GCSE electricity calculations are much easier when you know what each quantity means and choose the correct equation before substituting numbers. This guide covers the most useful relationships for current, potential difference, resistance, power, energy and charge.
The key electricity equations
V = I × R
P = V × I
E = P × t
Q = I × t
Here, V is potential difference in volts (V), I is current in amperes (A), R is resistance in ohms (Ω), P is power in watts (W), E is energy transferred in joules (J), t is time in seconds (s), and Q is charge in coulombs (C).
Worked example 1: using Ohm’s law
A resistor has a potential difference of 12 V across it and a current of 3 A through it. Find the resistance.
R = V ÷ I = 12 ÷ 3 = 4 Ω
Worked example 2: electrical power
A lamp operates at 6 V and draws a current of 0.5 A. Calculate its power.
P = V × I = 6 × 0.5 = 3 W
Worked example 3: energy transferred
A 60 W device operates for 120 seconds. Calculate the energy transferred.
E = P × t = 60 × 120 = 7200 J
Worked example 4: charge flow
A current of 2 A flows for 30 seconds. Calculate the charge that passes a point in the circuit.
Q = I × t = 2 × 30 = 60 C
A reliable calculation method
1. Write down the values and units given. This helps identify which quantities are involved.
2. Choose the equation before substituting numbers. Avoid trying to manipulate numbers first.
3. Rearrange if necessary. For example, V = IR becomes R = V ÷ I when resistance is unknown.
4. Convert units where needed. Minutes may need converting to seconds; milliamps may need converting to amps depending on the question.
5. Include the correct unit in the final answer.
Common electricity-calculation mistakes
Using the wrong equation: identify the known and unknown quantities first.
Forgetting unit conversions: check whether current is given in mA or time in minutes.
Rearranging incorrectly: write the original equation first, then isolate the unknown.
Dropping units: units are part of the answer and can help you spot whether the result makes sense.
Quick practice
1. A current of 4 A flows through a 5 Ω resistor. Find the potential difference.
2. A device uses 230 V and draws 2 A. Find its power.
3. A 100 W appliance runs for 50 s. Find the energy transferred.
4. A charge of 120 C flows in 40 s. Find the current.
Answers: 1. 20 V. 2. 460 W. 3. 5000 J. 4. 3 A.
Continue your GCSE Physics revision
Explore our Physics subject page, Year 10 Physics and Year 11 Physics. For broader exam support, visit our GCSE preparation page.
For structured teaching and feedback, view Redbridge Tuition classes, times and prices.
Frequently asked questions
What is Ohm’s law?
For an ohmic conductor at constant temperature, potential difference is proportional to current. The relationship used in calculations is V = I × R.
What is the unit of resistance?
Resistance is measured in ohms, symbol Ω.
How are power and energy different?
Power is the rate of energy transfer and is measured in watts. Energy transferred is measured in joules.