An electric dipole of moment P→ is lying along a uniform electric field E→ . The work done in rotating the dipole by 90° is

When an electric dipole is placed in an electric field E→ , a torque τ→=p→×E→ acts on it. This torque tries to rotate the dipole through an angle. If the dipole is rotated from an angle θ1 to θ2, then work done by external force is given by W=pEcosθ1-cosθ2                       (i)  Putting θ1=0°,θ2=90° in the Eq. (i), we get  W=pEcos0°-cos90° =p E(1-0)=p E