This Demonstration shows a conducting spherical shell surrounding a charge. We can determine the surface density of the charge . The magnitude of the field outside the conductor is given by , where is the total charge on the outer surface of the sphere, is the permittivity of free space and is the distance from the center of the sphere to the point of measurement. The electric field inside the cavity is given by , where is the charge inside the cavity. Drag the locator to vary the point of measurement.