K323 / 2027
Magnetism overview

Full chapter

Magnetism

All 4 topics and the revision summary on one page.

01

Magnet properties and observations

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.

Optional check 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?
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?

02

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?

03

Finding field direction with a compass

A small compass is a bar magnet free to turn. Its north-seeking end gives the magnetic field direction at the compass's position.

Magnetic forces act without contact. A magnetic field describes the magnetic effect around a source; the field can turn a small magnet until it aligns with the local direction.

Read the north-seeking end

Identify the compass's marked N end and allow the needle to settle. The direction in which that end points is the field direction. Its S end points the other way.

The compass responds to the field where it is placed. It need not point towards the centre of a nearby bar magnet or towards geographic north. The field direction describes the needle's alignment; it is not a rule for the direction in which the centre of the whole compass must move.

Read the north-seeking end at each position

These are four separate settled-compass observations around one bar. The magnet's field is assumed to dominate the background field.

Four compass directions around an N-left, S-right bar magnetA horizontal bar at the centre has N on its left and S on its right. A small compass on the axis to the left of N has its marked N end pointing left, away from N. A compass above the centre points its N end right. A compass on the axis to the right of S points its N end left, towards S. A compass below the centre points its N end right. Each teal half-needle ends at the side labelled N. Its direction is tangent to the ideal local field, not necessarily geographic north or the direction towards the bar's centre. No field-direction arrow is being used as a force on the whole compass.NSLeft of NNAbove centreNRight of SNBelow centreN

The marked N end indicates the local field direction. Reversing the bar reverses these four ideal directions.

This bar has N on the left and S on the right. With other fields neglected, the compass N end points left on the axis beyond either end of the bar, and right above and below its centre.

Read four local directions

The same bar, four observation points

  • Left of the N pole: the compass N end points left, away from that pole.
  • Above the centre: the N end points right.
  • Right of the S pole: the N end points left, towards that pole.
  • Below the centre: the N end points right.

These are local directions around one source. If the magnet is reversed while the positions stay fixed, the ideal directions reverse too.

Map a field using a plotting compass

  1. Place a bar magnet on paper on a non-magnetic surface. Draw its outline and label its poles. Keep the magnet and paper fixed.
  2. Check the marked N end and that the compass turns freely. Place it at a starting point near the magnet and wait for it to settle.
  3. Mark the positions of the needle ends, identifying N. Draw a short arrow from its S-end mark towards its N-end mark.
  4. Move the compass so its S end occupies the previous N-end mark. Let it settle and mark the new N-end position.
  5. Repeat in small steps, then join the marks with a smooth curve and add arrows in the recorded direction.
  6. Start again at several other positions to build the field pattern.

A small compass better approximates the direction at one position. Record a settled needle rather than a moving or stuck one, and keep nearby magnets and magnetic objects away from the map.

The needle responds to the resultant field, including Earth's field. An ideal bar-magnet diagram neglects that background. Make observations where the bar's effect is strong enough to dominate; far from it, the background can change the observed direction.

What iron filings show

Iron filings on a sheet near a magnet become magnetised and align, revealing the rough shape of the pattern. The visible chains do not identify an arrow direction by themselves. Use the compass N-end convention to add the arrows.

Field lines are drawn representations. The filings help reveal their shape; they are not threads that already existed around the magnet.

Optional check A small compass settles at a marked position near a bar magnet. Its north-seeking end points right. What is the magnetic field direction at that position?
A small compass settles at a marked position near a bar magnet. Its north-seeking end points right. What is the magnetic field direction at that position?

04

Magnetic field patterns

A field pattern combines shape and direction. Identify every pole first, then draw smooth lines with arrows that agree with the compass convention.

The north-seeking end of a settled compass points along the local field. Outside a magnet, field lines leave N and enter S.

Follow the exterior field from N to S

Teal arrows show field direction. These idealised patterns neglect background fields. Internal return paths through the magnets are omitted.

One bar: exterior field from N to S

One bar: exterior field from N to SOne horizontal bar has N on its left and S on its right. Three curves above and their three reflected counterparts below leave the north region, curve around outside the bar and enter the south region. Teal arrowheads are tangent to those curves and point from N to S outside the magnet. The opaque bar hides the unshown interior return paths. The drawing represents a symmetric qualitative field, with background fields neglected; it has no numerical field-strength scale.NS

Outside the bar, arrows leave its north region and enter its south region. A compass's N end aligns with the local tangent.

Unlike facing poles: field crosses the gap

Unlike facing poles: field crosses the gapTwo horizontal bars both have S on the left and N on the right. The left bar therefore presents N to the gap and the right bar presents S. Five inner field curves, including the central straight line, run from the facing N towards the facing S. Other curves return to the outer south poles, with arrows following the same exterior N-to-S rule. In particular, a large outer curve runs from the far-right N to the far-left S. No field curves cross. These field arrows are distinct from the two forces of attraction.SNSN

The gap's arrows run from the facing N to S. Outer curves obey the same rule, following the other labelled poles.

