at constant temperature 



rfH-dW = (T5-Si -p)dv. 



Hence dH^tdW unless Tf ^^ j — p remains zero ; that is, unless 



p=T:f(v) 



where f{v) means any function of the volume alone.^ 



Take the case of steam formed at a constant temperature of 100° C. 

 Per gram we have 



AH =540 cal/gram 



, .1650X10'^ ,n 1,1 

 ^ ^^ 42><iO«"' roughly. 



Hence AH=-t^AW. 

 40 



This is an example in which AH is much greater than AW. 

 Group II. — Surface tension, a. 



dll=CJT-T^^ dA, dW=—adA. 



For water ^ 4-m^.L at 300'^ Abs. 

 CT dT 500 



„ dB. 300 3 



Hence jxij~£n^~i,- 

 dW 500 5 • 



Group III. — Magnetism, 



.9/* 



dR=CidT-T^L dl. dW=—}idI. 



aT.i 



■^ Van der Waal's equation is p = — _ 



T^' =^ =■»+ " 

 c7l\v v-b ^ ifi 



Hence dH — dW = --dt) at const, temp. 



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