Two concentric circular coils A and B have radii $$20 \mathrm{~cm}$$ and $$10 \mathrm{~cm}$$ respectively lie in the same plane. The current in coil A is $$0.5 \mathrm{~A}$$ in anticlockwise direction. The current in coil B so that net field at the common centre is zero, is

Two concentric circular coils of 10 turns each are situated in the same plane. Their radii are $$20 \mathrm{~cm}$$ and $$40 \mathrm{~cm}$$ and they carry respectively $$0.2 \mathrm{~A}$$ and $$0.3 \mathrm{~A}$$ current in opposite direction. The magnetic field at the centre is ($$\mu_0=4 \pi \times 10^{-7}$$ SI units)

A coil of '$$n$$' turns and radius '$$R$$' carries a current '$$I$$'. It is unwound and rewound again to make another coil of radius $$\left(\frac{\mathrm{R}}{3}\right)$$, current remaining the same. The ratio of magnetic moment of the new coil to that of original coil is

An electron makes a full rotation in a circle of radius $$0.8 \mathrm{~m}$$ in one second. The magnetic field at the centre of the circle is $$\left(\mu_0=4 \pi \times 10^{-7}\right.$$ SI units)