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An electric dipole of dipole moment p⃗ is aligned parallel to a uniform electric field E⃗. The energy required to rotate the dipole by 90° is
- pE²
- p²E
- pE
- infinity
Correct answer: pE
Solution
The potential energy of a dipole in a uniform electric field is given by U = -pEcosθ. Initially, the dipole is aligned parallel to the field (θ = 0°), so U_initial = -pE. After rotating it by 90° (θ = 90°), U_final = 0. The energy required is the change in potential energy, ΔU = U_final - U_initial = 0 - (-pE) = pE.
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