Investigating Magnetism and Magnetic Fields
Sprinkle iron filings on a sheet of paper over a bar magnet and the magnetic field appears. It is the best image in Grade 10 physics.
But it only tells you half the story. Iron filings show the shape of a field. They do not show which way it points. For that you need a compass, and that is why this chapter has two experiments rather than one.
Poles, and why you can never have just one
Every magnet has two poles, north and south. Like poles repel, opposite poles attract.
Cut a magnet in half and you do not get a north piece and a south piece. You get two smaller magnets, each with both poles. Cut those and it happens again, all the way down.
This is the sharpest difference between magnetism and electric charge, and it is worth holding onto because the very next chapter is about charge:
| Can you isolate one on its own? | |
|---|---|
| Electric charge | Yes. A positively charged object is a real, ordinary thing |
| Magnetic pole | No. Every fragment of a magnet has both |
What a magnetic field actually is
A magnetic field is a region of space where a magnet or a ferromagnetic material will experience a force.
It fills three dimensions, though we always draw it flat on paper. Field lines run from north to south outside the magnet, they never cross, and the closer together they are the stronger the field.
Put two magnets side by side and the fields add. That is superposition, and if you have already done transverse pulses on a spring it is exactly the same idea in an entirely different context.
Where this fits in the curriculum
| Subject | Physical Sciences |
|---|---|
| Grade | 10 |
| Term | 3 |
| Topic | Electricity and magnetism |
| Status | Class experiments. Not a formal assessment |
| Marks | 40 on our worksheet. None is prescribed |
The apparatus
| Item | Qty |
|---|---|
| Bar magnets with keeper, a pair | 1 pr |
| Compasses | 4 |
| Iron filings, in a shaker | 1 |
| Plain paper | Several sheets |
Four compasses, not one. With one you move it to twenty positions and draw twenty arrows. With four the class sees the whole field at once, which is the point.
And keep the magnets away from the compasses when you are not using them. A bar magnet held against a compass can reverse the needle's polarity permanently, after which that compass reads backwards and the whole experiment gives exactly the wrong answer.
Experiment 1: attraction, repulsion and the field patterns
Part A: two magnets
- One magnet flat on the table. Bring one end of the second towards it
- Watch what the first magnet does. Record it
- Turn the second magnet round and repeat
One way they attract and pull together. The other way the first magnet slides away.
Do not let them snap together. Alnico is brittle and it chips.
Part B: three patterns in iron filings
- One magnet under a sheet of paper. Sprinkle filings on top and tap gently
- Two magnets a few centimetres apart, north facing north. Paper on top, sprinkle again
- Turn one round so north faces south. Sprinkle again
| Arrangement | What appears |
|---|---|
| One magnet | Loops running from one pole round to the other, densest at the poles |
| North facing north | A clear gap between them, with a bare patch in the middle |
| North facing south | Lines running straight across the gap, joining the two magnets into one |
Sprinkle, never pour. Far less is needed than anyone expects, and too many filings give you a grey smear instead of a pattern.
Never dip a magnet into the jar. The filings weld on and are almost impossible to remove. Always work through paper.
The neutral point, which is the best thing in this practical
In the north-facing-north pattern, look at the middle of the gap. There is a spot where the filings do not line up in any direction at all. They lie flat, or the paper stays bare.
That is a neutral point. At that exact place the field from one magnet and the field from the other are equal in strength and opposite in direction, so they cancel completely. The resultant field is zero, and with no field there is nothing to line a filing up.
It is superposition, made visible, in iron. If you have already watched two pulses cancel where they met on a spring, this is the same principle in a form you can photograph.
You can find it with a compass too. Move a compass into the gap and there is a position where the needle cannot decide and swings freely.
Experiment 2: which way does it point?
Ten minutes, and it is the half most classes never do.
- Bar magnet on a clean sheet of paper
- Put a compass near it and draw an arrow next to it in the direction the needle points
- Move it around, or use all four at once, until you have arrows all round the magnet
- Look at the pattern of arrows
Every arrow points away from the north pole and towards the south pole. So outside the magnet, the field runs from north to south.
A compass needle is itself a small bar magnet, free to rotate. It is not detecting the field with some sensor; it is being turned by it until it lines up. Saying that out loud stops learners treating the compass as a black box.
If it does not work
| What you see | What caused it |
|---|---|
| A compass points the wrong way | Its polarity has been reversed by storage against a magnet. Check every compass before the lesson |
| The filings are a grey smear, not a pattern | Too many. Sprinkle, do not pour, and tap the paper gently |
| Filings stuck all over the magnet | The magnet was dipped in the jar or used without paper. Very hard to remove |
| The magnets barely attract each other | Stored without the keeper. Alnico weakens over months with its poles open |
| No clear gap in the north-north pattern | The magnets are too far apart or too close together. A few centimetres |
| A magnet chipped | They snapped together. Ease them, never release them |
| All the compasses point the same way whatever you do | They are reading the Earth's field. The magnet is too far away or too weak |
The failure worth keeping
A group with a reversed compass will draw a complete, neat, internally consistent field pointing entirely the wrong way.
That is what makes it a good lesson. A systematic instrument error produces tidy wrong data, not messy data, so nothing about the result looks suspicious. Ask them to check the compass against a known direction and they find it themselves.
The keeper is not packaging
A pair of Alnico bar magnets ships with a small steel bar. That is the keeper, and it is part of the equipment.
Stored with their poles open, Alnico magnets gradually lose strength. The keeper bridges the poles and stops that. Most schools throw it away with the box, and then wonder a year later why the magnets are weak.
How the 40 marks are made up
| Section | Marks |
|---|---|
| Attraction and repulsion | 6 |
| Field patterns, three sketches | 12 |
| The neutral point, explained | 6 |
| Direction, from the compasses | 8 |
| Field line rules | 5 |
| Conclusion | 3 |
No mark allocation is prescribed. The worksheet and this split are ours.
The mark most often dropped is leaving the arrows off the field lines. A field drawn without direction is half a field, and supplying that direction is the entire purpose of Experiment 2.
Close behind: drawing field lines that cross. They never do.
If you have time
Find the neutral point with a compass. Two north poles facing each other, then move a compass into the gap until the needle stops committing. That locates it rather than inferring it.
Test what blocks a magnetic field. Paper, plastic, aluminium foil, a hand, a steel ruler. Only the steel makes a difference, which surprises everybody who expects the foil to work.
Cut a fridge magnet strip in half with scissors. Each half still has both poles. So does each half of those.
Free worksheet and marking memo
Both free, no sign up, straight to the PDF.
- Learner worksheet, 40 marks, with the attraction table, three boxes for the field sketches, the neutral point questions and the compass arrows
- Marking memorandum, with what each sketch must show, and the five places learners most often drop marks
Related practicals
- Electrostatics, Grade 10. The next chapter, and charge behaves differently from poles
- Transverse pulses and waves, Grade 10. Superposition, in a completely different context
- Series and parallel circuits, Grade 10. Same term
Buy this experiment
We are putting together a Magnetism Kit for Grade 10 with a pair of Alnico bar magnets and their keeper, four compasses, iron filings in a shaker and paper, plus a printed teacher guide and the marking memo. Coming shortly.
Bar magnets come in pairs with a keeper and we hold them in five lengths from 37 mm to 150 mm, in the magnetism range. A pair is what this practical needs, because half of Experiment 1 is about what two magnets do to each other.
Buy the compasses in fours. One compass turns Experiment 2 into twenty separate measurements; four turns it into one picture.