Magnets: properties, magnetisation and the domain theory: WASSCE Physics

A compass needle swings to point north, a fridge magnet holds a calendar, and a scrap-metal crane lifts a car. Magnetism is one of the oldest known forces — and the key to motors, generators and every loudspeaker.

Learning objectives

By the end of this lesson you should be able to:

distinguish magnetic and non-magnetic materials, and hard and soft magnetic materials
state the law of magnetic poles
describe methods of magnetisation and demagnetisation
explain magnetism using the domain theory

Magnetic materials

Magnetic (ferromagnetic) materials are strongly attracted by magnets and can be magnetised: iron, steel, nickel, cobalt and their alloys.
Non-magnetic materials are not attracted: copper, aluminium, brass, wood, plastic, glass, paper.

Hard magnetic materials (e.g. steel, alnico)	Soft magnetic materials (e.g. soft iron, mumetal)
hard to magnetise but keep their magnetism	easy to magnetise but lose it easily
used for permanent magnets (loudspeakers, compasses, motors)	used for temporary magnets: electromagnet cores, transformer cores, relays

Properties of magnets

A freely suspended bar magnet comes to rest pointing roughly north–south. The end pointing north is the north-seeking pole (N).
Magnetism is strongest at the poles, near the ends.
Poles always come in pairs: cutting a magnet in half gives two smaller magnets, each with N and S poles.

Definition

Law of magnetic poles: like poles repel; unlike poles attract.

Testing for a magnet: a magnet attracts both poles of a compass needle on one end and repels one of them. Only repulsion proves that a bar is a magnet (an unmagnetised iron bar is attracted by both poles).

Making magnets (magnetisation)

Single touch (stroking): stroke a steel bar repeatedly from one end to the other with one pole of a magnet, lifting it away at the end each time. The end where the stroke finishes gets the opposite pole to the stroking pole.
Divided (double) touch: stroke from the middle outwards with two magnets of opposite poles at the same time. Gives a stronger, more even magnet.
Electrical method (best): place the steel bar inside a solenoid (coil) and pass a large direct current for a moment. The polarity depends on the direction of the current.
Induction: an iron object near a magnet becomes a temporary magnet (this is why a chain of nails can hang from a magnet).

Destroying magnetism (demagnetisation)

Heating strongly (above about 770 °C for iron, the Curie point).
Hammering the magnet, especially when it lies east–west.
Electrical method (best): place the magnet in a solenoid carrying alternating current, and slowly withdraw it while the current flows (in an east–west direction).
Rough handling, dropping, or storing magnets without keepers.

The domain theory of magnetism

Definition

A ferromagnetic material is made up of tiny regions called domains, each of which is a small magnet with all its atomic magnets lined up.

In an unmagnetised bar, the domains point in random directions, so their effects cancel.
Magnetising the bar aligns the domains in one direction.
When all the domains are aligned, the magnet is saturated: it cannot be made any stronger.

The theory explains: why breaking a magnet gives two magnets; why heating or hammering destroys magnetism (the domains are shaken back into disorder); and why there is a limit (saturation) to how strong a magnet can be.

Storing magnets

Magnets slowly weaken as their poles repel the aligned domains at their ends (self-demagnetisation). They are stored in pairs, unlike poles side by side, with soft-iron keepers across the ends. The keepers become magnetised by induction and complete a closed loop, keeping the domains aligned.

In Sierra Leone

Old radio and loudspeaker magnets salvaged at Freetown's electronics repair shops are kept stacked with steel pieces across them — keepers that stop them losing their strength.

Key points

Iron, steel, nickel and cobalt are magnetic; steel makes permanent magnets, soft iron temporary ones.
Like poles repel, unlike poles attract; repulsion is the sure test for a magnet.
Magnetise by stroking or with d.c. in a solenoid; demagnetise by heating, hammering or a.c. in a solenoid.
Domain theory: magnetisation aligns domains; saturation when all are aligned.
Store magnets with keepers.

WASSCE examination focus

WAEC tests this

Describe methods of making and destroying magnets (especially the electrical methods), explain the domain theory and use it to explain saturation or loss of magnetism, distinguish soft and hard magnetic materials with uses, and explain why keepers are used.

Quick check

Why is soft iron, not steel, used for the core of an electromagnet?
What is the best way to demagnetise a magnet?
According to the domain theory, what is a saturated magnet?

Answers: 1. Soft iron is easily magnetised and loses its magnetism as soon as the current is switched off. 2. Place it in a solenoid carrying alternating current and slowly withdraw it. 3. One in which all the domains are aligned in the same direction.

Magnets: properties, magnetisation and the domain theory