Circuit calculation practice
SERIES CIRCUITS: To solve simple circuit calculations you will need to
use Ohm’s law, which can be represented mathematically as V = I x R.
The simplest problems will expect you to use the formula in this
arrangement. More difficult questions will require you to rearrange
the formula.
If you are not confident rearranging calculations use the formula
triangle to help you find the correct formula
V = Voltmeter reading
A = Ammeter Reading
R = Resistor value
V (volts)
[V] A (amps)
[I] Ω (ohms)
[R]
2
5
1.5
2
3
6
9
3
12
2
24
6
More difficult circuit problems will include more than one resistor connected in series. The
key to being able to solve these problems is remembering that total resistance is equal to the
value of both resistors added together. You can then use Ohm’s law (V = I x R) to calculate
the unknown values. Sometimes you may need to use Ohm’s law first.
Remember: if you are not confident rearranging calculations use the formula triangle to help
you find the correct formula
Resistor 1
value
(Ω)
Resistor 2
value
(Ω)
Total
resistance
(Ω)
Voltage
Current
(V)
(A)
R1
R2
R1 + R2
V
I
2
2
12
1
4
25
3
5
6
1
12
0.5
10
0.2
2
0.9
24
3.6
Circuit calculation practice
PARALLEL CIRCUITS: These circuits have two or more paths for current to flow through. Often
the key to solving parallel circuit problems is knowing that voltage is the same across each
component of the parallel circuit. Often ammeters are shown in the circuit, these can usually
be ignored as they do not affect the operation of the circuit.
The second rule to follow when solving parallel circuit problems is that the sum of the
currents through each path is equal to the total current that flows from the source or back
to the source.
V
R1
R2
I1
I2
7.5
5
2.5
36
9
6
10
5
5
60
2
3
1.5
2
6
60
200
I3
0.6
100
0.2
0.3
The most complicated circuits you will be expected to solve will
combine series and parallel circuits together. You will be
expected to make use of all the circuit rules you have met so far.
 total resistance is in a series circuit is equal to the value of
both resistors added together
 Voltage is the same across each component of the parallel
circuit.
 The sum of the currents through each path is equal to the
total current that flows from the source or back to the source.
 Also the total voltage of more than one cell is equal to the
individual cell voltages added together
V1
(V)
V2
(V)
2
Total
voltage
R1
(Ω)
R2
(Ω)
R3
(Ω)
2
20
10
10
1.5
1.5
15
15
15
4
6
2
4
6
10
5
5
3
5
10
4
12
I1
(A)
I2
(A)
2
2
1
0.5
I3
(A)
5
2
3
3
2
6
10
1
3
R2 and R3
Combined
Resistance
20