A student wraps a long wire into a coil and connects it to a battery to create a solenoid. The student then wants to make the solenoid as strong a magnet as possible. Describe four factors the student could change to increase the strength of the magnetic field produced by the solenoid, and explain why inserting an iron core further increases the field strength.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A solenoid is a wire wrapped into the shape of a coil. When current flows through it, the resulting magnetic field is similar in shape to that of a bar magnet. The strength of this field can be altered in several ways.
Model answer (5 marks)
1. Increase the magnitude of the current through the coil, because a larger current produces a stronger magnetic field.
2. Increase the number of turns of wire, as more turns add more magnetic field contributions.
3. Increase the cross‑sectional area of the coil (or alternatively increase its length), which raises the magnetic field per unit length.
4. Use a longer coil, which increases the total magnetic field produced.
5. Inserting an iron core further increases the field because iron is easier for magnetic field lines to pass through than air, concentrating the field inside the solenoid, and the iron becomes magnetised (induced magnet) adding to the overall magnetic field strength.
2. Increase the number of turns of wire, as more turns add more magnetic field contributions.
3. Increase the cross‑sectional area of the coil (or alternatively increase its length), which raises the magnetic field per unit length.
4. Use a longer coil, which increases the total magnetic field produced.
5. Inserting an iron core further increases the field because iron is easier for magnetic field lines to pass through than air, concentrating the field inside the solenoid, and the iron becomes magnetised (induced magnet) adding to the overall magnetic field strength.
Examiner tips
- Use the exact wording from the mark scheme (e.g., "increase the magnitude of the current", "increase the number of turns", "cross‑sectional area" or "length"), and give a brief physical reason for each. Explain the core effect in two sentences: iron concentrates lines and becomes an induced magnet. Keep each point concise to fit the 5‑mark limit.
Common mistakes
- Confusing the effect of increasing coil length with increasing current; length only affects field per unit length, not magnitude. Failing to mention that the iron core becomes magnetised, or only stating "iron concentrates the field" without the induced magnet part.
Mark scheme (5 marks)
- Increase the size (magnitude) of the current through the coil
- Increase the number of turns (coils) of wire
- Any one of: increase the cross-sectional area of the coil OR increase the length of the coil (1 mark for either)
- Iron is easier for magnetic field lines to pass through than air, so inserting an iron core concentrates / strengthens the magnetic field inside the solenoid
- The iron core becomes an induced magnet (it is magnetised by the solenoid's field), adding to the overall magnetic field strength
Key terms in this question
solenoid · magnetic field · iron core
Related
- All OCR A-Level Physics B: Advancing Physics (H557) revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
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