Induced Magnetism and Field Patterns

What this lesson covers in the specification

Incandio is aligned to this specification. It is not published by, endorsed by or affiliated with Pearson, and it reproduces none of Pearson's wording — the statement numbers are given so you can check every lesson against your own copy.

• 6.5 — Magnetism induced in some materials placed in a magnetic field
• 6.6 — Practical: investigate the field pattern of a bar magnet and between two bar magnets (required practical)
• 6.7 — Using two permanent magnets to produce a uniform field

1 · Understand it

No exam language yet. The only question this section answers is: do I actually understand what is happening?

Hang a steel paperclip from a magnet and a second paperclip will hang from the first, and a third from that. The magnet is only touching the top one, so what is holding the chain together? The answer is INDUCED MAGNETISM: a magnetic material placed in a magnetic field becomes a magnet itself, for as long as it stays there.

Think of it like a room full of weathervanes

Imagine thousands of tiny weathervanes scattered across a field, all pointing in random directions, so that from a distance they average out to nothing at all. Now the wind gets up, and every one of them swings round to point the same way. Nothing new has arrived and nothing has been added — the same weathervanes are simply lined up, and now the field has an obvious direction. That is what happens inside iron. It is full of minute regions that are already magnetic, but pointing every which way, so the bar as a whole shows nothing. Put it in a magnetic field and they turn to line up, and the bar becomes a magnet. Take the field away and, if the iron is soft, they fall back out of step and the magnetism goes.

Why an induced magnet is always ATTRACTED, never repelled

1. Bring the NORTH pole of a magnet near an iron nail.
2. The nail becomes magnetised, and the end nearest the magnet becomes a SOUTH pole — because that is the arrangement the field produces.
3. North and south attract, so the nail is pulled towards the magnet.
4. Now turn the magnet round and present its SOUTH pole instead.
5. The nail re-magnetises the other way, so the near end is now a NORTH pole, and again the poles are unlike and the nail is attracted.
6. Whichever pole you offer, the nail arranges itself to be attracted. It has no choice, which is exactly why induced magnetism can never produce repulsion.

So statement 6.5 explains the ambiguity that the previous lesson made you careful about. An unmagnetised magnetic material is attracted by either pole of a magnet, because the pole it is offered decides which way the material magnetises. A permanent magnet cannot do that — its poles are fixed — so a permanent magnet is repelled when it is presented the wrong way round. This also explains the paperclip chain: each clip is magnetised by the one above it, becoming a magnet in turn, which is why the chain gets weaker further down and collapses the moment the top magnet is removed.

Now to seeing the field, which is the required practical. Two methods are used and they do different jobs. IRON FILINGS sprinkled onto a card over a magnet give the whole pattern instantly, because each filing becomes an induced magnet and lines up along the field — but the filings carry no arrows, so they show the shape and not the direction. A PLOTTING COMPASS gives the direction, because its north end points along the field line, but it gives you the pattern only slowly, one small step at a time. The sensible experiment uses filings for the shape and a compass for the arrows.

The patterns between two magnets are the examined part, and there are two cases. Place UNLIKE poles facing each other — north opposite south — and the lines run straight across the gap from one to the other, because a free north pole placed in the gap would be pushed away from the north and pulled towards the south, and both effects point the same way. Place LIKE poles facing each other and the lines from each magnet curve away from the gap; between them is a point on the centre line where the two fields exactly cancel and the field is ZERO. That point is called a NEUTRAL POINT, and a plotting compass placed there does not know where to point.

Statement 6.7 uses the first of these cases deliberately. Put the north pole of one magnet opposite the south pole of another, a small distance apart, and in the region between them — away from the edges — the field lines are straight, parallel and evenly spaced. Straight and parallel means the direction is the same everywhere; evenly spaced means the strength is the same everywhere. A field with the same magnitude and direction at every point is called a UNIFORM FIELD, and it is what the motor effect and the generator both assume.

Reading a field pattern for both of its meanings

In a plotted field pattern, the lines beside a bar magnet are 3 mm apart near the pole and 12 mm apart at the far edge of the card. What does that tell you, and roughly by what factor does the field differ?

