How to create a Tangential potential field?

Dear Matlab users,

I'm looking for generating a tangential potential field around a given point as shown in the figure. </matlabcentral/answers/uploaded_files/26516/Tangential%20potential%20field.png>

This was taken from "Goodrich, Michael A. "Potential fields tutorial." Class Notes (2002)".

I just want to know the set of mathematical equation to generate such a field.

Thanks