Question

An electric dipole of dipole moment $p$ is aligned parallel to a uniform electric field $E$. The energy required to rotate the dipole by $90^\circ$ is :

• A. $p^2 E$
• B. $p E$
• C. infinity
• D. $p E^2$

Answer 1

Answer: (B)

$p E$

Answer 2

Potential energy of dipole,
$U =- pE(\cos \theta_2 - \cos \theta_1)$
Here, $\theta_1 = 0^\circ , \theta_2 = 90^\circ$
$\therefore U = -pE(cos 90^\circ - \cos0^\circ)$
$= -pE(0 - 1) = pE$