At <distance> $r\gg w$, the strip is indistinguishable to leading order from a straight line current. <Ampère's law> on a large circular contour gives \b[$B\simeq\mu_0I/(2\pi r)$], directed tangentially around the strip's longitudinal axis by the <right-hand rule>. On the perpendicular centreline this also follows by expanding $\tan^{-1}(w/(2z))\simeq w/(2z)$.

Solution

Past exam of the physics B course of the University of Cambridge / 2016 / Paper 1 / Section A / A1 / i / Solution

 Solution (source code)