How can I get enough space for fractions?

Question

\begin{center}
\begin{tabular}{|c|c|c|}
\hline
& polarity & $\displaystyle \frac{\textrm{charge}}{\textrm{mass}}$\\
\hline
\textbf{A} & negative & $\displaystyle \frac{E}{g}$\\
\textbf{B} & negative & $\displaystyle \frac{g}{E}$\\
\textbf{C} & positive & $\displaystyle \frac{E}{g}$\\
\textbf{D} & positive & $\displaystyle \frac{g}{E}$\\
\hline
\end{tabular}
\end{center}

Answer 1

I suggest using a combination of (a) increasing the value of \arraystretch from 1 (the default) to 2 and (b) inserting "struts" in the denominator terms of the \frac expressions in rows 1, 3, and 5. (In these rows, the letters in the denominators don't contain descenders.) If you don't insert the struts, you'll need to increase the value of \arraystretch.

Separately, do use \textnormal rather than \textrm for the fraction terms in the header cell.

\documentclass{article}
\usepackage{array} % for "\newcolumntype" macro
\newcolumntype{C}{>{$\displaystyle}c<{$}}
\newcommand\myfrac[2]{\frac{#1}{#2\mathstrut}}
\begin{document}

\begin{center}
\renewcommand\arraystretch{2}
\begin{tabular}{|c|c|C|}
\hline
& polarity & \myfrac{\textnormal{charge}}{\textnormal{mass}}\\
\hline
\textbf{A} & negative & \frac{E}{g}  \\
\textbf{B} & negative & \myfrac{g}{E}  \\
\textbf{C} & positive & \frac{E}{g}  \\
\textbf{D} & positive & \myfrac{g}{E} \\
\hline
\end{tabular}
\end{center}

\end{document}