An electric dipole with dipole moment $4 \times 10^{-9} \mathrm{C} \mathrm{m}$ is aligned at $30^{\circ}$ with the direction of a uniform electric field of magnitude $5 \times 10^{4} \mathrm{~N} \mathrm{C}^{-1}$. Calculate the magnitude of the torque acting on the dipole.
Electric dipole moment, $p=4 \times 10^{-9} \mathrm{C} \mathrm{m}$
Angle made by $p$ with a uniform electric field, $\theta=30^{\circ}$
Electric field. $E=5 \times 10^{4} \mathrm{~N} \mathrm{C}^{-1}$
Torque acting on the dipole is given by the relation,
$T=p E \sin \theta$
$=4 \times 10^{-9} \times 5 \times 10^{4} \times \sin 30$
$=20 \times 10^{-5} \times \frac{1}{2}$
$=10^{-4} \mathrm{Nm}$
Therefore, the magnitude of the torque acting on the dipole is $10^{-4} \mathrm{~N} \mathrm{~m}$.