Question: Q. 5. An electric dipole of length $4 \mathrm{~cm}$, when placed with its axis making an angle of $60^{\circ}$ with a uniform electric field, experiences a torque of $4 \sqrt{3} \mathrm{Nm}$. Calculate the potential energy of the dipole, if it has charge $\pm 8 \mathrm{nC}$.
A [Delhi I, II, III 2014]
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Solution:
Ans. Torque
$$ \begin{aligned} \tau & =p E \sin \theta \ 4 \sqrt{3} & =p E \sin 60^{\circ}=p E \frac{\sqrt{3}}{2} \ 4 \sqrt{3} & =p E \frac{\sqrt{3}}{2} \end{aligned} $$
Hence,
Potential energy,
$$ p E=8 $$
$$ \text { or } U=-8 \times \cos 60^{\circ}=-4 \mathrm{~J} $$
[CBSE Marking Scheme, 2014]
