An engineer designs an electromagnet to be used in a scrapyard to lift heavy steel objects. Explain how the engineer could increase the strength of the magnetic field produced by the electromagnet, and explain why steel objects are attracted to it when it is switched on.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
An electromagnet consists of a solenoid with an iron core. It is connected to a power supply and used to sort magnetic materials at a recycling facility.
Model answer (5 marks)
Increasing the current through the solenoid increases the magnetic field strength, because B∝I.
Increasing the number of turns (N) of wire in the solenoid also increases B, since B∝N.
Adding an iron core raises the field strength, as iron has a high magnetic permeability and allows field lines to pass through it more easily than air.
When the electromagnet is switched on, the magnetic field induces a magnetic moment in the steel object, turning it into a temporary magnet.
Induced magnetism always produces an attractive force, so the steel is drawn towards the electromagnet.
Increasing the number of turns (N) of wire in the solenoid also increases B, since B∝N.
Adding an iron core raises the field strength, as iron has a high magnetic permeability and allows field lines to pass through it more easily than air.
When the electromagnet is switched on, the magnetic field induces a magnetic moment in the steel object, turning it into a temporary magnet.
Induced magnetism always produces an attractive force, so the steel is drawn towards the electromagnet.
Examiner tips
- Use the proportionality symbols B∝I, B∝N to show relationships. Show the role of magnetic permeability for the iron core. Explain induction and attraction clearly. Keep each point concise to fit 5 marks.
Common mistakes
- Confusing the role of the iron core with a permanent magnet. Forgetting that the attraction is due to induced magnetism, not a permanent field. Using vague terms like "more current" without showing the proportionality.
Mark scheme (5 marks)
- Increasing the current increases the strength of the magnetic field
- Increasing the number of turns/coils on the solenoid increases the field strength
- The iron core increases the strength of the magnetic field (because field lines pass through iron more easily than air)
- When the electromagnet is switched on, the steel objects become induced magnets
- Induced magnetism always produces an attractive force, so the steel is attracted to the electromagnet
Key terms in this question
electromagnet · magnetic field
Related
- All OCR A-Level Physics B: Advancing Physics (H557) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Electrical circuits questions
- A student builds an electric motor using a coil of wire placed between two perma…
- A technician is investigating a solenoid connected to a battery. She notices tha…
- A student is building a simple electromagnet for a school project. She wraps cop…
- A student wraps a length of wire into a coil to make a solenoid and connects it …