Suppose the field inside a large piece of magnetic material is b0, so that h0 = (1/μ0)b0 − m, where m is a "frozen-in" magnetization. (a) now a small spherical cavity is hollowed out of the material (fig. 6.21). find the field at the center of the cavity, in terms of b0 and m. also find h at the center of the cavity, in terms of h0 and m. (b) do the same for a long needle-shaped cavity running parallel to m. (c) do the same for a thin wafer-shaped cavity perpendicular to m.