Conservation of Charge

The law of conservation of charge states that in any given physical process, electric charge may be TRANSFERRED from one part of a system to another, but the TOTAL charge of the system can never be created out of nothing, nor destroyed. Equivalently: for an isolated system (one that exchanges no charge with its surroundings), the total charge always remains exactly constant over time.

The classic illustration is charging a glass rod by rubbing it with silk: careful measurement always shows that the positive charge that appears on the glass rod is EXACTLY equal in magnitude to the negative charge that appears on the silk. Before rubbing, both the glass rod and the silk were individually neutral, so the combined glass-plus-silk system had zero net charge; after rubbing, the glass and silk have separated, equal-and-opposite charges, but the COMBINED system -- glass plus silk taken together -- still has exactly zero net charge, exactly as before. Nothing was created; charge was merely redistributed, moving from the silk onto the glass (or vice versa, depending …