A dipole consists of two equal and opposite point charges $(\pm q)$ separated by a distance $d .$ (a) Write an expression for the electric field at a point $(0, y)$ on the dipole axis. Specify the direction of the field in all four regions: $y>\frac{1}{2} d, 0<y<\frac{1}{2} d,-\frac{1}{2} d<y<0,$ and $y<-\frac{1}{2} d .$ (b) At distant
points $(|y| \gg d),$ write a simpler, approximate expression for the field. To what power of $y$ is the field proportional? Does this conflict with Coulomb's law? [Hint: Use the binomial approximation $\left.(1 \pm x)^{n} \approx 1 \pm n x \text { for } x \ll 1 .\right]$