CHAPTER II 

 ALTERNATING-CURRENT CIRCUITS 



12. Alternating-current Power. The power in a direct-current 

 circuit under steady conditions is always given by the product 

 of the volts across the circuit and the current in amperes flowing 

 in the circuit. This same rule applies to alternating-current 

 circuits, provided that only instantaneous values of amperes and 

 volts are considered. The average power, however, is not neces- 

 sarily the product of the effective volts and effective amperes, 

 the values which are ordinarily measured with instruments. 



FIG. 19. Power curve; current and voltage in phase. 



Figure 19 shows a voltage wave and a current wave in phase 

 with each other. To obtain the power at any instant, the am- 

 peres and the volts at that instant are multiplied together and a 

 new curve P may be plotted, the ordinates being the instantane- 

 ous products of E and /. The curve P then gives the power in 

 the circuit at any instant. The ordinates of this power curve 

 will always be positive when E and / are in phase, because the 

 voltage and the current are both positive together during the 

 first half cycle and are both negative together during the second 

 half-cycle, and the product of two negative quantities is positive. 

 That is, the current and the voltage act in conjunction through- 

 out the cycle and the ordinates of the power curve are always 

 positive. 



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