tikz-node
I want to put the text A into the right bottom corner of the node rectangle without changing any other stuff. Please help me.
\documentclass[10pt]{standalone}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}
\draw [help lines] (0,0) grid (40pt,40pt);
\node [draw,minimum height = 30pt, minimum width = 30pt, inner sep = 0] at (15pt,15pt) {$A$};
\end{tikzpicture}
\end{document}
The closest to what you like to obtain with one node:
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}
\node at (0,0) [draw,
text height=1cm, text depth=1ex,
minimum width=4cm, inner sep=-0.2ex] (a) {Lorem Ipsum};
\draw (-2,-2) rectangle (2,-1);
\node at (0,-1.8) {Lorem Ipsum};
\end{tikzpicture}
\end{document}
The node above is slightly taller.
I'm not sure I've understood correctly, but do you want something like this?
\documentclass[border=10pt,multi,tikz]{standalone}
\usetikzlibrary{arrows.meta}
\begin{document}
\begin{tikzpicture}
\node (circle) [draw, circle, minimum size = 20mm, label=above:$I$] at (0, 0) {};
\draw (circle.center) [{Rays[]}-|] -- (circle.east) node[below, midway] {$r$};
\draw (circle.east) -- (2, 0) node[below, midway] {$r$};
\draw (2, 0) [|-{Circle[]}] -- (3, 0) node[below, midway] {$r$} node[above] {P};
\end{tikzpicture}
\end{document}
Remember that a node is not the same as a path, so you cannot label it in the same way.
In effect, your original code places two nodes at the origin: first the circular one; then the $I$. This works because \node is equivalent to \path node. So you have \path node ... node ...; Hence, the second node is placed above the path, which consists of the point (0,0).
Addressing the clarified question, just put the node above the line's starting point. For example,
\documentclass[border=10pt,multi,tikz]{standalone}
\usetikzlibrary{arrows.meta}
\begin{document}
\begin{tikzpicture}
\node (circle) [draw, circle, minimum size = 20mm] at (0, 0) {};
\draw (circle.center) [{Rays[]}-|] node [above] {$I$} -- (circle.east) node[below, midway] {$r$};
\draw (circle.east) -- (2, 0) node[below, midway] {$r$};
\draw (2, 0) [|-{Circle[]}] -- (3, 0) node[below, midway] {$r$} node[above] {P};
\end{tikzpicture}
\end{document}
There are other ways to do this, but this seems as straightforward to me as any.
Best Answer
You can use a
labelto place some text inside the node: