Drawing is not centered

[Daniel Dilts-2]

I just started using TikZ last night, and I love it.

I am creating a diagram for class and I am having troubles with two  
things.

1. The drawing isn't centered on the page.  I have the drawing wrapped  
in a figure section and the caption for the figure is centered, but  
the drawing is not.

\begin{figure}[h]
   \begin{tikzpicture}[scale=.2]
   %outer circle
   \draw (0,0) circle (18cm);
   \end{tikzpicture}
   \caption{Forces on Rotating Disk}
\end{figure}

2. I can't get a label to locate at the center of an arc.  The  
examples in the manual show how to place a label at the center of a  
line but I can't figure out how to get it at the center of an arc.  
Currently I have

\begin{figure}[h]
   \begin{tikzpicture}[scale=.2]
   %Theta line
   \draw[->] (0,18.5cm) -- (0,20cm) arc (90:45:20cm) node[above=1em]{$
\Theta$};
   \end{tikzpicture}
   \caption{Forces on Rotating Disk}
\end{figure}

I have tried \draw[->] (0,18.5cm) -- (0,20cm) arc node{$\Theta$}  
(90:45:20cm); but that seems to lock up or crash.

Does anyone have any ideas what I am missing?

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[GGBBRR]

For your first problem you could explicitly specify that you want the tikzpicture centred, like this:

\begin{figure}[h]
\begin{center}
   \begin{tikzpicture}[scale=.2]
   %outer circle
   \draw (0,0) circle (18cm);
   \end{tikzpicture}
\end{center}
   \caption{Forces on Rotating Disk}
\end{figure}

For your second problem. Theta is drawn above the endpoint of the arc.
You could specify a path to the middle of the arc and put theta above the endpoint of that path, like this:

\begin{figure}[h]
\begin{center}
   \begin{tikzpicture}[scale=.2]
   \draw[->] (0,18.5cm) -- (0,20cm) arc (90:45:20cm);
   \path (0,20cm) arc (90:67.5:20cm)  node[above=0.1em]{$\Theta$};
   \end{tikzpicture}
\end{center}
   \caption{Forces on Rotating Disk}
\end{figure}

Regards,
Gerben