Given:

Number of turns, n = 100

Radius of coil, r = 8 cm

Current through the coil, I = 0.40 A

Magnitude of magnetic field at centre of coil, B = ?

Using Biot Savart law, we find that magnetic field at the centre of a coil is given by,

…(1)

Where,

B = Magnetic field strength

n = total number of turns

I = current through the coil

μ₀ is the permeability of free space.

μ₀ = 4 × π × 10^-7 TmA^-1

r = radius of coil

Now, by putting the values in equation (1), we get

⇒ |B| = 3.14 × 10^-4T

∴ Magnitude of magnetic field at the centre of the coil is 3.14 × 10⁴T.