The magnitude of the torque ($\tau$) exerted on an electric dipole in a uniform electric field is given by the formula $\tau = pE \sin\theta$, where $p$ is the dipole moment, $E$ is the electric field magnitude, and $\theta$ is the angle between them.

Given: $p = 4 \times 10^{-9}$ C m $E = 5 \times 10^4$ NC$^{-1}$ $\theta = 30^\circ$

Calculation: $\tau = (4 \times 10^{-9}) \times (5 \times 10^4) \times \sin(30^\circ)$ $\tau = 20 \times 10^{-5} \times 0.5$ $\tau = 10 \times 10^{-5} = 10^{-4}$ N m

(See NCERT Physics Class 12, Exercise 1.10).