Circuit RL série avec circuitikz — composants, courant, équation différentielle

\documentclass[border=10pt]{standalone}
\usepackage{tikz}
\usepackage[europeanresistors, americaninductors]{circuitikz}

\begin{document}
\begin{circuitikz}[american voltages, scale=1.2, transform shape]
  \draw
    (0,0) to[battery1, l=$U_0$]                    (0,3)
          to[short]                                 (2,3)
          to[switch, l=$K$, mirror]                 (4,3)
          to[short]                                 (6,3)
          to[L=$L$]                                 (6,0)
          to[short]                                 (4,0)
          to[R, l=$R$]                              (2,0)
          to[short]                                 (0,0);

  % Annotation courant
  \draw[->, very thick, red] (3, 3.5) -- (5, 3.5)
        node[midway, above, red] {$i(t)$};

  % Légende
  \node[draw=gray!50, fill=gray!5, inner sep=4pt, font=\footnotesize, align=left,
        anchor=north west] at (-0.2, -0.6) {%
    Circuit \textbf{RL série} :\\
    $\displaystyle U_0 = R\,i(t) + L\,\frac{\mathrm{d}i}{\mathrm{d}t}$
  };
\end{circuitikz}
\end{document}