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Question: Can you give me some simple information about electromagnets? 



Answer: Electro-magnets always have a soft iron core which increases the strength of the magnet. The core has to be soft - this means magnetically soft- so that when the current is turned off the magnetism disappears with it.

There are hundreds of different kinds of jobs that are done by using electro-magnets. Here are some other uses:

You can slide the bolts of locks and latches, tap out telegraphic dots and dashes, open and close switches in distant electric circuits. The rotors of all electric motors are spun by electro-magnetism, for example, washing machines and electric lawnmowers.

A relay is a device which uses a low current circuit to switch high current circuit on/off. A large relay system is used in cars for switching the starter motor, because it draws a very big current.

When the switch in the low current is closed it turns the electro magnet on, this attracts the iron rocker. The rocker pivots and closes the contacts in the high current circuit. When the low current switch is opened the electro-magnet stops pulling, the rocker returns and the high current circuit is broken again.

Electro-magnets have been used in hospitals to remove particles of metal ( those which are magnetic- because not all common metals are magnetic, e.g. aluminium, copper, brass, gold, silver), which may have lodged in people's eyes after an accident.

Some large cranes in scrapyards are electromagnetic. The electro-magnet is made up of a big coil of wire, with many turns and a soft iron core.

The crane lifts large pieces of metal when the current is switched on and drops its load when the current is switched off.

A circuit breaker, also called a resettable fuse is electro-magnetic. This is placed on the incoming live wire. If the current gets too high the magnetic field in the coil pulls the iron rocker which "trips" the switch and breaks the circuit. It can be reset manually but will always switch itself off if the current is too high.

A loudspeaker uses electromagnets. An electromagnet consists of an iron centre which has coils of wire tightly wrapped round it. This is known as a solenoid. When the current flows through the wires of the solenoid it produces a magnetic field round it. the iron core increases the strength of the magnetic field. A solenoid can behave like a normal bar magnet, with a north and south pole. Which is north and which south depends on the way the electricity goes through. When the electricity stops, the magnetism stops too. Electromagnets are useful because their magnetism can be turned on and off by turning the electricity supply on and off.

To understand how a loudspeaker works, it is best to start with a microphone. A microphone turns sound into electricity. A microphone has grains of carbon behind a disc. Wire carrying electricity is connected to the carbon. When someone speaks into a microphone, this makes sound waves, which make the disc tremble. When the disc trembles, it pushes the grains of carbon in and out slightly. This movement affects the strength of the electricity in the wire. Loud noises move the disc most and cause the biggest change in electricity. Soft sound hardly move the disc at all and cause only a weak change. Speaking into a microphone sends a changing electric current along its wire. Earphones turn the changing electric current back into sound waves. There is an electromagnet in the earphone which has a metal disc in front of it. Different strengths of electricity make the magnetism weaker or stronger. the changes in magnetic strength make the disc tremble, which makes sound waves in the air. These are an exact copy of the sound waves that went into the microphone, and so an exact copy of the sound is heard.

In the case of loudspeakers, the sound waves can be made extra clear by joining the metal disc to a cone of cardboard. The whole cone trembles when the disc does. This moves more air and makes clearer and louder sound waves. Radios, cassette and CD players all have loudspeakers inside them.




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