A long thin hollow metallic cylinder of radius R has a current i ampere. The magnetic induction B away from the axis at a distance ' r ' from the axis varies as shown in

Step-by-step solution

Using ampere circuital law, ∮ B → · d l → = μ 0 I If an ampere loop is taken inside the hollow cylinder then, ∮ B → · d l → = μ 0 × 0 = 0 If an ampere loop is taken of radius r   such that r ≥ R   then, ∮ B → · d l → = μ 0 I B 2 πr = μ 0 I B = μ 0 I 2 πr As it can be clearly seen that there is no magnetic field inside the hollow cylindrical metal and outside the cylinder B ∝ 1 r