## Electric field of a spherical shell. Ka-Tex rendered [Take II].

In a previous post I attempted to use the katex plugin to render an old post instead of using Mathjax. It seems that was not actually rendered with KaTex, but (I think) it was rendered with the latex keyword handling in the Jetpack plugin, which I also had installed. I’ve customized the katex plugin I have installed to use a different keyword (katex instead of latex).

This is a test of KaTex, the latex rendering engine used for Khan academy. They advertise themselves as much faster than mathjax, but this speed comes with some usability issues.

Here’s a rerendering of an old post, with the latex rendered with WP-KaTeX instead of MathJax-LaTeX.

# The post

### Problem:

Calculate the field due to a spherical shell. The field is

$$\mathbf{E} = \frac{\sigma}{4 \pi \epsilon_0} \int \frac{(\mathbf{r} - \mathbf{r}')}{{{\left\lvert{{\mathbf{r} - \mathbf{r}'}}\right\rvert}}^3} da',$$

where $\mathbf{r}'$ is the position to the area element on the shell. For the test position, let $\mathbf{r} = z \mathbf{e}_3$.

### Solution:

We need to parameterize the area integral. A complex-number like geometric algebra representation works nicely.

\begin{aligned}\mathbf{r}' &= R \left( \sin\theta \cos\phi, \sin\theta \sin\phi, \cos\theta \right) \\ &= R \left( \mathbf{e}_1 \sin\theta \left( \cos\phi + \mathbf{e}_1 \mathbf{e}_2 \sin\phi \right) + \mathbf{e}_3 \cos\theta \right) \\ &= R \left( \mathbf{e}_1 \sin\theta e^{i\phi} + \mathbf{e}_3 \cos\theta \right).\end{aligned}

Here $i = \mathbf{e}_1 \mathbf{e}_2$ has been used to represent to horizontal rotation plane.

The difference in position between the test vector and area-element is

$$\mathbf{r} - \mathbf{r}' = \mathbf{e}_3 {\left({ z - R \cos\theta }\right)} - R \mathbf{e}_1 \sin\theta e^{i \phi},$$

with an absolute squared length of

\begin{aligned}{{\left\lvert{{\mathbf{r} - \mathbf{r}' }}\right\rvert}}^2 &= {\left({ z - R \cos\theta }\right)}^2 + R^2 \sin^2\theta \\ &= z^2 + R^2 - 2 z R \cos\theta.\end{aligned}

As a side note, this is a kind of fun way to prove the old “cosine-law” identity. With that done, the field integral can now be expressed explicitly

\begin{aligned} \mathbf{E} &= \frac{\sigma}{4 \pi \epsilon_0} \int_{\phi = 0}^{2\pi} \int_{\theta = 0}^\pi R^2 \sin\theta d\theta d\phi \frac{\mathbf{e}_3 {\left({ z - R \cos\theta }\right)} - R \mathbf{e}_1 \sin\theta e^{i \phi}} { {\left({z^2 + R^2 - 2 z R \cos\theta}\right)}^{3/2} } \\ &= \frac{2 \pi R^2 \sigma \mathbf{e}_3}{4 \pi \epsilon_0} \int_{\theta = 0}^\pi \sin\theta d\theta \frac{z - R \cos\theta} { {\left({z^2 + R^2 - 2 z R \cos\theta}\right)}^{3/2} } \\ &= \frac{2 \pi R^2 \sigma \mathbf{e}_3}{4 \pi \epsilon_0} \int_{\theta = 0}^\pi \sin\theta d\theta \frac{ R( z/R - \cos\theta) } { (R^2)^{3/2} {\left({ (z/R)^2 + 1 - 2 (z/R) \cos\theta}\right)}^{3/2} } \\ &= \frac{\sigma \mathbf{e}_3}{2 \epsilon_0} \int_{u = -1}^{1} du \frac{ z/R - u} { {\left({1 + (z/R)^2 - 2 (z/R) u}\right)}^{3/2} }. \end{aligned}

Observe that all the azimuthal contributions get killed. We expect that due to the symmetry of the problem. We are left with an integral that submits to Mathematica, but doesn’t look fun to attempt manually. Specifically

$$\int_{-1}^1 \frac{a-u}{{\left({1 + a^2 - 2 a u}\right)}^{3/2}} du = \frac{2}{a^2},$$

if $a > 1$, and zero otherwise, so

$$\boxed{ \mathbf{E} = \frac{\sigma (R/z)^2 \mathbf{e}_3}{\epsilon_0} }$$

for $z > R$, and zero otherwise.

In the problem, it is pointed out to be careful of the sign when evaluating $\sqrt{ R^2 + z^2 - 2 R z }$, however, I don’t see where that is even useful?

