A metal rod rests on two parallel conducting rails that are connected to a galvanometer. The rod is pushed along the rails so that it moves at a steady speed through a uniform magnetic field directed vertically downward. The galvanometer shows a steady deflection. The rod is then pushed at a greater speed through the same magnetic field. Explain what happens to the induced EMF when the speed is increased, and describe two other changes a student could make to the setup to increase the induced EMF further.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A metal rod is placed on two horizontal conducting rails connected to a galvanometer. The rod can slide freely along the rails. A uniform magnetic field is directed vertically downward through the region between the rails.
Model answer (5 marks)
Increasing the speed increases the rate at which the rod cuts through magnetic field lines.
A greater rate of cutting field lines produces a greater induced EMF.
To increase the induced EMF further a student could:
1. Use a stronger magnet or increase the magnetic field strength.
2. Use a longer rod (increase the length of the conductor in the magnetic field).
Alternatively, the rod could be wrapped into a coil with more turns or a coil used instead of a single rod.
A greater rate of cutting field lines produces a greater induced EMF.
To increase the induced EMF further a student could:
1. Use a stronger magnet or increase the magnetic field strength.
2. Use a longer rod (increase the length of the conductor in the magnetic field).
Alternatively, the rod could be wrapped into a coil with more turns or a coil used instead of a single rod.
Examiner tips
- Show the relationship EMF = Bℓv and explain how v appears in the equation.
- Mention that B and ℓ can also be increased to raise EMF.
- Use the word ‘induced’ and ‘EMF’ correctly.
- Give two distinct, valid changes – magnet strength, rod length, or coil turns.
Common mistakes
- Confusing the direction of the magnetic field with the direction of the induced current.
- Forgetting to state that the EMF is proportional to the speed (v).
- Listing the same change twice (e.g., increasing both B and ℓ but not recognising they are separate options).
Mark scheme (5 marks)
- Increasing the speed increases the rate at which the rod cuts through magnetic field lines
- A greater rate of cutting field lines produces a greater induced EMF
- Use a stronger magnet / increase the strength of the magnetic field
- Use a longer rod (increase the length of the conductor in the magnetic field)
- Any second valid change: wrap the conductor into a coil with more turns / use a coil instead of a single rod (accept: increase number of turns in coil)
Key terms in this question
Related
- All Eduqas A-Level Physics revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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