K323 / 2027
Magnetism overview

Topic 2 of 4

Induced magnetism and material choice

A magnetic material can become magnetised in a magnetic field. What remains after that field is removed helps determine the material's use.

Like magnetic poles repel and unlike poles attract. An unmagnetised magnetic material can acquire poles; it does not have to start as a permanent magnet.

Near a strong magnet

Place an initially unmagnetised iron rod near a strong magnet. The rod becomes magnetised without needing to touch the magnet. Its nearer end develops the opposite pole to the nearby inducing pole.

An external magnet induces an opposite near pole

The iron rod was initially unmagnetised. The pole labels below show its induced state while the strong magnet is nearby.

An N pole near end A induces S at A and N at end BA strong magnet on the left has S at its left end and N at its right. A separate iron rod lies to the right with a clear gap. End A of the rod is nearest the magnet, and end B is farthest away. During induction A is S and B is N, so the near iron end is attracted to the inducing N pole. The rod is not touching the magnet. No electric charge transfer is shown or implied.StrongmagnetIron rodSNSNABInduced poles

A is the opposite pole to the nearby inducing pole. Reversing the inducing magnet reverses the rod's induced poles.

The inducing magnet has N on its right. The rod's nearer end A becomes S and its farther end B becomes N, so the near ends attract.

If the inducing pole is changed from N to S, A becomes N and B becomes S. Reversing the field reverses the induced poles. This process is induced magnetism; it does not require electric charge to transfer from the magnet to the rod.

Compare before, during and after

  1. Before: the initially unmagnetised rod has little or no retained attraction for a small test piece of iron.
  2. During: with the inducing magnet nearby, the rod becomes magnetised and can attract the test piece near its far end.
  3. After: move the inducing magnet far away and test the rod again. Suitable soft iron loses much of the induced effect.

The inducing magnet can also pull directly on the test piece. A comparison with the rod absent, keeping the magnet and test position fixed, helps distinguish that direct effect from the rod's contribution. Keep the geometry and test piece consistent when comparing observations.

After losing most of its magnetism, the iron remains a magnetic material: it can be magnetised again.

Inside a current-carrying solenoid

A solenoid is a coil with many turns of wire. Current is the rate of flow of charge. When current flows in the coil, it creates a magnetic field that can magnetise a suitable material placed inside.

The core sits inside the current-carrying coil

For this example, the coil's supplied on-state polarity is N on the left, S on the right. The core is magnetised in the same orientation.

Current on: the coil magnetises the core

Current on: the coil magnetises the coreA soft-iron core lies inside a continuous many-turn wire coil. The winding connects by two separate leads to a supply box labelled on. Its pole orientation is supplied: the coil is N at the left end and S at the right. The core becomes magnetised in the same orientation, N left and S right. The two core pole labels are outside the wound section so they remain visible. Current direction and supply polarity are not being inferred from this illustration. Back half-turns are drawn behind the core and front half-turns in front of it. The two leads return to the same supply box; they do not end as an open circuit in the on-state.NSSoft-iron coreCoilSupply on

The soft-iron core becomes a temporary magnet: N left and S right, matching the coil's supplied polarity.

Current off: most induced magnetisation is lost

Current off: most induced magnetisation is lostThe same soft-iron core remains inside the same coil, whose supply box is now labelled off. The main inducing field from the coil has been removed. The induced N and S core labels have been removed to indicate that suitable soft iron loses most of its induced magnetisation. This does not mean that the iron has become a non-magnetic material, or that every residual field is exactly zero. Back half-turns are drawn behind the core and front half-turns in front of it. The two leads return to the same supply box; they do not end as an open circuit in the on-state.Soft-iron coreCoilSupply off

Suitable soft iron loses most of its induced magnetisation when the main inducing field is removed. It is still a magnetic material.

The supplied coil orientation is N at the left and S at the right while current flows. The soft-iron core has the same N-left/S-right orientation. With current off, the main inducing field is removed and the core loses much of its magnetism.

The magnetised soft-iron core adds to the coil's magnetic effect, producing a stronger temporary electromagnet. Here the core is inside the coil's field, so its pole orientation corresponds to the solenoid's supplied orientation.

Switching off removes the main magnetising influence. Soft iron has low retention, so the core's magnetic effect largely disappears. A suitable steel sample magnetised in the coil can instead retain its poles after the current stops.

Choose a material for what should happen afterwards

Soft iron: a temporary magnet
It is readily magnetised and loses much of its magnetisation when the inducing field is removed. This makes it useful as the core of a switched lifting electromagnet that must release its load.
Suitable steel: a permanent magnet
It is harder to magnetise than soft iron in this comparison, but retains magnetism once magnetised. A compass needle or permanent bar magnet needs that retention so it works away from a magnetising coil.

Match the material to the task

A device picks up and releases small iron pieces

The core should become magnetised when switched on and lose much of that effect when switched off. Soft iron fits both conditions.

Using a material with high retention could leave the pieces attracted after switch-off. A compass has a different requirement: its needle should stay magnetised, so a suitable permanent-magnet material fits that task.

Temporary does not mean the effect is always exactly zero after switch-off, and permanent does not mean impossible to demagnetise. The distinction concerns how readily the material gains and retains magnetism.

Compare retention fairly

Use comparable specimen dimensions, positions and magnetising conditions. Remove the external field before comparing the retained attraction with the same test method. Repeating the comparison helps reveal variation.

The number of pieces lifted also depends on their weight, shape and contact. It can provide evidence about the magnetic effect under those conditions, but is not a direct numerical measurement of field strength or a universal material constant.

Optional check A switched device must pick up small iron pieces when current flows and release them after the current is switched off. Which material choice and reason best fit its core?
A switched device must pick up small iron pieces when current flows and release them after the current is switched off. Which material choice and reason best fit its core?