ELECTRIC CHARGE AND THE CONDENSER. 133 



certain amount. Similarly the electromotive force which acts 

 on a condenser has three important effects upon the dielectric of 

 the condenser, namely, (a) a certain amount of electric current 

 "soaks" through the dielectric, as it were, because the dielectric 

 is an electrical conductor although a very poor one, (b) the 

 dielectric has a certain amount of electrical "elasticity" (induc- 

 tivity as it is properly called), and it "yields" a little to the 

 electromotive force and allows a certain amount of charge to be 

 drawn out of one plate and forced into the other plate of the con- 

 denser, and (c) the dielectric has a certain ultimate strength and 

 it will be ruptured if the electromotive force exceeds a certain 

 amount. 



PROBLEMS. 



1. A variable current which is represented by i = ai? flows 

 through a circuit. Find the charge which passes through the 

 circuit between t = o and t = 10 seconds; the value of a 

 being such that the current will have reached a value of 18 am- 

 peres when / is 3 seconds. Ans. 667 coulombs. . 



2. A variable current is represented by i = I sin co/. (a) Find 

 the amount of charge which passes through the circuit from 

 cot = o to ut = TT. (b) Find the average value of the current 

 during the time o>/ = o to ut = TT. Ans. (a) 2l/u coulombs, 

 (b) 2//7T amperes. 



3. Two condensers of which the capacities are C farads and c 

 farads respectively are connected in parallel as shown in Fig. 3. 

 What is their combined capacity? 



Note. An electromotive force E draws cE coulombs out of one plate and 

 forces it into the other plate of c\ and the same electromotive force draws CE 

 coulombs out of one plate and forces it into the other plate of C. Therefore E 

 draws ce + CE coulombs out of one side of Fig. 3 and forces it into the other 

 side, and from this the total capacity of the arrangement is easily found. 



4. Two condensers of which the capacities are C = 10 micro- 

 farads and c = 2 microfarads are connected in series as shown 

 in Fig. 4. What is the combined capacity of the arrangement? 

 Ans. 1.67 microfarads. 



