The mutual inductance $(M)$ of two coils is $3 \ H$. The self-inductances of the coils are $4 \ H$ and $9 \ H$ respectively. The coefficient of coupling between the coils is

$A$ small square loop of wire of side $l$ is placed inside a large circular loop of radius $r$. The loops are coplanar and their centers coincide. The mutual inductance of the system is proportional to

The coefficient of mutual induction is $2 \text{ H}$ and the induced e.m.f. across the secondary is $2 \text{ kV}$. The current in the primary is reduced from $6 \text{ A}$ to $3 \text{ A}$. The time required for the change of current is:

The induction coil works on the principle of

The coefficient of mutual induction is $2 \ H$ and the induced e.m.f. across the secondary coil is $2 \ kV$. The current in the primary coil is reduced from $6 \ A$ to $3 \ A$. The time required for the change of current is:

Two coaxial solenoids are made by winding thin insulated wire over a pipe of cross-sectional area $A = 10 \ cm^2$ and length $\ell = 20 \ cm$. If one of the solenoids has $N_1 = 300$ turns and the other has $N_2 = 400$ turns,their mutual inductance is (given $\mu_0 = 4\pi \times 10^{-7} \ T \ m \ A^{-1}$):