A bicycle wheel of diameter ' $D$ ' has ' $N$ ' number of spokes. Wheel is rotating at the rate of ' $x$ ' revolutions per minute, perpendicular to the horizontal component of earth's magnetic field ' $\mathrm{B}_{\mathrm{H}}$ '. The e.m.f. induced between the rim and the centre of the wheel will be

If current ' i ' is passing through the solenoid of diameter ' $d$ ' having number of turns per unit length ' $n$ ', then the inductance per unit length near the middle of a long solenoid is directly proportional to

The flux linked with the coil at any instant ' t ' is given by $\phi=12 t^2-60 t+275$. The magnitude of induced e.m.f. at $\mathrm{t}=3 \mathrm{~second}$ is

A coil of area $12 \mathrm{~cm}^2$ has 250 turns. Magnetic field of $0.2 \mathrm{~Wb} / \mathrm{m}^2$ is perpendicular to the plane of the coil. The field is reduced to $0.1 \mathrm{~Wb} / \mathrm{m}^2$ in 0.1 second. The magnitude of induced e.m.f. in the coil is