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\documentclass[border=2pt]{standalone}
\usepackage[european]{circuitikz}% lädt TikZ

\usetikzlibrary{arrows.meta}

\tikzset{
    voltage drop arrow/.style={currarrow, sloped, allow upside down, pos=1}
}

\begin{document}
\begin{circuitikz}
\draw
    (0,6) to [short, o-] (2,6)
    (2,6) to [R, l=$R$, name=R] (2,4)
    (2,4) to [R, l=$R_L$, name=RL] (2,2)
    (2,2) to [L, l=$L$, name=L] (2,0)
    (0,0) to [short, o-] (2,0)
;
\draw
    (0,5.8) -- node [left] {$U$} node [voltage drop arrow] {} (0,0.2)
    (R) ++ (-0.5,0.5) -- node [left] {$U_1$} node [voltage drop arrow] {} + (0,-1)
    (RL) ++ (-0.5,0.5) -- node [left] {$U_2$} node [voltage drop arrow] {} + (0,-3)
    (RL) ++ (1,0.5) -- node [right] {$U_{R_L}$} node [voltage drop arrow] {} + (0,-1)
    (L) ++ (1,0.5) -- node [right] {$U_L$} node [voltage drop arrow] {} + (0,-1)
;
\end{circuitikz}
\begin{tikzpicture}[
    >=Stealth,
    thick,
    line cap=round,
    line join=round
]
%\draw[help lines,lightgray] (0,0) grid (4,4);
\begin{scope}[->, auto=right]
\draw (0,0) -- node {$U_1$} (2,0);
\draw (2,0) -- node {$U_{R_L}$} (4,0);
\draw (4,0) -- node {$U_L$} (4,4);
\draw (0,0) -- node [swap] {$U$} (4,4);
\draw (2,0) -- node [left] {$U_2$} (4,4);
\end{scope}
\end{tikzpicture}
\end{document}

\documentclass[
  11pt,        % Schriftgröße
  border=2pt,  % Aüßeren Rand lassen (nicht zwingend nötig)
  tikz         % Grafikpaket »«PGF/TikZ« laden
]{standalone}
\usepackage[T1]{fontenc}

\usepackage{lmodern}
\usepackage{microtype}

\begin{document}
  \begin{tikzpicture}[
    >=stealth,
    thick,
    line cap=round,
    line join=round
  ]
%    \draw[help lines,lightgray] (0,0) grid (4,4);
    \begin{scope}[->]
      \draw (0,0) -- (2,0) node[midway,below] {$U_1$};
      \draw (2,0) -- (4,0) node[midway,below] {$U_{R_L}$};
      \draw (4,0) -- (4,4) node[midway,right] {$U_L$};
      \draw (0,0) -- (4,4) node[midway,above left] {$U$};
      \draw (2,0) -- (4,4) node[midway,left] {$U_2$};
    \end{scope}
  \end{tikzpicture}
\end{document}