Question

A current i ampere flows in a circular arc of wire whose radius is r, which subtends an angle 3 / 2  radian at its centre. The magnitude of magnetic field at the center is

Answer 1

Here is your ans i hope you got the method

Answer 2

b = u•i/2r(Φ/2π)
=> u•i/2r({3π/2 x 2π}
=> 3u•i/8r