The nonuniform electric field is radial, and its magnitude varies as 1/r The dipole consists of -Q and Q The force on-Q is inward Qer The force on Q is outward Q So the net force is de dE f=QEr-QEr+dr=Q-s=p-i Let m be the number of dipoles per unit volume. then the electric force per unit volume is de de F1=Nf=Np,=P where P= Np is the polarization