Hi all,(adsbygoogle = window.adsbygoogle || []).push({});

I've been having a problem with understanding some of the van der Waals forces. I understand that the polarizability of a induced-fixed dipole interation is:

\begin{equation}\alpha =4\pi\epsilon r^{3} \end{equation}

Which leads to the potential field (assuming two different substances):

\begin{equation}

U(r)=-\displaystyle\frac{\mu_{1}^{2}\alpha_{2}+\mu_{2}^{2}\alpha_{1}}{16\pi^{2}\epsilon^{2}r^{6}}

\end{equation}

and therefore the force between them.

My problem lies with the fixed dipole-dipole interaction. Now I understand that the polarizability has to take into account the orientation of the molecule/atom to give:

\begin{equation}\alpha =4\pi\epsilon r^{3} + \displaystyle\frac{\mu^{2}}{3k_{B}T} \end{equation}

This is where I get confused. I have read in "Scanning force microscopy-Dror Sarid" that the potential field between two fixed dipoles does not take into acount there polarizability

\begin{equation}

U(r)=-\displaystyle\frac{\mu_{1}^{2}\mu_{2}^{2}}{48\pi^{2}\epsilon^{2}KTr^{6}}

\end{equation}

Why is that?

Sorry for the lengthy post and thankyou in advance.

Nick

**Physics Forums | Science Articles, Homework Help, Discussion**

Join Physics Forums Today!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

# Polarizability of Fixed Dipole Interactions

**Physics Forums | Science Articles, Homework Help, Discussion**