The number of coil turns (n) is 100.

Each turn’s radius (r) is 8 cm (0.08 m).

The magnitude of the current (I) passing through the coil is 0.4 A.

The magnitude of the magnetic field at the coil’s centre may be calculated using the following equation:

$|\overline B | = \frac{{{\mu _0}2\pi nI}}{{4\pi r}}$

Where ​${\mu _0}$ is the permeability of free space $ = 4\pi  \times {10^{ – 7}}Tm{A^{ – 1}}$

hence,

$|\overline B | = \frac{{4\pi  \times {{10}^{ – 7}}}}{{4\pi }} \times \frac{{2\pi  \times 100 \times 0.4}}{{0.08}}$

$ = 3.14 \times {10^{ – 4}}T$

The magnitude of the magnetic field is $3.14 \times {10^{ – 4}}T$