A student builds an electric motor using a coil of wire placed between two permanent magnets. When current flows through the coil, it begins to rotate. Explain how the electric motor produces rotation.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
An electric motor consists of a current-carrying coil of wire positioned between the poles of two permanent magnets. The two sides of the coil that are perpendicular to the magnetic field experience forces, causing the coil to rotate continuously.
Model answer (5 marks)
The current in the coil creates a magnetic field around it.
The magnetic field of the coil interacts with the field of the permanent magnets.
The two sides of the coil that are perpendicular to the magnetic field experience forces in opposite directions.
Using Fleming’s left‑hand rule, the direction of the force on each side can be found.
The opposing forces on the two sides cause the coil to rotate about its axis.
The magnetic field of the coil interacts with the field of the permanent magnets.
The two sides of the coil that are perpendicular to the magnetic field experience forces in opposite directions.
Using Fleming’s left‑hand rule, the direction of the force on each side can be found.
The opposing forces on the two sides cause the coil to rotate about its axis.
Examiner tips
- Use the exact terms: magnetic field, forces, Fleming’s left‑hand rule, rotation about its axis.
- Show the sequence: current → magnetic field → interaction → forces → rotation.
- Include the direction of forces (opposite on each side).
Common mistakes
- Confusing Fleming’s right‑hand rule with the left‑hand rule.
- Failing to mention that the forces are on the sides perpendicular to the field.
- Not explaining that the forces are opposite and cause rotation.
Mark scheme (5 marks)
- The current-carrying coil produces a magnetic field (around it)
- The magnetic field of the coil interacts with the magnetic field of the permanent magnets
- The two sides of the coil perpendicular to the magnetic field experience forces in opposite directions
- Fleming's left-hand rule can be used to determine the direction of the force on each side of the coil
- The opposing forces on the two sides cause the coil to rotate (about its axis)
Key terms in this question
electric motor · permanent magnet
Related
- All OCR A-Level Physics B: Advancing Physics (H557) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Electrical circuits questions
- A technician is investigating a solenoid connected to a battery. She notices tha…
- A student is building a simple electromagnet for a school project. She wraps cop…
- A student wraps a length of wire into a coil to make a solenoid and connects it …
- An engineer designs an electromagnet to be used in a scrapyard to lift heavy ste…