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Methods of magnetisation and demagnetisation

Electromagnets

An electromagnet is a temporary magnet formed using a coil of wire, called a solenoid.

When direct current flows through a solenoid, it creates a constant magnetic field.

If a core of magnetic material is placed inside the solenoid, the strength of the electromagnet will increase. A relatively small current can create a strong electromagnet.

A core made from a soft magnetic material will lose its magnetism quickly when the electricity is turned off. Electromagnets made using soft iron cores are useful for lifting and dropping magnetic materials.

Electromagnets are used to move steel objects in scrap yards.

The strength of an electromagnet can also be increased by increasing the number of turns of the coil and increasing the current though the coil.

These electromagnets are part of a simple door bell.

Electrical method of magnetisation

When direct current flows through a solenoid, a magnetic field is created.

If a core made from a magnetic material is placed inside a solenoid, the core will become magnetised. A relatively small current can create a strong magnet.

A core made from a hard magnetic material will keep its magnetism after the current has been turned off. This is the electrical method of magnetisation.

The magnetic poles can be determined using the right hand grip rule:

• Curl the fingers of your right hand in the direction of the conventional current in the solenoid and extend your thumb.
• The thumb points from the south pole to the north pole .

Electrical method of demagnetisation

An alternating current (A.C.) flowing through a solenoid creates a magnetic field that is constantly changing.

The north pole and the south pole swap positions with each other many times every second.

Placing a magnet inside a solenoid as shown below causes the magnet to become demagnetised.

This is because the individual magnetic fields (domains) inside the magnet become "confused" as they try to align themselves with the constantly changing magnetic field.

After some time, the domains will be pointing in random directions and cancel each other out.

This is known as the electrical method of demagnetisation.

Electrical demagnetisation method. The poles switch rapidly causing the magnetic domains to become muddled.