PRINCIPLES OF ALTERNATING CURRENT 



203 



47. Phase Displacement Power Power Factor. If an 



alternating e.m.f. is applied to a circuit, a current will flow 

 which will, in general, be of the same shape as the e.m.f. 

 wave. There are some cases where this is not true but they 

 will not be considered until later. 



Although the current and e.m.f. will, in general, be of 

 the same shape, they will not usually be in the same phase. 

 In Fig. 122 are shown the curves of e.m.f. and current in 

 two different circuits, one taking a leading current and the 

 other a lagging current. 



Time i\1 



w 



N- 'Leading currents f 



Time 



Laggingr current 



FIG. 122. Curves Showing Leading Current and Lagging Current. 



in both of these circuits, the current is said to be out 

 of phase with the voltage; the phase difference is given 

 by the angle <j> in both cases. A current in phase with the 

 voltage would have its maximum and minimum values 

 at the same time as the voltage. Such a current is shown in 

 Fig. 123. 



Lead and Lag on Vector Diagram. The idea of lead 

 and lag is expressed in a vector diagram by the relative 

 position of the two vectors used to represent the current 

 and e.m.f. In Fig. 121 is shown a voltage (vector OA) 



