Step 1: Recall the expression for torque (\( \tau \)) acting on a magnetic dipole (\( M \)) placed in a uniform magnetic field (\( B \)):
\[ \tau = MB \sin\theta \]
where \( \theta \) is the angle between the magnetic dipole axis and the direction of the magnetic field.
Step 2: Determine when torque is zero.
The torque will be zero if:
\[ \tau = MB \sin\theta = 0 \]
This occurs when:
\[ \sin\theta = 0 \quad \Rightarrow \quad \theta = 0^\circ \text{ or } 180^\circ \]
Step 3: From the given options, angle \(0^\circ\) satisfies the above condition.
Final Conclusion:
The torque on the magnetic dipole is zero when the angle between the dipole axis and the magnetic field is zero degrees (0°).
The number of turns of the coil of a moving coil galvanometer is increased in order to increase current sensitivity by $50 \%$ The percentage change in voltage sensitivity of the galvanometer will be: