Describe a straight-wire magnetic field investigation
Plan a low-voltage controlled current experiment using filings for pattern and a compass for arrow direction.
Section P4.5.3 · Magnetic effect of a current
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
- Draw the straight-wire field model.
- Distinguish alignment from directed arrows.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which part of a compass indicates local field direction?
What you will learn
- Plan a low-voltage controlled current experiment using filings for pattern and a compass for arrow direction.
- Describe controlled low-voltage observations that distinguish field pattern, arrow direction and background effects.
Separate pattern from direction evidence
Pass a straight section vertically through a horizontal card and connect it to a suitable teacher-supervised low-voltage, current-limited circuit with a switch. Keep the wire and card fixed. With current on, gently distributed iron filings can reveal roughly circular alignment around the wire after gentle tapping.
Filings show an alignment axis, not arrow direction. Use a small compass at several marked positions to identify its north-tip direction and add tangential field arrows. Record the conventional-current direction relative to the card before using a grip-rule comparison.
Include controls and physical limits
Record compass behaviour with current off to identify background fields, then on under the same geometry. Keep the return lead and other magnets away enough to support the intended straight-section comparison. Repeat readings after the needle settles and account for a resultant field if the background is not negligible.
The teacher selects appropriate current limits and short observation times to avoid overheating the wire. Never use mains or a learner’s body to complete the circuit. Paper method work checks experimental reasoning; it does not establish practical competence or accurate apparatus use.
Sources for this lesson
Pause and explain
Filings align in circles but have no arrows. What additional observation sets arrow sense?
Worked example
Why are both filings and a compass useful in the wire investigation?
Show the worked solution
- Filings reveal an approximate pattern axis.
- The compass north tip gives local arrow direction.
- Together they distinguish circular alignment from its sense.
Answer Filings for pattern; compass for local direction.
Try a topic test
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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.
Write a straight-wire field investigation including apparatus, pattern observation and direction readings.
Give me a hint
Include a current-limited low-voltage loop and two different indicators.
Compare my reasoning
- Fix wire through card in the supervised current circuit.
- Observe filings alignment with current on.
- Use settled compass north-tip readings around the wire.
A controlled filings-plus-compass investigation of wire field pattern and direction.
Look for these in your work
- Describe both indicators.
- State the current and direction convention.
Explain why a current-off observation and controlled return-lead placement improve the investigation.
Give me a hint
The measured field can include more than the straight wire.
Compare my reasoning
- Current-off reveals persistent background.
- Return current also has a magnetic field.
- Holding its geometry controlled reduces confounding changes.
They help separate the investigated wire field from background and return-path contributions.
Look for these in your work
- Name both effects.
- Retain the resultant-field limitation.
A report uses filings only and claims clockwise arrow direction from their appearance. Critique it and suggest a correction.
Give me a hint
Alignment has two possible senses without a directional marker.
Compare my reasoning
- Filings show an axis along the pattern.
- That does not distinguish opposite arrow senses.
- Add compass north-tip readings with a stated current convention.
The direction claim is unsupported; add compass observations.
Look for these in your work
- Separate pattern from sense.
- Provide the missing direction evidence.
Common mistakes
- Filings alone do not identify arrow sense.
- Use a current-off background reading.
- Keep the return conductor’s field in view.
What can you explain now?
What does current-off compass observation provide?
Compare with the explanation
A background-field control
It helps identify contributions that remain when the investigated current is absent.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Describe controlled low-voltage observations that distinguish field pattern, arrow direction and background effects.
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.