Electricity and Magnetism
Electricity and magnetism are manifestations of a single underlying electromagnetic force. Electromagnetism is a branch of physical science that describes the interactions of electricity and magnetism, both as separate phenomena and as a singular electromagnetic force. Amagnetic field is created by a moving electric current and a magnetic field can induce movement of charges (electric current). The rules of electromagnetism also explain geomagnetic and electromagnetic phenomena by explaining how charged particles of atoms interact.
Before the advent of technology, electromagnetism was perhaps most strongly experienced in the form of lightning, and electromagnetic radiation in the form of light. Ancient man kindled fires that he thought were kept alive in trees struck by lightning. Magnetism has long been employed for navigation in the compass. In fact, it is known that Earth's magnetic poles have exchanged positions in the past.
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 forms, positive charges and negative charges. Like charges repel each other, and differing types attract.
The force that attract positive charges to negative charges weakens with distance, but is intrinsically very strong—up to 40 times stronger than the pull of gravity at the surface of the earth. This fact can easily be demonstrated by a small magnet that can hold or suspend an object. The small magnet exerts a force at least equal to the pull of gravity from the entire Earth.
The fact that unlike charges 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. Other less known particles 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. Conversely, when the electron binding is strong, the material resists electron flow and is 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 flow from the negative to the positive.
Electromagnetism is the theory of a unified expression of an underlying force, the electromagnetic force. This is seen in the movement of electric charge, that gives rise to magnetism (the electric current in a wire being found to deflect a compass needle), and it was Scottish physicist James Clerk Maxwell (1831–1879), who published a unifying theory of electricity and magnetism in 1865. The theory arose from former specialized work by German mathematician Carl Fredrich Gauss (1777–1855), French physicist Charles Augustin de Coulomb (1736–1806), French scientist André Marie Ampère
(1775–1836), English physicist Michael Faraday (1791–1867), American scientist and statesman Benjamin Franklin (1706–1790), and German physicist and mathematician Georg Simon Ohm (1789–1854). However, one factor that did not contradict the experiments was added to the equations by Maxwell 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, with the expected velocity, was found to be an example of this electro-magnetic 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 nor 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 don't 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 rays and gamma rays.
The unification of electricity and magnetism allows a deeper understanding of physical science, and much effort has been put into further unifying the four forces of nature (e.g., the electromagnetic, weak, strong, and gravitational forces. The weak force has now been unified with electromagnetism, called the electroweak force. There are research programs attempting to collect data that may lead to a unification of the strong force with the electroweak force in a grand unified theory, but the inclusion of gravity remains an open problem.
Maxwell's theory is in fact in contradiction with Newtonian mechanics, and in trying to find the resolution to this conflict, Einstein was lead to his theory of special relativity. Maxwell's equations withstood the conflict, but it was Newtonian mechanics that were corrected by relativistic mechanics. These corrections are most necessary at velocities, close to the speed of light.
Paradoxically, magnetism is a counter example to the frequent claims that relativistic effects are not noticeable for low velocities. The moving charges that compose an electric current in a wire might typically only be traveling at several feet per second (walking speed), and the resulting Lorentz contraction of special relativity is indeed minute. However, the electrostatic forces at balance in the wire are of such great magnitude, that this small contraction of the moving (negative) charges exposes a residue force of real world magnitude, namely the magnetic force. It is in exactly this way that the magnetic force derives from the electric. Special relativity is indeed hidden in Maxwell's equations, which were known before special relativity was understood or separately formulated by Einstein.
Electricity at high voltages can carry energy across extended distances with little loss. Magnetism derived from that electricity can then power vast motors. But electromagnetism can also be employed in a more delicate fashion as a means of communication, either with wires (as in the telephone), or without them (as in radio communication). It also drives motors and provides current for electronic and computing devices.
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