IGCSE Physics revision · Electricity & magnetism
Magnetism
The gateway to the electromagnetic-effects topic that follows. Almost every mark here comes from drawing field lines correctly and using the phrase 'induced magnetism' where students want to write 'attraction'.
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What the syllabus demands
- —Describe magnetic poles, attraction and repulsion, and induced magnetism
- —Distinguish magnetic hard (steel) and soft (iron) materials
- —Draw the magnetic field pattern of a bar magnet; know field direction is north to south
- —Describe the field patterns around a straight wire and a solenoid
- —Describe how to magnetise and demagnetise materials
Definitions that earn marks
Clear definitions to practise — check your course mark scheme
- Magnetic field
- The region around a magnet in which a magnetic pole experiences a force. Field lines point from north to south.
- Induced magnetism
- Magnetism produced in an unmagnetised magnetic material when it is placed in a magnetic field — always causing attraction.
- Soft magnetic material
- A material (e.g. iron) that magnetises and demagnetises easily — used for electromagnet cores.
- Hard magnetic material
- A material (e.g. steel) that is difficult to magnetise but keeps its magnetism — used for permanent magnets.
Where the marks die
Common mistakes to check
- 01
Saying 'only repulsion proves a magnet'. Correct — attraction can be induced magnetism in any magnetic material, so repulsion is the only test for a permanent magnet. This exact sentence is a recurring mark.
- 02
Drawing field lines that cross. Field lines never cross, and they must have arrows from north to south.
- 03
Choosing steel for an electromagnet core. Electromagnets need soft iron, which loses its magnetism the instant the current stops; steel would stay magnetised.
- 04
Saying stronger field where lines are far apart. Closer field lines mean a stronger field — the spacing is the strength.
One worked example, done properly
Question
A bar of metal is attracted to both poles of a magnet. Explain whether the bar is a magnet, a magnetic material, or cannot be determined.
Method
- 1.Attraction to both poles means the bar experiences induced magnetism at each pole.
- 2.A permanent magnet would be attracted to one pole and repelled by the other.
- 3.Since it is attracted to both, it is an unmagnetised magnetic material.
It is a magnetic material, not a magnet — repulsion is the only proof of a magnet.