\documentclass{standalone}

\usepackage[european,straightvoltages]{circuitikz}

\begin{document}

\begin{circuitikz}
    \draw (0, 0)  to [V=$e(t)$] (0, 3)
        to [R, l_=$R$, v^<=$u(t)$] (2, 3)
        to [cute inductor, l=$L$] (2, 0)
        to (0, 0)
        (2, 3) to (4, 3)
        to [C=$C$] (4, 0)
        to (2, 0);
    \node [scale=1.7] at (6, 1.45) {$\iff$};
    \draw (8, 0) to [V=$e(t)$] (8, 3)
        to [R, l_=$R$, v^<=$u(t)$] (10, 3)
        to [R, l=$\underline Z_{eq}$] (10, 0)
        to (8, 0);
\end{circuitikz}

\end{document}
