An engineer is designing a scrapyard crane that uses an electromagnet to lift steel beams. Explain how the electromagnet works and describe two changes the engineer could make to increase the strength of the magnetic field it produces.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A scrapyard crane uses an electromagnet consisting of a solenoid with an iron core connected to a power supply. When switched on, the electromagnet can lift heavy steel beams. When switched off, the steel beams are released.
Model answer (5 marks)
When the power supply is switched on, a current I flows through the solenoid. The current creates a magnetic field B around and inside the coil, described by B=µ₀nI (n = turns per metre). The iron core placed in the coil becomes a highly permeable material; it is magnetised by the field and acts as a permanent magnet, so the field inside the core is much larger than that of the coil alone.
The steel beams are ferromagnetic. The magnetic field of the core induces a magnetisation in the beams, giving them a north and south pole. The opposite poles attract, so the beams are pulled into the core and can be lifted.
To increase the field strength the engineer could:
1. Increase the current through the solenoid – this directly increases B.
2. Use a core with a larger cross‑sectional area – a larger area allows more magnetic flux to pass, raising the field at the surface of the core.
Both changes will make the electromagnet stronger and able to lift heavier beams.
The steel beams are ferromagnetic. The magnetic field of the core induces a magnetisation in the beams, giving them a north and south pole. The opposite poles attract, so the beams are pulled into the core and can be lifted.
To increase the field strength the engineer could:
1. Increase the current through the solenoid – this directly increases B.
2. Use a core with a larger cross‑sectional area – a larger area allows more magnetic flux to pass, raising the field at the surface of the core.
Both changes will make the electromagnet stronger and able to lift heavier beams.
Examiner tips
- Use the formula B=µ₀nI to show the relationship between current, turns and field strength. Mention the role of the iron core’s permeability. Give two distinct, realistic modifications – e.g. higher current and larger core area. Keep the answer concise and use exam terminology such as ‘induced magnetisation’ and ‘ferromagnetic material’.
Common mistakes
- Saying the core is a permanent magnet – it is only magnetised when the current is on. Confusing the number of turns with the total number of turns; use turns per metre or total turns. Giving the same modification twice (e.g. both changes are ‘increase current’).
Mark scheme (5 marks)
- When current flows through the solenoid (coil of wire), a magnetic field is generated around/inside it
- The iron core becomes an induced magnet, increasing the overall strength of the magnetic field
- The steel beams are magnetic materials that become induced magnets, so they are attracted to the electromagnet
- One valid change to increase field strength, e.g. increase the current / increase the number of turns (coils) on the solenoid / add or use a larger iron core
- A second distinct valid change to increase field strength, e.g. a different one from: increase the current / increase the number of turns / use a larger cross-sectional area core
Key terms in this question
electromagnet · magnetic field
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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