Like facing poles: lines bend away from the gap

Like facing poles: lines bend away from the gapThe left bar is S-left and N-right; the right bar is N-left and S-right, so N faces N. Field curves leave each facing north region, bend away from the central gap and return to the outer south region on that side. The left and right patterns are mirror images. No curve joins N to N, and no curves cross. The identical symmetric model has cancellation exactly at the midpoint between the facing poles; no finite direction arrow is drawn there. Background fields are neglected. The field-line pattern is not a picture of lines physically pushing one another.SNNS

The curves leave both facing north regions and bend away from the gap. They do not connect one N pole to the other.

Field lines form continuous loops and do not cross. Line spacing is qualitative here; counting the drawn curves in different panels does not measure or compare field strength.

The panels show a single bar, unlike facing poles and like facing poles. Arrows describe the magnetic field outside the magnets, rather than forces on the complete magnets.

One bar magnet

Draw lines that leave the north pole, curve through the space around the bar and enter the south pole. The compass needle lies tangent to the local line, with its N end following the arrow.

Magnetic field lines continue through the magnet to form loops. Inside the bar their return direction is S to N. Keep the word outside with the familiar N-to-S rule.

Unlike poles facing

With N on the left of the gap and S on the right, lines connect across the air gap with arrows left to right, from N to S. Lines farther from the axis curve around the magnets.

The magnets attract, but that force observation does not change the field-arrow convention. A force on a whole magnet and the local field direction are different things to label.

Like poles facing

For N facing N, lines leave both facing north poles and bend away from the central gap region. They do not join N directly to N. For the corresponding S-facing-S arrangement, reverse the arrows so that lines enter both south poles.

For two identical magnets arranged symmetrically with like poles facing, their fields can cancel at the exact midpoint if background fields are neglected. There is no nonzero field-direction arrow to draw at that cancellation point. A real compass there can still respond to Earth's field or another disturbance.

Check the representation

  • Lines do not cross. A crossing would assign two different field directions to one position.
  • Arrows agree along a line. They follow a compass N end; outside magnets they leave N and enter S.
  • Closer spacing indicates a stronger field qualitatively within one consistently drawn pattern. An illustrator's arbitrary line count does not give an exact field strength.
  • The lines represent the field. They are not physical strings pushing one another apart, nor paths that every moving magnet must follow.

Repair a drawing

The gap shape is right, but the arrows run S to N

For unlike facing poles, keep the connecting curves and reverse the arrows so that they run N to S across the gap. The correction follows the compass convention.

If one magnet is then turned around, identify the new facing poles before drawing again. The gap pattern may need to change as well as its arrows.

Optional check Two bar magnets have N facing S across an air gap, with N on the left of the gap. A drawing joins the facing poles with smooth non-crossing lines but points the arrows from right to left. What should be corrected?
Two bar magnets have N facing S across an air gap, with N on the left of the gap. A drawing joins the facing poles with smooth non-crossing lines but points the arrows from right to left. What should be corrected?

Revision summary

Poles and evidence

Like poles repel and unlike poles attract. A freely suspended magnet turns approximately north-south when nearby disturbances are small. Its north-seeking end points roughly towards geographic north.

A magnetic material can become magnetised; it need not already be a permanent magnet. An end that repels a known N pole is N in the simple end-to-end test. Attraction alone can also involve an initially unmagnetised magnetic material. Dividing an ordinary bar magnet produces smaller magnets with both poles.

Both ways to induce magnetism

Near a strong magnet
The nearer end of a magnetic rod develops the opposite pole to the inducing pole. In the example with N beside end A, A becomes S and the far end B becomes N. No contact is required.
Inside a current-carrying solenoid
The coil's field magnetises its core. In the supplied N-left/S-right coil orientation, the core is also N-left/S-right. A soft-iron core strengthens the temporary magnetic effect and loses much of its magnetism when current stops.
Soft iron and suitable steel
Soft iron is readily magnetised and has low retention, useful when a switched lifting magnet must release its load. Suitable steel is harder to magnetise but retains magnetism, useful for a compass needle or permanent magnet.

Compare retention after the inducing field has been removed, using comparable samples and a consistent test. The number of objects lifted also depends on their shape, weight and contact.

Compass direction and mapping

The marked N end of a settled compass gives the local field direction. It does not always point towards geographic north or the centre of the nearby magnet.

  1. Keep the bar magnet and paper fixed and label the pole positions.
  2. Let a small compass settle, then mark its needle ends and N-end direction.
  3. Move its S end to the previous N-end mark, let it settle and mark the next position.
  4. Join the marks smoothly with arrows in that direction; repeat from other starting positions.

Avoid nearby magnetic disturbances and stuck or moving needles. The compass responds to the resultant field, including Earth's. Iron filings show a rough shape; the compass supplies the arrow direction.

Draw shape and direction together

  • Single bar: outside it, lines leave N and enter S. Their return through the magnet is S to N.
  • Unlike facing poles: lines connect across the gap from N to S.
  • Like facing poles: lines bend away from the central gap region and do not connect N to N or S to S. Reverse the arrows when changing a corresponding N/N pattern to S/S.
  • Representation: lines do not cross. Closer spacing gives a qualitative strength comparison within one consistent drawing.
  • Meaning: a field arrow follows the local compass N end. It is distinct from a force arrow on a whole magnet or a moving object's path.
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