Reducing space between equation and number

Question

I would like to reduce the space between an equation and its corresponding number, since the equation is long enough that the number runs over into its own line when the document is compiled in a two-column format. I have used the flalign environment to reduce the amount of space on the left hand size, but there's still lots of space on the right-hand side. Any thoughts?

Here's a minimal working example:

\documentclass[twocolumn, a4paper,10pt]{article}
\usepackage{amsmath}

\begin{document}

\begin{flalign}
  \dot{m}_a\!\frac{dx}{L_x}c_{p,a}T_a & =\! -U_o(T_a-T_r)dA_o & \\
  \dot{m}_rdh_r &\! =\! -\dot{m}_ac_{p,a}\frac{dx}{L_x}\left[T_a(x,L_y)\! -\! T_a(x,0)\right], &  
\end{flalign}

\end{document}

Answer 1

You can make the longest (second) equation marginally shorter to make the content fit by adding a negative space, say \hspace{-.5em}:

\documentclass[twocolumn]{article}

\usepackage{amsmath,lipsum}

\begin{document}

\lipsum*[1]
\begin{flalign}
  \dot{m}_a\!\frac{dx}{L_x}c_{p,a}T_a &=\! -U_o(T_a-T_r)dA_o \\
  \dot{m}_rdh_r &= -\dot{m}_ac_{p,a}\frac{dx}{L_x}\bigl[T_a(x,L_y)\! -\! T_a(x,0)\bigr], \hspace{-.5em}
\end{flalign}
\lipsum[2]

\end{document}

Answer 2

Don't align at the equals sign, but left. Two column typesetting requires some compromises; in this case the left-hand side in the first equation is just too big.

\documentclass[twocolumn, a4paper,10pt]{article}
\usepackage{amsmath}
\usepackage{lipsum}

\begin{document}

\lipsum*[2]
\begin{align}
& \dot{m}_a\frac{dx}{L_x}c_{p,a}T_a = -U_o(T_a-T_r)dA_o \\
& \dot{m}_rdh_r = -\dot{m}_ac_{p,a}\frac{dx}{L_x}[T_a(x,L_y) - T_a(x,0)],
\end{align}
\lipsum

\end{document}

Alternatively, use gather.

\documentclass[twocolumn, a4paper,10pt]{article}
\usepackage{amsmath}
\usepackage{lipsum}

\begin{document}

\lipsum*[2]
\begin{gather}
\dot{m}_a\frac{dx}{L_x}c_{p,a}T_a = -U_o(T_a-T_r)dA_o \\
\dot{m}_rdh_r = -\dot{m}_ac_{p,a}\frac{dx}{L_x}[T_a(x,L_y) - T_a(x,0)],
\end{gather}
\lipsum

\end{document}