Electric voltage in a circuit

Plot of electric voltage along circuit. Also see this 3D figure, and for more related figure, see the Electric engineering category.

Simple circuit with battery/EMF and single resistor.

With internal resistance:

Two resistors in series:

Two resistors in parallel:

Edit and compile if you like:

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% Author: Izaak Neutelings (December 2021)
\documentclass[border=3pt,tikz]{standalone}
\usepackage{pgfplots}
\usepackage[siunitx]{circuitikz}
\usepackage[outline]{contour} % glow around text
\usetikzlibrary{arrows}
\usetikzlibrary{decorations.markings}
\tikzset{>=latex} % for LaTeX arrow head
\usepackage{xcolor}
\contourlength{1.5pt}
\colorlet{Icol}{blue!60!black}
\colorlet{Ccol}{orange!90!black}
\colorlet{Rcol}{green!50!black}
\colorlet{myred}{red!90!black}
\newcommand\EMF{\mathcal{E}}
\tikzstyle{EMF}=[battery1,l=$\EMF$,thin]
\tikzstyle{internal R}=[R,color=Rcol,Rcol,l=$r$,bipoles/length=25pt]
\tikzstyle{thick R}=[R,color=Rcol,thick,Rcol,l=$R$,bipoles/length=35pt]
\tikzstyle{small <->}=[{Latex[length=4,width=3]}-{Latex[length=4,width=3]},thick]
\def\tick#1#2{\draw[thick] (#1) ++ (#2:0.1) --++ (#2:-0.2)}
\begin{document}
% RESISTOR with EMF
\begin{tikzpicture}[scale=0.9]
  
  \def\Vmax{2.0}
  
  % FILL AREA
  \fill[Icol!15] (0.09,-1.7) rectangle (0.90,\Vmax+0.2);
  \fill[Rcol!15] (1.70,-1.5) rectangle (3.30,\Vmax+0.2);
  
  % AXIS
  \begin{scope}[shift={(-0.5,0)}]
    \draw[->,thick]
      (-0.3,0) -- (4.9,0);
    \draw[->,thick]
 
 
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