\documentclass[tikz]{standalone}\input{pre.tex}\begin{document}\begin{tikzpicture}
\draw[interface] (-2,0) rectangle (2,-0.25);
\draw[thick] (-2,0) -- (2,0);
\contourlength{1mm};
\draw (0,1) circle (1);
\draw[fill=magenta] (0,0) circle (2pt) node[above, yshift=3pt] {\contour{white}{$B$}};
\lineann[-1]{90}{1}{$R$}
\draw[fill=magenta] (-2,1) circle (2pt);
\draw[->] (-2,1) -- ++ (0.5,0) node[right] {$\vec{v}_0$};
\end{tikzpicture}\end{document}