APPENDIX 373 



transformations entropy is generated. Therefore the 

 Entropy of the Universe tends to a maximum. 



AVAILABLE AND UNAVAILABLE ENERGY 



Consider the Carnot engine as a perfect mechanism. 

 It takes heat-energy from a source at a temperature 

 Ta", and it gives up heat to a refrigerator at a tempera- 

 ture T°, Ti° being greater than T^". In the adiabatic 

 expansion 1^2 the gas continues to expand until its 

 temperature becomes equal to that of the refrigerator. 

 It cannot, then, expand and do work any longer, and 

 thus the proportion of the heat, Q.2, received from the 

 source, which can be converted into work, depends on 

 the difference of temperature 2^2° - T^°. The greater 

 is this difference the greater will be the proportion of 

 the heat-energy received which can be converted into 

 work. If the engine were a perfect one, and if the gas 

 were also a perfect one (that is a gas which would 

 continue to expand according to the equation for the 

 adiabatic expansion of gases), and if the refrigerator 

 were absolutely cold, then all the heat energy received 

 from the source could be converted into work. 



We cannot produce a refrigerator of absolute tem- 

 perature 0°, and therefore only a certain proportion 

 of the heat which is received by the engine can be 

 transformed into mechanical work. But this work 

 can be used to reverse the action of the engine, and 

 thus the same fraction of the total heat-energy which 

 was given to the refrigerator can be taken from it and 

 given back to the source. The perfect engine is there- 

 fore reversible without loss of available energy. 



Now consider still the engine as a mechanism which 

 takes heat from a source and gives it to a refrigerator, 

 but let it be an actual engine. Instead of giving up 



