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Induced magnetism and magnetic materials

Key idea: A magnetic field induces poles in magnetic materials, with the nearest pole opposite to the inducing pole. Soft iron loses magnetism readily; magnetically hard steel retains it.

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A magnet can pick up an iron nail even though the nail was not a magnet beforehand. The magnet turns the nail into a magnet while it is nearby: it induces magnetism in it. Whether the nail keeps that magnetism afterwards depends on what it is made of, and that decides whether a material suits a compass needle or the core of a crane’s electromagnet.

Induced magnetism

Put a piece of unmagnetised iron close to a strong magnet. The magnet’s field magnetises the iron: the iron gains north and south poles. This is induced magnetism. Soft iron loses almost all this magnetism when the magnetising field is removed; a suitable magnetically hard steel can retain it.

  • The end of the iron nearest the magnet becomes the opposite pole to the pole beside it. The far end becomes the same kind of pole.
  • Unlike poles now face each other, so the iron is attracted. This is why a magnet attracts iron whichever pole you use.
  • The effect passes on. A magnet can hold a chain of paper clips because each magnetised clip induces opposite poles at the neighbouring ends of the next clip. The clips farther along the chain need not touch the original magnet.

A current-carrying solenoid (a long coil of wire) also has a magnetic field. A soft-iron rod placed along the solenoid’s axis becomes magnetised in the same way. If the direct current is reversed, the field reverses and the induced poles swap readily. A steel rod that already retains magnetism needs a sufficiently strong reversed field to reverse its poles.

For a classroom demonstration, use a low-voltage teaching coil and supply within their current ratings, under teacher supervision. Switch off between observations to limit heating.

Induced poles and solenoid magnetisationTwo panels show opposite poles induced at the nearest ends of a magnet and iron bar, and a soft-iron core inside a solenoid whose north and south poles swap when direct current is reversed. The winding is schematic; no current magnitude is specified.A. A magnet brought near ironSNSNpermanent magnetiron barThe objects attract.Nearest end: opposite poleNear known N → induced SFar end → induced NEither magnet pole attractsunmagnetised iron.B. A soft-iron core in a solenoidBefore current reversalSNsoft-iron coreNSsoft-iron coreReversethe current.Applied field reverses → poles swap.
Induced poles and solenoid magnetisationTwo panels show opposite poles induced at the nearest ends of a magnet and iron bar, and a soft-iron core inside a solenoid whose north and south poles swap when direct current is reversed. The winding is schematic; no current magnitude is specified.A. A magnet brought near ironSNSNpermanent magnetiron barThe objects attract.Nearest end: opposite poleNear known N → induced SFar end → induced NEither magnet pole attractsunmagnetised iron.B. A soft-iron core in a solenoidBefore current reversalSNsoft-iron coreNSsoft-iron coreReversethe current.Applied field reverses → poles swap.
A nearby north pole induces south at the iron’s near end. Reversing solenoid current reverses the field and swaps the soft-iron core’s poles.

Check your understanding 1 Predict both induced poles

An unmagnetised iron nail is brought near the S pole of a strong magnet. Which pole is induced at the end of the nail nearest the magnet?
Which pole is at the far end of the same nail?
Show answer

Beside the strong magnet’s S pole, the unmagnetised iron nail’s near end becomes N and its far end becomes S. Opposite poles face each other, explaining attraction. The induced nail has both poles; it has not acquired an isolated north pole.

Check your understanding 2 A rod inside a coil

A soft-iron rod sits inside a solenoid. In two separate trials, an initially unmagnetised soft-iron clip is brought near the same end of the rod while a steady current flows. The currents have equal sizes but opposite directions. A student predicts attraction in one trial and repulsion in the other. Is the prediction correct?

Show answer

No. Reversing the current reverses the rod’s poles, but the rod induces the opposite pole at the nearby end of the clip in each trial. Unlike poles face each other in both trials, so the clip is attracted in both. Reversing the field does not leave this initially unmagnetised soft-iron clip with a fixed pole that would repel the rod.

