Q.

A dipole with two electric charges of 2 μC magnitude each, with separation distance 0.5 μm, is placed between the plates of a capacitor such that its axis is parallel to an electric field established between the plates when a potential difference of 5 V is applied. Separation between the plates is 0.5 mm. If the dipole is rotated by 30° from the axis, it tends to realign in the direction due to a torque. The value of torque is :          [2025]

1 5×109 Nm  
2 5×103 Nm  
3 2.5×1012 Nm  
4 2.5×109 Nm  

Ans.

(1)

E=Vd=55×104=104 V/m

τ=PE sin θ

Where P=qa=2×106×5×107=1×1012 C-m

τ=1×1012×104×12=5×109 N-m