Properties of magnetic materials

A material is described as magnetic if it is attracted to a magnet. Magnetic materials include the metals iron, nickel and cobalt. Steel is another common magnetic metal. Steel is a mixture that contains a large proportion of iron.

Magnetic materials can be magnetised. Magnetised means turned into a magnet. The magnets you have used were all made by magnetising magnetic materials.

The magnets you have used are called permanent magnets because they have a magnetic field that is always there. You cannot switch the magnetic field off and on again.

One way to magnetise a magnetic material is by using electricity. When this method is used, the magnet is called an electromagnet.

Electromagnets

An electromagnet is made by wrapping a wire around a magnetic material, such as iron. The wire that is wrapped around is a called a coil. The material in the middle of the coil is called the core.

When current passes through the coil, the magnetic material becomes magnetised.

When the current is switched off, the magnetic material loses most of its magnetism.

The diagram shows the simplest type of electromagnet.

Poles of an electromagnet

An electromagnet has two poles, similar to a bar magnet.

You can find out which pole is which in two simple ways.

• Use a magnetic compass. A magnetic compass points along magnetic field lines, so will point towards the south pole.
• Use a bar magnet with known poles. Opposite poles attract and like poles repel so, by bringing the bar magnet close to the electromagnet, you can detect which pole is which.

You can reverse the poles of an electromagnet in one of two ways.

• Wrap the coil around in the opposite direction.
• Reverse the connections on the cell or power supply.

Applications of electromagnets

Electromagnets are used in many applications where a permanent magnet would not be useful.

The fire door in the picture is held open with an electromagnet. The electromagnet is connected to the fire alarm. When the fire alarm is switched on, the magnet is switched off and the door closes.

Some types of scanners in hospitals use powerful electromagnets. An MRI scanner is used to produce images from inside the human body. MRI stands for magnetic resonance imaging.

Electromagnets can be used for sorting scrap metal. The electromagnet will attract iron and steel, leaving other non-magnetic metals behind. Common non-magnetic metals include copper, aluminium and zinc. When the magnetic metals have been lifted, they can be moved away and then dropped by switching off the magnet.

In a toaster, when the handle is pushed down, an electromagnet holds down a metal basket with bread in it. A timer turns the electromagnet off and the metal basket pops up with toast.

An electric bell uses an electromagnet to make the hammer move. When the electromagnet is on, the hammer is pulled onto the bell. The movement breaks the circuit and the hammer moves away from the bell. The circuit becomes complete again and the hammer is pulled back to the bell. This continues until the power supply is turned off. Electric bells are used in schools, as fire alarms and as door bells.

Electric motors use electromagnets to change electrical energy into kinetic energy.