Magnets and magnetic materials
A magnet is an object that pulls iron. Things a magnet pulls are magnetic materials: iron, steel, nickel and cobalt. Things it does not pull are non-magnetic: copper, aluminium, gold, wood, plastic, glass.
A permanent magnet keeps its magnetism for a long time (a fridge magnet). Magnets can be bars, horseshoes, rings or discs.
Uses
- Separating iron scrap from other rubbish
- Door catches, bag clasps, fridge doors
- Speakers, electric motors and generators
- Compasses for finding direction
Poles: like repel, unlike attract
The pull of a magnet is strongest at its ends. These ends are the poles: the north pole (N) and the south pole (S). A freely hanging magnet turns until its N pole points north.
- Like poles repel: N–N or S–S push apart.
- Unlike poles attract: N–S pull together.
- The force is stronger when the magnets are closer.
Poles always come in pairs. If you break a magnet, each piece gets its own N and S. You can never get a magnet with only one pole.
Repulsion is the sure test for a magnet. Iron is attracted by both poles, but only another magnet can be pushed away.
Magnetic field and the compass
The space around a magnet where its force can be felt is its magnetic field. We draw it with field lines:
- They come out of the N pole and go into the S pole (outside the magnet).
- They never cross each other.
- Where lines are close together, the field is strong (near the poles).
A compass is a tiny magnet (a needle) that can turn freely. Near a magnet, the needle lines up along the field line at that place. You can map a field by moving a compass around and marking which way its N end points. Iron filings sprinkled on paper over a magnet also show the pattern.
Why some things are magnets
Every atom has moving electrons. A moving charge is a tiny electric current, and a current loop acts like a tiny magnet. This idea is Ampère's hypothesis: magnetism comes from tiny current loops inside matter.
In iron, these tiny magnets group into small regions called domains. In ordinary iron the domains point every which way, so they cancel. When you magnetise iron (stroke it with a magnet or put it in a coil with current), the domains line up and the iron becomes a magnet. Heating or hammering a magnet mixes the domains up again and it loses magnetism.
Earth's magnetism, electromagnets and levitation
The Earth behaves like a giant bar magnet, made by moving molten iron in its outer core. Its magnetic south pole is near the geographic North Pole. That is why the N end of a compass needle points north. The magnetic poles are not exactly at the geographic poles, and they slowly wander. The Earth's field also shields us from fast charged particles from the Sun; where they leak in near the poles we see auroras. The Sun has its own, much more tangled magnetic field, seen in sunspots.
Electromagnets
A coil of wire carrying current makes a magnetic field like a bar magnet. Put an iron core inside and it becomes an electromagnet: strong, and it can be switched on and off. More turns or more current make it stronger. Uses: cranes in scrapyards, electric bells, relays, MRI scanners.
Magnetic levitation
Repulsion (or carefully controlled attraction) can hold an object in the air. Maglev trains float a few centimetres above the track, so there is almost no friction.
Try it
- Stroke a steel needle 30 times in one direction with one end of a magnet. Push it through a small piece of cork and float it in a bowl of water. It turns to point north–south: you made a compass!
- Bring a fridge magnet near a spoon, a coin, a key and a pencil. Predict first, then check which stick.
- In the 3D free-play step, flip the second magnet and slide it. When do they attract? When do they repel?
Key formulas and definitions
- Like poles repel; unlike poles attract
- Field lines: out of N, into S (outside the magnet)
- Magnetic materials: iron, steel, nickel, cobalt
- Earth's magnetic S pole lies near geographic north
- Electromagnet strength grows with current and number of turns
Worked examples
1. A magnet's N pole is brought near another magnet's S pole. What happens?
They attract, because unlike poles attract.
2. You have two identical-looking metal bars. One is a magnet, one is plain iron. How can you tell which is the magnet using only the two bars?
Touch the end of bar A to the middle of bar B. The middle of a magnet has almost no pull. If they stick strongly, A is the magnet; if not, B is. (Or: only a magnet can repel the other bar.)
3. Why does a compass needle point north?
The needle is a small magnet. The Earth's magnetic south pole lies near geographic north, and it attracts the needle's N pole.
4. A bar magnet is cut into three pieces. How many poles are there in total?
Each piece becomes a full magnet with N and S, so there are 3 × 2 = 6 poles.
5. How can you make an electromagnet stronger?
Increase the current, add more turns of wire, or use a soft iron core.
Common mistakes
- Thinking magnets attract all metals. Copper, aluminium, gold and silver are not attracted.
- Using attraction to test for a magnet. Iron is attracted too; only repulsion proves both are magnets.
- Believing a broken magnet gives a lone N and a lone S. Every piece has both poles.
- Saying the Earth's magnetic north pole is at geographic north. The pole there behaves like a magnetic S pole.