\begin{equation}: "Display math should end with $$" error?

Question

I'm pretty new to LaTeX, completely new actually, and there is one thing I can't quite understand.

I get the error "Display math should end with $$" when writing an equation like this:

\begin{equation}

$R_{2}$=$\frac{U_{R_2}}{I_2}$=$\frac{2,95\, \mathrm{V}$}{$0,03*10^{-3}\, \mathrm{A}$} = 7,10\,\mathrm{k}\Omega \,.

\label{eq:Bsp_OhmsLaw}\end{equation}

I tried a lot of things, I also skimmed these forums. Nothing seemed to help :-(

Thanks in advance!

Answer 1

In addition to removing the $ as cmhughes suggested, the blank lines in the equation environment confuse things. This may be why you added the dollar signs - with the blanks, LaTeX otherwise complains about missing $ symbols in the code.

Here's a working version:

\documentclass{article}

\begin{document}

\begin{equation}
  R_{2}=\frac{U_{R_2}}{I_2}=\frac{2,95\, \mathrm{V}}{0,03*10^{-3}\, \mathrm{A}} = 7,10\,\mathrm{k}\Omega \,.
  \label{eq:Bsp_OhmsLaw}
\end{equation}

\end{document}

Answer 2

Use the siunitx package for typesetting physical quantities:

\documentclass{article}

\usepackage{amsmath}
\usepackage[
  locale = DE % comma as decimal mark
]{siunitx}

\begin{document}

\begin{equation}
\label{eq:Bsp_OhmsLaw}
  R_{2}
  = \frac{U_{2}}{I_{2}}
  = \frac{\SI{2.95}{\volt}}{\SI{0.03e-3}{\ampere}}
  = \SI{7.10}{\kilo\ohm}.
\end{equation}
Ohm's law is used in equation~\eqref{eq:Bsp_OhmsLaw}.

\end{document}

Note that cfr has given the answer to why the initial code isn't compiling.

Update

You can make the code a bit shorter by use abbreviations for the physical units:

\documentclass{article}

\usepackage{amsmath}
\usepackage[
  locale = DE % comma as decimal mark
]{siunitx}

\begin{document}

\begin{equation}
\label{eq:Bsp_OhmsLaw}
  R_{2}
  = \frac{U_{2}}{I_{2}}
  = \frac{\SI{2.95}{\V}}{\SI{0.03e-3}{\A}}
  = \SI{7.10}{\kohm}.
\end{equation}
Ohm's law is used in equation~\eqref{eq:Bsp_OhmsLaw}.

\end{document}

(See page 37 of the manual.)