Module 2 : Electrostatics
Lecture 9 : Electrostatic Potential
\includegraphics{fig30.eps}
 
  If the distance $a$is small compared to $r$(i.e., if the point P is far away from the dipole), we may use
 
\begin{displaymath}r_2-r_1 \approx a\cos\theta \ \ \ \ r_1r_2\approx r^2\end{displaymath}
  where $\theta$is the angle between $\vec r$and the dipole moment vector $\vec p$. This gives
 
\begin{displaymath}\phi(\vec r)\approx \frac{qa\cos\theta}{4\pi\epsilon_0 r^2} = \frac{p\cos\theta}{4\pi\epsilon_0r^2}\end{displaymath}
   
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