If a wheel with 24 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, the emf induced between the axle and the rim of the wheel is $E$. If the number of spokes is made 12 and the wheel is rotated with a speed of $90 \mathrm{rev} / \mathrm{min}$ in the same field, the induced emf is

A metallic disc of radius 0.3 m is rotating with a constant angular speed of $60 \mathrm{rad} \mathrm{s}^{-1}$ in a plane perpendicular to a uniform magnetic field of $5 \times 10^{-2} \mathrm{~T}$. The emf induced between a point on the rim and centre of the disc is

A coil of 45 turns and radius 4 cm is placed in a uniform magnetic field such that its plane is perpendicular to the direction of the field. If the magnetic field increases from 0 to 0.70 T at a constant rate in a time interval of 220 s , then the induced emf in the coil is

A horizontal telegraph wire of length 30 m spread east to west fell down freely from a height of 20 m . If the resistance of the wire is $40 \Omega$ and the horizontal component of the Earth's magnetic field at the place is $2 \times 10^{-5} \mathrm{~T}$, then the induced current when the wire reaches the ground is

(Acceleration due to gravity $=10 \mathrm{~ms}^{-2}$ )