CHAPTER V. 



Expression for Power Measurement of Power. 



POWER GIVEN TO ALTERNATING-CURRENT CIRCUITS. 



28. The power given to a circuit by a continuous current is 

 the product of the current flowing in it and the P.D. between 

 its terminals, and is given in watts when the current is given in 

 amperes and the P.D. in volts. The power given to circuit by 

 an alternating current cannot be determined in this fashion, for 

 whereas in a continuous-current circuit the current and P.D. 

 always act in the same direction round the circuit, in an alternating- 

 current circuit there are, in general, times occurring periodically 

 when the current and P.D. act in opposition and the circuit is 

 giving back energy to the source. 



We have seen (see Chap. IV., 24) that when an alternating 

 P.D. c = E sin pt is applied between the terminals of a circuit 

 containing resistance, self-induction, and capacity in series, the 

 resulting current is given by 



E sin (pt - 9) 



= / sin (pt - 0) 



P L- C 



where tan = 



r 



the notation being the same as in Chapter IV. 



The power being given to the circuit at any instant of time t 

 is the product of the P.D. and corresponding current at that 

 instant, that is the product 



El sin pt sin (pt - 0) 



