52 ELECTRICAL ENGINEERING 



In. moving completely across the pole face the work done is 

 W = fl d<j> = 7$, 



where < is the flux from the pole. 



If 7 is expressed in absolute units and $ in maxwells, W is in ergs. 



If the motion through the distance dx takes place in time dt sec- 

 onds, the work done is 



. . . . (82) 



where e = -^ is the electromotive force generated in the con- 



ductor and I dt = dq is the quantity of electricity raised through 

 the difference of potential e. Therefore the mechanical work sup- 

 plied to move the conductor through the distance dx against the 

 force / is used up in doing the electrical work of raising a quan- 

 tity of electricity dq through a difference of potential e, or in driv- 

 ing a current 7 against an e.m.f . e for a time dt. Thus mechanical 

 energy is transformed into electrical energy. This is what takes 

 place in an electric generator. 



If electric power is supplied to drive the current 7 against the 



electromotive force e = -r- , f or a time dt energy is supplied 



the conductor exerts a force / = %&l I dynes through a distance dx 

 and does mechanical work, 



fdx = Z&lldx = Id<}>. 



This is the action of an electric motor. 



Electric Power. The electric power in a circuit is the rate at 

 which energy is being transformed in the circuit. It is the product 

 of the current and the electromotive force in the circuit. 



The practical unit of electric power is the watt. It is the power 

 in a circuit carrying one ampere when the electromotive force 

 across it is one volt. The kilowatt, which is one thousand watts, 

 is more commonly used where the amounts of power are large. 

 One horse power is equivalent to 746 watts. 



