Question

What is the change in electric potential energy when the dipole moment of a molecule changes its orientation with respect to e⃗ from parallel to perpendicular?

Answer 1

Electric potential energy of a dipole is given as

$U = - P.E$

$U = - PE cos\theta$

so change in the potential energy is given by

$U_f - U_i = (-PEcos\theta_2)- (-PEcos\theta_1)$

$\Delta U = PE (cos\theta_1 - cos\theta_2)$

here initially it was parallel to electric field while finally it is perpendicular to the electric field

so we have

$\Delta U = PE(cos 0 - cos 90)$

$\Delta U = PE$

so above is the change in potential energy of dipole