When the switch is closed in the primary circuit,the instantaneous induced current in the secondary circuit is:

The induced $emf$ can be produced in a coil by:
$A.$ Moving the coil with uniform speed inside a magnetic field.
$B.$ Moving the coil with non-uniform speed inside a uniform magnetic field.
$C.$ Rotating the coil inside a uniform magnetic field.
$D.$ Changing the area of the coil inside a uniform magnetic field.
Choose the correct answer from the options given below:

$A$ coil is placed in a magnetic field $\vec{B}$ as shown below. $A$ current is induced in the coil because $\vec{B}$ is:

Magnetic flux passing through a coil is initially $4 \times 10^{-4} \ Wb$. It reduces to $10 \%$ of its original value in $t$ seconds. If the induced emf is $0.72 \ mV$,then $t$ in seconds is:

If a coil of $40$ turns and area $4.0 \, cm^2$ is suddenly removed from a magnetic field,it is observed that a charge of $2.0 \times 10^{-4} \, C$ flows through the coil. If the resistance of the coil is $80 \, \Omega$,the magnetic flux density in $Wb/m^2$ is:

$A$ magnetic field $B$ is confined to a region $r \le a$ and points out of the paper (the $z$-axis),$r = 0$ being the centre of the circular region. $A$ charged ring (charge $= Q$) of radius $b$,$b > a$ and mass $m$ lies in the $xy$-plane with its centre at the origin. The ring is free to rotate and is at rest. The magnetic field is brought to zero in time $\Delta t$. Find the angular velocity $\omega$ of the ring after the field vanishes.