Relative units: field 100 = 1 cm beyond the N end of one magnet; force 100 = two magnets end to end, N facing S, 1 cm apart. Model: each pole acts like a point source of field (the “pole model”); magnets are 8 cm long.
Drag a magnet to move it. Tap one to select it (yellow outline).
Earth’s magnetic field is shaped much like the field of a bar magnet, tilted a little from the axis Earth spins on. A compass needle is a tiny magnet, so its N end turns to line up with the field. Drag the compass around the Earth (or use the slider) and watch where it points and how much it tilts.
The compass’s N end is attracted toward the Arctic. Unlike poles attract, so the magnetic pole in the Arctic, called the north magnetic pole because of where it is, is really a magnetic south pole. That is why the imaginary bar magnet inside Earth has its S end in the north. The magnetic poles are not at the geographic poles, and they wander slowly.
Model: a perfect magnetic dipole tilted 10° (the real tilt of Earth’s dipole is about 9–10°), seen in one slice through the Earth. Dip: tan(dip) = 2 × tan(magnetic latitude). Strength: 30 µT at the magnetic equator, 60 µT at the magnetic poles (the real field is about 25–65 µT and less regular). The real north magnetic pole (where a needle points straight down) is not exactly where the tilted dipole puts it.
Earth is not really a bar magnet: the core is far too hot to stay magnetized. The field is made by electric currents in the moving liquid iron of the outer core.
Pick an object and bring the magnet close. Only a few materials are attracted: iron, nickel, cobalt, most kinds of steel, and other magnets. Many metals are not magnetic at all.
“Not magnetic” means no pull you could notice. Very strong lab magnets push or pull very weakly on almost everything (even water), far too weakly to feel with a school magnet.
An electric current makes a magnetic field. Wind the wire into a coil and the fields of all the turns add up; put an iron core inside and the field gets much stronger. Change the number of turns, the current and the core, and count the paper clips it can lift.
Model (simple, stated): field inside a long coil with no core B = μ0NI ÷ L, with L = 5 cm. A soft iron core makes it 100 times stronger, a steel core 60 times. Paper clips lifted = turns × current (A) × core factor ÷ 10, rounded down (soft iron 1, steel 0.6, no iron 0.01); a steel core keeps about a quarter of its clips after switch-off. Real cores stop getting stronger when the iron is saturated.
Right-hand grip rule: curl the fingers of your right hand the way the current flows round the coil; your thumb points to the N end.
The diagram you draw in a test: the magnetic field lines around a bar magnet, with the poles, the field direction, a compass, and where the field is strong and weak. Switch between the labeled diagram, a blank one to test your memory, and a quiz where you place each label yourself. You can also download both versions or print a label worksheet.
Tap a word, then tap the numbered box it belongs to (on the diagram or in the list below). Tap a filled box to take the word back. On a phone, swipe the diagram sideways to see every number.
Ten questions from a bank of twenty: poles, materials, fields, Earth, electromagnets (and two high-school calculations).
Tap an object, then tap the box it belongs in. Check when all twelve are placed.
What happens when you bring these together? Sort the nine pairs.
Label the diagram, predict and test materials, attract or repel, Earth’s field, an electromagnet investigation, multiple choice, a high-school extension and an answer key.
Every magnet has a north-seeking pole (N) and a south-seeking pole (S). Hang a bar magnet from a thread and its N pole swings round to face north: that is how the poles got their names and how a compass works. Bring two magnets together: N and S pull together, N and N (or S and S) push apart. A magnet also attracts things made of iron, steel, nickel or cobalt, but not aluminum, copper, gold, plastic or wood. The force acts through paper, glass and water, and it gets weaker quickly as you move further away.
Sprinkle iron filings around a magnet and they line up in curved lines; a small compass does the same. We draw these as magnetic field lines. They come out of the N pole and go into the S pole, they never cross, and where they are close together (at the poles) the field is strongest. Earth has a field like this too, so a compass needle lines up with Earth’s field lines and its N end points toward the Arctic.
Magnetism comes from moving electric charge. In 1820 Hans Christian Ørsted saw a compass needle move next to a wire carrying a current. A coil of wire with a current is an electromagnet: more turns, more current and an iron core make it stronger, and it can be switched off. In iron, the atoms themselves are tiny magnets (from the spin and motion of their electrons) grouped in regions called domains; a magnet lines the domains up, which is why iron is attracted by either pole. Earth’s field comes from currents in its liquid iron outer core. At high school, the rule becomes exact: a charge q moving at speed v across a field B feels a sideways force F = qvB, and a wire carrying current I feels F = BIL.
