A1 (source code)
= A1
{c}
{scope}
Take a long strip along $y$, occupying $-w/2<x<w/2$ in $z=0$, with uniform surface current $K=I/w$. Above the strip's centre, the contributions odd in $x$ cancel, while the <Biot-Savart law> gives the <magnetic field of a uniformly conducting strip>
$$
B_x(0,z)=\frac{\mu_0I}{2\pi w}\int_{-w/2}^{w/2}\frac{z\,dx'}{x'^2+z^2}=\frac{\mu_0I}{\pi w}\tan^{-1}\frac{w}{2z},\qquad z>0.
$$
The field reverses below the strip. Its field lines wrap around the longitudinal current: nearly parallel to the faces near the centre, then curving round the edges and approaching circles at large <distances>. The sketch is an original field integration; the conductor thickness is idealized as negligible.
\Image[/past-exam-of-the-physics-b-course-of-the-university-of-cambridge/2016/paper-1-strip-field.png]
{title=Original magnetic field-line integration for a uniformly current-carrying thin strip}
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