4.- A thick material shell (inner radius a, outer radius b) is made of dielectric material with a "frozen-in" polarization given by: k P(r) = r where k is a constant and r is the distance from the center as shown in the figure. (Hint: there is no free charge in the problem.) Find the electric field in all the three regions by two different methods P P b a. Compute the bound charges and use Gauss's law to calculate the field it produces b. Use Gauss's law in the context of dielectric to compute the Electric Displacement and the compute the electric field. c.- Which method is direct? Could you explain in which cases use each method?