I have a piece of copper in static condition. Since copper is a conductor, within the copper there is no electric field. Strange comes when I bring this copper in contact with a piece of zinc: in a tiny interval of time, certain amount of electrons would flow from copper to zinc. I suspect that during this interval, electric field would be created and in order to move electrons against this field, there must be a non-electrical force. So the question is: what's this force? I have searched this topic and answer are chemical force/potential etc. I would be very appreciated if anyone could give a force which the Newton 3rd Law could apply to, such as the Coulomb force, gravitational force.