RESISTANCES 



ELEMENTARY ELECTRICAL CIRCUIT 



If a constant-voltage generator of e.m.f. E is connected to a resistance R, 

 the current which flows (Figure 2.1) is 



E 



/is in amps if Eis measured in volts and R in ohms. This is the relationship 

 usually called Ohm's law, though in fact Ohm only said that / would be 

 proportional Xo Eif R were kept constant. 



If a generator of constant current / is connected to a resistance R {Figure 2.2) 

 a difference of potential Kis caused to appear across the resistance. V = IR, 

 therefore / = VI R. V is in volts if / is measured in amps and R in ohms. 



Figure 2.1 Figure 2.2 



Though E and V are both voltages the distinction between them is theoreti- 

 cally important, since £■ is a cause and V an effect. Whilst it is possible to 

 preserve the distinction when discussing very simple circuits it is not so easy 

 in more complicated cases ; this is because electronic circuits are arranged in 

 causal chains. Thus a voltage which is V for the «th link in the chain will 

 be E for the « + 1th. In such cases we shall simply speak of 'the voltage' 

 between such-and-such a pair of points, using the symbol V. 



In either Figure 2.1 or Figure 2.2 the quantity of electricity passing any 

 point in the circuit in time /is 



Q = It (coulombs, amps, seconds) 



The rate of expenditure of energy in the resistance is called the 'power' and 

 is 



y2 



P — VI ov PR or -^ (watts, volts, amps, ohms) 

 R 



The expenditure of energy in time / is 



W = Pt (joules, watts, seconds) 

 6 



