116 



ELECTRICAL ENGINEERING 



and is equal to the in-phase component of the e.m.f. multiplied 

 by the current or is equal to the square of the current multiplied by 

 the resistance of the circuit as found in equation 171; thus, all 

 the power consumed in the circuit is consumed by the resistance. 

 Reactance or self-inductance does not consume power since the 

 energy stored while the current is increasing is given back while 



I 



70 /3GO C 



uo- 



FIG. 83. Power in an inductive reactance. 



it is decreasing and the e.m.f. consumed by self-inductance is 

 a wattless e.m.f. Similarly condensive reactance does not con- 

 sume power since the energy stored while the e.m.f. is increasing 

 is given back to the circuit while the e.m.f. is decreasing and the 

 e.m.f. consumed lay condensive reactance is wattless. 



FIG. 84. Power in a condensive reactance. 



These results are illustrated in Fig. 83 and Fig. 84. 



In Fig. 83 are plotted the values of e, i and p for an inductive 

 circuit without resistance in which the current lags 90 degrees 

 behind the impressed e.m.f. The power curve cuts off equal 

 areas above and below the base line and therefore the average 

 power is zero. The area below the line represents the energy 



