Original Problem: "A sphere of radius a is made of a nonconducting material that has a uniform volume charge density [PLAIN]http://jkwiens.com/2007/10/24/answer-electric-field-of-a-nonconducting-sphere-with-a-spherical-cavity/d2606be4e0cd2c9a6179c8f2e3547a85_2.gif. [Broken] A spherical cavity of radius b is removed from sphere which is a distance z from the center of the sphere. Assume that a > z + b. What is the electric field in the cavity?" I understand how to solve an ordinary problem like this, but what if the sphere has dielectric constant ε which is different from the cavity whose permittivity is ε0? When solving this, should I treat the imaginary small sphere with -ρ to have permittivity ε or ε0?