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

A closely wound solenoid of 1200 turns and area of cross-section $5 \mathrm{~cm}^2$ carries a current. If the magnetic moment of the solenoid is $1.2 \mathrm{JT}^{-1}$, then the current through the solenoid is

If the magnetic field inside a solenoid is $B$, then the magnetic energy stored in it per unit volume is ( $c=$ speed of light in vacuum and $\varepsilon_0$ is permittivity of free space)