An electric motor drives a sample carousel. An engineer tests two versions of the motor. Each has a rectangular coil mounted on an axle between the poles of a permanent magnet. The motors use the same battery, magnet and coil.
| Motor | Contacts on the axle | Result when connected to the battery |
|---|---|---|
| P | a split-ring commutator and brushes | the carousel rotates clockwise continuously |
| Q | two complete metal rings and brushes, so each end of the coil remains connected to the same battery terminal | the carousel turns, rocks backwards and forwards, then stops |
Explain the different results for motors P and Q.
The engineer replaces the complete rings in motor Q with a split-ring commutator and connects it identically to motor P. Before any further changes, it would rotate clockwise. The engineer then reverses the battery connections and exchanges the poles of the permanent magnet.
Explain the direction in which the carousel now rotates.
86 exam-style questions on Edexcel GCSE Physics 12.3 The motor effect, covering 12.3.1 The motor effect, 12.3.2 Fleming's left-hand rule, 12.3.3 Force on a conductor, and 12.3.4 Electric motors. Each one has a worked solution and a mark scheme showing where the marks go.