Magnets and magnetic fields: poles, field lines and the planet-sized magnet under your feet
By the end of this lesson you will be able to
- State which materials are magnetic and distinguish a permanent magnet from a magnetic material using the repulsion test.
- Define a magnetic field and draw the field lines around a bar magnet, labelled N and S, with arrows showing the direction a free north pole would move.
- Describe how to plot a field with a compass and with iron filings, and interpret line spacing as field strength.
- Explain induced magnetism and why soft iron and steel behave differently.
- Describe the Earth’s magnetic field, explain why a compass needle points north, and calculate a true bearing from a magnetic one.
- Design a valid investigation into a magnetic force, with variables, values and instruments.
You met this before
- A non-contact force acts across a gap: gravity, magnetism and static electricity all act through a field, a region where an object feels the force (Year 2).
- Magnets have two poles, north-seeking (N) and south-seeking (S); like poles repel and unlike poles attract; the force is strongest at the poles (Year 2).
- A force is measured in newtons (N) with a newton meter; weight = mass × g with g = 9.81 N/kg on Earth (Year 1 and Year 3).
- A fair test changes one independent variable, measures one dependent variable with a named instrument and keeps every other variable fixed at a stated value (Year 1).
- Newton’s third law: if a magnet pulls a steel ball with 0.40 N, the ball pulls the magnet with 0.40 N the other way (Year 3).
A steel ball of mass 0.050 kg hangs from a newton meter which reads 0.49 N. A magnet is brought below the ball and the reading rises to 0.73 N. Calculate the magnetic force on the ball.
Reveal
The newton meter now supports the weight plus the magnetic pull.
Magnetic force = 0.73 − 0.49 = 0.24 N, downwards on the ball (and by Newton’s third law 0.24 N upwards on the magnet).
The weight check: W = 0.050 kg × 9.81 N/kg = 0.49 N, so the meter was reading correctly before the magnet arrived.
Revise it in full: Year 2 · Interactions · Lesson 3 — Non-contact forces: gravity, magnets and static electricity · Year 1 · Being a scientist · Lesson 3 — Variables and the fair test
The puzzle
Hold a compass anywhere in Botswana and the red end settles pointing north. Nothing is pulling on it that you can see; the needle turns inside a sealed plastic case, indoors, at night, in a moving car.
Now the awkward part. The red end is a magnetic north pole, and unlike poles attract — so whatever sits under the Arctic must be a magnetic south pole. And the red end does not point exactly at the geographic North Pole either: from Gaborone it points roughly 18° west of it, and the offset drifts by about a quarter of a degree every year.
What is the compass actually feeling, why is the pole “the wrong way round”, and why does it move?
A bar magnet is cut in half across the middle. Predict what you find at the cut ends, and whether you could ever separate a single north pole from a single south pole by cutting.
Compare your answer
Each half is a complete magnet with an N and an S: a new south pole appears on one cut face and a new north pole on the other. Cut again and again — down to a single atom — and every piece still has two poles.
An isolated north pole (a “monopole”) has never been found, which is why the field lines in Section 1.3 always form closed loops and why “free north pole” is a thought experiment, not something you can put in a box.
The science — structured notes
1.1 Magnetic materials and the repulsion test
Only a few materials are attracted by a magnet: iron, steel (an alloy of iron), nickel, cobalt, and alloys made from them — including the two you have most probably held: neodymium magnets (Nd₂Fe₁₄B, the small grey discs that grip absurdly hard) and ferrite (the black ceramic in fridge magnets).
Everything else — copper, aluminium, brass, gold, glass, wood, plastic, water, you — is, for this course, non-magnetic. A magnet will lift a steel paper clip and ignore a copper coin of the same mass, however strong it is.
(A physicist would add that all matter responds very slightly, and that a strong enough field will levitate a frog, but the effect is thousands of times weaker and the examination calls those materials non-magnetic.)
| Material | Magnetic? | Behaviour | Used for |
|---|---|---|---|
| Soft iron (pure iron) | Yes | Magnetises strongly in a field, loses almost all of it when the field is removed — a temporary magnet | Cores of electromagnets, transformers, relays (Lessons 3 and 5) |
| Steel (iron + carbon) | Yes | Harder to magnetise, but keeps its magnetism — a permanent magnet | Compass needles, older bar magnets |
| Neodymium alloy, alnico, ferrite | Yes | Permanent; neodymium is the strongest, ferrite the cheapest | Motors, speakers, hard drives, fridge magnets |
| Nickel, cobalt | Yes | Magnetic metals; nickel alloys in coins are why some coins stick to a magnet | Alloys, coins, batteries |
| Copper, aluminium, brass, gold, silver | No | Not attracted, not repelled; an eddy-current effect appears only when they move in a field (Lesson 4) | Wires — because they conduct and do not stick to magnets |
| Plastic, wood, glass, paper, water | No | Field passes straight through them, undiminished | The card between a fridge magnet and a photo |
A magnet and a piece of soft iron look the same to a magnet: both are attracted. The only test that proves something is a magnet is repulsion. Bring one end of a bar magnet up to the unknown object; if either end is pushed away, the object is a magnet.
