Two point charges –4 and 4 , constituting an electric dipole, are placed at (–9, 0, 0) cm and (9, 0, 0) cm in a uniform electric field of strength . The work done on the dipole in rotating it from the equilibrium through 180° is [2025]

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Solution

Q.
Two point charges –4 $μ C$ and 4 $μ C$ , constituting an electric dipole, are placed at (–9, 0, 0) cm and (9, 0, 0) cm in a uniform electric field of strength $10^{4}$ ${NC}^{- 1}$ . The work done on the dipole in rotating it from the equilibrium through 180° is [2025]

1 14.4 mJ

2 18.4 mJ

3 12.4 mJ

4 16.4 mJ

Ans.
(1)

$U = - PE \cos θ$

$W_{ext} = ∆ U = U_{f} - U_{i} = - PE \cos 180 ° + PE \cos 0 °$

$W_{ext} = 2 PE = 2 \times (4 \times 10^{- 6}) (18) \times 10^{- 2} \times 10^{4}$

$= 144 \times 10^{- 4} = 14.4 mJ$

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