Explain how the phenomenon of self-inductance arises in a coil carrying a changing current, and explain why the self-induced EMF opposes the change in current that produces it.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
A changing current in a coil produces a changing magnetic flux through the coil itself.
By Faraday’s law this changing flux induces an EMF in the coil.
Lenz’s law states that the induced EMF acts in a direction that opposes the change in flux (and therefore the change in current) that produced it.
This opposition is required by conservation of energy – energy must be supplied against the back‑EMF to build up or remove the current and the magnetic field energy stored in the coil.
By Faraday’s law this changing flux induces an EMF in the coil.
Lenz’s law states that the induced EMF acts in a direction that opposes the change in flux (and therefore the change in current) that produced it.
This opposition is required by conservation of energy – energy must be supplied against the back‑EMF to build up or remove the current and the magnetic field energy stored in the coil.
Examiner tips
- Use the exact terms ‘changing magnetic flux’, ‘Faraday’s law’, ‘induced EMF’, ‘Lenz’s law’, and ‘opposes the change’.
- Show the causal chain: current → flux → EMF → opposition to current change.
- Mention energy conservation to justify the direction of the induced EMF.
Common mistakes
- Confusing self‑inductance with mutual inductance; not specifying that the flux is through the same coil.
- Using vague phrases like ‘magnetic field’ without linking to flux or EMF.
- Omitting the energy conservation argument or mis‑stating Lenz’s law direction.
Mark scheme (4 marks)
- A changing current in the coil produces a changing magnetic flux through/linking the coil itself.
- By Faraday's law, this changing flux induces an EMF in the coil.
- By Lenz's law, the induced EMF acts in a direction that opposes the change in flux (and hence the change in current) that caused it.
- This opposition is consistent with conservation of energy: energy must be supplied against the back-EMF to build up (or remove) current and the associated magnetic field energy stored in the coil.
Key terms in this question
Related
- All IB DP Physics Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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