 Solution

ID: 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 is

U = \frac{1}{4 π ϵ _{0} r ^{3}} [p_{1} \cdot p_{2} - 3 (p_{1} \cdot \hat{r}) (p_{2} \cdot \hat{r})] .

Here

p_{1} \cdot \hat{r} = 0

. For parallel axes,

p_{1} \cdot p_{2} = p^{2}

, hence

U = \frac{p ^{2}}{4 π ϵ _{0} r ^{3}} = 1.20 \times 1 0^{- 20} J = + 0.0748 eV .

This side-by-side parallel arrangement is repulsive in the point-dipole model.

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