Attraction proves only that it is magnetic material — a magnet attracts iron by turning it, temporarily, into a magnet with the opposite pole nearest (Section 1.4).
A student tests three grey rods. Rod A is attracted by both ends of her bar magnet; rod B is attracted by one end and repelled by the other; rod C is unaffected by either. She concludes: “A and B are magnets, C is aluminium.” Find the error and write the correct conclusion.
Reveal answer
Rod A is attracted by both poles, which is what a piece of iron or steel does — it is magnetic material, not proved to be a magnet. Only B, which is repelled, is proved to be a magnet.
C is non-magnetic but could be aluminium, copper, brass or plastic painted grey; the test cannot tell which. Correct conclusion: “B is a magnet; A is a magnetic material that is not a magnet (or a very weak one); C is non-magnetic.”
1.2 What a magnetic field is
A magnetic field is the region around a magnet (or around a current — Lesson 3) in which a magnetic material, another magnet or a moving charge experiences a force.
You cannot see it, but it has a definite direction at every point and a definite strength. The direction at a point is the direction of the force on a free north pole placed there — equivalently, the direction the N end of a small compass points.
The strength is called the magnetic flux density, symbol B, measured in tesla (T). The tesla is a large unit: the Earth’s field at the surface is about 5 × 10⁻⁵ T (50 microtesla, μT), a fridge magnet a few millitesla at its surface, a neodymium disc about 0.3–0.5 T at its face, and a hospital MRI scanner 1.5 or 3 T.
1.3 Field lines: drawing the invisible
The field is drawn with field lines (lines of magnetic flux). The rules are strict, and the examination tests them by asking you to draw the field of a bar magnet:
- Outside the magnet, lines run from the north pole to the south pole. Put an arrow on every line to show it — this is the direction a free north pole would move.
- Lines never cross. At a crossing the field would have two directions at once, which is impossible.
- Lines are closest together where the field is strongest — at the poles — and spread out where it is weaker.
- Lines are continuous closed loops: inside the magnet they run from S back to N. (You are not usually asked to draw the inside.)
- The lines show the direction of the field, not the “path” of anything. Nothing flows along them.
Two ways to make the lines visible. Plotting compass: place the magnet on paper, put a small compass near the N pole, mark a dot at each end of the needle, move the compass so its tail sits on the head dot, mark again, and repeat until you reach the S pole; join the dots and add an arrow.
Each chain is one field line, and the compass gives you the direction directly. Iron filings: lay a sheet of paper over the magnet, sprinkle filings, tap; each filing becomes a tiny magnet and lines up along the field, so the pattern appears at once — but filings show shape and spacing only, not direction. The examination asks for arrows; filings cannot give them.
Two bar magnets are placed end to end 3 cm apart, N facing N. Describe the field pattern between them and explain why there is a point between the poles where a compass needle spins freely.
Reveal answer
Lines leave each N pole and, since they cannot cross and cannot enter another N pole, they bend sharply away from the gap and curve round to the far S poles.
Midway between the two N poles the two fields are equal in strength and opposite in direction, so they cancel: the resultant field is zero. That is a neutral point, and a compass there feels no net turning force and points anywhere.
(Between N and S facing each other, by contrast, the lines run almost straight across the gap and the field is strong and nearly uniform.)
1.4 Induced magnetism: why a magnet picks up a nail that then picks up another
Bring a magnet’s N pole towards an unmagnetised iron nail and the nail becomes a magnet: the end nearest the N pole becomes an S pole, the far end an N pole.
This is induced magnetism, and it explains attraction — the induced S pole is closer to the magnet’s N than the induced N pole is, so the pull beats the push and the nail moves.
Hang a second nail from the first and it, too, is magnetised by the first, which is why a chain of clips hangs from one magnet in the lab of Lesson 3.
The picture behind it: in iron, each atom is a tiny magnet, and neighbouring atoms lock together in regions called domains, each a few hundredths of a millimetre across, in which all the atomic magnets point the same way.
In an unmagnetised nail the domains point in every direction and cancel. An external field turns the domains to point along it — the nail is magnetised.
In soft iron the domains turn back at random as soon as the field is removed, so the nail drops its clips; in steel the carbon atoms pin the domains, so they stay aligned and the steel keeps its magnetism (that pinning is also why steel is harder to magnetise in the first place).
Heat a magnet strongly or hammer it and the jostling atoms scramble the domains: it demagnetises. Iron loses its magnetism completely above its Curie temperature, 770 °C.
In the lab below a steel ball rests on a top-pan balance zeroed at 200.0 g. A neodymium magnet clamped 20 mm above the ball pulls it upwards without touching it and the balance reads 160.0 g. Calculate the magnetic force on the ball, and state the force on the magnet.
Change in reading: 200.0 g − 160.0 g = 40.0 g = 0.0400 kg.
Force: the balance reads less by the weight of 0.0400 kg that the magnet is now supporting.
F = m × g
F = 0.0400 kg × 9.81 N/kg = 0.392 N
(0.40 N with g = 10 N/kg, which is what the lab’s display uses.)
