Magnetism
The pull of a magnet
- A fridge magnet sticks; a compass needle always swings to point north.
- A magnet has two ends, called poles: a north (N) and a south (S).
- Let's see the rules magnets follow — and how to picture their invisible field.
Poles and magnetic materials
- The pole rule is like the charge rule:
- Same poles (N–N or S–S) repel (push apart).
- Different poles (N–S) attract (pull together).
- A magnet attracts magnetic materials: iron, steel, cobalt, nickel. Copper, plastic and wood are not magnetic.
Two north poles are brought close together. They:
Like poles repel. Only different poles (N and S) attract.
Which of these is a magnetic material (attracted by a magnet)?
Iron (also steel, cobalt, nickel) is magnetic. Copper, plastic and wood are not.
Induced, temporary and permanent magnets
- Put iron near a magnet and the iron becomes a magnet itself — induced magnetism. The near end gains the opposite pole, so they attract.
- Soft iron magnetises easily but loses it quickly → a temporary magnet (great for electromagnet cores).
- Steel is harder to magnetise but keeps it → a permanent magnet (great for bar magnets).
Which material is best for a permanent magnet?
Steel is hard to magnetise but keeps it, so it makes a permanent magnet. Soft iron loses magnetism quickly, so it suits temporary magnets and electromagnet cores.
Magnetic fields
- A magnetic field is a region where a magnetic pole feels a force.
- We draw it with field lines that come out of N and go into S.
- Where the lines are close together, the field is strong.
- Show the pattern with iron filings or a small plotting compass (the needle points along the field).
Magnetic field lines around a bar magnet point:
By convention, field lines come out of N and go into S; the closer together they are, the stronger the field.
Where the field lines are closer together, the magnetic field is stronger.
Closely spaced field lines mean a strong field — that is why the field is strongest near the poles.
Uses of magnets
- Permanent magnets: fridge catches, compass needles, loudspeakers.
- Electromagnets (magnetic only while a current flows): scrap-yard cranes, electric bells, relays.
- An electromagnet's big advantage: you can switch it on and off and make it stronger — a permanent magnet cannot.
What can an electromagnet do that a permanent magnet cannot?
An electromagnet is magnetic only while a current flows, so it can be switched on/off and made stronger by increasing the current or turns.
You've got it
- like poles repel, unlike poles attract; magnetic materials = iron, steel, cobalt, nickel
- soft iron = temporary magnet; steel = permanent magnet
- field lines go out of N, into S; closer lines = stronger field
- electromagnets can be switched on/off and made stronger