Two concentric circular coils of 'n' turns each are situated in the same plane. Their radii are ' a₁ ' and ' a₂ ^ (a₂>a₁ ) and they carry currents 'I ₁ ' and ' I ₂ ' respectively ( I ₁> I ₂ ) in opposite direction. The magnetic field at the centre is

Step-by-step solution

(D) Magnetic fields due to the two coils are in opposite directions and are given by array l B ₁= ₀ nI ₁ 2 a ₁ and B ₂= ₀ nI ₂ 2 a ₂ array aligned B &= B ₁- B ₁ &= ₀ nI 2 [ I ₁ a ₁ - I ₂ a ₂ ] &= ₀ nI 2 [ I ₁ a ₂- I ₂ a ₁ a ₁ a ₂ ] aligned