Force on the magnet: by Newton’s third law the ball pulls the magnet downwards with an equal force of 0.392 N — which is why the balance method works: the clamp stand feels an extra 0.392 N, and the balance feels 0.392 N less.
Check the physics: the ball is steel, magnetised by induction with its S pole uppermost, facing the magnet’s N pole; the attraction between the magnet’s N and the induced S is the force measured. Replace the ball by copper and the reading stays at 200.0 g.
The magnet is turned over so its S pole faces the ball. Predict the new balance reading and explain, using induced poles.
Reveal answer
Still 160.0 g. The ball has no permanent poles; whichever pole faces it induces the opposite pole in the nearer end of the ball — now an N pole uppermost — and the attraction is the same size. Only a permanent magnet on the balance would give a different reading (a repulsion, with the reading rising above 200.0 g) when the magnet is turned over.
1.5 The Earth’s magnetic field
The Earth behaves as though a huge bar magnet lay along its axis, tilted by about 10° from the rotation axis, with — and this is the puzzle’s answer — its magnetic south pole up in the Arctic and its magnetic north pole in the Antarctic. A compass needle’s N end points north because it is attracted by a south pole.
The pole in the Arctic is nevertheless called the “north magnetic pole” in everyday speech, meaning the place on the Earth’s surface where the field points straight down; keep the two uses apart in an answer by writing “the magnetic pole near geographic north is a south-seeking pole” or by drawing it.
Three things the diagram does not tell you. First, there is no bar magnet inside the Earth — the core is far above iron’s Curie temperature. The field is generated by electric currents in the churning liquid iron of the outer core, 2 900 km down, a self-sustaining geodynamo (you will see in Lessons 3 and 4 that currents make fields and moving conductors in fields make currents; the core does both at once).
Second, the field is not fixed: the “north magnetic pole” wanders — it moved from northern Canada towards Siberia at up to about 55 km a year in the 2000s — and the field has reversed polarity many times, most recently about 780 000 years ago, the record preserved in the magnetised minerals of cooled lava and the sea floor.
Third, the field is weak and uneven: about 25–65 μT at the surface, and weakest of all over the South Atlantic and southern Africa, where it is around 25–30 μT — the South Atlantic Anomaly, through which satellites pass with their electronics at risk.
Declination. Because the magnetic pole is not at the geographic pole, a compass points a little east or west of true north by an angle called the magnetic declination, which depends on where you are and changes slowly.
Over Botswana it is currently west everywhere — from about 8° W in the far north to about 18° W at Gaborone (the compass N end points west of true north); over parts of the Americas it is east. Maps print the local value and its annual change; a phone’s compass applies a model of the field — the World Magnetic Model, revised every five years — to convert automatically.
A geologist must walk from a borehole on a true bearing of 075° for 40 km to reach a survey marker (bearings are measured clockwise from north). The magnetic declination on her map is 15° W. (1) State the compass bearing she should walk.
(2) She forgets the correction and walks on a compass bearing of 075°. Calculate how far from the marker she ends up. (3) Explain why a map from 1990 could give her a different declination.
(1) The compass N end points 15° west (anticlockwise) of true north, so every compass bearing reads 15° larger than the true bearing of the same direction.
Compass bearing = true bearing + declination(W)
Compass bearing = 075° + 15° = 090°
(A quick check: walking due east, true 090°, she would need to read 105° on the compass.)
(2) On compass 075° her true bearing is 075° − 15° = 060°, an error of 15° to the left.
Sideways miss after 40 km = 40 km × sin 15°
Sideways miss = 40 × 0.259 = 10.4 km — in flat mopane scrub, a serious error.
(The direct distance from where she stands to the marker is 2 × 40 × sin 7.5° = 10.4 km as well; the two agree to three figures for small angles.)
(3) The field is generated by moving liquid iron, and the pattern drifts year by year (secular variation); over southern Africa the declination has recently been changing by roughly a quarter of a degree a year, so a 1990 value could be several degrees out — enough to matter over 40 km. She should use the map’s stated annual change or a current model.
A ship’s compass in the South Atlantic reads 040° where the declination is 20° W. Calculate the true bearing. Then state what a compass would read heading true north at a place where the declination is 8° E.
Reveal answer
True = compass − declination(W)
True = 040° − 20° = 020°
With 8° east declination the compass N end sits 8° clockwise of true north, so heading true north (000°) the compass reads 360° − 8° = 352°.
The rule:
True = magnetic + east declination
True = magnetic − west declination
“The Earth’s magnetic field is strongest at the equator, because that is where the field lines are straightest.”
Reveal answer
False. The lines are most crowded near the poles, where they plunge into or out of the ground, so the field is strongest there (about 60–65 μT) and weakest near the magnetic equator (about 25–35 μT). Straight, widely spaced lines mean a weak field; crowding means strength. The lowest surface values on Earth are in the South Atlantic Anomaly off Brazil and over southern Africa.
