<P> Since the mobile charges in the interior of a metal object are free to move in any direction, there can never be a static concentration of charge inside the metal; if there was, it would attract opposite polarity charge to neutralize it . Therefore in induction, the mobile charges move under the influence of the external charge until they reach the surface of the metal and collect there, where they are constrained from moving by the boundary . </P> <P> This establishes the important principle that electrostatic charges on conductive objects reside on the surface of the object . External electric fields induce surface charges on metal objects that exactly cancel the field within . </P> <P> The electrostatic potential or voltage between two points is defined as the energy (work) required to move a small charge through an electric field between the two points, divided by the size of the charge . If there is an electric field directed from point b (\ displaystyle \ mathbf (b)) to point a (\ displaystyle \ mathbf (a)) then it will exert a force on a charge moving from a (\ displaystyle \ mathbf (a)) to b (\ displaystyle \ mathbf (b)). Work will have to be done on the charge by a force to make it move to b (\ displaystyle \ mathbf (b)) against the opposing force of the electric field . Thus the electrostatic potential energy of the charge will increase . So the potential at point b (\ displaystyle \ mathbf (b)) is higher than at point a (\ displaystyle \ mathbf (a)). The electric field E (\ displaystyle \ mathbf (E)) at any point is the gradient (rate of change) of the electrostatic potential V (\ displaystyle V): </P> <Dl> <Dd> ∇ V = E (\ displaystyle \ nabla V = \ mathbf (E) \,) </Dd> </Dl>

What happens to the flow of charges when the potential difference between two points becomes zero