Chapter 21, Problem 67P

(III) (a) Show that at points along the axis of a dipole (along the same line that contains +Q and –Q), the electric field has magnitude

$E=\frac{1}{4\pi {ϵ}_0}\frac{2p}{r^3}$

for r >> ℓ (Fig. 21–45), where r is the distance from a point to the center of the dipole. (b) In what direction does $\stackrel{\to }{E}$ point?