An engineer is designing a transformer for use in the National Grid. The transformer has a primary coil and a secondary coil wound around an iron core. Explain why the transformer only works with alternating current in the primary coil, and describe how increasing the number of turns on the secondary coil compared to the primary coil affects the output potential difference.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Electricity is transmitted across the National Grid at very high voltages to reduce energy loss. Transformers are used to change the magnitude of the alternating voltage at different stages of transmission.
Model answer (5 marks)
An alternating current in the primary coil creates a changing magnetic field in the iron core. The changing field links the secondary coil, so a voltage is induced there. If the primary carried a direct current the field would be constant and no emf would be induced in the secondary.
The induced potential difference is proportional to the number of turns. Therefore, if the secondary has more turns than the primary the transformer is a step‑up device and the output voltage is increased.
The induced potential difference is proportional to the number of turns. Therefore, if the secondary has more turns than the primary the transformer is a step‑up device and the output voltage is increased.
Examiner tips
- State that AC gives a changing magnetic field and DC gives a constant field.
- Explain that the induced emf is proportional to turns.
- Mention that more secondary turns give a higher output voltage (step‑up).
Common mistakes
- Claiming that DC can induce voltage in the secondary.
- Confusing the role of the iron core with the coil turns.
- Failing to link the number of turns to the voltage ratio.
Mark scheme (5 marks)
- Alternating current in the primary coil produces a changing magnetic field
- The changing magnetic field cuts through / links with the secondary coil
- A direct current would produce a constant magnetic field so nothing would be induced in the secondary coil
- A potential difference (and alternating current) is induced in the secondary coil
- Having more turns on the secondary coil than the primary coil produces a step-up transformer, increasing the output potential difference
Key terms in this question
alternating current · transformer · potential difference · primary coil · secondary coil
Related
- All OCR A-Level Physics B: Advancing Physics (H557) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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