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.