CHAPTER III 

 ELECTRIC CIRCUITS 



51. Ohm's Law. When an electromotive force is applied 

 to the terminals of a conductor, a current is produced which is 

 directly proportional to the e.m.f. and is inversely proportional 

 to the resistance of the conductor; 



E 



1 r= amperes, (110) 



K 



where I is the current in amperes, 



E is the e.m.f. in volts, 

 R is the resistance in ohms. 

 This is Ohm's Law. 



A conductor has a resistance of one ohm, when an e.m.f. of 

 one volt is required to drive a current of one ampere through it. 



When therefore a current I flows through a resistance R, 

 electromotive force is consumed; 



E = IR volts. ...... (Ill) 



52. Joule's Law. Whenever a current flows through a re- 

 sistance, electric energy is transformed into heat energy. The 

 power or the rate at which energy is transformed in the circuit 

 is equal to the product of the current and the electromotive force 

 consumed in driving the current through the resistance of the 

 circuit. 



P = El watts, 

 but E = IR and, therefore, 



P = PR watts; (112) 



thus, the power consumed in the circuit is equal to the square of 

 the current multiplied by the resistance. This is Joule's Law. 



The power consumed in the resistance of circuits represents a 

 loss of power except in such cases as the incandescent lamp, where 

 it is utilized in producing light, or the electric heater, where the 

 heat developed is applied to a useful purpose. 



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