In this unit, we consider charges, electric fields, and the electric potential in and around conductors. Electric charge can move freely through conductors such as metal objects. The human body and the Earth are also conductors. When charges move through metals, it is the negatively charged electrons that are mobile. Wood, plastic, stone, and glass are all insulators. Charges cannot move easily in these materials. If charges are added to an insulator they stick in place. On the other hand, charges in conductors move freely. Any excess charge moves onto the surface of the conductor. Even if a conductor has a considerable amount of charge, there is no charge in the interior. The electric field inside a conductor is zero. If an electric field existed inside a conductor, the mobile charges would move and cancel out the electric field. In a later unit we will find that this does not hold for conductors connected to external agents, such as batteries, that maintain a voltage difference. A person inside a metal box would not be shocked, even if the box were to be charged to a high voltage. Since there is no electric field inside a conductor, the electric potential, V, has no gradient. Therefore, the electric potential is constant inside a conductor. The surface and interior of a conductor are at the same electric potential. As an analogy to the electric field and potential inside a conductor, consider the surface of a fluid. If the surface were not flat, gravity would eventually flatten the surface. An undisturbed fluid surface is flat. It is an equipotential surface with respect to gravity. Suppose we have two conductors, one charged and one uncharged. The surface of each is an equipotential. If we connect the two conductors with a conducting wire, the surfaces of the two conductors and the wire must be an equipotential. Charges must flow from the charged to the uncharged sphere. If we connect the two conductors with a conducting wire, the surfaces of the two conductors and the wire must be an equipotential. Consider a conductor of the shape shown. Since the surface of the conductor is an equipotential, it follows that the electric field is higher at the pointy parts of the conductor, like point B, than at smooth parts, like point A.

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