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Magnetism

12 min read

Magnets and their poles

A magnet attracts iron, steel, nickel and cobalt. These are magnetic materials. Every magnet has two poles: a north-seeking pole (N) and a south-seeking pole (S). The poles are where the force is strongest.

Poles of magnetsThree pairs of bar magnets. Unlike poles, north facing south, attract. Two north poles facing each other repel, and two south poles facing each other repel.SNSNN–S: attractSNNSN–N: repelNSSNS–S: repel

Magnetic fields

The space around a magnet where it exerts a force is its magnetic field. We draw it with field lines. They leave the north pole, curve round and enter the south pole. Where the lines are closer together the field is stronger. A small compass needle lines up with the field.

Magnetic field of a bar magnetField lines leave the north pole of a bar magnet, curve round and enter the south pole. They are closest together near the poles, where the field is strongest.SNfield lines go from N to Sclosest together at the poles: strongest field
The field lines of a bar magnet. A small compass placed anywhere lines up along the nearest field line.

Making a magnet

Inside iron and steel are tiny regions called domains, each like a small magnet. In an unmagnetised bar the domains point in all directions and cancel out. Stroking the bar with one pole of a magnet, always in the same direction, lines the domains up. More strokes line up more domains until all are aligned: this is saturation.

Domains in iron before and after magnetisingLeft: in an unmagnetised bar the tiny magnetic domains point in all directions and cancel out. Right: in a magnetised bar the domains are lined up in the same direction, so the bar has a north and a south pole.Not magnetised: domains mixedMagnetised: domains lined upSN
Try itStroking a steel bar

This is a simple model: each stroke lines up some of the domains that are still pointing the wrong way. The bar gets stronger quickly at first, then levels off.

Ways to demagnetise a magnet
MethodWhy it works
Hammeringshakes the domains out of line
Heating stronglyheat makes the domains move randomly
Dropping many timesjolts disturb the alignment
Placing in a decreasing alternating fieldscrambles the domains gradually
Two ways of making a magnetStroking a steel bar with one pole of a magnet in the same direction, many times, lines up its domains. Placing a bar inside a coil carrying direct current also magnetises it, and gives a north pole at one end and a south pole at the other.SNStroke with one pole, always thesame way, then lift well awayStrokingdirect currentSNCoil: turns round the barwith direct currentElectrical method

The Earth's magnetic field

The Earth behaves like a giant bar magnet. A compass needle points to the Earth's magnetic north, which is close to the geographic north. The angle between magnetic north and true north is called the declination.

The Earth as a magnetThe Earth behaves like a giant bar magnet. Its magnetic field lines come out near the geographic south pole and go into the Earth near the geographic north pole. A compass needle lines up with the field and points north.SNgeographic northgeographic southmagnetic S pole is near geographic northcompass

Uses of magnets

UseHow the magnet is used
CompassA magnetised needle lines up with the Earth’s field to show north.
Fridge door sealA strip of magnetic material holds the door shut.
LoudspeakerA magnet and a coil of wire move the cone to make sound.
Scrap-yard craneA powerful electromagnet lifts iron and steel and drops them when switched off.
Check yourself Which two will attract each other?
Check yourself Which of these is NOT attracted to a magnet?

Key terms

Pole
An end of a magnet where its force is strongest.
Magnetic field
The region around a magnet where it exerts force.
Domain
A tiny region inside a magnetic material that acts like a small magnet.
Magnetic material
A material attracted by a magnet, such as iron.

Summary

  • Like poles repel; unlike poles attract.
  • Field lines go from N to S and are closest where the field is strongest.
  • Stroking aligns domains to make a magnet; heating or hammering demagnetises it.
  • The Earth has its own magnetic field.
Current Electricity Forces in Equilibrium