51�Թ�

Electric charge is a basic property of elementary particles of matter. The protons in an atom, for example, have a positive charge, the electrons have a negative charge, and the neutrons have zero charge. In an ordinary atom, the number of protons equals the number of electrons, so the atom normally has no net electric charge. An atom becomes negatively charged if it gains extra electrons, and it becomes positively charged if it loses electrons; atoms with net charge are called ions. Every charged particle is surrounded by an electric field, the area in which the charge exerts a force. Particles with nonzero electric charge interact with each other by exchanging photons, the carriers of the electromagnetic force. The strength and direction of the force charged particles exert on each other depends on the product of their charges: they attract each other if the product of their charges is negative and repel each other if the product is positive. Thus two electrons, each with charge −1, will repel each other, since −1 × −1 = +1, a positive number. Static electricity consists of charged particles at rest, while electric current consists of moving charged particles, especially electrons or ions.