Two concentric circular coils having radii ' $r_1{ }^{\prime}$ and ' $r_2$ ' $\left(r_2 \ll r_1\right)$ are placed co-axially with centres coinciding. The mutual inductance of the arrangement is ( $\mu_0=$ permeability of free space) (Both coils have single turn)

An inductor coil of inductance $L$ is divided into two parts and both parts are connected in parallel. The net inductance is

Two coils have a mutual inductance 0.003 H . The current changes in the first coil according to equation $I=I_0 \sin \omega t$, where $I_0=8 \mathrm{~A}$ and $\omega=100 \pi \mathrm{rad} \mathrm{s}^{-1}$. The maximum value of e.m.f. in the second coil is

The current in LR circuit if reduced to half What will be the energy stored in it?