Chapter 21, Problem 62P

(II) Suppose both charges in Fig. 21–45 (for a dipole) were positive. (a) Show that the field on the perpendicular bisector, for r >> ℓ, is given by $(1/4\pi {ϵ}_0)(2Q/r^2)$. (b) Explain why the field decreases as 1/r² here whereas for a dipole it decreases as 1/r³.