Temporary and permanent magnets

Compare soft iron with magnetically hard steel suitable for making a permanent magnet. Their key difference is how much magnetism they keep after the magnetising field is removed.

How iron and steel compare as magnets
PropertyIron (soft iron)Magnetically hard steel
Ease of magnetisingEasyHarder
When the field is removedLoses almost all its magnetismKeeps its magnetism
Type of magnetTemporaryPermanent
Typical uses

Cores of electromagnets that must switch on and off: a scrapyard crane, an electric bell, a relay

Magnets that must stay magnetised: a compass needle, a permanent bar magnet

“Soft” and “hard” describe magnetic behaviour, not how the metal feels. Soft iron is easy to magnetise and easy to demagnetise. In this comparison, the selected steel is magnetically hard; this does not describe every steel alloy.

Magnetic retention after the applied field is removedTwo timelines compare soft iron and a suitable magnetically hard steel before magnetisation, in an applied magnetic field, and after the field is removed. Decision boxes link soft iron to switchable electromagnets and steel to permanent magnets. Soft iron magnetises readily; suitable magnetically hard steel is harder to magnetise.Compare the state after the magnetising field is removedSoft ironInitiallyIn the applied fieldAfter removalnot magnetisedApply field.NSRemove field.Loses most magnetismreadily after removal.Choose for a switchable electromagnet.Suitable magnetically hard steelInitiallyIn the applied fieldAfter removalnot magnetisedApply field.NSRemove field.Retains its polesafter removal.Choose when the object must remain magnetised.
Magnetic retention after the applied field is removedTwo timelines compare soft iron and a suitable magnetically hard steel before magnetisation, in an applied magnetic field, and after the field is removed. Decision boxes link soft iron to switchable electromagnets and steel to permanent magnets. Soft iron magnetises readily; suitable magnetically hard steel is harder to magnetise.Compare the state after the magnetising field is removedSoft ironInitiallynot magnetisedApply field.In the applied fieldNSRemove field.After removalLoses most magnetismreadily after removal.Choose for a switchable electromagnet.Suitable magnetically hard steelInitiallynot magnetisedApply field.In the applied fieldNSRemove field.After removalRetains its polesafter removal.Choose when the object must remain magnetised.
Soft iron loses most magnetism when the field is removed; suitable magnetically hard steel retains it better. Choose according to whether magnetism must switch off.

Spot the mistake 1

Choosing the core for a crane

About 4 min

Learner claim

“A scrapyard crane should use a steel core in its electromagnet, because steel makes a stronger, longer-lasting magnet.” Decide what the crane needs, then choose the core.

Try this before viewing the solution

What must happen when the crane operator switches the current off?
Which core material meets that need?
Show solution step by step

  1. State the job

    Method

    Say what the magnet must do when the current stops.

    Reason

    The choice depends on the job, not on which magnet is “better”.

    Working

    Answer

    The crane must release the scrap when switched off.

  2. Match the property

    Method

    Choose the material that loses its magnetism.

    Reason

    Soft iron keeps little magnetism after switch-off; the selected steel keeps much more.

    Working

    Answer

    Use soft iron for the core.

  3. Choose the core

    Working

    Answer

    Use soft iron: it magnetises when the current flows and loses almost all its magnetism when the current stops.

Check your understanding 3 A compass needle

Why is a compass needle made of magnetically hard steel rather than soft iron?

Show answer

A compass needle must stay a magnet so that it lines up with a field. Hard steel keeps its magnetism after the magnetising field is removed; soft iron loses almost all of it.

Common mistakes

  • Saying the iron must already have been a magnet. The nearby field induces poles in it, with the nearer end opposite to the pole beside it.
  • Saying a permanent magnet is always the better choice. A crane must drop its load when switched off, so it needs a core that loses its magnetism.
  • Thinking “soft” iron is soft to touch. “Soft” means easy to magnetise and easy to demagnetise.

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