1) In the Magnet Lab, switch on Iron filings and then Compass grid: do they show the same pattern? 2) Load N–N repel and find the neutral point between the magnets with the test compass. 3) Look at the graph with two magnets: what happens to the force when you double the distance? 4) In the Earth section, find the magnetic equator, where the needle lies level (dip 0°; in this model it is tilted 10° from the geographic equator), then move the compass to the north magnetic pole and watch the dip grow to 90°. 5) In the Electromagnet Lab, double the number of turns and count the clips; then try a steel core and switch off.
| Fact | Details |
|---|---|
| Poles | Every magnet has an N pole and an S pole. Cut a magnet in half and you get two smaller magnets, each with both poles. |
| Forces | Like poles repel (N–N, S–S); unlike poles attract (N–S). Magnetic materials are attracted by either pole. |
| Magnetic materials | Iron, nickel, cobalt and many of their alloys, such as most steels. Most metals (aluminum, copper, gold, silver, brass, zinc, lead) are not magnetic. |
| Field lines | Outside the magnet they go from N to S; inside from S to N, so each line is a closed loop. They never cross. Close together = strong field. |
| Strength | Strongest at the poles; weaker further away. Far from a bar magnet the field falls roughly with 1 ÷ distance³. |
| Through materials | Magnetic forces act through paper, plastic, wood, glass, water and aluminum. A thick sheet of iron or steel can shield (redirect) the field. |
| Temporary and permanent | Soft iron becomes a magnet only while it is near a magnet (induced magnetism). Steel keeps its magnetism: it can be made into a permanent magnet. |
| Destroying magnetism | Heating above the Curie temperature (770 °C for iron), hammering or dropping a magnet many times can jumble the domains and weaken it. |
| Fact | Details |
|---|---|
| Shape | Close to the field of a bar magnet tilted about 10° from Earth’s spin axis, with its S end in the north. |
| North magnetic pole | In the Arctic, a few hundred kilometers from the geographic North Pole and moving (in recent decades from northern Canada toward Siberia). Magnetically it is a south pole. |
| Strength at the surface | About 25 to 65 microtesla (µT); a fridge magnet is about a hundred times stronger close up. |
| Cause | Electric currents in the moving liquid iron of the outer core (the geodynamo). |
| Dip | A freely hanging needle tilts down toward the ground; it is level near the magnetic equator and vertical at the magnetic poles. |
| Reversals | The field has reversed many times; the last full reversal was about 780,000 years ago. Rocks on the sea floor record these flips. |
| Protection | The field steers charged particles from the Sun; near the poles they hit the air and make the aurora. |
| Quantity | Formula | Notes |
|---|---|---|
| Force on a moving charge | F = qvB sin θ (vector: F = qv × B) | θ = angle between v and B. Largest at 90°, zero along the field. Always at right angles to v, so it does no work. |
| Circular motion | r = mv ÷ (|q|B), T = 2πm ÷ (|q|B) | The period does not depend on the speed (non-relativistic): the idea behind the cyclotron. |
| Force on a wire | F = BIL sin θ (F = IL × B) | The motor effect. Direction: Fleming’s left-hand rule or the right-hand rule. |
| Field of a long straight wire | B = μ0I ÷ (2πr) | Circles round the wire (right-hand grip rule). |
| Field in a long solenoid | B = μ0nI, n = N ÷ L | Uniform inside, nearly zero outside an ideal solenoid; an iron core multiplies it. |
| Constants and units | μ0 ≈ 4π × 10−7 T·m/A; 1 T = 1 N/(A·m) | Earth ≈ 50 µT; fridge magnet a few mT; strong neodymium magnet up to about 1 T at its surface; hospital MRI 1.5–3 T. |
Proton in a field. A proton (q = 1.6 × 10−19 C, m = 1.67 × 10−27 kg) moves at 1.0 × 106 m/s at right angles to a 0.50 T field. F = qvB = 1.6 × 10−19 × 1.0 × 106 × 0.50 = 8.0 × 10−14 N. Radius r = mv ÷ (qB) = 1.67 × 10−27 × 106 ÷ (8.0 × 10−20) ≈ 0.021 m = 2.1 cm. Period T = 2πm ÷ (qB) ≈ 1.3 × 10−7 s.