1. Spacing encodes strength: closer lines mean a stronger field.
2. The lines near the pole are four times closer together than those at the edge (12 ÷ 3 = 4).
3. So the field near the pole is roughly four times stronger than at the edge of the card.
4. Direction is a separate reading: at every point the field acts ALONG the line, in the direction of its arrow.
5. Note the word 'roughly'. Field-line spacing is a genuine but approximate guide to strength, and no exam question will ask for a precise value from it.

Answer: The field near the pole is about four times stronger, and the direction at any point is along the line there.

2 · Grade 9 Notes

A different job from the section above. You have already understood it; this is the precise set of things to LEARN — definitions to reproduce word for word, processes in order, equations with units, and the answers that score full marks.

Learn this definition · Induced magnetism

Magnetism produced in a magnetic material when it is placed in a magnetic field. The material becomes a magnet while it remains in the field, and a magnetically soft material loses that magnetism when the field is removed.

Learn this definition · Uniform field

A field that has the same magnitude and the same direction at every point. It is shown by field lines that are straight, parallel and evenly spaced.

Required practical 6.6 — plotting the field of one magnet and of two

1. Place a bar magnet on a bench and lay a sheet of white card or paper over it, marking the outline of the magnet on the card.
2. Sprinkle iron filings thinly and evenly over the card from a shaker, then tap the card gently until the filings settle into the pattern.
3. Record the pattern by sketching it, and note where the filings are most densely packed.
4. For the directions, clear the filings and place a plotting compass near one pole; mark a dot at each end of the needle.
5. Move the compass so that its tail sits on the dot the head just made, mark the new head position, and repeat across the card.
6. Join the dots into a smooth line and put an arrow on it in the direction the compass north pointed.
7. Repeat the whole procedure with two magnets, first with UNLIKE poles facing across a gap, then with LIKE poles facing, marking any neutral point where the compass will not settle.

Variables

Independent (changed) — The arrangement of the magnets — one magnet, unlike poles facing, or like poles facing
Dependent (measured) — The shape and direction of the field pattern recorded on the card

Sources of error

Iron filings against a plotting compass

The three patterns you must be able to draw

• ONE BAR MAGNET — lines from N round to S, crowded at the poles, never crossing
• UNLIKE POLES FACING — lines run straight across the gap from the N of one to the S of the other
• LIKE POLES FACING — lines curve away from the gap, and there is a NEUTRAL POINT on the centre line where the field is zero
• Between unlike poles, away from the edges, the lines are straight, parallel and evenly spaced — a UNIFORM field

Plotting one field line with a compass

1. Place the compass near a pole and mark a dot at the head and a dot at the tail of the needle.
2. Move the compass so its TAIL sits on the dot the head just made.
3. Mark the new head position.
4. Repeat until the compass reaches the other pole or the edge of the card.
5. Join the dots into a smooth curve and add an arrow in the direction the north end pointed.

Model answer [4 marks]

Explain why an unmagnetised iron nail is attracted to either pole of a magnet, and is never repelled by either. [4]

When the nail is placed in the magnet's field it becomes magnetised by induction, and the pole induced in the end nearest the magnet is always the opposite of the magnet's pole. Unlike poles attract, so the nail is pulled towards the magnet. If the magnet is turned round, the nail simply magnetises the other way, so the near end is again the opposite pole and the nail is attracted once more. Because the nail's poles are induced by the magnet rather than fixed, it always arranges itself to be attracted and can never be repelled.

Model answer [4 marks]

Describe how two permanent magnets can be used to produce a uniform magnetic field, and state how the pattern shows the field is uniform. [4]

Place the two magnets a short distance apart with the north pole of one facing the south pole of the other. In the region between them, away from the edges, the field lines run straight across the gap from the north pole to the south pole. The lines are straight and parallel, which shows that the direction of the field is the same at every point, and they are evenly spaced, which shows that the strength of the field is the same at every point. A field with the same magnitude and direction everywhere is a uniform field.

Not this: Iron filings show which way a magnetic field points.

This: Filings show only the SHAPE of the pattern. Each filing is a tiny induced magnet with two ends and no arrow, so it lines up ALONG a field line without indicating which way along it. Only a plotting compass gives the direction.

Mark-losing trap. Induced magnetism can only ATTRACT — the material always magnetises the way that pulls it in.

