OHAPTBK XVIL 



THERMODYNAMICS. 



The Second Law of Thermodynamics The Indicator Diagram Isothermals 

 Adiabatics or Isentropics Heat Engines Carnot's Reversible Heat Engine 

 Carnot's Cycle Conditions for Reversible Working Examples of Reversible 

 Processes Of Irreversible Processes Efficiency of an Engine Absolute or 

 Work Scale of Temperature Efficiency expressed on the Absolute Scale 

 Comparison of the Absolute with the Air Scale Water-wheel Analogue 

 Reversible Cycles in general Entropy Entropy-Temperature Diagram 

 Quantities Analogous to Entropy Entropy tends to increase Dissipation of 

 Energy Intrinsic Energy Available Energy Possible efficiency of a Steam 

 Engine. 



WE shall now investigate the conditions under which the transformation 

 from heat into other forms of energy occurs, and the extent of the trans- 

 formation under given conditions. 

 We shall make use of a general 

 principle, which we may con- 

 veniently state here, known as 

 The Second Law of Thermo- 

 dynamics. We cannot transform 

 heat into work merely by cooling a 

 body already at the lowest available 

 temperature. 



The significance of this state- 

 ment will be appreciated better 

 when we apply the law to re- 

 versible heat engines. Being a 

 negative statement, it is hardly 

 founded on direct experiment, but 

 rather on the general observation 

 that when we seek to transform 

 heat to work we usually derive the 

 heat from a hotter body, we trans- 

 form some of it, and are obliged to 

 give some of it to a colder body, 



Volume 

 FIG. 147. Indicator Diagram. 



actually heating that colder body. 

 Any apparent exception will, on examination, be found to involve some- 

 thing beyond mere cooling of the already coldest body. For instance, if 

 a quantity of air be under great pressure it may be allowed to do a con- 

 siderable amount of work in expanding, and its temperature will fall 

 very appreciably, perhaps far below that of the surroundings. But it is 

 not merely cooled. It has not given up its heat by conduction. It has 

 changed its volume while doing the work. If the air could be put 