1.6 Field strength, distance and the force on a magnetic object
The force a magnet exerts on a piece of iron falls off rapidly with distance — much faster than the inverse square law of gravity. The reason is that a magnet has two poles which pull opposite ways on the iron; far away their effects nearly cancel, so the field of a bar magnet (a dipole) falls roughly as 1 ÷ distance³, and the force on an object it magnetises falls faster still.
You are not expected to know these laws, but you are expected to test a relationship for proportionality and to describe it honestly. The lab below gives data to do exactly that: force against gap for a neodymium magnet above a steel ball.
| Gap / mm | Force / N (model) | Force × gap / N mm | Force × gap² / N mm² |
|---|---|---|---|
| 5 | 1.64 | 8.2 | 41 |
| 10 | 0.95 | 9.5 | 95 |
| 20 | 0.40 | 8.0 | 160 |
| 30 | 0.20 | 6.0 | 180 |
| 50 | 0.075 | 3.8 | 190 |
Neither product is constant, so the force is neither inversely proportional to the gap nor to its square: it is “a curve that falls steeply and then flattens”, and the honest description in a conclusion quotes values — “the force falls from 1.64 N at 5 mm to 0.075 N at 50 mm, by a factor of 22 for a tenfold increase in gap” — without claiming a law the data do not support.
(The model behind the lab uses the distance from the magnet’s centre, about 20 mm behind its face, which is why the simple products drift.)
Using the table, a student claims “doubling the gap halves the force”. Test the claim at 5 → 10 mm and at 10 → 20 mm, and write one sentence she could defend.
Reveal answer
5 → 10 mm: 1.64 → 0.95 N, a factor of 0.58, not 0.5. 10 → 20 mm: 0.95 → 0.40 N, a factor of 0.42. The claim fails both tests and fails them in opposite directions, so no single power law fits the raw gap.
Defensible: “The force decreases as the gap increases; doubling the gap cuts the force to roughly a half at small gaps and to well under a half at larger gaps, so the relationship is not inverse proportion.”
A neodymium magnet holds a 10-page document to a steel fridge door. Explain, using induced magnetism and the idea of field strength falling with distance, why it will not hold 40 pages.
Reveal answer
The magnet magnetises the steel door by induction, inducing an opposite pole in the steel directly under it; the attraction between the magnet and that induced pole clamps the paper, and friction between paper, magnet and door supports the weight.
Paper is non-magnetic, so the field passes through it undiminished, but each extra page increases the gap between magnet and steel. The field at the steel falls steeply with distance, so the induced pole is weaker and the clamping force falls — with 40 pages the gap is four times larger, the force is far less than a quarter of what it was, and friction can no longer hold the greater weight.
The Earth’s field at Gaborone is about 28 μT, and a hospital MRI magnet produces 1.5 T. (1) Express both in tesla in scientific notation and calculate how many times stronger the MRI field is.
(2) The MRI’s field has fallen to about 0.5 mT (5 × 10⁻⁴ T) at a distance of 5 m from the centre of the scanner — the “5-gauss line” beyond which people with pacemakers may safely stand.
Explain why the field must fall far faster than 1 ÷ distance between 0.5 m and 5 m, using a calculation.
Reveal answer
(1) 28 μT = 2.8 × 10⁻⁵ T
1.5 T = 1.5 × 10⁰ T
Ratio = 1.5 ÷ 2.8 × 10⁻⁵ = 5.4 × 10⁴ — about 54 000 times the Earth’s field.
(2) If the field fell as 1 ÷ distance, going from 0.5 m to 5 m (ten times further) would divide it by 10, giving 0.15 T — 300 times the stated 5 × 10⁻⁴ T.
The actual fall is a factor of 1.5 ÷ (5 × 10⁻⁴) = 3 000 over a factor of 10 in distance, faster than 1 ÷ d² (100) and about the 1 ÷ d³ (1 000) of a dipole far from the magnet.
It is steeper still because the scanner’s iron and active shielding are designed to pull the 5-gauss line in close to the room.
Try it — how does the magnetic force change with distance?
The lab clamps a magnet above a steel ball on a top-pan balance. The balance is zeroed with the ball on it and the magnet far away; as the magnet is lowered the reading drops by the mass the magnet is supporting, and the engine converts that to a force with g = 10 N/kg.
The independent variable is the gap between magnet face and ball, 5 to 50 mm; you can change the magnet (neodymium, alnico, ferrite) and the object (steel ball, iron nail, copper ball).
Suppose your mean forces were: 5 mm → 1.62 N; 10 mm → 0.96 N; 20 mm → 0.41 N; 30 mm → 0.20 N; 40 mm → 0.12 N; 50 mm → 0.074 N.
(a) Describe the relationship, quoting values, and state with a test whether it is inverse proportion.
(b) Between which two gaps is the force falling fastest per millimetre? Show a calculation.
(c) A student says the copper ball “proves copper is repelled by magnets, because the reading did not fall”. Evaluate.
