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Kosmoi.com Science Physics Electromagnetism :

There is a fundamental connection between electrity - moving electrons - and magnetism. Like electricity and gravity, magnetism is a fundamental force. Magnetism and electricity are closely related and are regarded as two expressions of a single force, the electromagnetic force. Moving electrons have magnetic fields, and magnetic fields make electrons travel curved paths. This is how television and computer monitors 'scan' a picture onto the screen. An electrical current causes the deflection of a compass needle because magnetic effects are produced by a moving electrical charge.

Magnetism is seen whenever electrically charged particles are in motion, either from movement of electrons in an electric current, resulting in electromagnetism, or from the constant subatomic movement of electrons, resulting in what are known as 'permanent magnets'.

Electromagnets

Magnetic force can be created by electricity flowing through a conductor, e.g. a wire.

Any conductor that carries a current is surrounded by a magnetic field whose lines of force form circles with the conductor at the centre. The strength of the field is proportional to the current. If the conductor is wound into a coil (called a solenoid) the field is concentrated along the inside, and the coil behaves like a bar magnet. An electromagnet is some suitable core material around which is wrapped a current-carrying coil. When an electric current is passed through the winding, the electromagnet produces a strong magnetic field. Unlike a permanent magnet, the magnetic field of an electromagnet may be switched on and off. The force can be further increased by coiling it around some iron.

Electromagnets are useful in cases where a magnet must be switched on or off; for instance, large cranes to lift junked automobiles. The electromagnet is the basis of many electromechanical devices, such as relays, solenoids, electric bells, and electric motors. Much of modern technology is based on electromagnetism, such as door bells, telephones, loudspeakers, cassette recorders, television monitors, electric clocks, electric motors, etc etc.

Electromagnetic Waves

If the charge moving through the conductor accelerates, then the magnetic field changes along with the associated electric field. By making the fields fluctuate electromagnetic radiation can be produced, in which the fluctuations are propagated through space at a speed of nearly 300,000 km/186,000 mi per second. Wave-like fluctuations in electric and magnetic fields travelling through free space or a material medium. This radiation is produced by the acceleration of electric charges. The electromagnetic waves travel at a constant speed in a vacuum, called the speed of light, which is close to 300,000 km/s (186,000 m.p.s.): this may be slower if the waves are travelling in a material medium. The (limitless) range of possible wavelengths and frequencies of electromagnetic waves, which can be thought of as making up the electromagnetic spectrum, includes radio waves, infrared radiation, visible light, ultraviolet radiation, X-rays, and gamma rays.

The unification of electricity and magnetism into a single theory of electromagnetism is due to James Clerk Maxwell, who in 1864 predicted the existence of electromagnetic radiation and its ability to travel through space at the speed of light. Maxwell's equations describe the origin and behavior of the fields that govern these forces. In 1888 the German physicist Heinrich Hertz used oscillating electrical circuits to produce radio waves which travelled at the velocity of light, thereby providing experimental support for Maxwell's work. In 1901 Italian inventor Guglielmo Marconi constructed a device to translate radio waves into electrical signals and achieved reception of a signal in Newfoundland, Canada, transmitted from Cornwall, England across the Atlantic Ocean.

The expression of the velocity of light in terms of fundamental constants suggested to Einstein that it should always be the same for all observers. This conclusion is central to his theory of special relativity (1905), which in turn explains more fully the relationship between electric and magnetic effects.