Faster proton. At 10% of the speed of light (3.0 × 107 m/s) in a 1.0 T field, r = 1.67 × 10−27 × 3.0 × 107 ÷ (1.6 × 10−19 × 1.0) ≈ 0.31 m.
Wire in a field. A wire 0.20 m long carrying 5.0 A at right angles to a 0.50 T field: F = BIL = 0.50 × 5.0 × 0.20 = 0.50 N.
Solenoid. 50 turns on a 5.0 cm coil with 2.0 A: B = μ0NI ÷ L = 4π × 10−7 × 50 × 2.0 ÷ 0.050 ≈ 2.5 × 10−3 T (2.5 mT), about 50 times Earth’s field, before adding an iron core.
A magnetic field pushes on electric charges only when they move, and always sideways: at right angles to both the motion and the field. A charged particle is bent into a circle; a wire carrying a current is pushed (the motor effect). Field into the page is drawn as ×, out of the page as •.
For F = qv × B: point the fingers of your right hand along v, curl them toward B, and your thumb gives F for a positive charge (reverse it for a negative charge). For a wire use the current direction instead of v. Because F is always at right angles to v, the magnetic force changes the direction of a particle but never its speed.
Values: proton 1.673 × 10−27 kg, +1.602 × 10−19 C; electron 9.109 × 10−31 kg, −1.602 × 10−19 C; alpha particle 6.645 × 10−27 kg, +3.204 × 10−19 C. The circle is drawn to fit the screen; the readout gives the true radius.
Magnetism is a force made by magnets and by moving electric charges. It can pull or push other magnets and attract magnetic materials such as iron and steel without touching them, through the magnetic field around the magnet.
Key takeaway: a non-contact force from magnets and moving charges.The two ends where the magnetic force is strongest. A freely hanging magnet turns so that its north-seeking pole (N) faces north. Like poles repel and unlike poles attract.
Key takeaway: N and S; like repel, unlike attract.No. Only a few materials are attracted to a magnet: iron, nickel, cobalt and many of their alloys, such as most steels. Aluminum, copper, gold, silver, brass and most other metals are not magnetic, and neither are wood, plastic, glass, rubber or paper.
Key takeaway: iron, nickel, cobalt (and most steel); most metals are not magnetic.The magnetic field is the region around a magnet where its force acts. Field lines show the direction a compass needle would point: outside the magnet they go from the N pole to the S pole. Where the lines are close together the field is strong.
Key takeaway: lines from N to S; close together = strong.The needle is a small magnet that lines up with Earth’s magnetic field. Its N end is attracted toward Earth’s magnetic pole in the Arctic, which is magnetically a south pole. That pole is near, but not exactly at, the geographic North Pole.
Key takeaway: the needle lines up with Earth’s field.Earth’s magnetic pole in the Arctic is called the north magnetic pole because of where it is, but magnetically it behaves as a south pole: it attracts the N end of compass needles. It is a few hundred kilometers from the geographic North Pole and moves slowly.
Key takeaway: in the Arctic sits a magnetic S pole.Put more turns of wire on the coil, increase the current, or put a soft iron core inside the coil. Reversing the current swaps the N and S poles, and switching the current off turns a soft-iron electromagnet off.
Key takeaway: more turns, more current, iron core.Yes. A magnet attracts a paper clip through paper, card, plastic, glass, wood, water and even aluminum. A thick sheet of iron or steel can shield the field, because the field is drawn into the iron instead.
Key takeaway: it passes through non-magnetic materials.Every electron acts like a tiny magnet. In most materials these tiny magnets pair up and cancel. In iron, nickel and cobalt they do not cancel, and neighboring atoms line up in regions called domains. A magnet lines up the domains, so the material is attracted by either pole.
Key takeaway: aligned atomic magnets (domains) in iron, nickel, cobalt.F = qvB sin θ, at right angles to both the velocity and the field (F = qv × B). Because it is always sideways, it does no work: a magnetic field can bend a charged particle into a circle of radius r = mv ÷ (|q|B) but cannot change its speed. A wire carrying a current feels F = BIL sin θ, the basis of electric motors.
Key takeaway: F = qvB, sideways; r = mv ÷ (qB); F = BIL for a wire.