Reveal answer
(a) As the gap increases from 5 mm to 50 mm the force falls from 1.62 N to 0.074 N, steeply at first (1.62 to 0.96 N between 5 and 10 mm) and then slowly (0.12 to 0.074 N between 40 and 50 mm). Test: force × gap = 8.1, 9.6, 8.2, 6.0, 4.8, 3.7 N mm — not constant, so it is not inverse proportion; the relationship is a decreasing, non-linear curve.
(b) 5 → 10 mm: (1.62 − 0.96) ÷ 5 = 0.13 N per mm; 10 → 20 mm: 0.55 ÷ 10 = 0.055 N per mm; 20 → 30 mm: 0.021 N per mm. Fastest between 5 and 10 mm — the gradient of the curve is steepest nearest the magnet.
(c) Invalid. A reading that does not change means no force either way; repulsion would make the reading rise above 200.0 g. The result shows that copper is non-magnetic — it is neither attracted nor repelled — which is the correct conclusion.
Common mistakes to avoid
- Field lines with no arrows, or arrows pointing S → N outside the magnet. Outside, N → S — the way a free north pole would move.
- Lines that cross or touch. Never. Redraw with wider spacing.
- Lines equally spaced everywhere. Crowd them at the poles; the spacing is how the drawing shows strength.
- “Attraction proves it is a magnet.” Only repulsion does; iron is attracted too.
- Calling the pole in the Arctic a magnetic north pole in an explanation. The compass N end points there because it is attracted, so the pole is south-seeking. Say so explicitly.
- Writing “inversely proportional” for any falling curve. Test with force × distance; quote values.
- Confusing field strength with force. Field in tesla is a property of the place; force in newtons depends on what you put there.
- Aluminium and copper listed as magnetic because they are metals. Only iron, steel, nickel, cobalt and their alloys.
Study smart: how this is examined
Study routine. Today: draw the bar-magnet field from memory three times, checking the five rules each time; write the magnetic-metals list on a card. In two days: do the declination EXAMPLE and the balance EXAMPLE with the book closed, then explain aloud why the Arctic pole is a south-seeking pole. In a week: run the lab as a full investigation, test the data for inverse proportion, and take the quiz.
How we found out — from a stone that pointed to a planet that reverses
The mineral magnetite (Fe₃O₄), the “lodestone”, was known to the Greeks — Thales of Miletus, around 600 BCE, is said to have described its pull on iron — and Chinese writers of the Han dynasty, around 2 000 years ago, described a spoon carved from it that, spun on a bronze plate, settled pointing south.
By the eleventh century Chinese navigators were using a magnetised needle floating in water, and the scholar Shen Kuo, in 1088, recorded not only the compass but the awkward fact that it did not point exactly to the pole star: the first written note of declination.
The idea reached the Mediterranean within a century. In 1269 the French scholar Petrus Peregrinus, writing from a siege camp, laid a needle on a lodestone sphere, traced the lines it made, found they met at two points he named poles, and showed that like poles repel and that a broken magnet has two new poles — the field-line drawing in Section 1.3, seven and a half centuries early.
The planet came next. Mariners knew by the 1500s that the needle also dipped — its N end tilting downwards in the northern hemisphere — and in 1600 William Gilbert, physician to Queen Elizabeth I, published De Magnete: he had carved lodestones into spheres, moved a small needle over them, and found it dipped exactly as ships’ compasses did on the real Earth.
His conclusion, “the Earth itself is a great magnet”, was the first physical property assigned to the whole planet by experiment. Edmond Halley mapped declination across the Atlantic from a ship in 1698–1700 and printed the first chart with lines of equal declination in 1701.
In 1832 Carl Friedrich Gauss showed how to measure the field’s strength in absolute units, and the network of observatories he set up with Wilhelm Weber recorded that the field was slowly changing everywhere.
Two twentieth-century discoveries turned the bar-magnet picture into a dynamo. In 1906 Bernard Brunhes found volcanic rocks in France magnetised backwards — N where S should be — and in 1929 Motonori Matuyama showed from Japanese lavas that the field had reversed at a definite time in the past; the boundary, 780 000 years ago, bears both their names.
The pattern of reversed stripes on the ocean floor, explained by Fred Vine and Drummond Matthews in 1963, proved both that the field reverses and that the sea floor spreads — magnetism became the key evidence for plate tectonics.
Meanwhile Walter Elsasser and Edward Bullard in the 1940s and 1950s worked out that convection in the liquid-iron outer core could generate the field as a self-exciting dynamo, and in 1995 Gary Glatzmaier and Paul Roberts ran the first computer simulation of the core that produced a field like the Earth’s — and, unasked, a reversal.
Gilbert’s terrella experiment convinced people that the Earth was a magnet. Explain what evidence it provided that the earlier idea — that compasses were attracted by the pole star, or by a magnetic mountain in the far north — could not explain.
Reveal answer
Attraction by a distant star or a single mountain would make a needle point towards it but would not make it dip: a distant point source pulls in one direction only.
Gilbert’s needle over the lodestone sphere dipped more and more steeply towards the sphere’s pole, matching the dip sailors measured at different latitudes on the real Earth — a pattern only a magnet with poles inside the sphere produces, because the field lines curve down into the surface near the poles.
