Motors

Overview

This lesson shows how the motor effect becomes a useful machine. You are taking the force on one current-carrying conductor and extending it to a whole coil so that the force becomes a turning effect.

What You Need to Know

• In a magnetic field, the two opposite sides of a current-carrying coil experience forces in opposite directions.
• Because the forces act on opposite sides of the coil, they produce a turning effect.
• The turning effect becomes larger if the current is increased, the magnetic field is stronger, or the coil has more turns.
• A split-ring commutator reverses the current every half-turn so the coil keeps turning in the same direction.
• Brushes keep electrical contact with the rotating part of the motor.

How to Work Through It

1. Begin with a single-conductor motor-effect example so you can see where each force comes from.
2. Transfer that idea to a rectangular coil and identify the pair of forces that causes rotation.
3. Add the split-ring commutator and brushes to explain how continuous turning is maintained.
4. Practise explaining the full motor in stages using a labelled diagram.

Check Your Understanding

• Why do the forces on the two sides of the coil produce rotation instead of simple movement?
• Which three changes increase the turning effect in a motor?
• Why is the split-ring commutator needed in a d.c. motor?

Common Mistakes

• Saying the current stays the same way round in the coil. It must reverse every half-turn.
• Describing the commutator as the power source. It is the part that swaps the coil connections.
• Forgetting that the two forces act in opposite directions on opposite sides of the coil.

Next Steps

• Use the worksheet to practise explaining the motor from a labelled diagram.
• Keep the turning-effect idea secure because the next applications of electromagnets depend on the same link between current and magnetic force.