Determine magnetic field direction using a north pole
Use the north-seeking compass tip as a local direction indicator and distinguish it from the south end.
Section P4.1 · Simple phenomena of magnetism
Exam scope and assessment notes
Mapped to the named 0973 section using teaching from the corresponding single-science course. A 0973-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. This qualification reports two grades on the 9–1 scale; confirm the entry route for your zone and exam series.
Before you begin
- Recall the external bar-magnet pattern.
- Identify the labelled north tip of a compass.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which pole is used in the magnetic-field direction definition?
What you will learn
- State the north-pole force definition of field direction.
- Read a compass needle tangent to a local field line.
Use the defined test-pole direction
The magnetic field direction at a point is the direction of the force on a north pole placed there. The south pole experiences the opposite force in the simple test-pole model. Always name the test pole; saying only 'a magnet points this way' leaves the convention ambiguous.
A small freely turning compass aligns with the local resultant field. Its north-seeking tip indicates the field direction along the tangent to a field line. The needle should not be drawn pointing toward the geographic top of every page irrespective of the nearby magnet.
Read the local position
Near the outside of a bar magnet's north end, the field points away from N. Near its south end, it points toward S. Above a bar labelled N left and S right, a suitable middle location has a field direction toward the right in the schematic pattern.
A real compass also responds to Earth's field and nearby magnetic objects. The ideal bar-magnet sketch assumes the stated magnet dominates. At a point where the ideal resultant field is zero, there is no uniquely defined direction from that zero field alone.
Sources for this lesson
Pause and explain
A north compass tip points left. Which way is the local magnetic field?
Worked example
At a point the north tip of a small compass points east in the stated dominant field. Give the local field direction.
Show the worked solution
- The north tip indicates the defined direction.
- It points east.
- The local field direction is east.
Answer East; the force on a north test pole would be east.
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.
State the definition of magnetic field direction.
Give me a hint
Specify the pole whose force determines the direction.
Compare my reasoning
- Consider a magnetic test pole at the point.
- Use its north pole.
- The force direction on that north pole is the field direction.
The direction of force on a north pole at that point.
Look for these in your work
- Name north.
- Keep the location explicit.
A compass north tip points south-west. Give the field direction and the south-tip orientation.
Give me a hint
Read the named tip before reversing for the other end.
Compare my reasoning
- The field follows the north tip.
- It is south-west.
- The opposite south tip points north-east.
Field south-west; south tip north-east.
Look for these in your work
- Use the correct tip.
- Distinguish field and opposite end.
An ideal pair of fields cancels at one point. Does its field alone specify where a compass must point?
Give me a hint
A zero vector has no preferred direction.
Compare my reasoning
- The stated resultant is zero.
- Zero has no unique direction.
- Other real fields may still orient an actual compass.
No; the ideal zero field has no unique direction.
Look for these in your work
- Use the ideal condition.
- Separate other real influences.
Common mistakes
- Use the north tip.
- Read the local tangent, not the page orientation.
- A zero field has no unique direction.
What can you explain now?
Does the south tip define magnetic field direction?
Compare with the explanation
No.
The convention uses the force on a north pole; the south end points oppositely.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Use the north-pole convention and read local compass directions with the stated field conditions.
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.