Past exam of the physics course of the year 4 of the natural sciences course of the University of Cambridge / 2014 / Paper 1 / A10 / a / Solution

The point-dipole interaction isHere ${\mathbf{p}}_1\cdot \hat{\mathbf{r}}=0$. For parallel axes, ${\mathbf{p}}_1\cdot {\mathbf{p}}_2=p^2$, henceThis side-by-side parallel arrangement is repulsive in the point-dipole model.