Read a magnetic field map
You will be able to: Interpret magnetic field direction, closed field lines and compass alignment.
What does a compass tell you about a field?
Place a small compass beside a bar magnet. Its north-seeking end tends to point along the local magnetic field. Move the compass and the direction changes: a field assigns a vector to each location.
A useful starting point: Current and conventional direction →
Words and symbols before equations
- Magnetic field B
- A vector field measured in teslas (T). Its direction is the direction a small compass’s north end tends to point.
- Dipole
- A magnetic arrangement with both north and south polarity.
- Field line
- A drawing whose tangent shows local field direction, not a particle trajectory.
- Tangent
- Direction along a curve at a particular point.
What this picture assumes
Qualitative bar-magnet schematic: external lines connect N to S and continue S to N inside. Spacing and arrow lengths are not a calibrated field-strength scale.
Read the picture in three steps
- Locate the labeled sources, system boundary or graph axes. Read the units before comparing values.
- N left, S right. Exterior arrows follow N to S; interior return direction is leftward. Schematic only, not a field-strength map.
- Check what the picture assumes below. Use the Explore task to predict one change before moving a control.
Connect the picture to the physics
Outside a bar magnet, field lines run from its north end toward its south end. Inside the magnet they continue from south toward north, making closed loops. Cutting a magnet produces smaller dipoles, not separate isolated poles.
Field arrows represent local directions. Stronger fields may be represented by denser field lines in a consistently drawn map, but the number of drawn lines is a drawing choice. Our schematic illustrates direction only and does not provide field strength from spacing.
A compass aligns by rotating; that does not mean the whole compass must travel along the line. Earth’s large-scale field is approximately dipolar, so a compass responds to Earth as well as nearby magnets and currents.
A worked example, step by step
A bar magnet has N on the left and S on the right. Predict the field direction along the line just outside each end.
- Use the rule: outside, away from N and toward S.
- Immediately left of the N end, the field points left.
- Immediately right of the S end, it also points left, toward S.
- Inside the magnet, the return direction is from S toward N; the lines close.
Magnetic field lines do not start at north and stop forever at south; they form closed loops.
What happens if you cut the magnet in half?
Compare with an explanation
Each piece still has both north and south polarity.
Predict. Change one thing. Explain.
Reverse the magnet’s poles and predict every arrow reversal. Trace one full loop through the outside and inside portions.
On narrow screens, swipe or scroll diagrams sideways to read all labels.
N left, S right. Exterior arrows follow N to S; interior return direction is leftward. Schematic only, not a field-strength map.
Qualitative bar-magnet schematic: external lines connect N to S and continue S to N inside. Spacing and arrow lengths are not a calibrated field-strength scale.
Explain what you noticed: Which quantity changed? Which stayed fixed? Use the relevant field, force, flux or energy relationship to justify your prediction.
Apply the idea to a fresh problem Practice →Show what you understand.
Two original questions are a starting check, not proof of mastery. Explain your choice before revealing the answer.
Original written challenge
4 points · self-check · not an official AP questionDraw a bar magnet with N left and S right. (a) Add an outside field arrow above it. (b) Add the return arrow inside. (c) State how a compass aligns. (d) Explain what cutting the magnet does.
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Compare with the answer and four-point rubric
- 1 point: Above the magnet, the outside path goes from left N toward right S.
- 1 point: Inside the magnet the return arrow goes right S toward left N.
- 1 point: Compass north end aligns with local B.
- 1 point: Each cut piece is another dipole with both poles.
Accept equivalent correct methods and explanations. This is a Refresh Kid teaching rubric, not an official AP scoring guideline.
Retrieve it before you reveal it.
RECALL 1What unit measures B?
Tesla, T.
RECALL 2Does a field line show a charged particle’s path?
No. It shows the field’s local direction.
RECALL 3How is Earth’s field approximated here?
As a magnetic dipole on large scales.
Revisit these tomorrow and a week later. Try a fresh problem and explain why the method applies.
Read a magnetic field map
- Outside a bar magnet: N → S.
- Inside: S → N; field lines form closed loops.
- A compass north end tends to align with local B.
Remember: Magnetic field lines do not start at north and stop forever at south; they form closed loops.
Conditions: Qualitative bar-magnet schematic: external lines connect N to S and continue S to N inside. Spacing and arrow lengths are not a calibrated field-strength scale.
Refresh Kid · AP Physics 2 Unit 4 (official Unit 12) · Objectives 12.1.A · Review edition
Framework, scope and review status
Mapped to College Board CED, Topic 12.1, objectives 12.1.A. CED effective Fall 2024, current PDF ©2026; checked September 16, 2026. Refresh Kid calls this the fourth AP Physics 2 unit; College Board numbers it Unit 12; the first unit in this course is official Unit 9. The lesson breakdown and questions are original Refresh Kid work, not official topic subdivisions.
Implementation and automated checks are separate from independent teacher review and observation of students. Both human review stages remain pending. This is a review edition, not a certified or validated assessment.
Optional further resource: College Board’s released questions and scoring guides. Papers can combine units; this link is an archive, not an assignment of every question to this lesson.
Our learn, explore, practice and recall sequence is informed by the IES learning guide. The exact Refresh Kid implementation has not been evaluated for learning effectiveness.
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