Induce voltage by changing the field linking a coil
Explain induction with a stationary conductor and a changing linked field, including a stationary-field control.
Exam scope and assessment notes
Mapped to the named 0973 section using teaching from the corresponding single-science course. A 0973-specific review of every Core and Supplement objective is still in progress. Written practice is self-checked; it does not demonstrate supervised laboratory competence or award an official Cambridge grade. This qualification reports two grades on the 9–1 scale; confirm the entry route for your zone and exam series.
Before you begin
- Describe a magnetic field around a magnet or coil.
- Distinguish a steady state from a changing state.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What does a stationary bar magnet provide around it?
What you will learn
- Explain induction with a stationary conductor and a changing linked field, including a stationary-field control.
- Describe the actual change in linked magnetic field, including controls where everything stays fixed.
The conductor need not move
A changing magnetic field linking a conductor can induce an e.m.f. even if the conductor itself is stationary. Moving a bar magnet toward or away from a fixed coil changes the linked field. A nearby electromagnet with changing current can provide a field change without moving either coil.
A magnet held still in a fixed position beside a stationary coil gives an approximately steady linked field, so no sustained induced e.m.f. occurs in that unchanged arrangement. Field strength being large is not by itself sufficient: the linking must change.
Name the change, not just the object
Compare three states: magnet approaching, magnet held still, and magnet withdrawing. Approaching and withdrawing can induce e.m.f.; holding still after the movement stops gives zero in the ideal steady-field case. Changes in coil orientation or effective linked area can also alter linking even when a field source is fixed.
An induced e.m.f. drives current only through an appropriate complete path. This is induction between circuits or between a moving magnet and coil; it is not charge directly travelling from the magnet into the coil. Supplement work later explains the induced direction.
Sources for this lesson
Pause and explain
A strong magnet and coil are held fixed. What is expected after transients have ended?
Worked example
A magnet approaches a fixed coil, is held stationary, then is withdrawn. Identify the induction states.
Show the worked solution
- Approach changes the linked field.
- Holding both fixed leaves it steady.
- Withdrawal changes it again.
Answer Induction during approach and withdrawal; none sustained during the fixed hold.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Classify a fixed coil with magnet approaching, held still and withdrawing.
Give me a hint
Identify linked-field change in each stage.
Compare my reasoning
- Approach changes linking.
- A fixed hold keeps it steady.
- Withdrawal changes linking again.
Approach: induction; fixed hold: none sustained; withdrawal: induction.
Look for these in your work
- Include all stages.
- State the steady arrangement assumption.
A nearby source electromagnet and receiving coil are both fixed. Explain why increasing source current can induce e.m.f. in the receiving coil.
Give me a hint
Changing current changes the source magnetic field.
Compare my reasoning
- Source current increases.
- Its linked magnetic field changes.
- The receiving coil can develop induced e.m.f.
Changing source current can induce e.m.f. without mechanical movement.
Look for these in your work
- Name the field change.
- Do not transfer charge between separate circuits.
A student says induction always requires the receiving wire to move. Use two counterexamples.
Give me a hint
Change the linked field while keeping the receiver fixed.
Compare my reasoning
- Move a magnet toward a fixed coil.
- Change current in a nearby fixed electromagnet.
- Both change the field linking the stationary receiver.
A moving magnet or changing source current can induce in a stationary receiving coil.
Look for these in your work
- Provide two distinct changes.
- Keep the receiving conductor stationary.
Common mistakes
- A strong steady linked field is not enough.
- Changing source current can induce without motion.
- Charge does not transfer from magnet to coil.
What can you explain now?
Can a stationary coil have induced e.m.f.?
Compare with the explanation
Yes, if its linked magnetic field changes
Induction can follow a changing linked field even though the conductor remains still.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Describe the actual change in linked magnetic field, including controls where everything stays fixed.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.