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  This force has long been familiar to humankind. Possibly the first to learn about its properties were the Chinese, who discovered that a magnetized needle always points north. Later, static electricity and electric sparks fascinated a generation, and the discovery of current electricity produced primitive electric motors. It soon became obvious that electricity and magnetism were inextricably linked; a moving charge, or current, creates a magnetic field, and moving charges can be deflected with the use of magnets.

In the late 19th century, James Maxwell unified all these phenomena in four equations, known as Maxwell\'s equations, which described the entire gamut of electricity and magnetism, and moreover, showed that light, infrared radiation, ultraviolet radiation, radiowaves and x-rays are all exactly the same kind of waves, differing only in wavelength. They are electromagnetic waves, composed of an electric and a magnetic wave oscillating at right angles to each other. Maxwell\'s equations predicted the speed of light from other well known constants. This triumphant theory was perhaps the first example of the unification of forces, showing that electricity and magnetism are merely different aspects of the same force. Unification of all known forces is one of the ultimate aims of 20th-century physics.

There is an asymmetry in the equations, due to the non-existence of a magnetic monopole. Positive and negative electric charges may exist on their own, but we never see the north end of a magnet without the south end. If a magnet is broken in two, it merely forms two new north and south poles in the middle, so that one end never exists without the other. However, some modern theories suggest that magnetic monopoles may exist, and so the magnetic monopole term is sometimes included in the equations.

We now believe that the electromagnetic force operates by exchange of photons—the photon is the field carrier for the electromagnetic force. Current theories suggest that, just as electricity and magnetism may be unified as the electromagnetic interaction, so may the weak force and the electromagnetic force, as the electroweak interaction. The weak interaction has the Z and W bosons as its field carriers. At very high energies, such as those produced in particle accelerators, the photon and the Z boson look very similar.

Electromagnetic phenomena pervade our lives. All electrical apparatus, static electricity, magnets, motors, dynamos and natural phenomena such as lightning may be understood by the use of Maxwell\'s equations. JJ



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