(i) Define mutual inductance of a pair of coils. Write its SI unit. (ii) A long solenoid of radius $R$ and length $L$ has $n$ turns per unit length. A circular loop of radius $r (<R)$ is placed inside at the centre of the solenoid such that its axis coincides with the axis of the solenoid. Obtain the mutual inductance of the solenoid and the loop. OR Two long straight parallel conductors A and B carrying steady currents $I_a$ and $I_b$ in the same direction are separated by a distance $d$. Deduce the expressions for the force acting on length $L$ of conductor B due to conductor A and show it in figure. Write the expression for the force acting on length $L$ of conductor A due to conductor B and show that it follows Newton's third law.