Magnetism and Magnetic Fields
A short, high-yield topic. The rules are simple, but the drawing conventions for field lines carry marks on their own, and one common misconception derails the whole subject.
1. Poles and forces
Every magnet has a north pole and a south pole.
LIKE poles REPEL. UNLIKE poles ATTRACT.
Magnets have POLES, not charges. Saying a magnet has “charges” was a specific recorded error. Magnetism and electric charge are different phenomena — a magnet has north and south poles, while charge is positive and negative and belongs to electrostatics. Keep the vocabulary separate.
Poles always come in pairs. Cut a magnet in half and you get two smaller magnets, each with its own north and south — you can never isolate a single pole.
Testing for a magnet: the only reliable test is repulsion. A magnet will attract any magnetic material, but only another magnet will be repelled.
2. Magnetic materials
Magnetic (ferromagnetic) materials: iron, steel, cobalt, nickel — these are attracted to magnets and can be magnetised.
Non-magnetic: copper, aluminium, brass, plastic, wood, glass — not attracted, even though some are excellent electrical conductors.
Being a good electrical conductor does not make a material magnetic. Copper and aluminium conduct superbly and are not attracted to magnets. Confusion between material types was recorded here.
Hard and soft magnetic materials
| Soft (e.g. iron) | Hard (e.g. steel) | |
|---|---|---|
| Magnetises | easily | with difficulty |
| Loses magnetism | easily | retains it |
| Used for | electromagnet cores, transformer cores | permanent magnets |
Soft iron for electromagnet cores; hard steel for permanent magnets. Soft iron is chosen precisely because it loses its magnetism as soon as the current stops — which is what makes an electromagnet switchable.
3. Induced magnetism
Induced magnetism is when a magnetic material becomes a temporary magnet while it is near or touching a permanent magnet.
This is why a magnet attracts an unmagnetised iron nail: the nail becomes magnetised, with the pole nearest the magnet becoming the opposite pole — so attraction always results.
Induced magnetism explains why attraction is the default. A magnet attracts any magnetic material, whether or not it was magnetised to begin with — which is why only repulsion proves you have two magnets.
A soft material loses induced magnetism when removed; a hard one keeps some.
4. Magnetic fields
A magnetic field is a region where a magnetic material experiences a force.
Drawing field lines — the conventions, each of which is worth marks:
1. Field lines run from NORTH to SOUTH outside the magnet (and south to north inside it). 2. Every line has an ARROW showing that direction. 3. Field lines NEVER CROSS. 4. Lines are CLOSER TOGETHER where the field is STRONGER — nearest the poles. 5. In a UNIFORM field, lines are PARALLEL and EQUALLY SPACED.
Field lines never cross, because at any point the field has one direction — and a crossing point would mean two. This was flagged explicitly in lessons.
Uniform field lines must be drawn parallel to each other, running north to south — also flagged. A uniform field is found between the poles of a horseshoe magnet or two opposite poles facing each other.
Field patterns to know:
- Bar magnet: lines emerge from N, curve round, and enter S; densest at the poles
- Two unlike poles facing (N–S): lines run straight across from one to the other — a uniform field in the middle
- Two like poles facing (N–N): lines repel and curve away, with a neutral point between them where the fields cancel
Plotting a field experimentally: use iron filings sprinkled on paper over the magnet (shows the shape), or a plotting compass moved step by step (shows the shape and the direction).
Only a plotting compass gives the direction. Iron filings show the pattern but not which way the field points.
5. The Earth’s magnetic field
The Earth behaves like a giant bar magnet. A freely suspended magnet aligns with it — which is how a compass works.
A compass needle’s north-seeking pole points towards the Earth’s geographic north, which must therefore be a magnetic south pole.
6. Magnetic shielding
Soft iron placed around an object shields it from a magnetic field, because the field lines are concentrated through the iron and diverted around the space inside.
This is how sensitive instruments are protected.
7. Magnetising and demagnetising
To magnetise: stroke repeatedly with one pole of a magnet in the same direction, or place inside a solenoid carrying direct current.
Use DIRECT current to magnetise. An alternating current would keep reversing the field and would demagnetise instead. Confusion over induced magnetism and the role of direct current was recorded.
To demagnetise: heat it strongly, hammer it, or place it in a solenoid carrying alternating current and slowly withdraw it.
8. Mistakes that cost marks
Saying magnets have “charges” instead of poles.
Confusing magnetic materials with electrical conductors.
Drawing field lines without arrows.
Drawing field lines that cross.
Drawing lines from south to north outside the magnet.
Uneven spacing in a uniform field.
Saying attraction proves something is a magnet — only repulsion does.
Mixing up hard and soft materials and their uses.
Using a.c. to magnetise.
Frequently asked questions
What are the rules for magnetic poles? Like poles repel; unlike poles attract.
Do magnets have charges? No — they have poles. Charge belongs to electrostatics.
Which materials are magnetic? Iron, steel, cobalt and nickel.
Is copper magnetic? No, even though it conducts electricity well.
What is induced magnetism? A magnetic material becoming a temporary magnet when near a permanent one.
How do I test whether something is a magnet? Check for repulsion — attraction alone is not proof.
Which way do field lines point? From north to south outside the magnet.
Why can’t field lines cross? Because the field has only one direction at each point.
What is a uniform field? One with parallel, equally spaced field lines — found between two opposite poles.
Why is soft iron used in electromagnets? It magnetises and demagnetises easily, so the magnet can be switched off.
Quick revision checklist
- I know like poles repel and unlike attract
- I know magnets have poles, not charges
- I can list the magnetic materials
- I know conductivity and magnetism are unrelated
- I can explain induced magnetism
- I know only repulsion proves something is a magnet
- I can draw field lines N to S, with arrows, never crossing
- I show stronger fields with closer lines
- I can draw a uniform field with parallel equal spacing
- I can draw the patterns for a bar magnet and for like and unlike poles
- I know how to plot a field with filings and a compass
- I know hard vs soft materials and their uses
- I know to use d.c. to magnetise
- I can explain magnetic shielding
These notes cover simple phenomena of magnetism in the Cambridge IGCSE Physics (0625) syllabus and are written for Grade 9–11 / Year 10–11 students. They are based on teaching patterns observed across a large set of one-to-one IGCSE Physics lessons, with particular attention to the errors students make most often and the wording examiners reward. Always check the current syllabus and formula list for your own exam series.
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