A circular arc of wire of radius of curvature $r$ subtends an angle of $\dfrac{\pi}{5}$ radian at its centre. If current $i$ is flowing in it then the magnetic induction at its centre is ($\mu_0$ = permeability of free space)

A solenoid of 1000 turns is wound uniformly on a glass tube 4 m long and $0.3$ m in diameter. The magnetic intensity at the centre of the solenoid when a current of 4 A flows through it is

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