I need help understanding how induction works with a spherical conductor. Say there is a spherical conductor having a spherical cavity and a positive charge in the center of the cavity. Negative charges are induced on the inner surface of the sphere and equal positive charges are induced on the outer surface. My worry is if we consider the two surfaces of the sphere as one inner and one outer ,each surface will have two surfaces one inner and one outer. Why are four forms of inductions are not taken? I.e., negative charge on the inner surface of the inner sphere, positive charges on the outer surface of the inner sphere, negative charges on the inner surface of the outer sphere and positive charges on the outer surface of the outer sphere.
If we take two surfaces only, why does the potential of a point lying outside the sphere depend only the induced positive charges of the outer sphere? The book says the electric field at the point is zero due to positive charge in the cavity and the induced negative charge on the inner surfaces. How can this be possible?