The Motor Effect and Force on a Conductor — AQA GCSE Physics
When a current flows through a wire in a magnetic field, the wire experiences a force — the motor effect.
The motor effect
When a current-carrying conductor is placed in a magnetic field, the magnetic field of the wire interacts with the field of the magnet, and the conductor experiences a force. This is the motor effect.
- The force is greatest when the wire is at 90° (perpendicular) to the magnetic field, and zero when the wire is parallel to the field.
- Reversing the current or the magnetic field reverses the direction of the force.
Fleming's left-hand rule
The direction of the force is found using Fleming's left-hand rule:
- First finger = Field (N → S),
- seCond finger = Current,
- thuMb = Motion (force).
Calculating the force (Higher Tier)
For a conductor at 90° to the field: $$F = B \times I \times L$$
- F = force (N), B = magnetic flux density (tesla, T), I = current (A), L = length of conductor in the field (m).
Uses
The motor effect is used in electric motors and loudspeakers, which convert electrical energy into movement/sound.
Exam tips
- The motor effect: a current-carrying wire in a magnetic field feels a force.
- Force is greatest at 90°, zero when parallel.
- Use Fleming's left-hand rule (Field, Current, Motion).
- Learn F = BIL; B is in tesla.