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Learn4PH1Paper 1P · Paper 1 (core content)Magnetism and electromagnetismElectromagnets and motors
Magnetism and electromagnetism · Subtopic 3 of 4

Electromagnets and Motors

A current produces a magnetic field: circles round a straight wire, and a bar-magnet pattern from a solenoid. Wind a solenoid round soft iron and you have an electromagnet. A current-carrying wire in a magnetic field feels a force, and in a d.c. motor that force turns a coil, with a split-ring commutator reversing the current every half turn.

Cambridge 5054 Paper 1Cambridge 5054 Paper 2Cambridge 0625 Papers 2, 4Cambridge 0625 Papers 1, 3Edexcel 4PH1 Paper 1PEdexcel 4PH1 Paper 2P≈ 30 minRevision 5 · updated 2026-10-27
Section 1 of 14

Learning objectives

  • Describe the magnetic field of a straight wire and of a solenoid.
  • Explain how electromagnets work and give uses.
  • Describe the force on a current-carrying conductor and use Fleming's left-hand rule.
  • Explain how a d.c. motor works and how to make it turn faster.
Pause and think

A loudspeaker has no moving parts except a cone attached to a coil, sitting near a magnet. How can an electrical signal make it move back and forth?

Section 2 of 14

Core idea

Start hereA jumping wire

Lay a wire between magnet poles and switch on a current: the wire jumps sideways. Current and field together make a force. Bend the wire into a coil and the forces make it spin: an electric motor.

The big idea

Current makes a field. Current in a field feels a force (Fleming's left-hand rule). A coil turns; a split-ring commutator keeps it turning.

1. Fields from currents

Circular field around a current-carrying wire
•current out of the page: field anticlockwise
Right-hand grip rule: thumb along the current, fingers curl the way the field goes. The field is weaker further from the wire.
  • Straight wire: circles (right-hand grip rule).
  • Solenoid: like a bar magnet; iron core strengthens it.

2. The motor effect

A d.c. motor: forces on the two sides make a turning effect
F upF downNSsplit-ring commutator reverses the current every half turn
Current goes one way along the left side and the other way along the right, so the forces are opposite: the coil turns.
  • Left hand: First finger Field, seCond finger Current, thuMb Motion.
  • Commutator reverses the current every half turn so the turning force keeps the same sense.
Worked exampleImproving a motor

Make it faster; reverse it.

Goal
Changes.
Strategy
Force depends on current, field, turns.

  1. Faster: more current, stronger magnets, more turns.

  2. Reverse: swap the current direction or the field.

AnswerAs above.
✓ CheckReversing both leaves the direction unchanged ✓.
Common misconceptionRight hand for motors Left hand for motors; the right-hand grip rule is for the field around a wire.
Pause and think

Why does a wire parallel to the field feel no force?

Summary

  • Current → field.
  • Field + current → force.
Key words
Commutator
Reverses current in a motor coil.
HistoryØrsted's compass. In 18201820, during a lecture, Hans Christian Ørsted noticed a compass needle twitch when he switched on a current in a nearby wire. It was the first evidence that electricity and magnetism are connected; within a year Faraday had built the first simple motor.
Fun factMotors are everywhere. A typical home contains over 5050 electric motors: in the fridge, washing machine, fans, hair dryer, hard drives, toothbrushes, and the vibrating motor in your phone. Electric motors use nearly half of all the world's electricity.
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