An electric motor turns the drum of a seed dispenser continuously clockwise. The motor has a rectangular coil mounted on an axle between permanent magnet poles. A split-ring commutator reverses the current in the coil after every half turn.
Which explains why the drum continues to rotate clockwise?
Reversing the current also reverses the permanent magnet field, so the forces remain in their original directions.
Reversing the current makes both long sides carry current in one direction, so their forces turn the axle together.
Reversing the current reverses both forces just as the long sides swap sides of the axle, so the turning effect stays clockwise.
Reversing the current keeps both forces pointing the same way as before, so the turning effect stays clockwise.
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.