Plot bar-magnet field lines with a compass
Build a continuous path from repeated local compass readings and include pole direction, controls and recording limits.
Core and Extended; Physics 0625, 2026–2028. Follow the selected magnetism or charge section. Field lines are explanatory models, not material threads or measured force values. Written practice is self-checked.
Before you begin
- Use the compass north-tip convention.
- Draw local tangents rather than arbitrary straight joins.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which compass feature supplies a field direction?
What you will learn
- Describe a plotting-compass method around a bar magnet.
- Explain how repeated readings determine both pattern and direction.
Record a sequence of local directions
Place a bar magnet on paper, trace its position and label its poles. Put a small plotting compass at a chosen point, allow it to settle, and mark the positions of both needle ends. Record which marked end is the north tip so the reading retains direction.
Move the compass so its tail is near the previous north-tip mark and repeat. Join the successive marks with a smooth curve and add arrows following the north tip. Start at several different positions to build a pattern rather than tracing one line and inventing the others.
Control the surroundings and sampling
Keep the magnet fixed while collecting a path. Remove nearby magnetic objects and compare the local pattern with the background compass direction where appropriate. A compass needs room to settle; touching or crowding it against the magnet can distort the measurement.
Large step sizes may miss curvature. Several small local readings give stronger evidence for a smooth path than a long straight join between distant points. The written method explains a practical task; actual competent use requires supervised apparatus work.
Pause and explain
Why should the magnet remain fixed during a plotted path?
Worked example
Explain how a compass reading is used to continue a plotted line.
Show the worked solution
- Mark both needle ends after settling.
- Move the compass toward the previously marked north-tip position.
- Repeat and join the successive directions smoothly.
Answer Successive local north-tip readings build the directed field line.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
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.
Outline a plotting-compass method for a bar magnet.
Give me a hint
Include tracing, settling, marking and repeated movement.
Compare my reasoning
- Trace and label the fixed magnet.
- Place the compass and mark its settled needle ends.
- Move in small steps, repeat and draw directed smooth curves.
Use repeated local north-tip readings around a fixed labelled magnet.
Look for these in your work
- Give a repeatable sequence.
- Include direction arrows.
A learner marks directions only at two far-apart points and joins them straight. Explain a limitation.
Give me a hint
A field line can curve between sampled points.
Compare my reasoning
- Only two local directions were recorded.
- Intermediate curvature is not resolved.
- More closely spaced readings are needed.
The straight join may miss curvature; take more local readings.
Look for these in your work
- Identify missing information.
- Suggest a relevant improvement.
The magnet is rotated halfway through plotting. Can all marks be treated as one field line?
Give me a hint
Check whether the source arrangement stayed the same.
Compare my reasoning
- Rotation changes the field pattern relative to the paper.
- The later readings sample a different arrangement.
- Repeat with a fixed magnet rather than merging the paths.
No; the readings belong to two source arrangements.
Look for these in your work
- Identify the changed variable.
- Keep a single consistent arrangement.
Common mistakes
- Keep the magnet fixed.
- Record which tip is north.
- Use enough steps to resolve curvature.
What can you explain now?
Do compass readings reveal direction as well as a pattern?
Compare with the explanation
Yes.
The identified north tip supplies the local direction convention.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Describe a complete compass-plotting method and explain why local sampling and controls matter.
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.