O-Level and SEC G3 Physics K323
P18: Electromagnetism
Connect current, magnetic field, force, motors, relays, and control factors using the required direction rule.
Reviewed for the 2027 cohort on 19 July 2026. The official syllabus remains authoritative for assessable wording and paper details.
Core notes
Electromagnetism connects current to magnetic fields, applications of electromagnets, forces on currents and charged particles, Fleming's left-hand rule, and the turning action of a d.c. motor.
Fields from currents and electromagnets
A current in a straight wire produces concentric magnetic field lines. Reversing current reverses field direction, while increasing current increases field strength. A solenoid produces a field resembling a bar magnet, with direction set by current direction.
An electromagnet becomes stronger with greater current, more turns per unit length and a suitable soft-iron core. Applications such as circuit breakers use current-controlled magnetism to produce a mechanical action.
Motor effect and direction
A current-carrying conductor in a magnetic field can experience a force because its field interacts with the external field. Reversing current or external field reverses force; reversing both leaves force direction unchanged.
Fleming's left-hand rule relates field, conventional current and force when they are mutually perpendicular. A beam of charged particles is also deflected, with direction depending on charge sign and motion direction.
D.C. motor
Opposite forces on the two sides of a current-carrying coil form a couple and produce a turning effect. More turns or greater current increases the effect. A soft-iron cylinder strengthens the field through the coil.
The split-ring commutator reverses current in the coil every half-turn, so the torque continues in the same rotational sense. Brushes maintain electrical contact with the rotating commutator.
Formulae and relationships
This chapter is assessed mainly through models, field patterns and explanations. Build the causal chain before adding any calculation.
Worked examples
Example 1: A wire carrying current to the right lies in a magnetic field into the page. Use Fleming's left-hand rule to state the force direction.
- Point the first finger into the page for magnetic field.
- Point the second finger to the right for conventional current.
- The thumb then points upward for force.
Answer: Upward.
Chapter checkpoint
Use these three moves to organise the topic before attempting a mixed or practical question.
- Relate current direction to the magnetic field around a wire and solenoid.
- Use the motor-effect direction rule with conventional current, field, and force assigned correctly.
- Explain how current, field strength, coil turns, and geometry change an electromagnet or motor effect.
Official outcome coverage
K323 P18: 6 mapped outcomes, references P18(a), P18(b), P18(c), P18(d), P18(e), P18(f). Check the official K323 syllabus.
The outcome wording is not reproduced here. The relevant official syllabus remains authoritative for exact assessable scope.
Practical and data connection
Vary one factor in an electromagnet or motor-effect setup, measure a defensible response, and keep heating and contact quality under control.
Exam traps and retrieval check
Avoid these traps
- Using electron flow instead of conventional current in Fleming's left-hand rule.
- Saying a commutator supplies current rather than reversing coil connection.
- Predicting force without fixing field and current directions.
Check from memory
What happens to force if both current and field reverse?
The force direction remains unchanged.
How can an electromagnet be strengthened?
Increase current, increase turns, or add a suitable soft-iron core.
Why does a motor coil keep rotating in one sense?
The split-ring commutator reverses coil current every half-turn.
Pure versus Combined scope
Combined Physics revisits part of this core under CP15 Magnetism and Electromagnetism, but with reduced outcome scope. Combined students should follow the component checklist rather than assume every K323 outcome is assessable.
Shared explanation source
Eclat has a related explanation in its existing IP library. It can help with the shared concept, but its IP extensions and school-sensitive scope are not automatically part of K323. Open the related IP explanation.