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Better Equipped Practical Teaching Guides
An enhanced classroom guide to investigating magnetic fields using iron filings and a plotting compass, with scientific background, apparatus, detailed methods, field comparisons, troubleshooting, evaluation, differentiation, exam support and teacher preparation guidance.
Teacher note: This resource is provided as a practical support guide to accompany laboratory equipment. Teachers should adapt procedures and risk assessments to suit their curriculum requirements, examination board specifications and local laboratory policies.
Magnetic fields are invisible, but they underpin technologies ranging from electric motors and loudspeakers to generators, MRI scanners and compasses. This investigation gives students an opportunity to make a magnetic field visible and then determine its direction using two complementary techniques.
Students first place a sheet of paper or transparent material over a magnet and gently sprinkle iron filings across the surface. Light tapping allows the filings to become temporarily magnetised, form chains and align with the local magnetic field. Students then repeat the investigation using a plotting compass to establish magnetic field direction and trace representative external field lines.
Finally, students compare the fields produced by a single bar magnet, attracting poles, repelling poles and, as an optional extension, a horseshoe magnet.
A magnetic field is the region around a magnet where magnetic forces act. Iron filings become temporarily magnetised when placed in a magnetic field. Each filing behaves like a tiny magnet and rotates to align with the local field, forming chains that reveal the overall magnetic field pattern.
Iron filings do not, by themselves, show the direction of the magnetic field. Direction is established using a plotting compass. The north-seeking end of the compass needle points in the direction of the magnetic field at that point.
Repeat the investigation with two opposite poles facing, two like poles facing and, optionally, a horseshoe magnet.
For a fair comparison:
| Magnet arrangement | Shape of field | Strongest region |
|---|---|---|
| Bar magnet | Curved lines from N to S outside the magnet | Lines closest near the poles |
| Unlike poles | Lines connect across the gap from N to S; many lines pass through the gap | Between the magnets |
| Like poles | Lines bend away and do not join across the gap. For equal, symmetrically placed magnets, a neutral point may occur at the midpoint. | The strongest regions remain close to the pole faces |
| Horseshoe magnet, optional | Lines are approximately parallel in the central part of the gap and curve outside it | Between the poles |
Important clarification: for two identical like poles positioned symmetrically, the fields can cancel at the exact midpoint to produce a neutral point. This does not mean that the surrounding magnetic field is zero.
Students should record exactly what they see before attempting to explain it. Good observations use clear labels, accurate diagrams and repeat observations where appropriate.
Orientation of the facing poles: N–S compared with N–N or S–S.
Observed shape and direction of representative magnetic field lines, recorded using a compass plot and labelled diagram.
Repeat compass measurements at selected positions and compare the recorded direction.
The whole plot can also be repeated without moving or replacing the magnet arrangement to see whether a similar field pattern is obtained.
For the two-bar-magnet comparison, use the same magnets, separation, paper and plotting method so that only the facing-pole arrangement changes.
Expected conclusion: iron filings reveal the overall shape of a magnetic field pattern but do not establish its direction. The north-seeking end of a plotting compass points in the direction of the field. Outside a magnet, field-line arrows point from North to South. On a conventional field-line diagram, closer spacing represents a stronger field, although the number of lines drawn is not itself a measurement. Unlike-pole fields join across the gap, whereas like-pole fields bend apart and may produce a neutral point between equal, symmetrically positioned magnets.
Directly supported:
Later curriculum connections:
“Magnets only attract metal.”
Magnets can attract or repel other magnets. They also attract certain magnetic materials, including iron, steel, nickel and cobalt.
“Iron filings show magnetic force itself.”
Iron filings reveal the overall magnetic field pattern but do not show which direction the field points.
“Field lines are real objects.”
Field lines are a model used to represent a magnetic field.
“Field lines can cross.”
Field lines never cross because the magnetic field has only one direction at any point.
“More lines drawn means a stronger magnet.”
The number of lines is chosen by the person drawing the diagram. It is the relative spacing within a correctly drawn field-line model that represents field strength.
