two concentric circular coils of ten turns each are situated in the same plane.their radii are 20cm and 40cm and they carry respectively 0.2 and 0.3 ampere current in opposite direction.the magnitude in weber/m2 at the center in terms of permiabitity in vaccum is

1]35/4

2]1/80

3]7/80

4]5/4

 NI/2R_o            The formula of magnetic field at the center is μ            As the direction of current is opposite in both of the coils, they will generate opposite magnetic field.            So,                         B = B1 - B2)₂/R₂ – I₁/R₁N/2)(I_o                        = (μ10/2)(0.2/0.2 – 0.3/0.4)_o                        = (μ/4_o                        = 5μ            So, the answer is option #4

Radius of first coil, R₁ = 20cm

            Radius of second coil, R₂ = 40cm

            Number of turns = 10

            Current in first coil, I₁ = 0.2 A

            Current in first coil, I₂ = 0.3 A