O Level: Temporary and permanent magnets

Iron as a temporary magnet:

• Iron can be easily magnetised or demagnetised (soft magnetic material. It can even be magnetized by a weak magnetic field. it is therefore suitable to be used in temporary magnets.
• When mixed with other metals (e.g. Ni, Cu, Mn, Si), powerful temporary magnets can be made.
• These temporary magnets are used to make temporary electromagnets. Electromagnets lose its magnetism when it is removed from magnetising fields. Electromagnets are very useful because they can be turned on and off and their strengths can be varied.
• In order to shield or contain any magnetic effects, soft permeable iron is also used as effective magnetic shields. (magnetic keepers)

E.g. Electromagnets can be used for such tasks as moving cars or sorting metals from other landfill materials. Other applications are in circuit breakers, magnetic relays, electric bells, audio and video tapes transformers etc.

Steel as a permanent magnet

• Compared to iron, steel cannot be easily magnetised or demagnetised (hard magnetic material). It can only be magnetized by a strong magnetic field. But, steel has the ability to retain its magnetism once it is magnetized. This trait allows steel to be suitable to be used in permanent magnets.
• Steel is typically mixed with other magnetic material to ensure structural stability. In this way, strong permanent magnets are made.

E.g. Permanent magnets are used in compasses, magnetic door catches, moving coil galvanometers, d.c. motors, a.c. generators, loudspeakers, and for many other purposes.

Note: Theoretical limit for a permanent magnetic field is 5 Tesla. Electromagnets made with ordinary wires can produce steady fields of 34 Tesla.

The difference between magnetic properties of iron and steel can be summarised in the table below: