Electromagnets and motors

A current in a wire makes a magnetic field around it. Coiling the wire makes an electromagnet, which can be switched on and off. A current-carrying wire in a magnetic field feels a force: that’s how motors work.

Key facts

  • Field around a wire: circles; right-hand grip rule gives the direction.
  • Electromagnet strength: more current, more turns, iron core.
  • Motor effect: F = BIL (wire at right angles to the field).
  • Fleming’s left-hand rule: thumb force, first finger field, second finger current.

Worked examples

Example 1

A 0.2 m wire carries 3 A at right angles to a 0.5 T field. Find the force.

  1. 0.5 × 3 × 0.2 = 0.3 N

Answer: 0.3 N

Example 2

Give two ways to make an electromagnet stronger.

  1. Increase the current.
  2. Add more turns, or use an iron core.

Answer: More turns, more current (or an iron core)