The Electric Field and Surface Charge at a Conductor
Learning Goal: To understand the behavior of the electric field at the surface of a conductor, and its relationship to surface charge on the conductor.
A conductor is placed in an external electrostatic field. The external field is uniform before the conductor is placed within it. The conductor is completely isolated from any source of current or charge.
Part A
Which of the following describes the electric field inside this conductor?
 It is in the same direction as the original external field.It is in the opposite direction from that of the original external field.It has a direction determined entirely by the charge on its surface.It is always zero.
The net electric field inside a conductor is always zero. If the net electric field were not zero, a current would flow inside the conductor. This would build up charge on the exterior of the conductor. This charge would oppose the field, ultimately (in a few nanoseconds for a metal) canceling the field to zero.
Part B
The charge density inside the conductor is: