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Cambridge IGCSE Physics · 0625

Chapter 4: Electricity and magnetism — Part 15

Topic 4.5.5 · The d.c. motor

Turning effect on a coil

A current-carrying coil in a magnetic field can experience a turning effect (opposite forces on the two sides).

The turning effect is increased by increasing:

  • the number of turns on the coil
  • the current
  • the strength of the magnetic field
Simple d.c. motor: a current-carrying coil in a magnetic field with opposite forces on the two sides producing a turning effect.
Diagram 1: Simple d.c. motor: a current-carrying coil in a magnetic field with opposite forces on the two sides producing a turning effect.

Split-ring commutator

In a d.c. motor, a split-ring commutator and brushes reverse the current in the coil every half-turn. That keeps the turning effect in the same sense, so the coil keeps rotating instead of stopping aligned with the field.

Split-ring commutator and brushes reversing the coil current every half-turn so the turning effect keeps the same sense.
Diagram 2: Split-ring commutator and brushes reversing the coil current every half-turn so the turning effect keeps the same sense.

Exam Traps

  • Do not confuse the split-ring commutator (d.c. motor, reverses coil current) with slip rings (a.c. generator, do not reverse the connections each half-turn).

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