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    				| 1. | Figure shows a circuit having a coil of resistance `R = 2.5 Omega` and inductance `L` connected to a conducting rod of radius `10 cm` with its center at `P`. Assume that friction and gravity are absent and a constant uniform magnatic 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^(-1)`. Find the magnitude of this torque (in `mNm`) when current reaches half of its maximum value. Neglect the self inductance of the loop formed by the circuit. | 
| Answer» Correct Answer - `(5)` Induced `EMF = (1)/(2) B omega l^(2)` Maximum current: `i_(0) = (B omegal^(2))/(2R)` Torque about the hinge `P` is `tau = underset(0) overset(l) int i(dx)Bx rArr tau = (1)/(2) iBl^(2)` Putting `i= i_(0)//2`, we get: `tau = (B^(2)Omega l^(4))/(8R) = 5mNm` | |