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18.4 Series and parallel circuits
Example 8 A network of resistors
The resistors are connected to an unknown
voltage supply as shown. The current flowing
through the main circuit is 2 A.
10 
A
B
2A
unknown
voltage
supply
20 
30 
C
(a) What is the
equivalent
resistance of the
network of
resistors?
18.4 Series and parallel circuits
Let R be the equivalent resistance of the 20 
and 30  in parallel.
10 
1
1
1
=

R 20  30 
2A
1
5
unknown
=
voltage
R 60 
R
supply
R = 12 
Equivalent resistance of
the network of resistors
= 12  + 10 
= 22 
18.4 Series and parallel circuits
10 
A
B
2A
unknown
voltage
supply
20 
30 
(b) What is the
voltage of the
supply?
IR
= 2 A  22 
= 44 V
C
(c) What is the voltage across (i) AB, (ii) BC?
voltage across AB
= 2 A  10 
= 20 V
voltage across BC
= voltage of the supply –
voltage across AB
= 44 V – 20 V
= 24 V
18.4 Series and parallel circuits
10 
A
unknown
voltage
supply
20 
B
30 
(d) What is the
current through
(i) the 20 
resistor, (ii) the
30  resistor?
C
voltage across the 20  resistor
= voltage across the 30  resistor = 24 V
(i) current through
(ii) current through
the 20  resistor
the 30  resistor
V 24 V
= 1.2 A
= =
R 20 Ω
V 24 V
= 0.8 A
= =
R 30 Ω