A security system uses an electromagnet to hold a heavy door shut. The electromagnet is made from a solenoid with an iron core. Explain how the electromagnet produces a magnetic field and describe three ways the engineer could increase the strength of this magnetic field.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
An electromagnet in a door-locking system consists of a coil of insulated wire wound around an iron core, connected to a direct current power supply.
Model answer (5 marks)
A current flowing through the coil of the solenoid creates a magnetic field.
The iron core increases the field strength because magnetic field lines are attracted to the core, so the core’s high permeability concentrates the field.
To make the field stronger the engineer could:
1. Increase the number of turns of wire on the solenoid.
2. Increase the current flowing through the coil.
3. Use a core material with higher magnetic permeability or increase the core’s cross‑sectional area.
The iron core increases the field strength because magnetic field lines are attracted to the core, so the core’s high permeability concentrates the field.
To make the field stronger the engineer could:
1. Increase the number of turns of wire on the solenoid.
2. Increase the current flowing through the coil.
3. Use a core material with higher magnetic permeability or increase the core’s cross‑sectional area.
Examiner tips
- Use the exact terms "current", "solenoid", "iron core", "permeability". Show the three methods in separate points. Mention that the core material or cross‑section is a valid third method.
Common mistakes
- Writing that the field is produced by the iron core alone. Forgetting to mention the current. Listing a method that does not increase field strength, e.g. changing wire colour.
Mark scheme (5 marks)
- Current flowing through the solenoid (coil of wire) produces a magnetic field
- The iron core increases the strength of the magnetic field because field lines pass through iron more easily than air
- Increase the number of turns/coils of wire on the solenoid
- Increase the current flowing through the coil
- Any one further valid method: use a core material that is more easily magnetised / increase the cross-sectional area of the coil
Key terms in this question
electromagnet · solenoid · magnetic field · iron core
Related
- All OCR A-Level Physics A (H556) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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