ELECTRIC CHARGE AND THE CONDENSER. 129 



be equal to 500 X 10 inches X 10 inches or 322,500 square 

 centimeters. 



80. The work done by an electromotive force E in pushing 

 a given amount of charge, Q, through a circuit. When Q 

 coulombs of electric charge flows through a battery of which 

 the electromotive force is E, the amount of work W done by 

 the battery is EQ joules. That is: 



W = EQ (i) 



This is evident from the following considerations. Imagine a 

 current I flowing through the battery; then El watts is the 

 rate at which the battery does work, and Elt joules is the 

 amount of work done in / seconds. But the product // is the 

 amount of charge Q (in coulombs) which has been pushed 

 through the circuit. Therefore the work done, namely Elt joules, 

 is expressible as EQ joules. Therefore we have equation (i). 



81. The potential energy of a charged condenser. A charged 

 condenser represents a store of potential energy in much the 

 same way that a stretched spring represents a store of potential 

 energy, and before considering the amount of potential energy 

 in a charged condenser it is helpful to consider the amount of 

 potential energy in a stretched spring. 



stretched spring 



Fig. 87. 



Let q represent the elongation of a spring due to a stretching 

 force e as shown in Fig. 87. As is well known q is proportional 

 to e\ therefore if we plot corresponding values of g and e as 

 10 



