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charge (electric charge)

In physics, charge, also known as electric charge, electrical charge, or electrostatic charge and symbolized q, is a characteristic of a unit of matter that expresses the extent to which it has more or fewer electrons than protons. In atoms, the electron carries a negative elementary or unit charge; the proton carries a positive charge. The two types of charge are equal and opposite.

In an atom of matter, an electrical charge occurs whenever the number of protons in the nucleus differs from the number of electrons surrounding that nucleus. If there are more electrons than protons, the atom has a negative charge. If there are fewer electrons than protons, the atom has a positive charge. The amount of charge carried by an atom is always a multiple of the elementary charge, that is, the charge carried by a single electron or a single proton. A particle, atom, or object with negative charge is said to have negative electric polarity; a particle, atom, or object with positive charge is said to have positive electric polarity.

In an object comprised of many atoms, the net charge is equal to the arithmetic sum, taking polarity into account, of the charges of all the atoms taken together. In a massive sample, this can amount to a considerable quantity of elementary charges. The unit of electrical charge in the International System of Units is the coulomb (symbolized C), where 1 C is equal to approximately 6.24 x 1018 elementary charges. It is not unusual for real-world objects to hold charges of many coulombs.

An electric field, also called an electrical field or an electrostatic field, surrounds any object that has charge. The electric field strength at any given distance from an object is directly proportional to the amount of charge on the object. Near any object having a fixed electric charge, the electric field strength diminishes in proportion to the square of the distance from the object (that is, it obeys the inverse square law).

When two objects having electric charge are brought into each other's vicinity, an electrostatic force is manifested between them. (This force is not to be confused with electromotive force, also known as voltage.) If the electric charges are of the same polarity, the electrostatic force is repulsive. If the electric charges are of opposite polarity, the electrostatic force is attractive. In free space (a vacuum), if the charges on the two nearby objects in coulombs are q1 and q2 and the centers of the objects are separated by a distance r in meters, the net force F between the objects, in newtons, is given by the following formula:

F = (q1q2) / (4or2)

where o is the permittivity of free space, a physical constant, and is the ratio of a circle's circumference to its diameter, a dimensionless mathematical constant. A positive net force is repulsive, and a negative net force is attractive. This relation is known as Coulomb's law.

This was last updated in June 2015

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