Magnets and their fields
Every magnet has two poles, a north pole and a south pole. The basic law of magnetism is: like poles repel, unlike poles attract. Only a few materials are magnetic — iron, steel, nickel and cobalt — while copper, aluminium and brass are not.
Key idea
A magnetic field is the region where a magnet exerts a force. It is drawn with field lines that point from the north pole to the south pole outside the magnet. The field is strongest where the lines are closest together — at the poles. Field lines never cross.
Temporary and permanent magnets
- Soft iron is easy to magnetise and demagnetise, so it is used for temporary magnets and electromagnet cores.
- Steel is hard to magnetise but keeps its magnetism, so it is used for permanent magnets.
A material can be magnetised by induction when placed in a field, and demagnetised by heating, hammering, or applying a decreasing alternating current.
Electromagnets
Passing a current through a coil (solenoid) produces a magnetic field. An electromagnet can be made stronger by increasing the current, adding more turns, or using a soft iron core. Its great advantage is that it can be switched on and off.
Remember
- Like poles repel; unlike poles attract.
- Soft iron = temporary; steel = permanent.
- A plotting compass points along the field, its north end towards the magnet's south pole.