The network shown in figure is a part of a complete circuit. If at a certain instant, the current $i$ is $4 \mathrm{A}$ and is increasing at a rate of ${10}^3 \mathrm{A} {\mathrm{s}}^{-1}.$ Then $V_B-V_A$ will be

The ratio of initial to final current through the battery when the switch is closed in the figure is

Two coils $A$ and $B$ are wound on the same iron core as shown in figure. The number of turns in the coil $A$ and $B$ are $N_A$ and $N_B$ respectively. Identify the correct statement

A small square loop of wire of side $l$ is placed inside a large square loop of wire $L\left(L\gg l\right)$. Both loops are coplanar and their centres coincide at point $O$ as shown in figure. The mutual inductance of the system is