The work done in rotating a dipole placed in a uniform electric field E, from initial angle q1 to final angle
q2 is W = q1∫q2 tdq
= pE (cosq1 - cosq2)
where U2, U1 are the potential energy of the dipole in the two orientations. Potential energy
U2 – U1 = – pE cos q2 – (–pE cos q1)
= –pE (cos q2 – cosq1)
The zero of the potential is taken at q = 90° . Thus, potential energy of the dipole is
U = -®p.®E
= - pE cosq
Umin = -pE, Umax = + pE
Work done in rotating a dipole from q = 0 (aligned parallel to E), to q = 180° (aligned antiparallel to E) is W = 2pE
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