188 ELEMENTS OF ELECTRICITY AND MAGNETISM. 



region of zero potential and the given point. The product Eq 

 is equal to the work required to carry charge q from the region 

 of zero potential to the given point, and it is therefore equal to 

 the potential energy of the charge q when it is placed at the 

 given point 



The idea of potential is important in the mathematical theory 

 of electricity and magnetism, but its use by students who are 

 beginning the study of the subject of electricity and magnetism 

 tends to turn the attention away from physical realities. 



. PROBLEMS. 



115. During 0.03 second a charge of 1 5 coulombs passes 

 through a circuit. What is the average value of the current dur- 

 ing this time ? Ans. 500 amperes. 



116. Suppose the strength of a current in a circuit to increase 

 at a uniform rate from zero to 50 amperes in 3 seconds. Find 

 the number of coulombs of charge carried through the circuit by 

 the current during the 3 seconds. Ans. 75 coulombs. 



117. A condenser of which the capacity is known to be 5 

 microfarads, is charged by a Clark standard cell of which the 

 electromotive force is 1.434 volts and then discharged through a 

 ballistic galvanometer. The throw of the ballistic galvanometer is 

 observed to be 15.3 scale divisions. What is the reduction factor 

 of the galvanometer ? Ans. 469 x I o~ 9 coulombs per division. 



118. A condenser of unknown capacity is charged by 10 Clark 

 cells in series, giving an electromotive force of 14.34 volts, and 

 then discharged through the ballistic galvanometer specified in 

 problem 117; the throw of the ballistic galvanometer is observed 

 to be 1 8. 6 divisions. What is the capacity of the condenser? 

 Ans. 0.608 microfarad. 



119. An electromotive force acting on a condenser increases at 

 a uniform rate from zero to 100 volts during an interval of 1/200 

 of a second. The capacity of the condenser is 20 microfarads. 

 Find the value of the current during the 1/200 of a second. 

 Ans. 0.2 ampere. 



