Question-42 An electric dipole having each charge of magnitude 2\ \mu C is placed in an electric field of intensity 8\times10^4\ N/C. If the maximum torque acting on the dipole is 4\times10^{-3}\ N-m, the length of the dipole is
(A) 10\ mm
(B) 25\ mm
(C) 15\ mm
(D) 20\ mm
Answer : (B)
Explanation:
τ = pE \sin \theta
∴ τ_max = pE sin 90° ….(∵ Dipole moment p = q × l)
τ_max = q × l × E × 1
4 × 10^{-3} = 2 × 10^{-6} × l × 8 × 10^{4} × 1
∴ l = \frac{4 \times 10^{-3}}{2 \times 10^{-6} \times 8 \times 10^{4}} = 25 \times 10^{-3} = 25 mm