A relay is an electrical switch that uses an electromagnet to control a second, separate circuit. Explain how a relay uses electromagnetism to switch on a high-current device such as a motor.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Relays are widely used in cars to allow a low-current switch on the dashboard to safely control high-current devices such as starter motors.
Model answer (4 marks)
When a low‑current control circuit energises the relay coil, current flows through the wire and the coil becomes an electromagnet, producing a magnetic field.
The magnetic field magnetises the iron core, which is attracted to the core and pulls the armature towards it.
The moving armature closes the normally open contacts, completing the high‑current circuit that powers the motor.
When the control current is removed the magnetic field collapses, the armature is released (often by a spring) and the contacts open, breaking the high‑current circuit.
The magnetic field magnetises the iron core, which is attracted to the core and pulls the armature towards it.
The moving armature closes the normally open contacts, completing the high‑current circuit that powers the motor.
When the control current is removed the magnetic field collapses, the armature is released (often by a spring) and the contacts open, breaking the high‑current circuit.
Examiner tips
- Show the sequence: coil → magnetic field → core → armature → contacts → motor
- Use the exact terms: electromagnet, iron core, armature, contacts, high‑current circuit
Common mistakes
- Confusing the coil with the motor; describing the motor as the electromagnet
- Omitting the step that the armature closes the contacts
- Using vague language such as ‘magnetic field’ without linking it to the iron core
Mark scheme (4 marks)
- When current flows through the coil of wire, it creates a magnetic field around the coil / the coil becomes an electromagnet
- The magnetic field magnetises the iron core, making it a stronger magnet / the iron core is attracted and pulls the armature
- The attracted armature closes the contacts / completes the second (high-current) circuit
- When the current in the coil is switched off, the magnetic field disappears and the armature returns / the second circuit is broken again (because an electromagnet is only magnetic when current flows)
Key terms in this question
Related
- All Eduqas GCSE Physics revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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