[" 18.Let "P(r)=(Q)/(pi R^(4))" r be the charge density distribution "],[" for a solid sphere of radius "R" and total charge "Q" ."],[" For a point p'inside the sphere at distance "r_(1)" from "],[" the centre of the sphere,the magnitude of electric "],[" field is : "[qquad (Qr_(1)^(2))/(3 pi epsilon_(0)R^(4)),0],[" 19.A disk of radius a/4having a une,ri "]]