A model that reproduces the quantitative pattern (dip against latitude) beats one that reproduces only the direction.
Where it is used now
- Finding minerals from the air. Ore bodies containing magnetite, and the rocks that host diamonds and copper, disturb the Earth’s field by a few nanotesla. Aeromagnetic surveys — a magnetometer towed behind a light aircraft flying a grid — have mapped most of Botswana beneath its Kalahari sand, guiding exploration for copper–nickel and diamonds where nothing shows at the surface.
- Recycling and mining. A conveyor belt running over a magnetic drum lifts steel out of mixed scrap and household waste; iron ore is concentrated by magnetic separation. Neither works on aluminium cans, which need the eddy-current separators of Lesson 4.
- Medicine. An MRI scanner’s 1.5–3 T field lines up the protons in the body’s water so that radio pulses can map soft tissue with no ionising radiation — the reason the waiting room has a “no steel objects” sign, because a steel oxygen cylinder becomes a projectile in a field that strong.
- Everyday holds. Fridge-door seals are flexible ferrite strips; laptop lids, phone cases and cupboard catches use small neodymium discs; the magnetic strip on a bank card stores data as patterns of magnetised particles.
- Animal navigation. Loggerhead turtles, salmon and migrating birds sense the Earth’s field — its strength and its dip — and use it as a map; the mechanism, still being worked out, seems to involve light-sensitive proteins in the eye and iron-based particles.
- Space weather. The field deflects the solar wind, and when a solar storm compresses it — as in May 2024, the strongest geomagnetic storm in two decades, with aurorae seen from South Africa and Namibia — induced currents in long power lines and pipelines can trip transformers. Grid operators watch the forecasts.
Science now — and the scientist you need to be
What is happening in 2026
The Earth’s field is being watched more closely than ever, because it is doing two things that matter to technology.
It is weakening overall — by roughly 9 % over the past two centuries — and the weak patch over the South Atlantic and southern Africa is growing and splitting; satellites crossing it, from the International Space Station to the low-orbit constellations that carry internet traffic, record more memory errors and sometimes shut instruments down while they pass.
The European Space Agency’s three Swarm satellites, in orbit since 2013, map the field to a fraction of a nanotesla and feed the World Magnetic Model, revised in December 2024, which every phone compass and aircraft navigation system uses for declination.
Meanwhile the north magnetic pole’s dash towards Siberia has slowed from about 55 km a year to nearer 35; nobody predicted either the speed-up or the slow-down, and researchers are now testing machine-learning methods, trained on satellite and observatory records, alongside physics-based core models to forecast how the field will change.
AI is being tried for space weather too: in 2023 a NASA-supported research model (DAGGER) used solar-wind data from spacecraft to predict geomagnetic disturbances around 30 minutes ahead — the kind of warning that the transformer operators of Lesson 5 need.
The mindshift
What you need to be able to do
Draw the field of a bar magnet correctly every time — it is the single most reliable mark in this unit. Describe a field in words that separate direction from strength.
And, for Criteria B and C, design a magnetic-force investigation with a measurable force (from a balance or newton meter), a set gap and three named controls with values, then describe the result as a curve with quoted numbers rather than a law you have not tested.
See an example
Landmark at 083° magnetic, 068° true: a declination of about 15° W, as the map said. The assistant explained the attraction correctly, said the field comes from currents in the liquid outer core, and — when pushed — admitted that “like a bar magnet” describes the shape of the field, not its cause.
Key terms
| Magnetic material | A material attracted by a magnet: iron, steel, nickel, cobalt and their alloys. |
| Permanent / temporary magnet | Keeps its magnetism (steel, neodymium, ferrite) / magnetised only while in a field (soft iron). |
| Poles | The regions of a magnet where the field is strongest; north-seeking (N) and south-seeking (S). Like poles repel, unlike attract. |
| Magnetic field | The region around a magnet or current in which a magnetic material, another magnet or a moving charge experiences a force. |
| Field line | A line whose direction at every point is the direction of the force on a free north pole; N → S outside the magnet; never crossing; closest where the field is strongest. |
| Magnetic flux density, B | The strength of a magnetic field, in tesla (T); Earth’s surface field ≈ 5 × 10⁻⁵ T, MRI ≈ 1.5 T. |
| Induced magnetism | Magnetism produced in a magnetic material by a nearby magnet; the nearer end gets the opposite pole, so it is attracted. |
| Domain | A region in iron in which the atomic magnets point the same way; magnetising aligns the domains. |
| Neutral point | A point where two fields cancel to give zero resultant field. |
| Declination | The angle between the direction a compass points (magnetic north) and true north at a place; about 8–18° W across Botswana (about 18° W at Gaborone). |
| Geodynamo | The electric currents in the Earth’s liquid-iron outer core that generate its magnetic field. |
Think like a scientist
Criterion B A manufacturer of magnetic cupboard catches wants to know how the thickness of a non-magnetic spacer between a neodymium disc magnet and a steel plate affects the force needed to pull them apart, because the catches are sold with plastic covers of different thicknesses.
