When a current $i$ through a solenoid is increasing at a constant rate, then the induced current is

In a pair of adjacent coils, if the current in one coil changes from 10 A to 2 A in a time 0.2 s , an emf of 120 V is induced in another coil. The mutual inductance of the pair of the coils is

A wheel of 20 metallic spokes each 40 cm long is rotated with a speed of $180 \mathrm{rev} / \mathrm{min}$ in a plane normal to the horizontal component of earth's magnetic field $H_c$ at a place. If $H_c=0.4 \mathrm{G}$ (Gauss) at that place, the induced emf between the axle and the rim of the wheel is

A metal disc of radius 30 cm rotates with a constant angular velocity $\omega=100 \mathrm{rad} / \mathrm{s}$ about its axis. Find the magnitude of potential difference between the centre and the rim of the disc of the external uniform magnetic field on induction $B=4 \mathrm{mT}$ is directed perpendicular to the disc.