A molecule of a substance has permanent dipole moment $p$. A mole of this substance is polarised by applying a strong electrostatic field $E$. The direction of the field is suddenly changed by an angle of $60^{\circ} .$ If $N$ is the Avogadro's number the amount of work done by the field is
NEET 2017, Medium
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$(\vec{U})=-\vec{p} \cdot \vec{E}=-p E \cos \theta$
$W=\Delta U$
$W=-N p E\left(\cos 60^{\circ}-\cos 0^{\circ}\right)=\frac{1}{2} N p E$
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