How to draw parallel capacitor plates

Question

I am trying to create a drawing of part of a circuit with 3D perspective like so:

How can this be done?

The main body of the capacitors has now been created in latex, all that remains is to create the line from either side of the shaded circle.

Answer 1

The following should get you started.

Edit: you can control the opacity of draw and fill elements independently, using the draw opacity and fill opacity keys, respectively. See Tikz: use opacity for fill, yet leave draw (lines) untouched for more details.

\documentclass{standalone}
\usepackage{tikz}
\usetikzlibrary{decorations.markings,arrows,patterns,shapes}

\tikzset{
  ->-/.style={decoration={
    markings,
    mark=at position .3 with {\arrow{triangle 90}}},postaction={decorate}},
}

\begin{document}
\begin{tikzpicture}
\def\Cplanesep{3} % <--- adjust the distance between the planes here
\def\Cwirelen{4}  % <--- adjust the wire length here
\path[use as bounding box]
  ({3-\Cwirelen}, -3) rectangle ({3.2+\Cplanesep+\Cwirelen}, 3.5);
  % necessary for cropping the picture
  % because control points lie way out
  % (dimensions adjusted manually...)
\newcommand\Cplane[1][0]
{%
  \begin{scope}[xshift=#1 cm]
  \filldraw[thick,draw=black,fill=gray,fill opacity=.3]
      (2.5,-2.5) --
      (2.5,1)    --
      (4,3)      --
      (4,-0.5)   --
      cycle;
  \end{scope}
}
\node
[
  draw,
  thick,
  ellipse,
  minimum height = 1.5cm,
  minimum width  = 1cm,
  decorate,
  pattern = north east lines,
  fill opacity = .7,
  rotate       = -20
] (hc) at (1.5,0) {};
\draw[thick,->-]
  ({3-\Cwirelen},0) -- ++(\Cwirelen,0)
  node[pos=.3,anchor=north] {$I$};
\Cplane
\Cplane[\Cplanesep]
\draw[thick] ({3.2+\Cplanesep},0) -- ++(\Cwirelen,0) ;
\draw
  (hc.north)
  .. controls (6,10) and (5, -9) ..
  (hc.south);
\end{tikzpicture}
\end{document}

Answer 2

This code uses hobby tikz library to draw the envelope:

\documentclass[tikz,border=10pt]{standalone}
\usetikzlibrary{decorations.markings, arrows,calc,patterns,shapes,hobby,backgrounds}

\tikzset{
    single/.style={draw,
        decoration={markings, mark=at position .5 with {\arrow{triangle 45}}},
        postaction={decorate},
    },
    }
\begin{document}
  \begin{tikzpicture}[font=\tiny]
    \coordinate (a) at (0,0);
    \coordinate (b) at (2,0);
    \coordinate (c) at (2,2);
    \coordinate (d) at (0,2);
    %
    \coordinate (a1) at (0.65,0.5);
    \coordinate (b1) at (2.65,0.5);
    \coordinate (c1) at (2.65,2.5);
    \coordinate (d1) at (0.65,2.5);
    %
    \draw[thick,fill=gray!30] (d) -- coordinate(da) (a) -- (a1) -- (d1) -- cycle;
    \draw[thick,fill=gray!30] (b) -- (b1) -- (c1) -- (c) -- cycle;
    \draw[single] (-2,1) -- (da)node[midway,above] {I};
    \draw(2.3,1) -- (4,1);
    \node
        [
            draw,
            %thick,
            ellipse,
            minimum height = 1cm,
            minimum width  = 0.5cm, 
            pattern = north east lines,            
            fill opacity = .5,
            rotate       = -10
        ] (el) at (-0.5,1) {};
    \begin{scope}[on background layer]
    \draw (el.north)
        to[curve through={(-0.3,1.7) .. (0.5,2.6) .. (0.6,2.6) ..
                  (0.9,1)   .. (0.75,0) ..
                  (0.1,-0.3)  
                  }
            ]
            (el.south);
    \end{scope}
  \end{tikzpicture}
\end{document}

Answer 3

With PSTricks.

\documentclass[pstricks,border=30pt,12pt]{standalone}
\usepackage{pstricks-add}
\def\plate{\pspolygon(-1,-2)(1,-1)(1,3)(-1,2)}

\begin{document}
\begin{pspicture}[fillstyle=solid,fillcolor=gray,arrowscale=2,opacity=.3](9,5)
\rput(3,2){\pcline[ArrowInside=->](-3,0)(0,0)\nbput{$I$}\plate}
\rput(6,2){\plate\psline[ArrowInside=->](3,0)}
\end{pspicture}
\end{document}