Use iron filings to reveal a magnetic field pattern
Explain the alignment of temporarily magnetised filings and why their pattern does not itself provide arrow direction.
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
- Describe induced magnetism in iron.
- Use a compass to identify field direction.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Why can iron filings align near a magnet?
What you will learn
- Describe the iron-filings method for a bar magnet.
- Distinguish an observed pattern from a directed field map.
Make the pattern visible
Place a sheet over a fixed bar magnet and spread a thin layer of iron filings across it. Gentle tapping helps the filings turn and form a pattern. In the magnetic field the filings become temporarily magnetised, and their alignment follows the local field pattern.
The sheet separates the filings from the magnet and keeps the arrangement manageable. A thin layer is easier to interpret than a dense heap. Large piles and interactions between filings can obscure the pattern, so the visible strands are an approximate experimental aid.
Add direction separately
A filing alignment shows an axis, not which way along it is north-to-south. Use a compass with a known north tip to add direction arrows. Do not claim that sprinkling filings alone establishes the sense of every field line.
Filings are material test pieces that respond to the field; they are not the field itself. A photograph with missing filings in one place does not prove the field there is zero. Poor distribution, friction and limited sensitivity can also leave an apparently empty patch.
Sources for this lesson
Pause and explain
What do filings alone fail to identify?
Worked example
An iron-filings photograph shows curves around a bar magnet but no arrows. What measurement adds direction?
Show the worked solution
- The filings show the aligned pattern.
- A known north compass tip gives the local sense.
- Add N-to-S external arrows using that reading.
Answer A compass reading, with the north tip identified.
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Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
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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 basic iron-filings method.
Give me a hint
Include the fixed magnet, separating sheet and gentle movement.
Compare my reasoning
- Place a sheet over the fixed magnet.
- Spread a thin layer of filings.
- Tap gently and observe the alignment pattern.
Thin filings on a sheet over the magnet reveal an approximate pattern after gentle tapping.
Look for these in your work
- Include the arrangement.
- Explain that filings align.
Explain why a compass is useful after obtaining the filing pattern.
Give me a hint
Distinguish a line's axis from its arrow direction.
Compare my reasoning
- Filings indicate alignment.
- They do not identify which way is the field direction.
- The compass north tip supplies that direction.
The compass supplies the direction missing from the filing pattern.
Look for these in your work
- Name the north tip.
- Separate pattern and direction.
A filing photograph has a blank patch. Is a zero field proven there?
Give me a hint
Consider distribution and response as well as the actual field.
Compare my reasoning
- Filings may not cover the patch evenly.
- Friction or limited response may conceal alignment.
- A blank patch alone is insufficient evidence of zero field.
No; the material distribution and detection limits must be considered.
Look for these in your work
- Give another explanation.
- Avoid treating the photograph as a numerical field map.
Common mistakes
- Filings show pattern without arrow sense.
- Avoid interpreting clumps as a calibrated field map.
- No visible filing is not proof of zero field.
What can you explain now?
Are iron filings themselves magnetic field lines?
Compare with the explanation
No.
They are material pieces responding to the field and representing its pattern approximately.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Explain the filing method and why direction and measurement limits need separate evidence.
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.