A circular disc of radius 20 cm is placed in uniform magnetic field of induction $\dfrac{7}{22}\ \text{Wb/m}^2$ in such a way that its axis makes an angle of $60^\circ$ with $\vec{B}$. The magnetic flux linked with the disc is $(\cos 60^\circ = 0.5)$

An aeroplane having a wing span of 30 m flies due north with the speed of 170 m/s. Magnetic field $B = 3.6 \times 10^{-5}$ T. The potential difference between the tips of the wings will be

The self inductance of an air-core inductor (solenoid) is $0.03$ mH. By introducing an iron core into inductor the self-inductance increases to $30$ mH. The relative permeability of the core used is

A coil of effective area $3$ m$^2$ is placed at right angles to a magnetic field of induction $0.05$ Wb/m$^2$. If the field is decreased to $20\%$ of its original value in $10$ second, the e.m.f. induced in the coil will be