The region between two concentric conducting spheres with radii a and b is filled with a conducting material with resistivity rho. (a) Show that the resistance between the spheres is given by R=rho4π (1a−1b) (b) Derive an expression for the current density as a function of radius, in terms of the potential difference Vab between the spheres. (c) Show that the result in part (a) reduces to Eq. (25.10) when the separation L=b−a between the spheres is small.