An electric motor is used in a food blender to spin the blades. Explain how the motor produces continuous rotation, referring to the forces acting on the coil of wire inside the motor.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A food blender uses an electric motor to spin its blades at high speed. Inside the motor, a coil of current-carrying wire sits between two permanent magnets.
Model answer (5 marks)
A current‑carrying coil in a magnetic field experiences a force (motor effect). The two sides of the coil that are perpendicular to the field are in opposite directions, so the forces on them are opposite. These opposite forces create a torque that makes the coil rotate rather than translate. Fleming’s Left‑Hand Rule is used to find the direction of each force and therefore the direction of rotation. The coil keeps turning because the commutator (or the changing current direction) reverses the current each half‑turn so that the forces always act in the same rotational sense while current flows.
Examiner tips
- Use the phrase ‘motor effect’ to show understanding of the force on a current‑carrying wire in a magnetic field.
- Explain that forces on opposite sides are opposite, giving a torque that causes rotation.
- Mention Fleming’s Left‑Hand Rule to determine the force direction.
- Show why the motor keeps rotating – the commutator reverses current so forces stay in the same rotational sense.
Mark scheme (5 marks)
- The current-carrying coil experiences a force because it is placed in a magnetic field (motor effect / the two magnetic fields interact)
- 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 (rather than translate)
- Fleming's Left-Hand Rule can be used to determine the direction of the force on each side of the coil
- The motor continues to rotate because the current direction (or the connection via a split-ring commutator) ensures the forces always act in the same rotational sense / the coil keeps spinning as long as current flows
Key terms in this question
Related
- All OCR A-Level Physics A (H556) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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