A school laboratory has: neodymium disc magnets (12 mm diameter), a steel plate fixed to the bench, sheets of plastic card each 0.5 mm thick (measured with a micrometer), a newton meter reading 0–10 N in 0.1 N steps, a top-pan balance, a ruler, thread and tape. Design the investigation.
(a) Write a research question that names both variables and a range. [2]
(b) Formulate a hypothesis with a testable prediction and a scientific reason introduced by “because” — the reason must refer to the field and to induced magnetism. [3]
(c) State the independent and dependent variables, each with its unit and the instrument that sets or measures it, and explain why “how strongly it sticks” would not be accepted as the dependent variable. [4]
(d) Identify three control variables, each with the value or condition held and the reason it matters. [6]
(e) Write a numbered method another student could follow, including at least five spacer thicknesses, three trials at each, what is recorded, how the data are processed and graphed, and a safety point specific to strong magnets. [6]
(f) Suggest one extension that changes the independent variable, with its research question. [2]
Model answer
(a) How does the thickness of a plastic spacer, from 0 to 3.0 mm, between a 12 mm neodymium disc magnet and a steel plate affect the force needed to pull the magnet off the plate? [1 for both variables named, 1 for a measurable question with a range.]
(b) If the spacer thickness is increased from 0 to 3.0 mm, then the pull-off force will decrease, and by more than half each time the thickness doubles [1], because the field of the magnet weakens rapidly with distance from its face, so with a thicker spacer the field at the steel is weaker [1].
The steel is magnetised by induction in proportion to the field reaching it, and the force between the magnet and the weaker induced pole is therefore smaller — falling faster than the field itself because both the field and the induced magnetism drop [1].
(c) Independent variable: spacer thickness in mm, set by stacking 0, 1, 2, 4 and 6 sheets of 0.5 mm card and checked with the micrometer [1]. Dependent variable: the force in newtons at which the magnet separates from the plate, read on the newton meter attached to the magnet by thread and tape and pulled slowly at right angles to the plate [1].
“How strongly it sticks” is not a quantity — it cannot be read from an instrument and two students would judge it differently; the dependent variable must be a measured force in N [1].
The pull must be at right angles and slow, because a sideways or jerked pull gives a different, smaller reading [1].
(d) Any three [2 each — value/condition + reason]: the same magnet (one 12 mm disc, marked, used throughout) because different magnets have different field strengths; the same place on the steel plate (marked with a pencil cross) because plate thickness and surface finish change the induced magnetism.
The pull direction — thread at 90° to the plate, checked against a set square — because an angled pull reduces the separating force; temperature (room at about 22 °C, magnets not handled for long) because neodymium loses strength when warm; the same spacer material (plastic card, non-magnetic) because a magnetic spacer would change the field.
[“Same equipment” or “same magnet” with no reason scores 0 for that control.]
(e) [1 each] 1. Tape a loop of thread to the back of the magnet and hook the newton meter to it; mark a cross on the plate. 2. Place the magnet on the cross with no spacer; pull the newton meter slowly at right angles until the magnet separates; record the maximum reading to 0.1 N. 3.
Repeat twice more (three trials). 4. Insert 1 sheet (0.5 mm), then 2, 4 and 6 sheets (1.0, 2.0, 3.0 mm), three trials at each, wiping the plate between readings. 5.
Record in a table with thickness/mm and force/N in the headings; calculate the mean force at each thickness to 0.1 N, noting any anomaly. 6. Plot mean force (N) against thickness (mm) with a curve of best fit; test for inverse proportion by calculating force × thickness for the non-zero thicknesses.
| Spacer thickness / mm | Pull-off force trial 1 / N | Pull-off force trial 2 / N | Pull-off force trial 3 / N | Mean pull-off force / N |
|---|---|---|---|---|
| 0.0 | — | — | — | — |
| 0.5 | — | — | — | — |
| 1.0 | — | — | — | — |
| 2.0 | — | — | — | — |
| 3.0 | — | — | — | — |
Safety specific to the task: neodymium magnets snap together hard enough to pinch skin and shatter — keep them apart, keep them away from phones, cards and anyone with a pacemaker. [Level grid: 5–6 complete, repeatable, with values, repeats, processing and specific safety; 3–4 sequence correct but a quantity, repeat or the processing missing; 1–2 outline only.]
(f) Extension [1 for a new IV, 1 for its question]: change the magnet diameter (8, 12, 16, 20, 25 mm discs) — “How does the diameter of a neodymium disc magnet affect the pull-off force from a steel plate at a fixed spacer thickness of 1.0 mm?” Or change the plate thickness, or the spacer material (plastic vs steel shim). Repeating the same experiment with more trials is an improvement, not an extension.
Probe deeper — exam-style questions
Attempt every question before opening the answer. Marks in brackets show what a full answer is worth.
State the meaning of the term magnetic field, and state the direction in which the field lines around a bar magnet are drawn.
Reveal answer
A region (of space) in which a magnetic material, a magnet or a moving charge experiences a force [1]; from the north pole to the south pole outside the magnet — the direction a free north pole would move [1].
