A student places a straight wire connected to a battery inside a magnetic field. The wire lies at right angles to the field and experiences a force. The student then makes three separate changes to the setup. State what effect each of the following changes has on the magnitude of the force acting on the wire: (i) doubling the current through the wire; (ii) reversing the direction of the current; (iii) replacing the magnets with weaker ones.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A current-carrying wire is positioned at right angles to a uniform magnetic field and experiences a force due to the motor effect.
Model answer (4 marks)
(i) Doubling the current doubles the force.
(ii) Reversing the current reverses the direction of the force; the magnitude is unchanged.
(iii) Using weaker magnets decreases the force.
(ii) Reversing the current reverses the direction of the force; the magnitude is unchanged.
(iii) Using weaker magnets decreases the force.
Examiner tips
- Use the formula F=ILB to justify each change; mention proportionality to I or B.
- State the effect clearly for each part, not just the direction for part (ii).
Common mistakes
- Saying the force doubles for part (ii) instead of reversing direction.
- Confusing the effect of weaker magnets with no change in force.
Mark scheme (4 marks)
- (i) Doubling the current increases the magnitude of the force (the force doubles / the force increases).
- (ii) Reversing the current reverses the direction of the force / the force acts in the opposite direction; the magnitude of the force is unchanged.
- (iii) Using weaker magnets decreases the magnitude of the force (the force decreases / the force is smaller).
- Correct reasoning linked to either (i) or (iii): the magnitude of the force depends on / is proportional to the magnitude of the current OR the strength of the magnetic field.
Key terms in this question
magnetic field · current · force
Related
- All Pearson Edexcel International GCSE Physics (4PH1) revision notes →
- How to answer a "State" question →
- Decode the mark scheme abbreviations →
More Electromagnetism questions
- A student investigates the generator effect by pushing a bar magnet into a coil …
- A DC electric motor contains a rectangular coil of wire suspended between the po…
- A straight current-carrying wire is placed at right angles to a uniform magnetic…
- A loudspeaker contains a coil attached to a cone placed inside a magnetic field.…