- Slides: 21
Electromagnetism • A magnetic field is produced when an electric current flows through a coil of wire. This is the basis of the electromagnet. • Electricity & magnetism are different aspects of the same basic phenomenon: ‘Electromagnetism’ • We can make an electromagnet stronger by doing these things: • wrapping the coil around an iron core • adding more turns to the coil • increasing the current flowing through the coil.
A Simple Electromagnet: When an electric current passed through a wire it generates a magnetic field around the wire.
The magnetic field around an electromagnet is just the same as the one around a bar magnet. It can, however, be reversed by turning the battery around. Unlike bar magnets, which are permanent magnets, the magnetism of electromagnets can be turned on and off just by closing or opening the switch.
If the wire is made into a coil and connected into a circuit, a magnetic field is produced around the coil.
Using Electromagnets are very useful as they can be switched on and off, have their strength altered or their poles switched around. They are used in many things, especially when we want to create movement. Examples are : Motors Bells Loudspeakers The shape of the field around an electromagnet is similar to the field around a bar magnet.
Electric bells like the ones used in most schools also contain an electromagnet. › When the current flows through the circuit, the electromagnet makes a magnetic field. › The electromagnet attracts the springy metal arm. › The arm hits the gong, which makes a sound and the circuit is broken. › The electromagnet is turned off and the springy metal arm moves back. › The circuit is complete again.
The Relay A relay is a device that uses a small current to operate a switch to control a high current. This shows how a relay is used to operate a car starter motor. When the switch is closed, current flows through the coil and the iron core becomes magnetised. This attracts the iron armature. The top of the armature is pulled towards the electromagnet. The armature turns on the pivot, and the bottom part moves to the right, pushing the contacts together. The second circuit is now complete, and the large current will make the motor turn.
The Loudspeaker A loudspeaker produces sounds by vibrating quickly backwards and forwards. Signals fed to a loudspeaker pass through an amplifier which makes the amplitude of the vibrations larger. This makes the sound louder. Inside the speaker there is a permanent magnet and an electromagnet. The strength of the electromagnet is changed by the signal, and so the force between it and the permanent magnet also changes. One of the magnets is attached to the cone of the speaker. As the strength of the electromagnet changes, the cone vibrates and makes a sound.