ENERGETICS 103 



formed ; whereas the other lake whose water can run into the 

 sea realizes the conditions necessary for utilization, viz. the 

 transformability of its energy. The same may be said of all 

 forms of energy ; a heat engine can only act as a transformer, 

 change heat into work, if there is a difference of temperature 

 between its source and its sink ; an electric motor can only 

 work if there is a fall of potential between the entrance and 

 the exit of the electric current. 



Energy presents itself to us as the product of two factors, 

 weight and height in the waterfall, quantity and temperature 

 in the heat engine, current intensity and potential in the 

 electric motor. 



In considering these two factors we may note that one 

 factor is always a quantity (Q) and the other an intensity 

 (I). This latter expresses some sort of difference of position 

 or condition, the height of the weight, a difference of tempera- 

 ture in the heat engine, of pressure in the gas engine, or of 

 electric potential in the dynamo or electric furnace. There can 

 be no current of energy without this difference of potential, 

 and therefore no transformation from one form of energy to 

 another. 



The second law of thermodynamics, Carnot's law, may 

 therefore be enunciated thus : " Energy cannot be transformed 

 without, a fall of potential." 



We may also derive this principle from a consideration of 

 the formula of efficiency, the ratio of the work done by the 

 transformer to the work done on the transformer. 



._„, . energy transformed 



Efficiency = , -— T ~— —5 r—j- 



total energy absorbed 



's 



The total energy is the product QI, i.e. the product of the 

 total quantity by the total intensity at our disposal. The 

 transformed energy is Q(I — I'), the product of the total 

 quantity by the difference of intensity at the inlet and at the 

 outlet of the machine. The formula tor efficiency thus becomes 



Q(I 1)_ *~~ _ If I represents a temperature, then in order 



QI I ' 



that the efficiency may be positive I' must be less than I, 



