Q. A molecule with a dipole moment $p$ is placed in an electric field of strength E. Initially the dipole is aligned parallel to the field. If the dipole is to be related to be anti-parallel to the field the work required to be done by an external agency is
A) $- 2pE$
B) $- pE$
C) $pE$
D) $2pE$
Explanation
(d) Work done in rotating an electric dipole in an uniform electric field $(E)$ through an angle $\theta$ from the direction of field is
$W = pE\left( 1 - \cos\theta \right)$
Where p is electric dipole moment.
If the dipole is to be rotated to be and anti-parallel to the field $(\theta - 180{^\circ})$, then
$W = pE\left( 1 - \cos{180{^\circ}} \right)$
$\ \ \ \ \ \ = pE\left\lbrack 1 - ( - 1) \right\rbrack = 2pE$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.