A long wire carrying a steady current is bent into a single coil such that magnetic induction at centre is $B$. Then same wire is bent to form a coil of smaller radius of $4$ turns when magnetic induction at centre is $B\text{'}$. Then

An infinite straight conductor carrying current $2i$ is split into a loop of radius $r$ as shown in the figure. The magnetic field at the centre of the coil is

Magnetic field produced by electrons in atoms and molecules is due to their

In a moving coil galvanometer, a radial magnetic field is produced due to the

The scale of M.C.G. is linear because of

To make the field radial in a moving coil galvanometer,

A $100$ turns coil shown in the figure carries a current of $2 \mathrm{A}$ in a magnetic field $B=0.2 \mathrm{Wb} {\mathrm{m}}^{-2}$. The torque acting on the coil is