1. The problem statement, all variables and given/known data The region between two concentric spheres of radii a and b(>a) contains volume charge density ρ(r) = C / r where C is a constant and r is the radial distance, as shown in figure. A point charge q is placed at the origin, r= 0. Find the value of C for which the electric field in the region between the spheres is constant (i.e., r independent). A) q/πa2 B) q/π(a2+b2) C) q/2πa2 D) q/2πb2 2. Relevant equations ε∫E.dS = qfree 3. The attempt at a solution Let us consider a a gaussian surface, a concentric sphere at radius r . Applying Gauss law ε∫E.dS = q + 4πC(r2-a2) εE(4πr2) = q + 4πC(r2-a2) E = q/(4πr2ε) + C(r2-a2)/(εr2) Now for E to be constant , q-4πca2 = 0 or C=q/(4πa2) .This is not given as one of the options . I would be grateful if somebody could help me with the problem.