The equivalent inductance between A and B is equal to

A coil of ' $n$ ' turns and area ' $A$ ' is suddenly removed from a magnetic field, a charge ' $q$ ' flows through the coil. If resistance of the coil is ' $R$ ' then the magnetic flux density is (in $\mathrm{Wb} / \mathrm{m}^2$ )

A coil of n turns and resistance $\mathrm{R} \Omega$ is connected in series with resistance $R / 4$. The combination is moved for time $t$ second through magnetic flux $\phi$ to $\phi_2$. The induced current in the circuit is

Two identical coils of inductance $L$ joined in series are placed very close to each other such that the winding direction of one coil is exactly opposite to that of the other. The net inductance is