Poles, fields, magnetic materials, and how magnets are made and lost.
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A magnet attracts iron, cobalt and nickel. Every magnet has two poles, north and south, where the attraction is strongest, and the poles cannot be separated: breaking a magnet gives two smaller magnets. A freely suspended magnet settles along north–south. Unlike poles attract and like poles repel; repulsion is the sure test for a magnet, because attraction also occurs between a magnet and plain iron.
How can you tell whether a steel bar is a magnet or just iron?
Bring one end to the pole of a known magnet: repulsion shows it is a magnet; attraction alone does not.
A magnet is broken in two. How many poles does each piece have?
Each piece has its own north and south pole, so there are two poles per piece.
Why does a freely suspended magnet point north?
Earth acts like a giant magnet and its field turns the suspended magnet along the north–south line.
The region around a magnet where its force can be felt is the magnetic field. Field lines show it: they leave the north pole, enter the south pole outside the magnet, never cross, and are closer where the field is stronger. Iron filings sprinkled on paper over a magnet line up along the field lines. A compass needle points along the field.
What does a crowded set of field lines show?
A stronger magnetic field.
Why can two field lines never cross?
At a crossing point the field would have two directions, which is impossible.
How do iron filings show the field?
Each filing becomes a tiny magnet and lines up along the field direction.
Magnets can be made by single touch or double touch stroking with a magnet, or by passing current in a coil around the bar (electromagnet). Soft iron magnetises and demagnetises easily and is used in electromagnets; steel keeps its magnetism and is used for permanent magnets. A magnet loses magnetism when heated, hammered, dropped repeatedly or placed in an alternating-current coil. Magnets are stored in pairs with keepers, with unlike poles together.
Why is steel used for permanent magnets and soft iron for electromagnets?
Steel retains its magnetism; soft iron loses it quickly when the current stops.
Name two ways of demagnetising a magnet.
Heating it strongly or hammering it; also by an alternating current.
Why are bar magnets stored in pairs with keepers?
Unlike poles together close the field loops and stop the magnets from weakening.
1. Name three magnetic materials.
2. State the law of magnetic poles.
3. What is the sure test of magnetism?
4. Which of copper, nickel and wood is attracted by a magnet?
5. State two properties of field lines.
6. What happens if a magnet is cut in two?
7. Name one use of a compass.
8. Why is soft iron used in electromagnets?
9. Give two ways to destroy a magnet.
10. Why must magnets be stored with keepers?