A student sets up an electromagnet by wrapping a wire into a coil and connecting it to a power supply. The student wants to make the electromagnet as strong as possible. Describe four factors the student could change to increase the strength of the magnetic field produced by the electromagnet, and explain why inserting an iron core into the centre of the coil increases the field strength.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
An electromagnet is a solenoid with an iron core. The strength of the magnetic field it produces depends on several factors.
Model answer (5 marks)
1. Increase the current through the coil – a higher current produces a stronger magnetic field.
2. Increase the number of turns in the coil – more turns give a larger field.
3. Increase the length of the coil – a longer coil gives a larger field per unit length.
4. Increase the cross‑sectional area of the coil – a larger area increases the field.
5. Inserting an iron core concentrates the field: iron is easier for field lines to pass through than air, so the field is focused inside the coil, and the iron becomes an induced magnet whose magnetisation aligns with the solenoid field, adding to the overall field strength.
2. Increase the number of turns in the coil – more turns give a larger field.
3. Increase the length of the coil – a longer coil gives a larger field per unit length.
4. Increase the cross‑sectional area of the coil – a larger area increases the field.
5. Inserting an iron core concentrates the field: iron is easier for field lines to pass through than air, so the field is focused inside the coil, and the iron becomes an induced magnet whose magnetisation aligns with the solenoid field, adding to the overall field strength.
Examiner tips
- Use the exact terms ‘current’, ‘number of turns’, ‘length’, ‘cross‑sectional area’ and ‘iron core’.
- Explain the iron core in two parts: concentration of field lines and induced magnetisation.
- Keep each point brief – 1–2 sentences – to fit the 5‑mark limit.
- Use UK spelling (e.g. ‘area’).
Common mistakes
- Confusing the effect of coil length with coil diameter – both affect the field but in different ways.
- Failing to mention that the iron core adds to the field by becoming an induced magnet.
- Using vague phrases like ‘make it stronger’ without specifying the physical factor.
Mark scheme (5 marks)
- Increase the current (flowing through the coil) to increase field strength
- Increase the number of turns/coils on the solenoid
- Any one of: increase the length of the coil / increase the cross-sectional area of the coil
- The iron core is easier for magnetic field lines to pass through than air, so the field is concentrated/strengthened inside the coil
- The iron core becomes an induced magnet, aligning with the existing field and reinforcing/adding to the overall magnetic field of the solenoid
Key terms in this question
electromagnet · magnetic field
Related
- All OCR A-Level Physics A (H556) revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
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