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

Two coils having self-inductance $L_1 = 75 \text{ mH}$ and $L_2 = 48 \text{ mH}$ are coupled with each other. If the mutual inductance of the coils is $37.2 \text{ mH}$, then the coefficient of coupling will be:

$A$ varying current at the rate of $3 \, A/s$ in a coil generates an $e.m.f.$ of $8 \, mV$ in a nearby coil. The mutual inductance of the two coils is:

$X$ and $Y$ are two metallic coils arranged in such a way that when the steady change in current flowing in coil $X$ is $4 \ A$,the change in magnetic flux associated with coil $Y$ is $0.4 \ Wb$. The mutual inductance of the system of these coils is . . . . . . $H$.

$A$ circular coil of radius $r$ is placed on another circular coil whose radius is $R$. The current flowing through the larger coil is changing, and their centers coincide. Given $R \gg r$, if both coils are coplanar, then the mutual inductance between them is proportional to:

$O$ is the centre of two coplanar concentric circular conductors,$A$ and $B$,of radii $r$ and $R$ respectively as shown in the figure. Here $r \ll R$. The mutual inductance of the system of the conductors can be given by . . . . . . .