If a straight current carrying wire of linear density $0.12 \mathrm{~kg} \mathrm{~m}^{-1}$ is suspended in mid air by a uniform horizontal magnetic field of 0.5 T normal to the length of the wire, then the current through the wire is (Acceleration due to gravity $=10 \mathrm{~ms}^{-2}$, Neglect Earth's magnetic field)

Two concentric loops $A$ and $B$ of same radius $2 \pi \mathrm{~cm}$ are placed at right angles to each other. If the currents flowing through $A$ and $B$ are 3 A and 4 A respectively, then the net magnetic field at their common centre is

The magnetic field at a distance of 10 cm from a long straight thin wire carrying a current of 4 A is

A velocity selector is to be constructed to select ions with a velocity of $6 \mathrm{~km} \mathrm{~s}^{-1}$. If the electric field used is $400 \mathrm{~V} \mathrm{~m}^{-1}$, then the magnetic field to be used is