Mark-losing trap. A NEUTRAL POINT is where two fields cancel exactly, so the field is zero and a compass will not settle.

Mark-losing trap. Uniform means straight, parallel AND evenly spaced — all three, or it is not uniform.

3 · Prove it — the five questions

The five questions climb Grade 6 → Grade 7 → Grade 8 → Grade 9 → Grade 9 challenge, and are marked inside Incandio on your own device, by rule, with an authored diagnosis of the mistake you actually made. The mark schemes stay in the app so that the practice is worth doing; the questions themselves are here.

1. Grade 6 · State [2 marks] — A steel paperclip hangs from a magnet, and a second paperclip hangs from the first. What has happened to the first paperclip?
2. Grade 7 · Explain [3 marks] — Explain why iron filings sprinkled on a card over a magnet reveal the field pattern.
3. Grade 8 · Explain [3 marks] — Two identical bar magnets are placed on a bench with their north poles facing each other across a small gap. A plotting compass is placed exactly midway between them on the centre line. Explain what the compass does and why.
4. Grade 9 · Describe [6 marks] — Select every statement that belongs in a full-mark description of how to plot the magnetic field pattern of a single bar magnet using a plotting compass.
5. 9+ · Evaluate [6 marks] — A student sets up two bar magnets with unlike poles facing across a gap and claims that the field everywhere between them is uniform, because 'the lines go straight across'. Select every statement that belongs in a full-mark evaluation.

The people behind this science

Two ways into the same idea — the one who did this exact experiment thousands of times, and the one who read a magnetic field with nothing but a compass needle. Inside Incandio each of them answers knowing exactly which lesson you have just finished.

Michael Faraday — the one who did this exact experiment thousands of times

This page's practical was Faraday's daily method for decades, and his laboratory notebooks are full of the patterns. What makes him worth asking here rather than about the field in general is that he understood precisely what the filings were doing: each one is magnetised by induction and turns to lie along the field, which is why the pattern appears — and also why it has no direction in it. He is the person to ask why an experiment that shows you so much still needs a compass to complete it, and why he trusted a picture made of iron dust more than the mathematics of his contemporaries.

• “Why do iron filings arrange themselves into curves at all?”
• “What can the filings not tell you?”
• “What happens at a point where two fields cancel?”
• “How did you record a pattern before photography?”
• “Why did you keep repeating an experiment you already understood?”

Hans Christian Ørsted — the one who read a magnetic field with nothing but a compass needle

Every direction on this page is found with a plotting compass, and Ørsted is the person who established what a compass needle actually reports. In 1820 he found that a needle near a current-carrying wire does not point towards the wire or away from it but sets itself at right angles to it — a result so unexpected that he spent three months trying to destroy it before publishing. He is the right second figure here because he treated a small pivoted magnet as a scientific instrument rather than a navigation aid, and because he shows what it takes to trust a reading that contradicts what you expected to see.

• “What is a compass needle actually responding to?”
• “Why did the needle turn sideways instead of pointing at the wire?”
• “How do you know a needle is showing the field and not being pulled by something else?”
• “Why did you spend three months trying to disprove yourself?”
• “What would a compass do at a point where the field is zero?”

Then defend it

On Incandio a lesson is not finished when the questions come out right. You teach the idea back to Ember, an AI apprentice who asks the awkward question, and then you argue it against William Gilbert in a structured debate marked against descriptors you can read before you enter. Learn it, teach it, then defend it — all three happen on this page once the app loads.

Carry on through the course

• Previous lesson: Magnets and Magnetic Fields
• Next lesson: Electromagnets
• Magnets and Magnetic Fields — Why only repulsion proves you are holding a magnet, and what a field line is actually a picture of.
• Electromagnets — The four pages of Latin that showed a current makes a field — and why that field goes round the wire rather than along it.
• The Motor Effect — Put a current in a magnetic field and something moves — the effect behind every electric motor and every loudspeaker.
• Electromagnetic Induction — Move a magnet near a coil and a voltage appears from nowhere — provided something is changing.
• Transformers — Two coils that never touch, the reason the National Grid runs at 400 000 volts, and why none of it works on a battery.
• All of Physics · Incandio Science