A student sets up an electromagnet by wrapping a wire into a solenoid and connecting it to a power supply. The student then places a soft iron core inside the solenoid. Explain how the solenoid produces a magnetic field and why inserting the soft iron core increases the strength of that field.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
An electromagnet consists of a solenoid with a soft iron core. The student notices that the electromagnet can lift more paperclips when the iron core is inserted than when the solenoid has no core.
Model answer (5 marks)
Current flows through the wire, producing a magnetic field around each turn of the coil.
Coiling the wire causes the individual fields from each turn to combine and align, giving a single, stronger, almost uniform field along the centre of the solenoid.
The field shape of a solenoid is similar to that of a bar magnet, with a north and south pole.
When a soft iron core is inserted, it becomes an induced magnet due to the solenoid’s field.
Because magnetic field lines pass more easily through iron than through air, the core increases the overall magnetic field strength of the electromagnet.
Coiling the wire causes the individual fields from each turn to combine and align, giving a single, stronger, almost uniform field along the centre of the solenoid.
The field shape of a solenoid is similar to that of a bar magnet, with a north and south pole.
When a soft iron core is inserted, it becomes an induced magnet due to the solenoid’s field.
Because magnetic field lines pass more easily through iron than through air, the core increases the overall magnetic field strength of the electromagnet.
Examiner tips
- Use the command word ‘Explain’ to describe the process, not just state facts. Show the logical sequence: current → field → coiling → field strength → core induction → increased field. Mention the similarity to a bar magnet to demonstrate understanding of field shape.
Common mistakes
- Failing to mention that the core becomes an induced magnet. Confusing the role of the core with that of the solenoid’s own field. Using vague terms like ‘stronger’ without linking to the core’s magnetic permeability.
Mark scheme (5 marks)
- Current flowing through the wire produces a magnetic field around each turn of the coil
- Coiling the wire causes the individual magnetic fields from each turn to combine / align to produce a single, stronger, (almost) uniform magnetic field along the centre of the solenoid
- The overall field shape of a solenoid is similar to that of a bar magnet (with a north and south pole)
- The soft iron core becomes an induced magnet when placed inside the solenoid (magnetism is induced by the solenoid's field)
- It is easier for magnetic field lines to pass through iron than through air, so the iron core increases the overall magnetic field strength of the electromagnet
Key terms in this question
solenoid · electromagnet · magnetic field
Related
- All Edexcel A-Level Physics (9PH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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