Исходники иллюстрации

\documentclass[tikz]{standalone}\input{pre.tex}\begin{document}\begin{tikzpicture}[scale=1.5]
    \lineann[1]{0}{1}{$a$}
    \lineann[2]{0}{1.57}{$x$}
    \begin{scope}[xshift=1.5cm]
        \lineannn[-1]{0}{0.15}{$dx$}        
    \end{scope}
    \begin{scope}[xshift=1cm]
        \lineann[1]{0}{2}{$r$}      
    \end{scope} 
    \begin{scope}[xshift=1.57cm]
        \lineann[2]{0}{1.43}{$R$}       
    \end{scope}     
    \draw (0,-0.15) rectangle ++(2,0.3);
    \draw[fill=magenta] (1.5,-0.15) rectangle ++(0.15,0.3);
    % \draw (1.15,-0.1) node[below] {$dq$};
    % \draw (-4,0) circle (0cm);
    \vbLabel{0}{-0.1}{0};
    \draw[axis,->] (0,0) -- ++(4,0) node[right] {$+x$};

    \draw[fill=magenta] (3,0) coordinate (Y) circle (2pt);
    \draw[force,->] (Y) -- ++ (0.5,0) node[below] {$d\vec{E}$};

\end{tikzpicture}\end{document}