Recognise induced voltage in a moving conductor
Identify motion across a magnetic field and distinguish induced e.m.f. from current in a closed circuit.
Mapped to the named 0654 section using teaching from the corresponding single-science course. A 0654-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.
Before you begin
- Identify a magnetic field direction.
- Distinguish e.m.f. from current.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What is e.m.f. measured in?
What you will learn
- Identify motion across a magnetic field and distinguish induced e.m.f. from current in a closed circuit.
- Recognise the changing condition producing e.m.f. and separately check whether current has a complete path.
Crossing the field can induce e.m.f.
A conducting rod moving across a magnetic field can develop an induced electromotive force, e.m.f., between its ends. The moving charges in it respond to the magnetic field and separate until the conductor’s electrical conditions balance. Mechanical movement supplies the energy in this generator arrangement.
Relative direction matters. In the simple straight-rod case, movement parallel to the magnetic field does not give the same separation as crossing it. For the illustrated case the rod, field and movement are mutually perpendicular, so the induced end-to-end effect is clear. Merely being near a stationary magnet is not enough.
Voltage and current are separate claims
An induced e.m.f. can exist between the ends of a rod even when there is no external closed path for sustained current. Connecting a suitable complete conducting path allows an induced current. A sensitive voltage measurement and a current-detector measurement answer different questions.
The induced current depends on the complete circuit as well as its driving e.m.f. Do not equate a larger meter current with a larger e.m.f. unless the comparison holds relevant circuit conditions constant. This Core lesson identifies the phenomenon; specific three-direction rules are separately labelled Supplement.
Sources for this lesson
Pause and explain
Which simple rod motion is the stated induction arrangement?
Worked example
A rod moves across a uniform field with its ends unconnected externally. Does no external current prove no induced e.m.f.?
Show the worked solution
- Cross-field movement can separate charges.
- Its ends can have an induced e.m.f.
- No external closed loop prevents sustained external current.
Answer An induced e.m.f. can exist without sustained external current.
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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.
Compare a stationary rod and the same rod moving across a fixed magnetic field in the stated geometry.
Give me a hint
Identify what changes and whether the conductor crosses the field.
Compare my reasoning
- The stationary rod has no supplied changing condition.
- Cross-field movement can induce an end-to-end e.m.f.
- A complete external circuit is additionally needed for sustained external current.
Cross-field movement can induce e.m.f.; stationary proximity alone does not.
Look for these in your work
- Specify the geometry.
- Separate voltage from current.
Explain why an unconnected moving rod can show induced voltage but no sustained external current.
Give me a hint
Compare end charge separation with external path continuity.
Compare my reasoning
- Motion across field separates charges.
- An end-to-end e.m.f. develops.
- No closed external path exists for sustained loop current.
Induced e.m.f. does not require sustained external loop current.
Look for these in your work
- Name the voltage mechanism.
- Identify the missing path.
A rod moves parallel to a uniform field, and a student predicts the same effect as perpendicular cross-field motion. Correct the comparison.
Give me a hint
Motion direction is a controlled part of the arrangement.
Compare my reasoning
- The perpendicular arrangement crosses field direction.
- Parallel motion lacks that cross-field effect in the ideal case.
- Equal speed alone does not make their e.m.f.s equivalent.
The ideal parallel motion gives no corresponding end-to-end motional e.m.f.; geometry matters.
Look for these in your work
- Compare directions.
- Do not use speed alone.
Common mistakes
- Movement parallel to field is a different geometry.
- Stationary proximity alone is insufficient.
- E.m.f. and closed-loop current are distinct.
What can you explain now?
Can induction create a voltage without an external closed current path?
Compare with the explanation
Yes
The conductor can develop an end-to-end e.m.f. while no sustained external loop current flows.
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Make it stick
Recognise the changing condition producing e.m.f. and separately check whether current has a complete path.
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.