NEET 2025 · PhysicsDipole torquePrevious Year Question
An electric dipole with dipole moment 5 × 10⁻⁶ C m is aligned with the direction of a uniform electric field of magnitude 4 × 10⁵ N/C. The dipole is then rotated through an angle of 60° with respect to the electric field. The change in the potential energy of the dipole is:
- A.1.2 J
- B.1.5 J
- C.0.8 J
- D.1.0 J✓
Correct Answer
(D) 1.0 J
Solution & Explanation
Answer: (d) Solution: Potential energy of a dipole in a uniform field: U(θ) = −pE cosθ. Change in PE on rotating from θ₁ = 0° to θ₂ = 60°: ΔU = U(60°) − U(0°) = −pE cos60° − (−pE cos0°) = pE(cos0° − cos60°) = pE(1 − 0.5) = pE/2. ΔU = (1/2)(5 × 10⁻⁶)(4 × 10⁵) = (1/2)(2) = 1.0 J. Hence the change in potential energy is 1.0 J — option (d).
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