A toroid has a core of inner radius r₁ and outer radius r₂ , around which N turns of wire are wound. If the current in the wire is I then the magnetic field inside the toroid is ( ₀= permeability of free space)

Step-by-step solution

Considering Ampearian loop at the centre of toroid: B . d l = ₀ N I Since, B has radial symmetry & angle between B & d l is zero aligned & B d l=B 2 r₁+r₂ 2 = ₀ N I & B= [ ₀ N I (r₁+r₂ ) ] aligned