Question bank/JEE Main/Physics

A long, straight wire of radius a carries a current distributed
uniformly over its cross-section. The ratioof the magnetic fields due
to the wire at distance[IMAGE] and 2a, respectively from
the axis of the wire is n/3. Find n.

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Using Ampere's circuital law:
$B_{B} = \frac{\mu_{0}i}{2\pi(2a)}$
$\frac{B_{A}}{B_{B}} = \frac{4}{6} = \frac{2}{3}$

Source: Grass-G / P-20 Physcis Paper 12 Dec.docx