in physics, constancy of the total electric charge in the universe or in any specific chemical or nuclear reaction. The total charge in any closed system never changes, at least within the limits of the most precise observation. In classical terms, this law implies that the appearance of a given amount of positive charge in one part of a system is always accompanied by the appearance of an equal amount of negative charge somewhere else in the system; for example, when a plastic ruler is rubbed with a cloth, it becomes negatively charged and the cloth becomes positively charged by an equal amount.

Although fundamental particles of matter continually and spontaneously appear, disappear, and change into one another, they always obey the restriction that the net quantity of charge is preserved. When a charged particle changes into a new particle, the new particle inherits the exact charge of the original. When a charged particle appears where there was none before, it is invariably accompanied by another particle of equal and opposite charge, so that no net change in charge occurs. The annihilation of a charged particle requires the joint annihilation of a particle of equal and opposite charge.