Magnetism: Complete O Level Physics Cheatsheet
O Level Physics, Chapter 19 · Read time: ~7 minutes
These notes come from TGC Academy's O Level Physics tuition in Singapore.
Key Formulas
Properties of magnets
A magnet has a north pole and a south pole. Like poles repel, unlike poles attract.
Magnets attract magnetic materials (iron, steel, nickel and cobalt). Non-magnetic materials such as copper, aluminium and plastic are not attracted.
A freely suspended magnet comes to rest pointing north-south.
Trap: Attraction does not prove something is a magnet, because a magnet also attracts unmagnetised iron. **Repulsion** is the only sure test that both objects are magnets.
Induced magnetism
A magnetic material placed near a strong magnet, or inside a current-carrying solenoid, becomes magnetised itself. This is induced magnetism.
The end of the material nearest the magnet becomes the opposite pole, so it is always attracted, whichever pole is brought close.
Temporary magnets like soft iron are easily magnetised and lose their magnetism easily. They are used in electromagnets, relays and transformer cores.
Permanent magnets like steel are harder to magnetise but keep their magnetism. They are used in compasses, loudspeakers and fridge magnets.
Trick: Link the use to the property. If the device needs to switch its magnetism on and off, the answer is soft iron. If it needs to stay magnetised, the answer is steel.
Magnetic fields
A magnetic field is the region around a magnet where a magnetic material experiences a force.
Field lines point from north to south outside the magnet, never cross, and are closest together where the field is strongest, near the poles.
To plot a field, place a small plotting compass at points around the magnet. Its north pole points along the field direction at that point. Mark the direction and move the compass on, then join the marks.
Two magnets: between unlike poles (N facing S), lines run straight across from N to S. Between like poles (N facing N), the lines push away from each other, leaving a neutral point in the middle where the field is zero.
Worked Example
Two bar magnets are placed in a line with their north poles facing each other, 10 cm apart, on a sheet of paper.
(a) Describe the magnetic field pattern between the two north poles. [2]
Field lines leave both north poles and curve away from each other, turning outwards towards the south poles.
There is a neutral point midway between the two north poles where no field lines pass.
(b) A plotting compass is placed exactly at the neutral point. State and explain what it shows. [2]
The fields of the two magnets cancel there, so the resultant field from the magnets is zero.
The compass simply aligns with the Earth's magnetic field, pointing north-south.
(c) One of the magnets is replaced by an unmagnetised iron bar. State what happens to the iron bar and the other magnet. [2]
The iron bar becomes an induced magnet, with a south pole at the end nearest the magnet's north pole.
The iron bar and the magnet now attract each other.
Why this question is a good test of the topic: it checks field drawing, the idea of a neutral point, and induced magnetism, the three things most often tested in Magnetism.
Frequently Asked Questions
What is the only sure test of whether something is a magnet?
Repulsion. A magnet attracts both magnets and unmagnetised magnetic materials, but only another magnet can repel it.
Why is soft iron used in electromagnets?
It is easily magnetised and easily demagnetised, so the electromagnet can be switched on and off.
Why is steel used for permanent magnets?
Steel is harder to magnetise but keeps its magnetism for a long time.
Which direction do magnetic field lines point?
From north to south outside the magnet, the direction a compass needle's north pole points.
What is a neutral point?
A point where the magnetic fields of two magnets cancel, so the resultant field is zero, for example between two like poles.
Field patterns still confusing?
Small group O Level Physics classes at TGC Academy's Bishan, Bukit Timah and Potong Pasir centres, helmed by Andrew Seah, MOE Award-Winning Teacher and Marshall Cavendish textbook author.