Transformers
Master Transformers for GCSE Physics with this free worksheet and full mark scheme — Higher-tier exam-style questions with worked answers for AQA, Edexcel and OCR. A separate (triple) physics topic: a transformer changes the size of an alternating voltage according to the turns ratio, and for an ideal transformer power in equals power out.
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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.
This is a Higher tier only topic, so there's no Foundation paper — only the Higher worksheet and mark scheme below.
Topic overview
A transformer changes the size of an alternating potential difference. It consists of a primary coil and a secondary coil wound on an iron core. This is a Triple-only and Higher-only topic.
An alternating current in the primary coil produces a continually changing magnetic field in the iron core. That changing field induces an alternating potential difference in the secondary coil.
The equation is Vp ÷ Vs = np ÷ ns, where V is potential difference and n is the number of turns. A step-up transformer has more turns on the secondary; a step-down transformer has fewer. If the transformer is 100 per cent efficient, the power input equals the power output, so Vp × Ip = Vs × Is.
Revision notes
How a transformer works
An alternating current in the primary coil produces a continually changing magnetic field in the iron core.
The iron core carries this changing field to the secondary coil, where it induces an alternating potential difference. A transformer only works with alternating current, because a steady direct current would produce an unchanging field and induce nothing.
The turns ratio equation
Vp ÷ Vs = np ÷ ns, relating the potential differences to the numbers of turns.
A step-up transformer has more turns on the secondary coil, so it increases the potential difference. A step-down transformer has fewer turns on the secondary and decreases it.
Power and efficiency
If a transformer is 100 per cent efficient, the power input equals the power output.
So Vp × Ip = Vs × Is. This means that stepping the potential difference up steps the current down by the same factor, which is exactly what the National Grid requires.
Key points
- A transformer changes an alternating potential difference.
- It has primary and secondary coils on an iron core.
- A changing magnetic field induces the output.
- Transformers only work with alternating current.
- Vp ÷ Vs = np ÷ ns.
- If 100 per cent efficient, Vp × Ip = Vs × Is.
Worked examples
Example 1
A transformer has 100 turns on the primary and 500 on the secondary. The primary potential difference is 20 V. Calculate the secondary potential difference. [3 marks]
Model answer
Using Vp ÷ Vs = np ÷ ns: 20 ÷ Vs = 100 ÷ 500 (1 mark). 100 ÷ 500 = 0.2, so 20 ÷ Vs = 0.2 (1 mark). Vs = 20 ÷ 0.2 = 100 V (1 mark).
Example 2
Explain why a transformer does not work with a direct current supply. [3 marks]
Model answer
A steady direct current produces a magnetic field that does not change (1 mark). A changing magnetic field is required to induce a potential difference in the secondary coil (1 mark), so with direct current no potential difference is induced (1 mark).
Example 3
A 100 per cent efficient transformer has a primary power of 500 W. State the secondary power and explain your answer. [2 marks]
Model answer
The secondary power is also 500 W (1 mark), because in a 100 per cent efficient transformer the power output equals the power input (1 mark).
Common mistakes
Saying transformers work with direct current.
A changing magnetic field is needed, so only alternating current works.
Inverting the turns ratio.
Vp ÷ Vs = np ÷ ns — keep primary on top on both sides.
Saying a transformer increases power.
At 100 per cent efficiency the power is unchanged; only V and I change.
Confusing step-up and step-down.
Step-up has more turns on the secondary and increases potential difference.
Exam tips
- Keep primary quantities on the same side of both ratios.
- Explain dc failure through the unchanging magnetic field.
- Use Vp × Ip = Vs × Is for power questions.
- Remember this is Triple-only AND Higher-only.
Key terms
- Transformer
- A device changing the size of an alternating potential difference.
- Primary coil
- The input coil of a transformer.
- Secondary coil
- The output coil of a transformer.
- Turns ratio
- The ratio of the numbers of turns on the two coils.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.