$A$ long copper tube of inner radius $R$ carries a current $i$. The magnetic field $B$ inside the tube is

The current required to be passed through a solenoid of $15\,cm$ length and $60$ turns in order to demagnetize a bar magnet of magnetic intensity $2.4 \times 10^3\,A/m$ is $.........A$.

$A$ long solenoid has $n$ turns per meter and current $I \ A$ is flowing through it. The magnetic field at the ends of the solenoid is

$A$ long solenoid is formed by winding $70$ turns $cm^{-1}$. If $2.0\,A$ current flows,then the magnetic field produced inside the solenoid is $.......\times 10^{-4}\,T$ $\left(\mu_0 = 4\pi \times 10^{-7}\,TmA^{-1}\right)$

$A$ $50 \ cm$ long solenoid has a winding of $400$ turns. What current must pass through it to produce a magnetic field of induction $4 \pi \times 10^{-3} \ T$ at the centre (in $A$)?

$A$ toroid has a non-ferromagnetic core of inner radius $24 \ cm$ and outer radius $25 \ cm$,around which $4900$ turns of a wire are wound. If the current in the wire is $12 \ A$,the magnetic field inside the core of the toroid is: (in $mT$)