ELECTRIC dipole is placed in an electric field \[\vec{E},\] a torque \[\vec{\TAU }=\vec{p}\times \vec{E}\] ACTS on it. This torque tries to rotate the dipole through an angle. If the dipole is rotated from an angle \[{{\theta }_{1}}\] to \[{{\theta }_{2}}\] then work done by external force is given by \[W=pE(\COS {{\theta }_{1}}-\cos {{\theta }_{2}})\] ...(i) putting \[{{\theta }_{1}}={{0}^{o}},\,\,{{\theta }_{2}}={{90}^{o}}\] in the Eq. (i), we get \[W=pE(\cos {{0}^{o}}-\cos {{90}^{o}})\] \[=pE\,\,(1-0)\] \[=pE\]