Investigate electromagnetic induction
Explain how experiments with changing magnetic flux induce an e.m.f. and how speed, field strength and turns affect its magnitude.
Section 20 · Magnetic fields
A Level; Cambridge International Physics 9702, 2025–2027 syllabus, Topic 20. These original explanations and calculations are study aids; full timed assessment and human subject-teacher review remain outstanding.
Before you begin
- Use current, charge, force, velocity, vectors, area and SI units; distinguish conventional current from electron flow.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What must change to induce an e.m.f. in a coil?
What you will learn
- Describe an experiment showing induction from changing flux
- Explain how the direction of motion changes induced polarity
- Identify factors that increase induced e.m.f.
A changing flux induces an e.m.f.
Connect a coil to a sensitive galvanometer and move a bar magnet into or out of the coil. A deflection shows an induced e.m.f. and current when the circuit is closed. Holding the magnet still gives no sustained deflection because the flux through the coil is not changing.
Reverse the magnet's motion or swap its poles and the galvanometer deflection reverses. The experiment shows that it is the change in magnetic flux linkage, not merely the presence of a magnetic field, that induces e.m.f.
A faster flux change gives a larger response
The induced e.m.f. magnitude increases when flux linkage changes more rapidly. Move the magnet faster, use a stronger magnet, or increase the number of coil turns: each can make the flux-linkage change larger per unit time.
Keep the comparison controlled when explaining an experiment. For example, to test speed, use the same magnet and coil and vary only the speed of motion; compare the maximum galvanometer deflection as a qualitative measure of induced current and e.m.f.
Pause and explain
A magnet is held still inside a coil. What happens to the induced e.m.f. after insertion stops?
Worked example
A magnet is pushed into a coil, held stationary, and then pulled out. Describe the galvanometer response.
Show the worked solution
- Relate each stage to flux change.
- Use the galvanometer deflection as evidence of induced current.
- Compare the direction for entry and withdrawal.
Answer The galvanometer deflects while the magnet moves into the coil, returns to zero when it is held still, and deflects in the opposite direction when it is pulled out.
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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.
Design a simple experiment to show that changing flux induces e.m.f. and that faster motion increases its magnitude.
Give me a hint
Use a coil, magnet and galvanometer, and vary one factor at a time.
Compare my reasoning
- Connect the coil to a galvanometer and move a magnet through it; observe a deflection.
- Hold the magnet stationary and observe no sustained deflection.
- Repeat with the same magnet and coil at different speeds; faster motion gives a larger peak deflection.
A coil–galvanometer circuit and moving magnet show induction; controlled speed trials show larger deflection for faster motion.
Look for these in your work
- Induction setup described
- Stationary control included
- Speed comparison controlled
The same coil is used with a weak magnet moved slowly and a stronger magnet moved at the same speed. Which should produce the larger e.m.f. and why?
Give me a hint
Compare the flux-linkage change while holding speed and coil fixed.
Compare my reasoning
- The coil and speed are fixed.
- The stronger magnet produces a larger magnetic flux through the coil.
- For the same motion, the larger flux change produces a larger induced e.m.f.
The stronger magnet should produce the larger e.m.f. because it causes a larger change in flux linkage per time.
Look for these in your work
- Controlled variables identified
- Flux change compared
- E.m.f. magnitude explained
A magnet is moved into a coil and then withdrawn along the same path. Why do the induced-current directions differ?
Give me a hint
Compare whether flux linkage is increasing or decreasing.
Compare my reasoning
- As the magnet enters, flux linkage changes in one direction.
- As it leaves, flux linkage changes in the opposite direction.
- The induced e.m.f. and current reverse to oppose the respective changes.
The flux change reverses on withdrawal, so the induced polarity and current direction reverse.
Look for these in your work
- Entry change stated
- Withdrawal change compared
- Polarity reversal explained
Common mistakes
- Claiming a stationary magnet continuously induces e.m.f.
- Saying only field strength, not flux change, matters
- Claiming reversing motion leaves induced polarity unchanged
- Changing several variables in an experiment intended to test one factor
What can you explain now?
What must change to induce an e.m.f. in a coil?
Compare with the explanation
Magnetic flux linkage through the coil
A changing linkage produces an induced e.m.f.
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Make it stick
Why does a moving magnet induce an e.m.f. in a coil while a stationary magnet does not?
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