Electromagnetism



Electromagnetism is the force involving the interaction of electricity and magnetism. It is the science of electrical charge, and its rules govern the way charged particles of atoms interact. Electromagnetism is one of the four fundamental forces of the universe (gravity and the "strong" and "weak" forces that hold an atomic nucleus together are the other three). Because its effects can be observed so easily, electromagnetism is the best understood of these four forces.

Some of the rules of electrostatics, or the study of electric charges at rest, were first noted by the ancient Romans, who observed the way a brushed comb would attract particles. Until the nineteenth century, however, electricity and magnetism were thought to be totally different and separate forces. In 1820, a direct connection between the two forces was confirmed for the first time when Danish physicist Hans Christian Oersted (1777–1851) announced his discovery that an electric current, if passed through a wire placed near a compass needle, would make the needle move. This suggested that electricity somehow creates a magnetic force or field, since a compass needle moves by magnetism.

Shortly afterward, French physicist André Marie Ampère (1775–1836) conducted experiments in which he discovered that two parallel

A MAGLEV supertrain. (Reproduced by permission of Phototake.)
A MAGLEV supertrain. (Reproduced by permission of
Phototake
.)

wires each carrying a current attract each other if the currents flow in the same direction, but repel each other if they flow in opposite directions. He concluded that magnetism is the result of electricity in motion.

A decade after Oersted's experiments, English physicist Michael Faraday (1791–1867) observed that an electric current flowing in a wire created what he called "lines of force" to expand outward, inducing or causing an electric flow in a crossed wire. Since it was known from Oersted's work that an electric current always produces a magnetic field around itself, Faraday concluded from his experiments just the opposite: that a wire moving through a magnetic field will induce an electric current in the wire.

Finally, between 1864 and 1873, Scottish physicist James Clerk Maxwell (1831–1879) devised a set of mathematical equations that unified electrical and magnetic phenomena into what became known as the electromagnetic theory. He and his contemporaries now understood that an electric current creates a magnetic field around it. If the motion of that current changes, then the magnetic field varies, which in turn produces an electric field.

Words to Know

Electromagnetic radiation: Radiation (a form of energy) that has properties of both an electric and magnetic wave and that travels through a vacuum with the speed of light.

Electromagnetic spectrum: The complete array of electromagnetic radiation, including radio waves (at the longest-wavelength end), microwaves, infrared radiation, visible light, ultraviolet radiation, X rays, and gamma rays (at the shortest-wavelength end).

Frequency: The rate at which vibrations take place (number of times per second the motion is repeated), given in cycles per second or in hertz (Hz). Also, the number of waves that pass a given point in a given period of time.

Maxwell also discovered that the oscillation or fluctuation of an electric current would produce a magnetic field that expanded outward at a constant speed. By applying the ratio of the units of magnetic phenomena to the units of electrical phenomena, he found it possible to calculate the speed of that expansion. The calculation came out to approximately 186,300 miles (300,000 kilometers) per second, nearly the speed of light. From this, Maxwell theorized that light itself was a form of electromagnetic radiation that traveled in waves. Since electric charges could be made to oscillate at many velocities (speeds), there should be a corresponding number of electromagnetic radiations. Therefore, visible light would be just a small part of the electromagnetic spectrum, or the complete array of electromagnetic radiation.

Indeed, modern scientists know that radio and television waves, microwaves, infrared rays, ultraviolet light, visible light, gamma rays, and X rays are all electromagnetic waves that travel through space independent of matter. And they all travel at roughly the same speed—the speed of light—differing from each other only in the frequency at which their electric and magnetic fields oscillate.

Many common events depend upon the broad span of the electromagnetic spectrum. The ability to communicate across long distances despite intervening obstacles, such as the walls of buildings, is possible using the radio and television frequencies. X rays can see into the human body without opening it. These things, which would once have been labeled magic, are now ordinary ways we use the electromagnetic spectrum.

The unification of electricity and magnetism has led to a deeper understanding of physical science, and much effort has been put into further unifying the four forces of nature. Scientists have demonstrated that the weak force and electromagnetism are part of the same fundamental force, which they call the electroweak force. There are proposals to include the strong force in a grand unified theory, which attempts to show how the four forces can be thought of as a manifestation of a single basic force that broke apart when the universe cooled after the big bang (theory that explains the beginning of the universe as a tremendous explosion from a single point that occurred 12 to 15 billion years ago). The inclusion of gravity in the unified theory, however, remains an open problem for scientists.

[ See also Electricity ; Electromagnetic field ; Electromagnetic induction ; Magnetism ]



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