A conducting rod of length $l$  is hinged at point O. It is free to rotate in a vertical plane. There exists a uniform magnetic field $\overrightarrow{B}$  in horizontal direction. The rod is released from the horizontal position as shown. The potential difference between the ends of the rod when it has turned by $\text{θ}$ (as shown in figure) is proportional to

The diagram shows a circuit having a coil of resistance $R=2.5\Omega$ and inductance L connected to a conducting rod PQ which can slide on a perfectly conducting circular ring of radius 10 cm with its centre at 'P'. Assume that friction and gravity are absent and a constant uniform magnetic field of 5 T exists as shown in figure.

At t = 0, the circuit is switched on and simultaneously a time varying external torque is applied on the rod so that it rotates about P with a constant angular velocity 40 rad/s. Find magnitude of this torque (in milli Nm) when current reaches half of its maximum value. Neglect the self-inductance of the loop formed by the circuit.