An engineer designs a junkyard crane that uses an electromagnet to pick up scrap iron. The electromagnet is a solenoid with an iron core. The engineer wants to make the electromagnet as strong as possible. Describe four factors that affect the strength of the magnetic field produced by the electromagnet, and explain why replacing the iron core with an air core would make the electromagnet weaker.
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 its magnetic field depends on several factors including the current, the number of turns, and the core material.
Model answer (5 marks)
1. Increasing the current through the coil increases the magnetic field strength, because B∝I.
2. Increasing the number of turns of wire increases the field strength, because B∝N.
3. Increasing the length of the coil (i.e. decreasing the coil density) increases the field strength, because B∝1/l.
4. Using a core material with a high magnetic permeability (e.g. iron) increases the field strength, because B∝μ.
5. Replacing the iron core with an air core weakens the electromagnet because iron is magnetised and provides a much higher permeability, allowing more magnetic flux to pass through the core; air has μ≈μ₀, so the field is much weaker.
2. Increasing the number of turns of wire increases the field strength, because B∝N.
3. Increasing the length of the coil (i.e. decreasing the coil density) increases the field strength, because B∝1/l.
4. Using a core material with a high magnetic permeability (e.g. iron) increases the field strength, because B∝μ.
5. Replacing the iron core with an air core weakens the electromagnet because iron is magnetised and provides a much higher permeability, allowing more magnetic flux to pass through the core; air has μ≈μ₀, so the field is much weaker.
Examiner tips
- Use the exact symbols B, I, N, l, μ and state the proportionality; mention the core’s permeability; keep each point brief and separate; explain the air‑core effect in one sentence.
Common mistakes
- Confusing length of coil with number of turns; writing that a longer coil gives a stronger field without noting the 1/l dependence; stating that a larger cross‑sectional area increases B, which is incorrect for a solenoid; omitting the explanation of why an air core is weaker.
Mark scheme (5 marks)
- Any one from: increasing the current increases the field strength / increasing the number of turns increases the field strength / increasing the length of the coil increases the field strength / increasing the cross-sectional area increases the field strength
- A second distinct factor from the list above that also increases field strength
- A third distinct factor from the list above that also increases field strength
- A fourth distinct factor — the type of core used affects field strength / iron core gives a stronger field than an air core
- Explanation that it is easier for magnetic field lines to pass through iron than air, so removing the iron core reduces the strength of the field / iron is magnetised (becomes an induced magnet) and adds to the overall magnetic field, so removing it weakens the field
Key terms in this question
electromagnet · solenoid · iron core · 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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