From the list — copper, steel, aluminium, nickel, glass, iron — select the magnetic materials, and describe a test that would show whether a magnetic steel rod is itself a magnet.
Reveal answer
Steel, nickel, iron [1 for all three, none extra]; bring each end of a known magnet, in turn, to each end of the rod [1]; if any pairing repels, the rod is a magnet — attraction alone only shows it is magnetic material [1].
Describe how you would use a plotting compass to draw one field line of a bar magnet, and explain why iron filings could not have given the arrow direction.
Reveal answer
Place the compass near the N pole, mark dots at the two ends of the needle, move the compass so its S end sits on the previous N-end dot, mark again, and repeat to the S pole; join the dots [1].
The arrow points the way the needle’s N end pointed, N → S [1]; filings line up along the field but each filing is a tiny magnet that can lie either way round, so the pattern shows shape and spacing but not direction [1].
True or false, with a reason: “The Earth’s north magnetic pole, near the geographic North Pole, is a north-seeking pole.”
Reveal answer
False [1]: the N (north-seeking) end of a compass points towards it, and unlike poles attract, so the pole near geographic north is a south-seeking magnetic pole; “north magnetic pole” is a place name, not a description of its polarity [1].
A steel ball of mass 60 g rests on a balance zeroed at 0.0 g. A magnet held above it makes the balance read −24.0 g. Calculate the magnetic force on the ball (g = 9.81 N/kg), and state the size and direction of the force on the magnet.
Reveal answer
Mass supported = 24.0 g = 0.0240 kg [1]
F = 0.0240 × 9.81 = 0.235 N upwards on the ball [1]
0.235 N downwards on the magnet (Newton’s third law) [1]
(The ball’s own 60 g is irrelevant — the balance was zeroed with it in place.)
A hiker in Namibia, where the declination is about 13° W, wants to walk on a true bearing of 300°. State the compass bearing to use, and explain what would happen to her position over 20 km if she used 300° on the compass without correcting.
Reveal answer
Compass = true + west declination
Compass = 300° + 13° = 313° [1]
On compass 300° she walks true 287°, 13° anticlockwise (to the left) of her intended line [1].
After 20 km she is 20 × sin 13° = 4.5 km to the side of her target [1].
7. Two identical bar magnets are placed side by side, 2 cm apart, pointing the same way (both N ends to the right). Draw or describe the field pattern, identify where a neutral point lies, and explain how the pattern would change if one magnet were turned round.
Reveal answer
Lines leave both N poles on the right and enter both S poles on the left; in the gap between the magnets the field alongside each magnet runs back from N to S (right to left) for both magnets, so the two fields add and the pair behaves like one wider, stronger magnet [1].
The neutral points lie on the mid-line between the magnets, just beyond each pair of ends — there the returning right-to-left field alongside the magnets is cancelled by the outward field spreading from the two N poles (and, at the other end, by the field converging on the two S poles) [1].
Turned round (N beside S), short, crowded lines run across the gap from the N end of each magnet to the adjacent S end of the other, giving a strong field across the gap at each end [1]; but in the middle of the gap the field alongside one magnet points right and alongside the other points left, so they cancel: the neutral point moves to the centre of the gap between the magnets [1].
8. A satellite’s memory chips suffer more faults each time it crosses the South Atlantic Anomaly. Using what you know about the Earth’s field, explain why a weaker field over that region leads to more faults, and suggest why the problem is expected to get worse over the coming decades.
Reveal answer
The Earth’s field deflects charged particles from the Sun and from cosmic rays — a moving charge in a magnetic field experiences a force at right angles to its motion, so particles are steered away or trapped in belts above the atmosphere [1].
Where the surface field is weak the belts dip closer to the Earth and more energetic particles reach low orbits, so a satellite crossing the anomaly is struck by more particles [1]; a single particle depositing charge in a memory cell can flip a stored bit — a fault [1].
The field over the region has been weakening and the weak patch expanding and splitting for decades, so the belts will dip lower and the flux of particles at a given altitude will rise, unless the trend reverses, which the core models cannot yet predict [1].
Summary
- Iron, steel, nickel, cobalt and their alloys are magnetic; copper, aluminium and non-metals are not. Only repulsion proves an object is a magnet.
- A magnetic field is a region where a magnetic material, magnet or moving charge feels a force. Field lines run N → S outside a magnet, never cross, and are closest where the field is strongest; their direction is the direction a free north pole would move.
- A magnet magnetises nearby iron by induction, inducing the opposite pole nearest — which is why iron is attracted. Soft iron loses the magnetism; steel keeps it.
- The Earth’s field resembles that of a bar magnet tilted about 10° to the axis with a south-seeking pole in the Arctic; it is generated by currents in the liquid outer core, wanders, weakens and reverses. Declination corrects a compass bearing to a true one.
- The force from a magnet falls steeply and non-linearly with distance; test any claimed proportionality with the numbers before writing it.
Next time → Resistance and circuit rules: what current, p.d. and resistance really are, V = IR, series and parallel combinations, and why a lamp’s graph bends while a resistor’s stays straight.