# How can I draw an arc in 3d using tikzpicture, in the xy plane?

So I'm trying to draw an arc from the x axis to the y axis to simbolize the cross product between r and p, and I'm using the code below, but as you can see it doesn't give me the arc that I'd want. How can I do it?

\begin{tikzpicture}
\draw (0,0,0) node[above left] {};
\draw[thick, ->] (0,0,0) -- (2,0,0) node[anchor=north west]{$y$};
\draw[thick, ->] (0,0,0) -- (0,2,0) node[anchor=south west]{$z$};
\draw[thick, ->] (0,0,0) -- (0,0,2) node[anchor=south east]{$x$};

\draw[thick, ->,green] (0,0,0) -- (1,0,0) node[anchor=north east]
{$\vec{p}$};
\draw[thick, ->,red] (0,0,0) -- (0,1,0) node[anchor=north west]{$\vec{L}$};
\draw[thick, ->,blue] (0,0,0) -- (0,0,1) node[anchor=south east]{$\vec{r}$};
\draw[->] (0,0,0.5) arc (0:90:0.5cm);
\end{tikzpicture}


And here's the image: ## 1 Answer

One way is to load the tikz-3dplot package.

\documentclass[tikz]{standalone}
\usepackage{tikz-3dplot}
\begin{document}
\begin{tikzpicture}
\tdplotsetmaincoords{70}{110}
\draw (0,0,0) node[above left] {};
\draw[thick, ->] (0,0,0) -- (2,0,0) node[anchor=north west]{$y$};
\draw[thick, ->] (0,0,0) -- (0,2,0) node[anchor=south west]{$z$};
\draw[thick, ->] (0,0,0) -- (0,0,2) node[anchor=south east]{$x$};

\draw[thick, ->,green] (0,0,0) -- (1,0,0) node[anchor=north east]
{$\vec{p}$};
\draw[thick, ->,red] (0,0,0) -- (0,1,0) node[anchor=north west]{$\vec{L}$};
\draw[thick, ->,blue] (0,0,0) -- (0,0,1) node[anchor=south east]{$\vec{r}$};
\tdplotdrawarc{(0,0,0)}{0.5}{0}{110}{anchor=north}{$\phi$}
\end{tikzpicture}
\end{document} If you do not want to load that package, you can always fake the curve.

\documentclass[tikz,border=3pt]{standalone}
\begin{document}
\begin{tikzpicture}
\draw (0,0,0) node[above left] {};
\draw[thick, ->] (0,0,0) -- (2,0,0) node[anchor=north west]{$y$};
\draw[thick, ->] (0,0,0) -- (0,2,0) node[anchor=south west]{$z$};
\draw[thick, ->] (0,0,0) -- (0,0,2) node[anchor=south east]{$x$};

\draw[thick, ->,green] (0,0,0) -- (1,0,0) node[anchor=north east]
{$\vec{p}$};
\draw[thick, ->,red] (0,0,0) -- (0,1,0) node[anchor=north west]{$\vec{L}$};
\draw[thick, ->,blue] (0,0,0) -- (0,0,1) node[anchor=south east]{$\vec{r}$};
\draw[->] (0,0,0.6) arc [start angle=-90,end angle=0,x radius=0.8,y radius=0.24];
\end{tikzpicture}
\end{document} 