# Electromagnetism

## The Fundamental Role Of Special Relativity In Electromagnetism, Technological Uses Of Electromagnetism

Electromagnetism is a branch of physical science that involves all the phenomena in which electricity and magnetism interact. This field is especially important to electronics because a magnetic field is created by an electric current. The rules of electromagnetism are responsible for the way charged particles of atoms interact.

Some of the rules of electrostatics, the study of electric charges at rest, were first noted by the ancient Romans, who observed the way a brushed comb would attract particles. It is now known that electric charges occur in two different types, called positive and negative. Like types repel each other, and differing types attract.

The force that attracts positive charges to negative charges weakens with distance, but is intrinsically very strong. The fact that unlike types attract means that most of this force is normally neutralized and not seen in full strength. The negative charge is generally carried by the atom's electrons, while the positive resides with the protons inside the atomic nucleus. There are other less well known particles that can also carry charge. When the electrons of a material are not tightly bound to the atom's nucleus, they can move from atom to atom and the substance, called a conductor, can conduct electricity. On the contrary, when the electron binding is strong, the material is called an insulator.

When electrons are weakly bound to the atomic nucleus, the result is a semiconductor, often used in the electronics industry. It was not initially known if the electric current carriers were positive or negative, and this initial ignorance gave rise to the convention that current flows from the positive terminal to the negative. In reality we now know that the electrons actually run from the negative to the positive.

Electromagnetism is the theory of a unified expression of an underlying force, the so-called electromagnetic force. This is seen in the movement of electric charge, which gives rise to magnetism (the electric current in a wire being found to deflect a compass needle), and it was a Scotsman, James Clerk Maxwell, who published the theory unifying electricity and magnetism in 1865. The theory arose from former specialized work by Gauss, Coulomb, Ampère, Faraday, Franklin, and Ohm. However, one factor that did not contradict the experiments was added to the equations by Maxwell so as to ensure the conservation of charge. This was done on the theoretical grounds that charge should be a conserved quantity, and this addition led to the prediction of a wave phenomena with a certain anticipated velocity. Light, which has the expected velocity, was found to be an example of this electromagnetic radiation.

Light had formerly been thought of as consisting of particles (photons) by Newton, but the theory of light as particles was unable to explain the wave nature of light (diffraction and the like). In reality, light displays both wave and particle properties. The resolution to this duality lies in quantum theory, where light is neither particles or wave, but both. It propagates as a wave without the need of a medium and interacts in the manner of a particle. This is the basic nature of quantum theory.

Classical electromagnetism, useful as it is, contains contradictions (acausality) that make it incomplete and drive one to consider its extension to the area of quantum physics, where electromagnetism, of all the fundamental forces of nature, it is perhaps the best understood.

There is much symmetry between electricity and magnetism. It is possible for electricity to give rise to magnetism, and symmetrically for magnetism to give rise to electricity (as in the exchanges within an electric transformer). It is an exchange of just this kind that constitutes electromagnetic waves. These waves, although they do not need a medium of propagation, are slowed when traveling through a transparent substance.

Electromagnetic waves differ from each other only in amplitude, frequency and orientation (polarization). Laser beams are particular in being very coherent, that is, the radiation is of one frequency, and the waves coordinated in motion and direction. This permits a highly concentrated beam that is used not only for its cutting abilities, but also in electronic data storage, such as in CD-ROMs.

The differing frequency forms are given a variety of names, from radio waves at very low frequencies through light itself, to the high frequency x and gamma rays.

Many miracles 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. The remaining known forces are the so called weak, strong, and gravitational forces. The weak force has now been unified with electromagnetism, called the electroweak force. There are proposals to include the strong force in a grand unified theory, but the inclusion of gravity remains an open problem.