What happens with two concentric spheres, or with a point charge at the center of a spherical shell? The net field outside any of the objects is simply the vector sum of the fields from the different objects. The field inside any conductors is zero - charge on the conductor shifts to ensure this.

In the example above, a positive point charge Q is placed at the center of a conducting shell that has a net charge of -2Q. What is the field as a function of r, if the shell has inner radius R₁ and outer radius R₂?

For r < R₁ the field comes only from the point charge. The charge on the sphere does not produce a field in this region, so E = kQ/r² directed out from the center.

For R₁ < r < R₂ the field is zero because that's inside the conductor. For this to be true there must be a net -Q on the inside surface of the shell to stop all the field lines coming from the +Q charge at the center.

For r > R₂ the field once again looks like the field from a point charge equal to the total charge in the system, which is +Q - 2Q = -Q. Therefore the field here is also given by E = kQ/r², but now is directed in toward the center.