Radio frequency choke uses a core of

In the figure,the switch $S$ is closed so that a current flows in the iron-core inductor which has inductance $L$ and the resistance $R$. When the switch is opened,a spark is obtained at the contacts. The spark is due to

$A$ conducting loop having a capacitor is moving outward from the magnetic field. Which plate of the capacitor will be positive?

In the circuit shown in the figure,a conducting wire $HE$ is moved with a constant speed $V$ towards the left. The complete circuit is placed in a uniform magnetic field $\vec B$ perpendicular to the plane of the circuit and directed inward. The current in $HKDE$ is

In an $L-R$ circuit connected to a battery of constant $e.m.f.$ $E$,switch $S$ is closed at time $t = 0$. If $e$ denotes the magnitude of induced $e.m.f.$ across the inductor and $i$ the current in the circuit at any time $t$,then which of the following graphs shows the variation of $e$ with $i$?

Two different coils have inductances $L_1 = 4 \ mH$ and $L_2 = 2 \ mH$. At a certain instant,the current in the two coils is increasing at the same constant rate,and the power supplied to the first coil is four times that of the second coil. If $e$,$I$,and $U$ indicate the potential difference,current,and energy stored in the inductance respectively,then which of the following is correct?