Two circular coils have their centers at the same point. The mutual inductance between them will be maximum when their axes

The induction coil works on the principle of

Two coils $P$ and $S$ have a mutual inductance of $3 \times 10^{-3} \ H$. If the current in the coil $P$ is $I = 20 \sin(50 \pi t) \ A$,then the maximum value of the e.m.f. induced in coil $S$ is (in $V$)

The mutual inductance of a pair of coils,each of $N$ turns,is $M$ henry. If a current of $I$ ampere in one of the coils is brought to zero in $t$ seconds,the e.m.f. induced per turn in the other coil in volt is:

Two concentric circular coils having radii $r_1$ and $r_2$ $(r_2 \ll r_1)$ are placed co-axially with centres coinciding. The mutual induction of the arrangement is (Both coils have single turn,$\mu_0 =$ permeability of free space).

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