A basic property of elementary particles of matter. One does not define charge but takes it as a basic experimental quantity and defines other quantities in terms of it. The early Greek philosophers were aware that rubbing amber with fur produced properties in each that were not possessed before the rubbing. For example, the amber attracted the fur after rubbing, but not before. These new properties were later said to be due to “charge.” The amber was assigned a negative charge and the fur was assigned a positive charge.
According to modern atomic theory, the nucleus of an atom has a positive charge because of its protons, and in the normal atom there are enough extranuclear electrons to balance the nuclear charge so that the normal atom as a whole is neutral. Generally, when the word charge is used in electricity, it means the unbalanced charge (excess or deficiency of electrons), so that physically there are enough “nonnormal” atoms to account for the positive charge on a “positively charged body” or enough unneutralized electrons to account for the negative charge on a “negatively charged body.”
The rubbing process mentioned “rubs” electrons off the fur onto the amber, thus giving the amber a surplus of electrons, and it leaves the fur with a deficiency of electrons. See also: Electrostatics
In line with the previously mentioned usage, the total charge q on a body is the total unbalanced charge possessed by the body. For example, if a sphere has a negative charge of 1 × 10-10 coulomb, it has 6.24 × 108 electrons more than are needed to neutralize its atoms. The coulomb is the unit of charge in the SI system of units. See also: Coulomb's law; Electrical units and standards
The surface charge density σ on a body is the charge per unit surface area of the charged body. Generally, the charge on the surface is not uniformly distributed, so a small area ΔA which has a magnitude of charge Δq on it must be considered. Then σ at a point on the surface is defined by the equation below.
The subject of electrostatics concerns itself with properties of charges at rest, while circuit analysis, electromagnetism, and most of electronics concern themselves with the properties of charges in motion. See also: Capacitance; Electric current; Electricity