Charge is the basic property of elementary particles of matter. Charge is taken as the basic electrical quantity to define other electrical quantities such as voltage, current etc.
According to modern atomic theory, the nucleus of an atom has positive charge because of protons. Generally, when the word charge is used in electricity, it means excess or deficiency of electrons.
Charges may be stationary or in motion. Stationary charges are called static charge. The analysis of static charges and their forces is called electrostatics.
Example: If a hard rubber pen or a comb is rubbed on a sheet of paper, the rubber will attract paper pieces. The work of rubbing, resulted in separating electrons and protons to produce a charge of excess electrons on the surface of the rubber and a charge of excess protons on the paper. The paper and rubber give evidence of a static electric charge having electrons or protons in a static state i.e. not in motion or stationary charges.
The motion of charged particles in any medium is called current. The net transfer of charge per unit time is called current measured in ampere.
Charge of billions of electrons or protons is necessary for common applications of electricity. Therefore, it is convenient to define a practical unit called the coulomb (C) as equal to the charge of 6.25 x 1018 electrons or protons stored in a dielectric.
The symbol for electric charge is Q or q. A charge of 6.25 x 1018 electrons is stated as Q = 1 Coulomb = 1C. This unit is named after Charles A. Coulomb (1736-1806), a French physicist, who measured the force between charges.