A student investigates the magnetic field produced by a solenoid connected to a direct current (d.c.) supply. Explain how the magnetic field inside and outside the solenoid is similar to, and different from, the magnetic field of a bar magnet.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
The magnetic field outside a solenoid has the same shape as that of a bar magnet: field lines emerge from one end and re‑enter the other, giving the solenoid a north pole at one end and a south pole at the other.
Inside the solenoid the field is strong and uniform, unlike the field of a bar magnet which is weaker and varies with distance from the pole.
The field strength of a solenoid can be increased by raising the current or by adding more turns of wire, whereas a permanent bar magnet’s field cannot be altered in this way.
Finally, the direction of the solenoid’s field (and which end is north or south) can be reversed by reversing the current direction – a feature that permanent bar magnets lack.
Inside the solenoid the field is strong and uniform, unlike the field of a bar magnet which is weaker and varies with distance from the pole.
The field strength of a solenoid can be increased by raising the current or by adding more turns of wire, whereas a permanent bar magnet’s field cannot be altered in this way.
Finally, the direction of the solenoid’s field (and which end is north or south) can be reversed by reversing the current direction – a feature that permanent bar magnets lack.
Examiner tips
- Show the two key differences – uniform inside vs. non‑uniform outside; and the ability to change field strength and polarity by current. Use the words "uniform", "strength", "current", "reversed".
- Include the point about the field lines emerging from one end and re‑entering the other to link to the bar magnet shape.
Common mistakes
- Confusing the inside field of the solenoid with that of a bar magnet; forgetting that the solenoid’s field is uniform. Failing to mention that the field strength can be varied by changing current or turns. Using vague terms like "strong" without specifying that it is uniform and that it can be reversed.
Mark scheme (5 marks)
- The magnetic field outside the solenoid has the same shape as the field of a bar magnet (field lines emerge from one end and re-enter the other)
- The solenoid has a north pole at one end and a south pole at the other, just like a bar magnet
- The field inside the solenoid is strong and uniform, unlike the field of a bar magnet
- The strength of the solenoid's magnetic field can be increased by increasing the current or increasing the number of turns of wire, unlike a permanent bar magnet
- The direction of the magnetic field (and which end is north/south) can be reversed by reversing the direction of the current, which is not possible with a permanent bar magnet
Key terms in this question
Related
- All AQA A-Level Physics (7408) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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