Charge, Electrical
The ancient Greeks first conceived of the notion of an atom, which was their word for indivisible. Modern theory divides the atom into the proton, electron, and neutron. These particles all differ in weight and charge. The proton has a positive charge, the electron a negative charge, and the neutron a zero or negligible charge. Because the atom is ordinarily electrically neutral, the net positive charge due to the protons in the atom's nucleus must cancel out the negative charge of the atom's electrons, which is to say that the number of protons and electrons must be the same. Electric charge always comes as multiples of the elementary charge of the electron, i.e., 1.6 x 10-19 coulombs.
Electrons are about one six-million-millionth of an inch in diameter, and have a mass of about 0.9 billionth of a billionth of a billionth of a gram. (Actually, relativistic theory tells us that the electron's mass depends on its velocity; since mass increases with speed, the faster the electron moves, the more it weighs.) Electrons possess inertia, so remain at rest or in uniform motion in the same direction unless acted upon by some external force. All electrons are alike: they all carry the same negative charge, and they repel each other. Oppositely charged particles, such as a proton and an electron, attract.
To say that matter is positively charged or negatively charged is strictly meaningful only in relative terms. If one removes electrons from a body, that body becomes positively charged. Since, ordinarily, it is not possible to remove all the electrons from an object, an object's net positive charge is just an indication of how many electrons were removed. Simply stated, an object that has an excess of electrons is negatively charged; one that has a deficiency of electrons is positively charged.
The science that concerns itself with electric charges at rest is called electrostatics. When a glass rod and a piece of fur are rubbed together, a process referred to as charging takes place. Ordinarily, the electrons in the glass rod are distributed fairly uniformly, and the rod will not have any affect when brought into contact with small pieces of paper. But when the glass rod is rubbed with the fur, electrons are redistributed to the portion of the rod that was brought into contact with the fur, and that part of the rod acquires the ability to attract the pieces of paper. This is a simple example of contact electrification involving a redistribution of charge, without net charge transfer, so the glass rod as a whole remains electrically neutral.
When charges in a conductor are in motion, they produce an electric current, defined as the rate at which charge is transported past a given point in the conductor. Here the term conductor includes metals, alloys, semiconductors, electrolytes, ionized gases, imperfect dielectrics, and even vacuum (under certain conditions). In some conductors such as metallic solids, one or more of the electrons bound to each atom may be liberated and become free to wander about the material. In electrolytes, ions having atomic mass are free to move about, producing an electric current as they do so. In gases, atoms may become ionized, so that the resultant free electrons and ions are free to conduct electricity. In still other materials, i.e., semiconductors, some of the bound charges can be liberated by thermal vibrations, light, or the application of externally produced electric fields. That the current flowing in metals and the liberation of charges from matter can be measured in the laboratory provides a basis for the macroscopic description of electromagnetism. Macroscopic theory, however, provides us with no insight into why the basic unit of charge is fixed and why it is the same for all particles.
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