ELECTRIC CIRCUITS 117 



given back by the magnetic field while the current is decreasing 

 from its maximum value /o to zero and the area above the line 

 represents the energy stored in the magnetic field while the cur- 

 rent is increasing again to its maximum value. The amount of 



energy in each case is L -^- watt-seconds. 



In Fig. 84 are plotted the values of e, i and p for a circuit con- 

 taining a condensive reactance but without resistance. The cur- 

 rent leads the impressed e.m.f. by 90 degrees and the average 

 power is again zero, so that no energy is consumed in the circuit. 

 The positive area cut off by the power curve represents the energy 

 stored in the electrostatic field of the condenser while the e.m.f. 

 is increasing and the negative area represents the energy re- 

 turned to the circuit while the e.m.f. is decreasing. The maxi- 

 mum amount of energy in each case is C -~ watt-seconds where 



a 



EQ is the maximum e.m.f. 



Fig. 85 illustrates various methods of representing the power 

 in a circuit; in (a) it is the product of the impressed e.m.f., the 



I R 

 * -wvw 1 



E E,|X 



FIG. 85. Power in alternating-current circuits. 



current and the cosine of the angle of phase difference between 

 them, 



P = EXlXcos<f>; (174) 



in (b) it is the product of the current and the in-phase component 

 of the e.m.f., 



P = I X Jcos< = 7 X EI = PR', . . . (175) 



in (c) it is the product of the e.m.f. and the in-phase component ' 

 of the current, 



P = E X I cos (176) 



The apparent power in a circuit is the product of the impressed 

 e.m.f. and the current and is expressed in volt amperes or kilo- 

 volt amperes. 



