Heat expansion

Edit and compile if you like:

% Author: Izaak Neutelings (February 2019)
\documentclass[border=3pt,tikz]{standalone}
\usepackage{tikz,pgfplots}
\usepackage[outline]{contour} % glow around text
\usetikzlibrary{angles,quotes} % for pic (angle labels)
\tikzset{>=latex} % for LaTeX arrow head
\contourlength{1.4pt}

\tikzstyle{material}=[top color=blue!30,bottom color=blue!50,shading angle=10]
\tikzstyle{measure}=[fill=white,midway,inner sep=2]

\begin{document}


% ROD
\begin{tikzpicture}
  \def\W{0.3}
  \def\L{6}
  \def\dL{1}

  \draw[material] (0,-\W/2) rectangle ++(\L,\W);
  \draw[<->] (0,-1.4*\W) --++ (\L,0) node[measure] {$L_0$};
  \node[right=4pt] at (\L,0) {$T_0$};
  
  \begin{scope}[yshift=-30pt]
    \draw[material] (0,-\W/2) rectangle ++(\L+\dL,\W);
    \draw[<->] (0,-1.4*\W) --++ (\L,0) node[measure] {$L_0$};
    \draw[<->,below] (\L,-1.4*\W) --++ (\dL,0) node[measure] {$\Delta L$};
    \node[right=4pt] at (\L+\dL,0) {$T_0+\Delta T$};
  \end{scope}
  
\end{tikzpicture}


\end{document}