Draw the external field pattern around a bar magnet
Construct the curved field-line pattern with north-to-south arrows and consistent directions.
Section P4.1 · Simple phenomena of magnetism
Mapped to the named 0654 section using teaching from the corresponding single-science course. A 0654-specific review of every Core and Supplement objective is still in progress. Written practice is self-checked; it does not demonstrate supervised laboratory competence or award an official Cambridge grade.
Before you begin
- Name the magnet's two poles.
- Interpret a line as a field representation.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which information should be labelled before drawing the external curves?
What you will learn
- Draw a bar-magnet field pattern.
- Show external field direction from N toward S.
Build the pattern around the poles
Draw and label the bar magnet first. Representative external field lines leave its north-pole surface, curve around the bar and enter its south-pole surface. Draw lines on both sides of the magnet rather than a single straight arrow through empty space.
Put arrowheads on the curves to show N to S outside the bar. At a given point the tangent to the field line gives the local direction. Lines do not cross in the ordinary non-zero field: crossing would assign more than one field direction at one point.
Keep the complete direction convention
The external N-to-S convention is the part ordinarily requested in a bar-magnet sketch. The complete lines continue inside the magnet from S toward N, forming closed paths. Do not change the external arrows just because the internal continuation has the opposite direction.
A sketch is schematic, with no automatic numerical field scale. Avoid copying a curve that passes straight through the drawn bar as though it were an external path. Label poles and arrows clearly enough that another learner can follow the direction without guessing from colour.
Sources for this lesson
Pause and explain
Which external arrow convention is correct?
Worked example
A bar has N at the left and S at the right. State the direction along a line curving above it.
Show the worked solution
- The curve leaves the north end.
- It passes above the bar toward the south end.
- Its external arrow direction is N to S.
Answer Along the external upper curve, from the left N end toward the right S end.
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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.
Describe the essential features of a bar-magnet field sketch.
Give me a hint
Include pole labels, curved paths and arrows.
Compare my reasoning
- Label N and S on the bar.
- Draw non-crossing external curves around it.
- Place arrows from N toward S.
Labelled bar, curved non-crossing external lines, N-to-S arrows.
Look for these in your work
- Include both sides of the bar.
- Make directions explicit.
A sketch has correctly shaped curves but arrows from S to N outside. Identify the correction.
Give me a hint
Check direction separately from the curve shape.
Compare my reasoning
- The shape alone does not complete the answer.
- External lines must leave N and enter S.
- Reverse the external arrows.
Reverse the external arrows to N to S.
Look for these in your work
- Keep the valid curve pattern.
- Correct the direction rather than swapping unexplained colours.
Explain why crossing lines at a point with a non-zero field are not a valid ordinary field sketch.
Give me a hint
Use the tangent-direction interpretation at the intersection.
Compare my reasoning
- Each field line gives a local direction.
- Crossing curves would give two different directions there.
- A non-zero field has one resultant direction at that point.
A crossing would assign inconsistent local directions.
Look for these in your work
- Refer to the same point.
- Use the non-zero-field condition.
Common mistakes
- External arrows run N to S.
- Label poles as well as using colour.
- Do not cross non-zero field lines.
What can you explain now?
Which way do external lines enter a south pole?
Compare with the explanation
Toward the south pole.
External lines leave N and enter S in the field-direction convention.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Draw a labelled bar-magnet pattern with correct external arrows and explain the line convention.
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.