A student investigates the magnetic field produced by a current-carrying wire. She increases the current through the wire and also moves the wire closer to a plotting compass. Explain how the strength of the magnetic field around the wire changes in each case, and describe what effect this has on the deflection of the plotting compass needle.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A plotting compass is placed near a straight wire connected to a power supply. The student changes two variables separately: the size of the current in the wire and the distance between the wire and the compass.
Model answer (5 marks)
Increasing the current increases the magnetic field strength around the wire, because B∝I. A stronger field produces a larger torque on the compass needle, so the needle deflects more.
Moving the wire closer to the compass reduces the distance r between the wire and the needle. Since B∝1/r, the field at the needle becomes stronger, giving a larger deflection.
In both cases the needle aligns with the direction of the magnetic field produced by the current in the wire.
Moving the wire closer to the compass reduces the distance r between the wire and the needle. Since B∝1/r, the field at the needle becomes stronger, giving a larger deflection.
In both cases the needle aligns with the direction of the magnetic field produced by the current in the wire.
Examiner tips
- Use the proportionality B∝I and B∝1/r to justify changes; mention the torque on the needle; state that the needle aligns with the field direction.
- Show the two separate effects (current and distance) clearly and link each to deflection.
- Use the exact wording from the mark scheme where possible.
Common mistakes
- Confusing the direction of the field with the direction of the current; not stating that the needle aligns with the field.
- Lumping the two effects together instead of treating them separately; missing the 1/r dependence for distance.
Mark scheme (5 marks)
- Increasing the current increases the strength of the magnetic field around the wire
- A stronger magnetic field causes a greater deflection of the compass needle
- Moving the wire closer to the compass decreases the distance between the wire and the compass
- Moving the wire closer therefore causes a greater deflection of the compass needle
- The plotting compass needle aligns with / shows the direction of the magnetic field produced by the current in the wire
Key terms in this question
magnetic field · current · plotting compass
Related
- All Edexcel A-Level Physics (9PH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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