A solenoid has length 0.4 m, radius 1 cm and 400 turns of wire. If a current of 5 A is passed through this solenoid, what is the magnetic field inside the solenoid?
 

The magnetic field inside the solenoid is given by B $={\mu }_0{\mathrm{ni}}_0$
(n = number of turns per unit length, i = current in coil $\left.{\mu }_0=4\cdot \pi \times {10}^{-7}\mathrm{Tm}/A\right)$
where, $n=\frac{N}{L}\Rightarrow n=\frac{400}{0.4}\Rightarrow n=1000$
$B=4\times 3.14\times 1000\times 5\times {10}^{-7}=6.28\times {10}^{-3}T$