A student wraps a long wire into a coil and connects it to a battery. They notice that the magnetic field produced inside the coil is much stronger than the field produced by a single straight wire carrying the same current. The student then pushes an iron rod into the centre of the coil. Explain why the field inside the coil is stronger than that of a single straight wire, and describe how inserting the iron rod affects the magnetic field.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A student is investigating the magnetic field produced by a coil of wire connected to a battery. They compare this to the field produced by a single straight wire carrying the same current, and then investigate the effect of placing an iron rod inside the coil.
Model answer (5 marks)
A coil (solenoid) produces a stronger field because the magnetic field of each turn adds constructively; the fields are all in the same direction, giving a single, large, uniform field inside the solenoid rather than many weaker, separate fields.
The solenoid’s field is essentially that of a bar magnet, with a north and south pole at the ends.
When an iron rod is inserted into the centre, the rod becomes an iron core. Iron has a high magnetic permeability, so it is easier for magnetic field lines to pass through it than through air. This concentrates the field lines and increases the magnetic field strength inside the coil.
The solenoid’s field is essentially that of a bar magnet, with a north and south pole at the ends.
When an iron rod is inserted into the centre, the rod becomes an iron core. Iron has a high magnetic permeability, so it is easier for magnetic field lines to pass through it than through air. This concentrates the field lines and increases the magnetic field strength inside the coil.
Examiner tips
- Use the word ‘add constructively’ to show understanding of field addition. Mention the solenoid behaves like a bar magnet. Explain iron core as high permeability material. Keep answer concise and use correct terminology.
Common mistakes
- Confusing the coil with a single wire without explaining field addition. Forgetting to mention the core’s high magnetic permeability. Using vague terms like ‘more field’ instead of ‘higher field strength’.
Mark scheme (5 marks)
- Coiling the wire causes the individual magnetic fields from each turn to align / combine in the same direction
- This produces a single, large, uniform magnetic field inside the solenoid rather than many separate weaker fields
- The shape of the solenoid's magnetic field is similar to that of a bar magnet (with a north and south pole)
- Inserting the iron rod (iron core) increases the strength of the magnetic field
- Because it is easier for magnetic field lines to pass through iron than through air
Key terms in this question
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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