Electrical Power
Master Electrical Power for GCSE Physics with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA, Edexcel and OCR. Electrical power can be found using P = VI or P = I²R, and shows how quickly an appliance transfers energy.
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These worksheets and mark schemes are original, written for Virtus Academy and checked against the current AQA, Edexcel and OCR specifications. Every worksheet comes with a full mark scheme.
Topic overview
The power of an electrical device is the rate at which it transfers energy. Three equations connect power to the other electrical quantities.
Power = potential difference × current, or P = VI. Power = current² × resistance, or P = I²R. And power = energy transferred ÷ time.
Which equation to use depends on what the question gives you. If you have potential difference and current, use P = VI. If you have current and resistance, use P = I²R. Note that in P = I²R the current is squared, so doubling the current quadruples the power dissipated — which is why the National Grid transmits at high potential difference and low current.
Revision notes
The equations
Power = potential difference × current, P = VI.
Power = current squared × resistance, P = I²R. Power = energy transferred ÷ time. All give power in watts.
Choosing the right one
Use P = VI when given potential difference and current.
Use P = I²R when given current and resistance. If given potential difference and resistance, find the current first using V = IR, then use one of the power equations.
Why the current is squared
In P = I²R the current is squared, so doubling the current multiplies the power dissipated by four.
This is why the National Grid transmits at very high potential difference and correspondingly low current — it minimises the power wasted as heat in the cables.
Key points
- Power = potential difference × current.
- Power = current² × resistance.
- Power = energy transferred ÷ time.
- Power is measured in watts.
- Doubling the current quadruples the power in P = I²R.
- High potential difference means low current and less waste.
Worked examples
Example 1
A device operates at 230 V with a current of 4 A. Calculate its power. [2 marks]
Model answer
Power = potential difference × current = 230 × 4 (1 mark) = 920 W (1 mark).
Example 2
A current of 3 A flows through a 5 Ω resistor. Calculate the power dissipated. [3 marks]
Model answer
Power = current² × resistance = 3² × 5 (1 mark). 3² = 9, so 9 × 5 (1 mark) = 45 W (1 mark).
Example 3
Explain why doubling the current through a resistor quadruples the power dissipated. [2 marks]
Model answer
Power is given by P = I²R, in which the current is squared (1 mark), so doubling the current multiplies the power by 2² which is 4 (1 mark).
Common mistakes
Forgetting to square the current in P = I²R.
The current must be squared before multiplying by the resistance.
Using the wrong equation for the data given.
Match the equation to the quantities the question provides.
Saying doubling the current doubles the power.
It quadruples it, because the current is squared.
Confusing power with energy.
Power is the rate of transfer, in watts; energy is in joules.
Exam tips
- Choose the equation that matches the data given.
- Square the current as a separate step in P = I²R.
- Use the squaring to explain National Grid transmission.
- State watts as the unit.
Key terms
- Power
- The rate of energy transfer, in watts.
- Dissipated
- Transferred to the surroundings, usually as heat.
- Potential difference
- The energy transferred per coulomb, in volts.
- Resistance
- Opposition to current, in ohms.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.