78 



ALTERNATING CURRENTS 



Corresponding to the voltage E ab , Fig. 73 (a), the current 7^ 

 flows from a to b in virtue of this voltage. The current flowing 

 from 6 to a must be opposite in direction to that flowing from a 

 to b. Therefore I ba = Jab- This relation is illustrated in Fig. 

 73 (6), in which I ab differs in phase from I ba by 180. E ba is 

 180 from E ab . 



Figure 74 represents a circuit network abcde. The parts of the 



network, ab, be, etc., may be 

 either resistances, inductances, 

 capacitances, or sources of emf. 

 such as alternator or transformer 

 coils. It is obvious that the 

 voltage from a to c is equal to 

 the voltage from a to 6 plus the 

 voltage from b to c. That is, 

 ac = ab + b*'. It is to be 



FIG. 74. Circuit network. 



noted that when several voltages in series are being considered, 

 the first letter of each subscript must be the same as the last 

 letter of the preceding subscript. Figure 75 (a) shows vectorially 

 the voltage E ab and the voltage E&. To obtain the voltage E ac , 

 E bc is necessary. Therefore E cb is reversed giving E bc . E bc added 

 vectorially to E ab gives E ac . 



.E c > 



O) 



FIG. 75. Examples of symbolic notation. 



Currents may be treated in a similar manner, the principle 

 involved being Kirchhoff s first law. For example, in Fig. 74, the 

 current /& = he + hd. Figure 75 (b) shows currents I be and 

 Id b . Idh is reversed giving I b d and this is combined vectorially 

 with 7& c to obtain I ab > 



This notation not only distinguishes the various currents and 

 voltages but the directions in which they act as well, It is to 



