Permanent Magnets
A permanent magnet always has two poles:
- North pole (N)
- South pole (S)
The Rules of Attraction
Like poles repel (N–N or S–S push each other away) Unlike poles attract (N–S pull each other together)
You can never have a single magnetic pole (monopole) on its own. If you cut a magnet in two, each half becomes a complete magnet with its own N and S pole.
Magnetic Field Lines
The region around a magnet where magnetic forces act is called the magnetic field. We represent it with field lines:
- Lines run from the north pole to the south pole (outside the magnet)
- Where lines are closer together → field is stronger
- Lines never cross each other
- Lines always enter or leave a pole at right angles
Magnetic Materials
Only certain materials are magnetic:
- Iron, steel, nickel, cobalt are magnetic
- Gold, copper, aluminium, wood, plastic are NOT magnetic
Iron is attracted to a magnet but does not stay magnetised. Steel becomes a permanent magnet.
Electromagnets
An electromagnet is a coil of wire that becomes magnetic when an electric current flows through it.
Advantages over permanent magnets:
- Can be switched on/off
- Strength can be increased by:
- Increasing the current
- Adding more coil turns (loops)
- Adding an iron core inside the coil
Uses: electric bells, scrapyard cranes, MRI scanners, speakers
Real world: Compass navigation
A compass needle is a small permanent magnet that aligns itself with Earth's magnetic field. Earth acts like a giant bar magnet with magnetic poles near (but not exactly at) the geographic poles. Hikers, pilots, and sailors use compass bearings for navigation — the needle's north pole points toward Earth's magnetic north pole because opposite poles attract.
Real world: Scrapyard crane electromagnet
A scrapyard crane uses a large electromagnet to pick up steel car bodies. When current flows through the coil, the electromagnet switches on and lifts the metal. When the operator cuts the current, the electromagnet switches off and the metal drops precisely where needed. A permanent magnet could never release the load — the switchable nature of an electromagnet is essential.
Real world: Loudspeaker — turning electricity into sound
A loudspeaker uses a permanent magnet and an electromagnet coil (voice coil) attached to a cone. When a varying electrical signal flows through the coil, it becomes a magnet with alternating polarity that is attracted and repelled by the permanent magnet — pushing and pulling the cone rapidly to create sound waves in the air.
Practice Problems
Two magnets are placed so their north poles face each other. What happens?
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How can you increase the strength of an electromagnet?
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Why is iron used in the core of an electromagnet instead of steel?