In the following code, how do I align the n's?
Here is the MWE:
\documentclass[12pt]{article}
\usepackage{amsmath}
\begin{document}
\begin{align}
n \propto exp(-U/kT) \notag \\
\Rightarrow \quad n = A.exp(-U/kT) \notag
\end{align}
\end{document}

In the following code, how do I align the n's?
Here is the MWE:
\documentclass[12pt]{article}
\usepackage{amsmath}
\begin{document}
\begin{align}
n \propto exp(-U/kT) \notag \\
\Rightarrow \quad n = A.exp(-U/kT) \notag
\end{align}
\end{document}

Something like this. Put & right after n's.
Update: commented by barbara beeton. Thanks.

Code
\documentclass[12pt]{article}
\usepackage[paper size={10cm,3cm}]{geometry}
\usepackage{amsmath}
\begin{document}
\begin{align}
n& \propto \exp(-U/kT) \notag \\
\Rightarrow \quad n& = A \exp(-U/kT) \notag
\end{align}
\end{document}
\documentclass[preview,border=12pt,12pt]{standalone} % please change it back to your article class below
%\documentclass[12pt]{article}
\usepackage{amsmath}
\begin{document}
\abovedisplayskip=0pt\relax% don't use this line in your production
\begin{align*}
n & \propto \exp(-U/kT) \\
\Rightarrow \quad n &= A \exp(-U/kT)
\end{align*}
\end{document}

\Rightarrowto\implies. – Manuel Jul 19 '14 at 10:27\expis already provided byamsmathas an operator name, which should do the right thing. – barbara beeton Jul 19 '14 at 13:24