K323 / 2027
Magnetism overview

Lesson 1 of 4

Magnet properties and observations

  • I can state the properties of magnets.Syllabus

    Syllabus K323, 17(a). State the properties of magnets.

Make a guess. You are not marked.

A bar magnet is cut in half across its middle. What do you get?

  1. Two unmagnetised pieces, because cutting destroys the magnet's field.
  2. Two smaller magnets, each with its own north and south pole.
  3. A separate north pole in one piece and a south pole in the other.
Show the answer

Two smaller magnets, each with its own north and south pole.

Magnetic poles always come in pairs. Each half becomes a complete magnet, with a new pole formed at the cut face.

A magnet can attract or repel another magnet. The observation tells you about the poles involved, but attraction alone does not prove that an object was already a magnet.

A force is a push or pull. Magnetic forces can act without the objects touching: they are non-contact forces.

Identify poles and predict their interaction

An ordinary bar magnet has a north-seeking pole, N, and a south-seeking pole, S. Its magnetic effect is usually strongest near the poles.

Like poles repel. Unlike poles attract.N repels N, S repels S, and N attracts S.

Predict the force on each magnet

Purple arrows show forces on the named magnets. Their lengths are qualitative; no numerical force is supplied.

Unlike facing poles: attraction

Unlike facing poles: attractionThe left bar has S at its left end and N at its right end. The right bar also has S left and N right, so N and S face across the gap. Two separate purple arrows below the bars represent forces on the respective magnets: the left magnet is pulled right and the right magnet is pulled left. The arrows have equal length to show equal and opposite forces for the interaction, not a numerical force scale. They are force arrows, not magnetic field lines.Left magnetRight magnetSNSNForce onleft magnetForce onright magnet

Like facing poles: repulsion

Like facing poles: repulsionThe left bar has S left and N right. The right bar has N left and S right, so N faces N. Separate purple arrows below the bars represent forces on the respective magnets: the left magnet is pushed left and the right magnet is pushed right. The arrows have equal length to show equal and opposite forces for the interaction, not a numerical force scale. They are force arrows, not magnetic field lines.Left magnetRight magnetSNNSForce onleft magnetForce onright magnet

A field-direction arrow, used later, tells how a compass aligns at one position. It does not replace these two forces on the magnets.

Each arrow is a force on the magnet beside it. Unlike facing poles pull the magnets towards each other; like facing poles push them apart.

A freely suspended magnet, away from nearby magnetic disturbances, turns approximately north-south. Its north-seeking end points roughly towards geographic north. A nearby magnet can change this direction, so a compass needle does not always point geographically north.

Distinguish a magnet from a magnetic material

A magnetic material can become magnetised. A magnetised object has a magnetic effect at that time, whether it retains the effect itself or is being magnetised by a nearby source.

A magnet attracts suitable magnetic materials such as iron and the magnetic steel used in these examples. An initially unmagnetised piece can be attracted because the nearby magnet induces magnetism in it.

Being a metal is not sufficient. Ordinary copper and aluminium do not show the strong attraction of soft iron in this stationary-magnet comparison. Different steel alloys can also have different magnetic behaviour.

Use the observation as evidence

An unknown end X repels a known N pole

Like poles repel, so X is a north pole in this simple end-to-end test. The unknown bar is magnetised.

If X instead attracts the known N pole, two explanations are possible: X could be a south pole of a magnet, or it could be the nearer end of an initially unmagnetised magnetic material.

Attraction alone therefore does not establish that the unknown bar was already a permanent magnet. Repulsion identifies a magnetised end, but does not measure how long the material will retain its magnetism.

Smaller pieces still have both poles

Each smaller piece still has two poles

This is a conceptual division of an ordinary bar magnet. The new faces also become poles.

Dividing a bar gives two smaller N-to-S magnetsThe original horizontal magnet has N at its left end and S at its right. A dashed vertical guide marks a division across its middle. Below, the separated left piece has N at its original left end and a new S at its right cut face. The right piece has a new N at its left cut face and S at its original right end. Both smaller magnets therefore have N on the left and S on the right. No isolated north or south pole is shown, and the dashed guide is not a field line.Original magnetNSDivision at the dashed guideNSNSTwo smaller magnets, each with N and S
In this conceptual model, dividing an ordinary bar magnet gives smaller magnets, each with a north and a south pole. It does not isolate one north pole and one south pole.

The newly formed ends acquire poles too. Magnetic N and S labels describe magnetic poles; they are not labels for positive and negative electric charge.

Check your understanding

In a simple end-to-end test, end X of an unknown bar repels the known N pole of a reference magnet. What does this establish?

  1. X is a north pole and the unknown bar is magnetised.
  2. X is a south pole because unlike poles repel.
  3. The bar is unmagnetised iron, because magnetic materials repel a north pole brought near them.
Show the answer

X is a north pole and the unknown bar is magnetised.

Like poles repel, so X is N. This establishes magnetisation, although it does not by itself show how strongly or how long the bar will retain it.

Practise an exam question

Write your answer first, then tick the mark points it makes.

Exam-style question

A student has a steel bar and wants to find out whether it is a magnet. She has a known bar magnet. Describe what she should do, and explain why attraction alone does not prove the bar is a magnet. [2]

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