Explain why alternating current (AC) must be used in the primary coil of a transformer, and describe how a step-up transformer differs from a step-down transformer in both its structure and its effect on voltage.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Transformers are used throughout the National Grid to change the magnitude of an alternating voltage. They consist of an iron core with a primary coil and a secondary coil wound around opposite ends.
Model answer (5 marks)
AC is required in the primary coil because it creates a changing magnetic field in the iron core. The changing field links the secondary coil and induces a potential difference (or current) in it. If DC were used, the magnetic field would be constant, so no emf would be induced in the secondary.
A step‑up transformer has more turns on the secondary coil than on the primary. The larger turn ratio produces a higher potential difference in the secondary, so the output voltage is greater than the input.
A step‑down transformer has fewer turns on the secondary than on the primary. The smaller turn ratio gives a lower potential difference in the secondary, so the output voltage is less than the input.
A step‑up transformer has more turns on the secondary coil than on the primary. The larger turn ratio produces a higher potential difference in the secondary, so the output voltage is greater than the input.
A step‑down transformer has fewer turns on the secondary than on the primary. The smaller turn ratio gives a lower potential difference in the secondary, so the output voltage is less than the input.
Examiner tips
- Use the word ‘changing’ to describe the magnetic field – this shows you understand the need for AC.
- Mention the turn ratio explicitly when comparing step‑up and step‑down transformers.
- Show the effect on voltage (higher or lower) in the secondary.
Common mistakes
- Saying AC ‘creates a voltage’ instead of a changing magnetic field.
- Confusing the roles of primary and secondary coils in the description of the transformer type.
- Omitting the turn ratio or the resulting voltage change for step‑up/step‑down transformers.
Mark scheme (5 marks)
- AC in the primary coil creates a changing magnetic field (in the iron core)
- The changing magnetic field cuts through / links with the secondary coil, inducing a current (or potential difference) in it
- DC would produce a constant (not changing) magnetic field, so no current/pd would be induced in the secondary coil
- A step-up transformer has more turns on the secondary coil than the primary coil, resulting in a larger (higher) potential difference in the secondary coil
- A step-down transformer has fewer turns on the secondary coil than the primary coil, resulting in a smaller (lower) potential difference in the secondary coil
Key terms in this question
alternating current (AC) · transformer · step-up transformer · step-down transformer · primary coil
Related
- All Edexcel A-Level Physics (9PH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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