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**

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**