A circular coil of radius 25cm, carries a current of 50A. If it has 35 turns, the flux density at the centre of the coil will be _______ Wb / m².

Radius(r) = 25 cm = 25 x 10^-2 m,  current(I) = 50A.

Number of turns(n)= 35,  Permeability of free space, ${\mu }_0= 4\pi \times {10}^{-7} H/m$. 

The flux density at the centre of the coil is given as: 

$B={\displaystyle \frac{n{\mu }_{0}i}{2r}}$
$={\displaystyle \frac{35\times 4\pi \times {10}^{-7}\times 50}{2\times 25\times {10}^{-2}}}$

$B= 1.4 \pi \times {10}^{-3}$ $Wb/m^2$

Therefore, $1.4 \pi \times {10}^{-3}$ is the correct response.