Derivatives are sized differently with commath

Question

I'm writing a derivation in an upper level physics class and I use derivatives extensively. I'm using the commath package \od{}{} command and they are being sized differently in different places. Also, most of them are too small for my liking. I previously used physymb before it was deprecated and that didn't have this problem.

Here is part of the code:

\documentclass{article}  
\usepackage{nopageno}    
\usepackage{amsmath}    
\usepackage{commath}    
\usepackage{geometry}    
\geometry{margin=.75in}
\newcommand{\half}{\frac{1}{2}}
\title{\vspace{-1cm}Coulomb Scattering from a Fixed Point Charge, aka, Rutherford Scattering}
\date{}
\author{}
\begin{document}
    \maketitle
    Skipping irrelevant things...
From this we can get the equations of motion:

\begin{align}
\od{~}{t}\left(\pd{\mathcal{L}}{\dot{r}}\right)&=\pd{\mathcal{L}}{r} & \longrightarrow & &\od{~}{t}&\left(m\dot{r}\right)=mr\dot{\phi}^2+\frac{C}{r^2}\\ \od{~}{t}\left(\pd{\mathcal{L}}{\dot{\phi}}\right)&=\pd{\mathcal{L}}{\phi} 
 & \longrightarrow & &\od{~}{t}&\left(mr^2\dot{\phi}\right)=0 
\end{align}
where $C = \frac{Z_1Z_2e^2}{4\pi\epsilon_o}$.

The term $mr^2\dot{\phi}$ is simply the instantaneous angular momentum, and we see from Equation (2) that this is constant:
\[mr^2\dot{\phi}=L = mv_ob .\]
This yields\[\dot{\phi}=\frac{L}{mr^2}\text{ or }\frac{v_ob}{r^2}.\]

Using this in Equation (1) we can write
\begin{equation}
\od{~}{t}\left(m\dot{r}\right)=mr\frac{v_o^2b^2}{r^4}+\frac{C}{r^2}
\end{equation}
At this point, a substitution of variable is appropriate for $r$, so we let $r=\dfrac{1}{u}$. We will see after a bit of work that this helps quite a bit.

First we calculate $\dot{r}=\dfrac{-1}{u^2}\dot{u}$. Then we make use of the chain rule to change from time derivatives to $\phi$ derivatives:
\begin{align*}
\od{u}{t}&=\od{u}{\phi}\od{\phi}{t}\\
\dot{u}&=\od{u}{\phi}\dot{\phi}\\
\dot{u}&=\od{u}{\phi}v_o b u^2
\end{align*}
This gives us $\dot{r}=-v_ob\od{u}{\phi}$.

Using the same chain rule for the time derivative on the left side of Equation (3) we get
\[m\od{~}{\phi}\left(-v_ob\od{u}{\phi}\right)\left(v_o b u^2\right)=mv_o^2b^2u^3+Cu^2\]
\[-mv_o^2b^2u^2\od[2]{u}{\phi}= mv_o^2b^2u^3+Cu^2\]
\[\od[2]{u}{\phi}=-u-\frac{C}{2Eb^2}\]
where $E$ is the initial energy given above.



   Skipping more irrelevant stuff....

    \end{document}

When I compile in pdfLaTeX2e Equations 1 and 2 have small derivatives. Equation 3 does not. In the last three equations in the last align* environment there is a mixture of big and small. I prefer the big.

I have tried different fonts (default, tgschola, fouriernc) and also tried the document amsart. No changes in the derivatives. I am using TeXLive on Windows 7.

Please explain why there is a mixture of sizes and how I can easily force the derivative sizes to match the ordinary text.

Answer 1

Aha! A more careful reading of the commath documentation gives me a solution.

Instead of \od{ }{ } which "automatically" chooses font/style, I can use \dod{ }{ } which forces the default text font/style. Same thing with partial derivatives. Instead of using \pd{ }{ } I can use \dpd{ }{ }.

Here's an example of the difference:

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