$A$ transformer is based 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 $V$)

Two coils $X$ and $Y$ are placed in a circuit such that when a current changes by $2 \ A$ in coil $X$,the magnetic flux changes by $0.4 \ Wb$ in coil $Y$. The value of mutual inductance of the coils is .... $H$.

$A$ transformer works on the principle of

If the coefficient of mutual induction of the primary and secondary coils of an induction coil is $5\, H$ and a current of $10\, A$ is cut off in $5\times10^{-4}\, s$,the $emf$ induced (in $volt$) in the secondary coil is