Question

The dipole moment of a dipole in an uniform external field $ \vec{E} $ is $ \vec{P} $ . Then, the torque $ (\tau ) $ acting on the dipole is

• A. $ \vec{\tau }=\vec{P}\times \vec{E} $
• B. $ \vec{\tau }=\vec{P}.\vec{E} $
• C. $ \vec{\tau }=2(\vec{P}+\vec{E}) $
• D. $ \vec{\tau }=(\vec{P}+\vec{E}) $

Answer 1

Answer: (A)

Torque $(\tau)$ acting on the dipole in an uniform external field $E$
$\tau=$ either force $\times$ perpendicular distance
$=q E \times 2 a \sin \theta$
$=(q \times 2 a) \times E \sin \theta$
$ =P E \sin \theta$
or $\tau=P \times E$