# The energy of electric interaction between these dipoles? [closed]

I'm a physics tutor. This is not a homework problem. I'm unable to solve this problem.

The energy of electric interaction between these dipoles will be:  I tried taking P1=q1*d1 and P2=q2*d2 then calculated Potential energy between (q1,-q1), (q1,-q2), (q2,-q2), (-q1,q2) and used certain approximations but couldn't get to any of the results shown.

There are many ways of going about this (I'm only giving hints to the full method). One is by splitting the lower dipole into parallel and perpendicular components(its a vector, we can do that). Now use the formulae for field from a dipole at axial and equatorial positions (equatorial is $\frac{kp}{r^3}$, axial is double that), and calculate the change in potential while moving two charges $\pm q$ from infinity to a distance $r \pm d$.
Or, you can directly apply the formula $V(r,\theta)=\frac{k\vec{p}\cdot\vec{r}}{r^3}$, on the two charges at $r\pm d$.
You may use the electrostatic potential due to electric dipole $P_2$ (see, e.g., http://en.wikipedia.org/wiki/Dipole#Field_from_an_electric_dipole, although the system of units is different there) and find the energy of charges $+q_1$ at $r+d_1$ and $-q_1$ at $r$ under the condition that $d_1\ll r$.