equation number does not appear

Question

i wrote in an equation environment, however, the right equation number is not displayed! it displays (2.1), which is wrong because this is my first equation. I think it took the section number as a first value.

Any idea how to solve this?

my code:

\begin{equation}
      \overrightarrow { \rm \nabla }  \cdot [\varepsilon \overrightarrow{ \rm (r)} \nabla \varphi \overrightarrow{ \rm (r) }] = - \frac{1}{\varepsilon_0} \left [ \rho^{fixed} \overrightarrow{ \rm (r) } + e \sum_{i}^{n} C_{i}^{b} exp ( - \frac{ez_i}{kT} \varphi \overrightarrow{ \rm (r) } ) \right ]
 \end{equation}

Answer 1

I assume you either defined it in the beginning of your document or you use a document class that specifies it itself accordingly. Anyhow, you can use the chngcntr package to manually define how the counting is done.

Here an MWE with the numbering done without the section number:

\documentclass[12pt]{article}

\usepackage{chngcntr}
\counterwithout{equation}{section}

\begin{document}

\section{Section One}

This is some text in Section 1.

\section{Section Two}

This is some text in Section 2. And here comes your equation:
\begin{equation}
      \overrightarrow { \rm \nabla }  \cdot [\varepsilon \overrightarrow{ \rm (r)} \nabla \varphi \overrightarrow{ \rm (r) }] = - \frac{1}{\varepsilon_0} \left [ \rho^{fixed} \overrightarrow{ \rm (r) } + e \sum_{i}^{n} C_{i}^{b} exp ( - \frac{ez_i}{kT} \varphi \overrightarrow{ \rm (r) } ) \right ]
 \end{equation}

\end{document}

And here an MWE with the numbering done with the section number:

\documentclass[12pt]{article}

\usepackage{chngcntr}
\counterwithin{equation}{section}

\begin{document}

\section{Section One}

This is some text in Section 1.

\section{Section Two}

This is some text in Section 2. And here comes your equation:
\begin{equation}
      \overrightarrow { \rm \nabla }  \cdot [\varepsilon \overrightarrow{ \rm (r)} \nabla \varphi \overrightarrow{ \rm (r) }] = - \frac{1}{\varepsilon_0} \left [ \rho^{fixed} \overrightarrow{ \rm (r) } + e \sum_{i}^{n} C_{i}^{b} exp ( - \frac{ez_i}{kT} \varphi \overrightarrow{ \rm (r) } ) \right ]
 \end{equation}

\end{document}