Copyright © 2007 Dorling Kindersley
Electromagnetism is a two-way link between electricity and magnetism. An electric current creates a magnetic field, and a magnetic field, when it changes, creates a voltage. The discovery of this link led to the invention of the TRANSFORMER, electric motor, and generator. It also, after more than 50 years of further work, explained what light is and led to the invention of radio.
A wire carrying a current is surrounded by a magnetic field. If the wire is coiled, the fields from each turn of wire produce a stronger field. If the wire is wrapped around an iron core, the field gets stronger still. An electromagnet can be a single coil (called a solenoid) or bent double, with two coils.
Electromagnets make it easy to handle scrap metal. When the current is switched on, it creates strong magnetism that picks up a load of steel. The crane swings round, the current is switched off, the magnetism disappears, and the steel drops where it is wanted.
Travellers to Pudong Airport in China can ride at 430 kph (267 mph) on a train with no wheels. This Transrapid system, developed in Germany, uses electromagnets to suspend the train in thin air while a moving magnetic field from electromagnets in the track pushes the train along. Passengers have a smooth ride as the train floats above the magnetic guideway.
When the current is switched on, the electromagnet becomes magnetic. But this does not happen instantly. A magnetic field is a store of energy, and it takes time to feed enough energy into it. This effect, known as inductance, can be used in electronics to control the rate at which things happen.
It was this scientist who made the first, vital link between electricity and magnetism. Lecturing at the University of Copenhagen in 1820, he connected a battery to a wire that ran near a magnetic compass. The compass needle swung round, and Oersted realized that the current was producing magnetism. He published his revolutionary discovery in 1821.
When Oersted discovered that electricity produced magnetism, Faraday wondered if magnetism could produce electricity. In 1831 he showed that it can. He pushed a magnet into a coil of wire and found that a moving magnet created a current. American Joseph Henry discovered this around the same time.
A transformer uses electromagnetism to transfer power between two circuits. Power can take the form of high voltage and low current, or low voltage and high current. Transformers can convert one to the other, but only if the current is alternating, or continually reversing its direction. Low voltages for electronic circuits often come from mains voltage through a transformer.
Transformers are essential for moving electricity around cheaply and safely. This is why modern systems use the alternating current that transformers require. At a generating station, huge transformers step up the voltage to transfer power along cables efficiently. The voltage is stepped down at local substations to a safer level for home use.
A transformer has two coils (windings) of wire round the same iron core, so that they share the same magnetic field. The changing field produced by alternating current in the primary winding causes a voltage across the secondary winding. If the secondary coil has fewer turns of wire than the primary, its voltage will be lower – as in this transformer from a hi-fi.