The word electricity comes from the Greek word, electron, meaning amber. Amber is a translucent, yellowish mineral made of fossilized resin. When rubbed with a cloth, amber becomes charged with static electricity. We now know that such electrostatic effects arise whenever a body has an excess or deficiency of electrons or protons. These effects usually accompany the transfer of electrons, such as occurs when two dissimilar materials are rubbed together. A fundamental rule of electrostatics is that bodies of like charge repel each other, and bodies of opposite charge attract each other. The Greeks described these attractive and repulsive electrostatic forces as electric force.
At the time of the American Revolution, little more was known about electricity than at the time of the Greeks. Benjamin Franklin was one of the first persons to experiment with electricity. In 1821, Hans Oersted demonstrated that electricity and magnetism were interrelated. Later, James Clerk Maxwell synthesized all that was known about electricity and magnetism into a set of equations that explain why a moving electric charge gives rise to a magnetic field, and why a changing magnetic field produces an electric field.
In the classical picture of the atom, an electron cloud surrounds a small nucleus (composed of neutrons and protons). The proton and electron are considered the fundamental particles of electricity, the two particles having equal and opposite charges (convention assigns a positive charge to the proton, and a negative one to the electron). Under normal conditions, the number of electrons equals the number of protons, so the atom is electrically neutral.
Since its discovery at the end of the nineteenth century, the electron has been considered a fundamental building block of nature that cannot be decomposed into more basic particles. Electrons are intrinsically stable, and possess mass, charge, and spin (i.e., intrinsic angular momentum). In recent years, despite extensive experimental and theoretical research in elementary particle physics, many physicists have concluded that the electron is indeed an ultimate, indivisible constituent of matter that will not be found to have more fundamental parts. The electron is thus considered a basic unit of electricity and matter.
When electrons in a conducting material drift under the influence of an electric field, they produce an electric current. Electric currents can also be produced by the motion of positive nuclei, as well as by ions moving in a liquid or gaseous electrical discharge.
In order for electric current to flow, there must exist an excess of electrons and a conductor to carry the current. As electrons enter the conductor, they displace the electrons in the conductor's atoms. In turn, these displaced electrons displace other electrons. Thus, the electron that exits the conductor will not be the same one that entered it. When a current flows in a conductor, the charges are impeded by collisions with the atoms that make up the material. The charges give up energy to the atoms, often as heat.
The electrons in the atoms of the conductor have fixed orbits. The maximum number of electrons in any orbit is fixed. For all orbits except the first one, this maximum number is eight. The number of valence electrons in an atom is the number of electrons that would be required to fill the outer orbit. If an electron orbit is full and unable to accept electrons, there must be eight electrons in that orbit. When an outside electron enters the conductor, it joins the partially empty orbit of one of the conductor's atoms, giving the atom a net negative charge. In the presence of an electric field, the displaced electron moves on to another atom, and the process repeats itself.
It is the electrons in the outer orbit that are available for electron flow. These electrons determine whether a particular element is classified as a conductor, semiconductor, or insulator. Freeing one of the electrons in the inner orbits of the atom requires considerably more energy than required to displace one in the outer orbit, and usually does not occur.Metals are excellent conductors, especially silver, copper, and aluminum. Metals are sometimes represented as a gas of free electrons. In the alkali metals, the outer electrons are only loosely bound to the nucleus, so are free to move from ion to ion in the solid state. To describe the conducting properties of more complex metals, the interactions of the various atoms and the periodic potential in which the electrons move must be taken into account. In pure semiconductors, there are no free electrons at absolute zero temperature. As the temperature is increased, some of the electrons are excited to current-carrying states (the conducting band); the valence states that are left unoccupied (called holes) are also available to carry electric current. Insulators may be thought of as semiconductors with very large gaps between the valence and conduction bands. In some insulating solids such as the alkali halides, ions carry current by hopping between vacant atomic sites in the crystal.
Electricity as a natural phenomenon shows up in many forms, including piezoelectricity, static electricity, atmospheric effects, and cosmic rays. When a piezoelectric material such as Rochelle salt, quartz, or tourmaline is squeezed, mechanical energy is converted into electrical energy. The Earth resembles a large battery, with electrons continuously being lost to the atmosphere. Thunderstorms return the electrons that have been conducted away from the Earth's surface back to Earth. When cosmic rays strike the upper atmosphere, they release electrons, giving rise to such phenomena as the northern lights.
Although electricity has been known to man since ancient times, and scientists have identified many of its consistent and predictable characteristics, it is still not completely understood.
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