The other fundamental law in electromagnetism is the law of conservation of charge:
where is the current per unit area, or “current density.” What this law states is that the integral of the normal component of the current density over a closed surface is equal to the rate of change of the charge within the surface. This law establishes current as the flow of charge. Although this law seems trivial at first glance, it is actually a statement of the fundamental observation that charge can neither be created nor destroyed. Thus it is on the same level as the laws of conservation of mass and energy.
