A security door in a museum uses an electric motor to open and close automatically. The motor contains a coil of wire placed between two permanent magnets. Explain how the motor produces rotation when a current flows through the coil.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
An electric motor in the museum's security door contains a rectangular coil of wire mounted on an axle and positioned between the poles of two permanent magnets. When a current flows through the coil, it rotates continuously to drive the door mechanism.
Model answer (5 marks)
A current‑carrying wire in a magnetic field experiences a force (F = I\,L\,\times\,B). In the motor the two sides of the rectangular coil that are perpendicular to the field are in opposite directions, so the forces on them are equal in magnitude but opposite in direction. These opposite forces create a torque that causes the coil to rotate. Fleming’s left‑hand rule is used to find the direction of the force on each side of the coil. The magnitude of the force – and therefore the torque and rotation – depends on the magnetic flux density (B) and the current (I).
Examiner tips
- Use the formula F = I\,L\,B to show the force on each side of the coil.
- Explain that opposite forces give a torque that rotates the coil.
- Mention Fleming’s left‑hand rule to determine direction.
- State that force increases with higher B and higher I.
Mark scheme (5 marks)
- A current-carrying wire in a magnetic field experiences a force
- The two sides of the coil that are perpendicular to the magnetic field experience forces in opposite directions
- These opposite forces cause the coil to rotate
- Fleming's left-hand rule can be used to determine the direction of the force on each side of the coil
- The strength of the force (and hence the rotation) depends on the magnetic flux density and the magnitude of the current
Key terms in this question
Related
- All OCR A-Level Physics B: Advancing Physics (H557) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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