Q 12-01-034JEE MainJEE Main 2025 (23 Jan, Shift 2)Easy
Two point charges $-4\ \mu\text{C}$ and $4\ \mu\text{C}$, constituting an electric dipole, are placed at $(-9, 0, 0)\ \text{cm}$ and $(9, 0, 0)\ \text{cm}$ in a uniform electric field of strength $10^4\ \text{N C}^{-1}$. The work done on the dipole in rotating it from the equilibrium through $180^\circ$ is
Answer: (B) $14.4\ \text{mJ}$
Dipole moment: $p = q(2a) = 4\times10^{-6}\times0.18 = 7.2\times10^{-7}\ \text{C m}$.
Work to rotate from $\theta = 0$ (equilibrium) to $180^\circ$:
$$W = pE(\cos0^\circ - \cos180^\circ) = 2pE = 2\times7.2\times10^{-7}\times10^4 = 14.4\times10^{-3}\ \text{J} = 14.4\ \text{mJ}$$
Solution by Sreeraj P, M.Sc Physics