ELECTRIC CHARGE. THE CONDENSER. 



I/I 



the distance through which the end of the spring is moved dur- 

 ing the bending. 



In a similar manner, it may be shown that the potential energy 

 of a charged condenser is 



in which E is the electromotive force between the plates of the 

 charged condenser, and q is the amount of charge which has 

 been drawn out of one plate and pushed into the other. The 

 potential energy W is expressed in joules when E is expressed 

 in volts and q in coulombs. 



A weight is hooked to the lower end of a vertical spring, as 

 shown in Fig. 105, and then the weight is released. In this case 

 the full value of the weight acts 

 upon the spring from the start, 

 and the weight oscillates up 

 and down for some time before 

 coming to rest in its equilibrium 

 position. Let E be the pull 

 of the earth upon the weight 

 and let q be the distance from 

 the initial to the equilibrium 

 position, then the total work 

 done by gravity after the weight 

 has come to rest is equal to Eq 

 (pull of earth multiplied by dis- 



fanitial position 



-[equilibrium .position 

 ^ 



_ _y[extreme position 



tance through which the weight 

 has moved), but the potential 

 energy which is stored in the 

 spring after the weight comes 

 to rest in its equilibrium posi- 

 tion is equal to \Eq that is, 

 one half of the work which has been done on the weight by 

 gravity is stored in the spring as potential energy and the re- 

 mainder of the work has been dissipated by the oscillations of 

 the weight. 



Fig. 105. 



