Module 2 : Electrostatics
Lecture 9 : Electrostatic Potential
  Dipole in a uniform Electric Field
The net force on the dipole is zero. There is a net torque acting on the dipole. If $a$is the length of the dipole, the torque is
   
 
\includegraphics{fig31.eps}
   
  Click here for Animation
 

\begin{displaymath}\tau = (qE)\times a\sin\theta = pE\sin\theta\end{displaymath}

Expressing in vector form,

\begin{displaymath}\vec \tau = \vec p\times\vec E\end{displaymath}
  If $\theta=0^\circ$or $180^\circ$, (i.e. when the dipole is aligned parallel or antiparallel to the field) the torque vanishes and the dipole is in equilibrium. The equilibrium is stable if $\theta=0^\circ$and unstable if $\theta = 180^\circ$.
  Example 16
   
1
9