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Testing magnets

Key idea: Like magnetic poles repel and unlike poles attract. Attraction alone does not prove an object is a magnet, because unmagnetised iron is attracted too; repulsion distinguishes a magnet.

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Two grey bars lie on a bench. One is a magnet and the other is plain iron, and a magnet attracts both of them. So attraction cannot tell you which bar is the magnet. Only a magnet can repel another magnet, and that is the test that settles the question.

Properties of magnets

  • A bar magnet strongly attracts magnetic materials such as iron, nickel, cobalt and suitable steel. Some steel alloys are not strongly magnetic. Copper, aluminium, brass, plastic and wood show no noticeable attraction in this school test.
  • An ordinary bar magnet has two poles, where its attraction of small iron objects is strongest.
  • A bar magnet suspended so it can turn freely in a horizontal plane, away from other magnets and magnetic objects, aligns roughly north–south in Earth’s magnetic field. Its north-seeking end is the north pole (N); the other end is the south pole (S).
  • Like poles repel; unlike poles attract.
  • Cutting an ordinarily magnetised bar across its length does not separate N from S. Each piece is a smaller magnet with its own N and S poles.
Magnetic interactions and the sure testThree comparisons show unlike poles attracting, like poles repelling, and a magnet attracting an unmagnetised iron bar. A final panel shows that cutting a uniformly magnetised bar across its length produces two smaller magnets, each with north and south poles.Identify the facing ends, then predict both forcesUnlike polesSNSNForces point towardsthe other object.Like polesSNNSForces point away fromthe other object.Magnet and ironSNironEither pole attractsunmagnetised iron.Cutting across a uniformly magnetised barNScut hereNSNSEach piece has both a north and a south pole.Testing an unknown barUse the same known polenear each end in turn.Repulsion at either end:the bar is a magnet.Attraction at both ends:could be unmagnetised iron.
Magnetic interactions and the sure testThree comparisons show unlike poles attracting, like poles repelling, and a magnet attracting an unmagnetised iron bar. A final panel shows that cutting a uniformly magnetised bar across its length produces two smaller magnets, each with north and south poles.Identify the facing ends, then predict both forcesUnlike polesSNSNForces point towardsthe other object.Like polesSNNSForces point away fromthe other object.Magnet and ironSNironEither pole attractsunmagnetised iron.Cutting across a uniformly magnetised barNScut hereNSNSEach piece has both a north and a south pole.Testing an unknown barUse the same known polenear each end in turn.Repulsion at either end:the bar is a magnet.Attraction at both ends:could be unmagnetised iron.
Unlike poles attract and like poles repel. Attraction is inconclusive because an unmagnetised iron bar is also attracted; repulsion is the decisive observation in the ordinary bar-magnet test.

For the bars in this school test, attraction can mean either unlike magnet poles or a magnet beside unmagnetised iron. Attraction alone cannot distinguish them. Test both ends with the same known pole: repulsion at one end proves the unknown bar is a magnet.

In the figure, the arrows show forces on the objects. Each arrow points towards the other object for attraction and away for repulsion. These force arrows are different from the magnetic field arrows you will draw later.

Worked example 1

Deciding whether a bar is a magnet

Problem

The N pole of a known magnet attracts end A of an unknown metal bar. It repels end B. Is the bar a magnet? Name the poles at A and B.

Show full solution

  1. Weigh the attraction

    Method

    Ask what could cause the attraction at A.

    Reason

    A south pole would be attracted, but so would unmagnetised iron or steel.

    Working

    Answer

    Attraction at A alone does not show that the bar is a magnet.

  2. Use the repulsion

    Method

    Ask what could cause the repulsion at B.

    Reason

    Only like poles repel, so B must itself be a pole of the same kind as the known pole.

    Working

    Answer

    B is a north pole, so the bar is a magnet.

  3. Name the other pole

    Working

    Answer

    A magnet has a pole at each end, so A is the south pole. This also explains the attraction at A: unlike poles attract.

Check your understanding 1 Three bars that look the same

You have three bars that look the same: a magnet, an unmagnetised iron bar and an aluminium bar. Using only a known bar magnet, describe how to tell which is which.

Show answer

Bring the N pole of the known magnet to each end of each bar. The aluminium bar shows no noticeable attraction in this school test. The iron bar is attracted at both ends and never repels. The magnet repels at one end, which is its N pole, and attracts at the other.

Check your understanding 2 Cut a bar magnet

On paper, draw a bar with S at its left end and N at its right end. It is magnetised along its length. If it is cut across the middle, label all four ends. Can either piece have only one pole?

Show answer

Self-review the paper diagram: each piece has S at its left end and N at its right end. The new cut face is N on the left piece and S on the right piece; label those as well as the two original ends. Cutting does not produce an isolated pole.

Why iron is attracted at all is explained in induced magnetism and magnetic materials, and the field around a magnet is mapped in magnetic fields and field patterns.

Common mistakes

  • Saying a bar must be a magnet because it was attracted to a magnet. Unmagnetised iron is attracted too; only repulsion proves it.
  • Saying all metals are magnetic. Copper and aluminium show no noticeable attraction, and some steel alloys are only weakly magnetic.
  • Saying that cutting a magnet separates its poles. Each piece is a smaller magnet with its own north and south poles.

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