Math & Science ⇒ Undefined control sequence

[Bravus]

Typing this sentence as part of my Masters literature review in physics:

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Gauss' law for electricity can be stated as $\nabla$$\cdot$E = $\rho$\textsubscript{f}/$\varepsilon$ where E = electric field strength (Vm\textsuperscript{-1}), $\rho$\textsubscript{f} = electric charge density (Cm\textsuperscript{-3}) and $\varepsilon$ = permittivity of the medium (Fm\textsuperscript{-1}).

I get an '!Undefined control sequence' error that seems to relate to this bit: "$\nabla$$\cdot$E = $\rho$\textsubscript{f}/$\varepsilon$" (without the quotes)

Three Greek letters (plus an upside down one, I guess, a dot and a fraction...

Any help with what's breaking?

[Bravus]

Error message line breaks as follows:

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$\nabla$$\cdot$E = $\rho$\textsubscript
                                {f}/$\varepsilon$

[Bravus]

Hmm, looks like \textsubscript was both a longwinded and a plain bad way of doing it!

This worked:

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Gauss' law for electricity can be stated as $\nabla$$\cdot$E = {$\rho_{f}$/$\varepsilon$ where E = electric field strength (Vm\textsuperscript{-1}), $\rho_{f}$ = electric charge density (Cm\textsuperscript{-3}) and $\varepsilon$ = permittivity of the medium (Fm\textsuperscript{-1}).

[cgnieder]

The message tells you that the command \textsubscript doesn't exist. And it doesn't without special packages or another than one of the standard classes. However, it would be wrong to use it for math typesetting, anyway. Here's how I'd do it:

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\documentclass{article}
\usepackage{amsmath}% typesetting math
\usepackage{siunitx}% typesetting units
\begin{document}

Gauss' law for electricity can be stated as $\nabla \cdot E =
\rho_{f}/\varepsilon$ where $E$ = electric field strength
(\si{\volt\per\metre}), $\rho_{f}$ = electric charge density
(\si{\coulomb\per\metre\cubed}) and $\varepsilon$ = permittivity of the medium
(\si{\farad\per\metre}).

\end{document}

You may be interested in mathmode, amsmath and siunitx for further reading.

Regards

[Bravus]

Excellent, thank you. Ended up getting it, and it didn't look too dissimilar, though I like the way you've laid it out for clarity.

Thanks - still a newbie, teaching myself as I go along... no doubt it'll get a lot easier with more practice.