A circular coil of radius 40 mm consists of 100 turns of conducting wire and the current through it is 20 mA. The magnetic field in the centre of the coil will be : (µ₀ = 4π×10^-7 Hm^-1) 

Concept:

A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. 

Calculation:

B = \({N \mu_0 I \over 2r}\)

Given:

r = 40 mm

N = 100 turns

I = 20 mA

B = \(100 \times 4\pi \times 10^{-7}\times 20 \times 10^{-3} \over 2 \times 40 \times 10^{-3} \)

= 0.00003141592 T

= 3.14×10-5 T

The correct answer is option (1).