Trace the magnetic field around a straight current-carrying wire
Draw concentric field lines in a cross-section and use a stated conventional-current direction to set their arrow sense.
Section P4.5.3 · Magnetic effect of a current
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
- Use into-page/out-of-page symbols.
- Identify a compass north tip and a field direction.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What does a dot represent for a page-normal current?
What you will learn
- Draw concentric field lines in a cross-section and use a stated conventional-current direction to set their arrow sense.
- Draw circular tangents using a conventional-current and page convention, with other fields kept explicit.
Use a cross-section normal to the wire
A straight current-carrying wire produces a magnetic field around it. In a cross-section perpendicular to a sufficiently long straight section, the field lines are concentric circles centred on the wire. The local field direction is tangent to a circle, not radially outward from the conductor.
Use conventional current for the right-hand grip rule: point the right thumb along current and curled fingers give the field sense. Current out of the page, represented by a dot, gives anticlockwise field arrows; current into the page, represented by a cross, gives clockwise arrows. The page convention must be stated.
A compass gives local direction
A small compass’s north-seeking tip aligns with the local resultant magnetic field. If the wire field is dominant, its direction is approximately tangent to the circular pattern. Earth’s field and other sources can alter the reading, so a compass does not necessarily point along the ideal wire-only pattern in every arrangement.
Field lines are a representation, not circular material tubes or the paths of charges inside the wire. The straight-wire drawing applies to the stated section; bends and nearby return conductors can change the combined field. Strength comparisons are separately developed in Supplement lessons.
Sources for this lesson
Pause and explain
For current out of the page, what is field direction directly above the wire?
Worked example
Conventional current comes out of a page-normal wire. Give field arrow sense and direction at the point directly right of the wire.
Show the worked solution
- Use the dot convention for outward current.
- The right-hand grip gives anticlockwise circulation.
- At the right-hand point, the tangent points upward.
Answer Anticlockwise field; upward at the right-hand point.
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.
Sketch field lines and arrow sense for conventional current out of a page-normal straight wire.
Give me a hint
Use a dot and the right-hand grip.
Compare my reasoning
- Mark the wire by a dot.
- Draw concentric circles centred on it.
- Add anticlockwise arrows tangent to the circles.
Concentric anticlockwise field circles.
Look for these in your work
- Centre the circles correctly.
- Keep arrows tangent.
For outward current, state local field directions at right, top, left and bottom positions.
Give me a hint
Follow an anticlockwise tangent around the wire.
Compare my reasoning
- At right the tangent is up, and at top it is left.
- At left it is down.
- At bottom it is right.
Right: up; top: left; left: down; bottom: right.
Look for these in your work
- Use the fixed current convention.
- Identify all four local directions.
A compass near the wire does not exactly follow a circle, and a student says the wire model is disproved. Explain the missing field condition.
Give me a hint
The compass follows the sum of fields at its location.
Compare my reasoning
- The ideal drawing isolates the straight-wire field.
- Earth and other conductors can add contributions.
- The resultant can have a different local direction.
Account for other fields before rejecting the wire-only pattern.
Look for these in your work
- Distinguish model and observation.
- Name the resultant-field effect.
Common mistakes
- Field direction is tangential, not radial.
- Use conventional current, not electron drift.
- Real compass readings follow the resultant field.
What can you explain now?
Where do the straight-wire field circles have their centre?
Compare with the explanation
At the wire cross-section
The concentric representation is centred on the stated current-carrying straight section.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Draw circular tangents using a conventional-current and page convention, with other fields kept explicit.
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.