A rectangular coil $20 \mathrm{cm}\times 20 \mathrm{cm}$ has $100 \mathrm{turns}$ and carries a current of $1 \mathrm{A}$. It is placed in a uniform magnetic field $B=0.5 \mathrm{T}$ with the direction of magnetic field parallel to the plane of the coil. The magnitude of the torque required to hold this coil in this position is

A ring carrying current $I$ lies in $x-z$ plane as shown. A short magnetic dipole with dipole moment directed along $x$-axis is fixed at origin. If the ring is free to move anywhere, then it will

Value of $r$ for which magnetic field at point $P$ on the axis of a circular current carrying loop of radius $a$ is maximum as shown in figure ______