Describe magnetic fields and field lines
Identify sources of magnetic fields and represent their direction and pattern with field lines.
A Level; Cambridge International Physics 9702, 2025–2027 syllabus, Topic 20. These original explanations and calculations are study aids; full timed assessment and human subject-teacher review remain outstanding.
Before you begin
- Use current, charge, force, velocity, vectors, area and SI units; distinguish conventional current from electron flow.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which can produce a magnetic field?
What you will learn
- Recognise fields produced by moving charges and permanent magnets
- Represent magnetic fields using field lines
- Interpret field direction and relative strength
Moving charges and magnets produce magnetic fields
A magnetic field is a field of force. It can be produced by moving electric charges, including an electric current, and by permanent magnets. A compass needle turns to align with the local magnetic field direction.
A magnetic field can exert a force on another magnet, a moving charge or a current-carrying conductor. A stationary isolated charge does not experience magnetic force merely because it is in a magnetic field.
Field lines show direction and pattern
Magnetic field lines are drawn so that the tangent at a point gives the field direction. Outside a bar magnet, lines go from north to south; inside the magnet they return from south to north, forming continuous loops.
Closer lines indicate a stronger field in a qualitative diagram. Field lines do not cross: the field at one point has one direction. Their spacing represents relative strength, not a count of separate magnetic threads.
Pause and explain
What does the tangent to a magnetic field line show at a point?
Worked example
Describe the field-line pattern around a bar magnet and how a compass shows its direction.
Show the worked solution
- Follow the direction outside and inside the magnet.
- Use a compass's north-seeking end to define field direction.
- Mention line spacing as a qualitative strength indicator.
Answer Outside the magnet, lines leave north and enter south; they continue through the magnet to form loops. A compass's north-seeking end points along the local field, and closer lines indicate a stronger field.
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
A compass is placed at several points around a bar magnet. Explain how its north-seeking end can be used to map field direction.
Give me a hint
At each position, relate the compass orientation to the local field.
Compare my reasoning
- Place the compass at a point and allow the needle to settle.
- Mark the direction in which its north-seeking end points.
- Repeat at nearby points to trace a field-line pattern; the local tangent gives field direction.
The compass's north-seeking end aligns with the local field, allowing the direction to be marked point by point.
Look for these in your work
- Compass settling described
- North-seeking end interpreted
- Local direction mapped
Two regions in a magnetic-field diagram have different line spacing. What can be concluded from the closer spacing?
Give me a hint
Interpret field-line density qualitatively.
Compare my reasoning
- The lines are closer together in one region.
- Closer field lines represent greater field strength in the diagram.
- The comparison is qualitative unless a scale is supplied.
The region with closer lines represents the stronger magnetic field qualitatively.
Look for these in your work
- Spacing compared
- Relative strength inferred
- Qualitative limit stated
A student draws two magnetic field lines crossing. Explain why this representation is invalid.
Give me a hint
Consider the field direction at the crossing point.
Compare my reasoning
- Each line's tangent would specify a field direction at the crossing.
- Crossing lines would give two different directions at one point.
- A magnetic field has one local direction, so the lines cannot cross.
Field lines cannot cross because that would assign two directions to the magnetic field at one point.
Look for these in your work
- Tangent meaning stated
- Conflict identified
- Single direction applied
Common mistakes
- Drawing magnetic field lines that begin or end in empty space
- Treating field lines as paths that a compass physically follows
- Claiming a stationary charge alone is a source of magnetic field
- Allowing field lines to cross
What can you explain now?
Which can produce a magnetic field?
Compare with the explanation
Moving charges and permanent magnets
Both electric current and permanent magnets create magnetic fields.
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Make it stick
How do magnetic-field lines represent direction and relative strength?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.