Explain why a potential difference is induced across the ends of a straight wire when the wire is moved through a magnetic field, and describe two ways the magnitude of the induced potential difference could be increased.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
An engineer is testing a simple generator by moving a length of straight copper wire between the poles of a horseshoe magnet. A voltmeter connected to the ends of the wire shows a reading whenever the wire moves.
Model answer (4 marks)
When the straight copper wire moves through the magnetic field of the horseshoe magnet, the wire cuts across magnetic field lines. The free electrons in the wire experience a magnetic force (F=qv×B) which pushes them to one end of the wire, creating a charge separation and a potential difference between the ends.
The magnitude of the induced potential difference can be increased by:
1. Increasing the speed of the wire through the magnetic field.
2. Using a stronger magnet to increase the magnetic field strength.
The magnitude of the induced potential difference can be increased by:
1. Increasing the speed of the wire through the magnetic field.
2. Using a stronger magnet to increase the magnetic field strength.
Examiner tips
- Use the term ‘magnetic force on moving charges’ and ‘charge separation’ to show understanding of the mechanism.
- Mention the formula V=Blv or V=IBl to justify the two methods of increasing V.
- Show the two methods clearly as separate points.
Common mistakes
- Saying the wire is ‘cutting through the field’ without explaining the force on electrons.
- Confusing the induced EMF with the magnetic field itself.
- Failing to list both ways to increase the potential difference.
Mark scheme (4 marks)
- The wire cuts through magnetic field lines (as it moves), causing an induced potential difference
- Electrons in the wire experience a force, causing them to move / build up at one end, creating a potential difference
- Increase the speed at which the wire moves through the field
- Use a stronger magnet (to increase the magnetic field strength)
Key terms in this question
induced potential difference · magnetic field
Related
- All OCR GCSE Physics A: Gateway Science (J249) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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