The Physics Question Bank

A split-ring commutator is one of the clever parts of a simple DC motor. Its job is to make sure the coil keeps turning the same way.

In a motor, the coil sits inside a magnetic field. When a current passes through the coil, the two sides of the coil feel forces in opposite directions. One side is pushed up and the other side is pushed down. This creates a turning effect.

However, there is a problem. After the coil has turned halfway round, the forces would normally make it turn back the other way. The motor would not keep spinning smoothly.

The split-ring commutator solves this problem. It swaps the contacts over every half turn, which reverses the current in the coil. When the current reverses, the direction of the force on each side of the coil also reverses.

This happens at just the right moment. The side of the coil that was being pushed up is now pushed down, and the side that was being pushed down is now pushed up. The coil therefore keeps being pushed around in the same direction.

For AQA GCSE Physics, the key idea is

The split-ring commutator reverses the current every half turn, so the forces on the coil reverse at the right time and the motor continues to rotate in the same direction.

3 Marks Answer

1 mark: The split-ring commutator reverses the current in the coil every half turn.

2 mark: This reverses the direction of the forces on each side of the coil.

3 mark: The forces continue to push the coil around in the same direction, so the motor keeps rotating.