Class of phenomena arising from the existence of Charge. According to modern theory, most Elementary Particles of Matter possess charge, either positive or negative. Two particles of like charge,
both positive or both negative, repel each other; two particles of unlike charge
are attracted (see Coulomb's Law). The electric Force between two charged particles is much greater than the gravitational force
(see Gravitation) between the particles. Many of the bulk properties of matter are ultimately
due to the electric forces among the particles of which the substance is
composed. Materials differ in their ability to allow charge to flow through them.
Those that allow charge to pass easily are Conductors (see Conduction), whereas those that allow extremely little charge to pass through are called Insulators (see Insulation), or Dielectrics. A third class of materials, called Semiconductors, is intermediate. Electrostatics is the study of charges, or charged bodies,
at rest. When positive or negative charge builds up in fixed positions on
objects, certain phenomena can be observed that are collectively referred to as Static Electricity. The charge can be built by rubbing certain objects together, such as Silk and Glass or Rubber and fur; the friction between these objects causes Electrons to transfer from one to another with the result that the object losing
electrons acquires a positive charge and the object gaining electrons acquires a
negative charge. Electrodynamics is the study of charges in Motion. A flow of electric charge constitutes an electric Current. In order for a current to exist in a conductor, there must be an Electromotive Force (EMF), or Potential difference, between the conductor's ends. An electric Cell, a Photovoltaic Cell, and a Generator are all sources of emf. An emf source with an external conductor connected
from one of the source's two terminals to the other constitutes an Electric Circuit. Direct Current (DC) is a flow of current in one direction at a constant rate. Alternating Current (AC) is a current flow that increases in magnitude from zero to a maximum,
decreases back to zero, increases to a maximum in the opposite direction, decreases to
zero, and then repeats this process periodically. The number of repetitions of
the cycle occurring each Second is defined as the frequency, which is expressed in Hertz (Hz). The frequency of ordinary household current in the U.S. is 60 cycles per sec
(60 Hz), and electric devices must be designed to operate at this frequency.
In a solid (see States Of Matter), the current consists not of a few electrons moving rapidly but of many
electrons moving slowly; although this drift of electrons is slow, the impulse that
causes it moves through the circuit, when the circuit is completed, at nearly
the Speed of Light. The movement of electrons in a current is not steady; each electron moves in
a series of stops and starts. In a direct current, the electrons are spread
evenly through the conductor; in an alternating current, the electrons tend to
congregate along the conductor surface (See Skin Effect). In liquids, Gases, and semiconductors, current carriers may be positively or negatively
charged.