| Mistake | Consequence | How to fix it |
|---|---|---|
| Too many filings | Pattern unclear | Sprinkle lightly |
| Heavy tapping | Filings jump randomly | Tap gently |
| Compass touches the magnet, is not flat or has a damaged pivot | Needle cannot turn freely | Reposition it flat and check that it moves freely |
| Forgetting arrows | Direction is not shown | Always add arrows to plotted field lines |
| Joining points inaccurately | Incorrect field representation | Draw smooth curves following measured points |
| Problem | Possible cause | Solution |
|---|---|---|
| Iron filings do not form a clear pattern | Too many filings or paper not tapped gently | Use a thin layer and tap the paper lightly. |
| Iron filings clump together | Damp filings or filings have become magnetised | Use clean, dry filings from a lidded shaker. Replace contaminated or persistently clumped filings. Do not blow on them or separate them by hand. |
| Magnetic field pattern looks distorted | Magnet moved during the investigation | Keep the magnet still and secure it with tape if necessary. |
| Plotting compass needle will not settle | Damaged pivot, compass not flat, vibration, nearby steel or magnets, electrical current or a moving magnet arrangement | Check that the compass moves freely, place it flat and remove possible sources of interference. |
| Compass points in unexpected directions | Nearby magnetic material or another magnet affecting the field | Remove nearby magnetic objects and use only one magnet arrangement at a time. |
| Field lines are inaccurate | Compass marks joined incorrectly or arrows omitted | Mark compass positions carefully and draw smooth curves with arrows showing North to South. |
| Weak magnetic field observed | Magnet may be weak or damaged | Check the magnet away from the main setup using a known working compass and use the same suitable magnet throughout comparisons. |
| Paper moves during the investigation | Paper not secured | Secure the paper and magnet to a non-magnetic board using tape. |
| Iron filings spill onto the bench | Shaker used too vigorously | Shake gently over the centre of the paper and collect spillages with a soft brush. |
| Students cannot identify the strongest part of the field | Too few plotted lines or unevenly distributed filings | Use the filings pattern for qualitative comparison or a Hall probe/magnetometer for a quantitative extension. A plotting compass alone determines direction. |
| Filings stick to the magnet | Magnet exposed or paper damaged | Stop, cover the magnet before reuse and clean it using the approved technician method. |
Support: identify North and South poles and add correct arrows to a field diagram.
Core: trace four representative field lines and compare like-pole and unlike-pole arrangements.
Stretch: evaluate the limitations of iron filings and a compass, design an investigation using a Hall probe and explain the like-pole neutral point.
Why are field lines closest together near the poles?
Because the magnetic field is strongest there, and closer line spacing on a field-line diagram represents greater field strength.
Why do field lines never cross?
Because the magnetic field has only one direction at a particular point.
How does an electromagnet differ from a permanent magnet?
An electromagnet produces a magnetic field when current flows and can be switched off or varied, whereas a permanent magnet remains magnetic without a current.
Why does a compass align with the magnetic field?
The magnetic field exerts a turning effect on the magnetised compass needle.
Where would the magnetic field be strongest on a horseshoe magnet?
In the gap between the North and South poles, where the field lines are most closely spaced.
Iron filings: demonstrate the magnet beneath the paper, a thin layer of filings, gentle tapping, safe recovery and why direction arrows cannot be inferred from filings alone.
Plotting compass: demonstrate the magnet outline and pole labels, keeping the compass flat, marking the north-seeking end and using the head-to-tail method to trace a smooth N → S field line.
Two-magnet comparison: demonstrate a fixed gap using tape or spacers and remove unused magnets from the surrounding area.
| Activity | Suggested time |
|---|---|
| Introduction | 20 min |
| Iron filings investigation | 30–40 min |
| Compass plotting | 30 min |
| Compare different magnets | 20 min |
| Plenary | 10 min |
Support: provide a partially completed magnet outline and first compass position.
Core: students independently plot representative field lines.
Stretch: evaluate the limitations of the methods and propose a quantitative Hall-probe investigation.
Indicative residual risk: low when standard school bar magnets and small quantities of iron filings are used with suitable controls. The activity should be reassessed if high-strength or neodymium magnets are used, or where an implanted medical device may be affected.
Important: this model guidance supports, but does not replace, a local risk assessment by the school or department.
| Question | Expected answer |
|---|---|
| What do iron filings show? | The overall magnetic-field pattern, but not its direction. |
| Which way do field lines point? | From North to South outside the magnet and from South to North inside it. |
| Where is the field strongest? | Where field lines are closest together, often near the pole faces. Between close unlike poles, the gap may also be a strong-field region. |
| Why use a plotting compass? | To determine the direction of the magnetic field at a point. |
| Why don't field lines cross? | Because the magnetic field has only one direction at any particular point. |
If asked to draw a magnetic field, include smooth, non-crossing field lines, arrows showing North → South outside the magnet, and closer line spacing near stronger regions such as the pole faces.
The filings become temporarily magnetised and align with the local magnetic field, forming chains that reveal the overall shape of the field.
Iron filings reveal the overall field pattern, while the north-seeking end of a plotting compass establishes the direction of the field at selected points.
No. Filing concentration can provide a qualitative indication when the quantity and distribution of filings are controlled, but the number of filings is not a direct measurement of magnetic-field strength.
A tilted compass may prevent the needle from rotating freely and can produce inaccurate readings.
For two equal, symmetrically positioned like poles, the fields may cancel at the exact midpoint to produce a neutral point. The surrounding magnetic field is not zero.
No. A plotting compass establishes field direction. A Hall probe or magnetometer can be used if quantitative magnetic-field measurements are required.
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This guide was written and reviewed by Better Equipped's technical team, drawing on experience supplying practical science equipment to schools, colleges, laboratories and science departments throughout the UK. If you have feedback on this guide, please contact us.
This page reflects the expanded Magnetism: Mapping Magnetic Fields with Iron Filings and a Plotting Compass practical guide.
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