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How is a potential difference induced, and how do transformers work?

Induced potential and transformers: electromagnetic induction, the generator effect, how transformers change voltage, and the transformer equations (separate physics).

A focused answer to AQA GCSE Physics 4.7.3, covering electromagnetic induction and the generator effect, how alternators and dynamos produce a current, how transformers change the size of an alternating potential difference, and the transformer equations.

Generated by Claude Opus 4.89 min answerUpdated 2026-06-02

Reviewed by: AI editorial process; not yet individually human-reviewed

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What this dot point is asking
Electromagnetic induction
Generators
Transformers
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What this dot point is asking

AQA wants you to explain electromagnetic induction (the generator effect), describe how generators produce a current, explain how a transformer changes an alternating potential difference, and use the transformer equations. This covers parts of topic 4.7.3 of the AQA GCSE Physics (8463) specification and is separate physics only.

Electromagnetic induction

The induced potential difference is larger when the wire moves faster, the magnetic field is stronger, or there are more turns on the coil. Reversing the direction of movement reverses the induced potential difference. An important AQA point is that an induced current itself produces a magnetic field, and this field always acts to oppose the original change that caused it. In practice this means you have to do work to keep moving the wire or coil against this opposing force, and it is that work which is transferred into electrical energy. This is why a generator becomes harder to turn when it supplies a larger current.

Generators

As the coil rotates, the rate at which it cuts through the magnetic field lines changes through each turn, so the induced potential difference rises and falls. The difference between the two generator types lies in how the coil connects to the external circuit. The alternator's slip rings keep the same coil end connected to the same output terminal, so the output reverses each half turn, giving alternating current. The dynamo's split-ring commutator swaps the connections every half turn, so the output always comes out the same way, giving direct current. On an oscilloscope an alternator output is a wave crossing zero, while a dynamo output is a series of humps all on the same side.

Transformers

Worked example: a step-down transformer

A transformer has $1000$ turns on the primary coil and $100$ turns on the secondary coil. The input potential difference is $230\,V$ . We want to find the output potential difference and confirm whether this is a step-up or step-down transformer.

Step 1: Write down the known quantities

The turns ratio equation links voltages and turn counts: $\dfrac{V_p}{V_s} = \dfrac{n_p}{n_s}$ . We have three of the four quantities:

V_p = 230\,V, \quad n_p = 1000, \quad n_s = 100

Step 2: Rearrange for the secondary voltage

Multiplying both sides by $V_s$ and rearranging, the secondary voltage equals the primary voltage scaled by the turns ratio:

V_s = V_p \times \dfrac{n_s}{n_p} = 230 \times \dfrac{100}{1000}

Step 3: Calculate and interpret

V_s = 23\,V

Because the secondary has fewer turns than the primary, the output voltage is lower than the input, confirming this is a step-down transformer.

Final answer: the secondary voltage is $23\,V$ ; this is a step-down transformer.

Try this

Q1. State what is needed for a potential difference to be induced in a coil. [2 marks]

Cue. Relative movement between the coil and a magnetic field, or a change in the magnetic field through the coil.

Q2. A transformer has $200$ turns on the primary and $800$ on the secondary, with $V_p = 12\,V$ . Calculate $V_s$ . [3 marks]

Cue. $V_s = V_p \times \dfrac{n_s}{n_p} = 12 \times \dfrac{800}{200} = 48\,V$ .

Exam-style practice questions

Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

AQA 20205 marksA step-up transformer has

500

turns on the primary coil and an input potential difference of

230\,\text{V}

. The output potential difference is

11{,}500\,\text{V}

. Calculate the number of turns on the secondary coil, and, assuming the transformer is 100% efficient, calculate the output current when the input current is

20\,\text{A}

Show worked answer →

Use the turns equation $\frac{V_p}{V_s} = \frac{n_p}{n_s}$ , rearranged to $n_s = n_p \times \frac{V_s}{V_p} = 500 \times \frac{11500}{230} = 500 \times 50 = 25{,}000$ turns (2 marks). For an ideal transformer power is conserved: $V_p I_p = V_s I_s$ , so $I_s = \frac{V_p I_p}{V_s} = \frac{230 \times 20}{11500} = \frac{4600}{11500} = 0.40\,\text{A}$ (3 marks). Markers reward rearranging the turns ratio correctly, using power conservation, and recognising that stepping the voltage up steps the current down. A common error is to use the turns ratio for current as if it behaved like voltage.

AQA 20214 marksExplain how a transformer changes the size of an alternating potential difference, and explain why a transformer will not work with a direct current supply.

Show worked answer →

The alternating current in the primary coil produces a continuously changing magnetic field in the iron core (1 mark). This changing magnetic field passes through the secondary coil and, because it is changing, it induces an alternating potential difference across the secondary coil by electromagnetic induction (1 mark). The ratio of the potential differences equals the ratio of the numbers of turns, so a different number of turns on the secondary gives a different output voltage (1 mark). A direct current produces a steady (unchanging) magnetic field in the core, and because there is no change in the field there is no induced potential difference in the secondary, so the transformer does not work with d.c. (1 mark). Markers reward the changing field, induction in the secondary, and the explicit reason d.c. fails.

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Sources & how we know this

AQA GCSE Physics (8463) specification — AQA (2016)