An electric dipole of moment $\vec p$ is lying along a uniform electric field $\vec E.$ The work done in rotating the dipole by $90^o $ is
AIPMT 2006, Easy
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work done in deflecting a diapole through an angle $\theta $ is given by
$W{\text{ = }}\int\limits_0^\theta {pE} \sin \,\theta \,d\theta = pE(1 - \cos \,\theta )$
Since $\theta =90^o$
$\therefore \,W = pE(1 - cos\,{90^o})$ or, $W=pE$
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