If a current of $3.0$ $amperes$ flowing in the primary coil is reduced to zero in $0.001$ $second,$ then the induced $e.m.f$ in the secondary coil is $15000$ $volts$. The mutual inductance between the two coils is....$henry$

$A$ long straight wire is placed along the axis of a circular ring of radius $R$. The mutual inductance of this system is

A small circular loop of wire of radius $a$ is located at the centre of a much larger circular wire loop of radius $b$. The two loops are in the same plane. The outer loop of radius $b$ carries an alternating current $I = I_0\, cos\, (\omega t)$ . The emf induced in the smaller inner loop is nearly

In a $dc$ motor, induced $e.m.f.$ will be maximum

With the decrease of current in the primary coil from $2\,amperes$ to zero value in $0.01\,s$ the $emf$ generated in the secondary coil is $1000\,volts$. The mutual inductance of the two coils is......$H$

Two conducting circular loops of radii $R_{1}$ and $\mathrm{R}_{2}$ are placed in the same plane with their centres coinciding. If $R_{1}>>R_{2}$, the mutual inductance $M$ between them will be directly proportional to: