Explain why alternating current (AC) must be used in the primary coil of a transformer, rather than direct current (DC), in order for the transformer to work.
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 voltage. They rely on a changing magnetic field to induce a current in the secondary coil.
Model answer (4 marks)
AC in the primary coil creates a continuously changing magnetic field.
The changing field links with the secondary coil.
This induces a potential difference (and current) in the secondary coil.
DC would give a constant magnetic field, so no change occurs and no potential difference or current is induced in the secondary coil.
The changing field links with the secondary coil.
This induces a potential difference (and current) in the secondary coil.
DC would give a constant magnetic field, so no change occurs and no potential difference or current is induced in the secondary coil.
Examiner tips
- Use the exact phrase "continuously changing magnetic field" for full marks.
- Show the link between changing field and induced EMF in the secondary.
- Mention that DC gives a constant field and therefore no induction.
- Keep the answer to four short points, matching the mark scheme.
Common mistakes
- Confusing AC with DC as the source of voltage rather than the cause of a changing magnetic field.
- Forgetting to state that a constant magnetic field produces no induced EMF in the secondary.
Mark scheme (4 marks)
- AC in the primary coil produces a continuously changing magnetic field
- This changing magnetic field cuts through / links with the secondary coil
- This induces a potential difference / current in the secondary coil
- DC produces a constant magnetic field, so no change occurs and no potential difference / current is induced in the secondary coil
Key terms in this question
alternating current · direct current · primary coil · transformer
Related
- All Edexcel GCSE Physics (1PH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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