# KaTex commentary

1. Conditional patterns, such as:
\left\{
\begin{array}{l l}
\frac{\sigma (R/z)^2 \mathbf{e}_3}{\epsilon_0}
& \quad \mbox{if $z > R$ } \\
0 & \quad \mbox{if $z < R$ }
\end{array}
\right.


messed up KaTex, resulting in render errors like:

Using $$...$$ within math mode instead of $...$ also messed things up. Example:

\left\{
\begin{array}{l l}
\frac{\sigma (R/z)^2 \mathbf{e}_3}{\epsilon_0}
& \quad \mbox{if $z > R$ } \\
0 & \quad \mbox{if $z < R$ }
\end{array}
\right.


This resulted in a messed up parse like so:

It looks like it's the mbox that messes things up, and not the array itself, so \text could probably be used instead.

2. The latex has to be all in one line, or else KaTex renders the newlines explicitly. Example:

Having to condense all my latex onto a single line is one of the reasons I switched from the default wordpress latex engine to mathjax. It was annoying enough that I started paying for my wordpress hosting, and stopped posting on my old free peeterjoot.wordpress.com blog. Using KaTex and having to go back to single line latex would suck!

3. The rendering looks great, just like mathjax.
4. The Mathjax-Latex wordpress plugin has some support for equation labeling and references. I don't see a way to do those with the WP-KaTex plugin.
5. I can have a large set of macros installed in my default.js matching a subset of what I have in my .sty files. I don't see a way to do that with the WP-KaTex plugin, but perhaps there is just no documented mechanism. KaTex itself does have a macro mechanism.
6. The display isn't left justified like the wordpress latex, and looks decent.

## Electric field of a spherical shell. Ka-Tex rendered

This is a test of KaTex, the latex rendering engine used for Khan academy. They advertise themselves as much faster than mathjax, but it looks like the reason for that is because they generate images that look crappy unless the browser resolution is matched to the images just right.

Here’s a rerendering of an old post, with the latex rendered with WP-KaTeX instead of MathJax-LaTeX.

# The post

### Problem:

Calculate the field due to a spherical shell. The field is

$\mathbf{E} = \frac{\sigma}{4 \pi \epsilon_0} \int \frac{(\mathbf{r} - \mathbf{r}')}{{{\left\lvert{{\mathbf{r} - \mathbf{r}'}}\right\rvert}}^3} da',$

where $\mathbf{r}'$ is the position to the area element on the shell. For the test position, let $\mathbf{r} = z \mathbf{e}_3$.

### Solution:

We need to parameterize the area integral. A complex-number like geometric algebra representation works nicely.

\begin{aligned}\mathbf{r}' &= R \left( \sin\theta \cos\phi, \sin\theta \sin\phi, \cos\theta \right) \\ &= R \left( \mathbf{e}_1 \sin\theta \left( \cos\phi + \mathbf{e}_1 \mathbf{e}_2 \sin\phi \right) + \mathbf{e}_3 \cos\theta \right) \\ &= R \left( \mathbf{e}_1 \sin\theta e^{i\phi} + \mathbf{e}_3 \cos\theta \right).\end{aligned}

Here $i = \mathbf{e}_1 \mathbf{e}_2$ has been used to represent to horizontal rotation plane.

The difference in position between the test vector and area-element is

$\mathbf{r} - \mathbf{r}' = \mathbf{e}_3 {\left({ z - R \cos\theta }\right)} - R \mathbf{e}_1 \sin\theta e^{i \phi},$

with an absolute squared length of

\begin{aligned}{{\left\lvert{{\mathbf{r} - \mathbf{r}' }}\right\rvert}}^2 &= {\left({ z - R \cos\theta }\right)}^2 + R^2 \sin^2\theta \\ &= z^2 + R^2 - 2 z R \cos\theta.\end{aligned}

As a side note, this is a kind of fun way to prove the old “cosine-law” identity. With that done, the field integral can now be expressed explicitly

\begin{aligned} \mathbf{E} &= \frac{\sigma}{4 \pi \epsilon_0} \int_{\phi = 0}^{2\pi} \int_{\theta = 0}^\pi R^2 \sin\theta d\theta d\phi \frac{\mathbf{e}_3 {\left({ z - R \cos\theta }\right)} - R \mathbf{e}_1 \sin\theta e^{i \phi}} { {\left({z^2 + R^2 - 2 z R \cos\theta}\right)}^{3/2} } \\ &= \frac{2 \pi R^2 \sigma \mathbf{e}_3}{4 \pi \epsilon_0} \int_{\theta = 0}^\pi \sin\theta d\theta \frac{z - R \cos\theta} { {\left({z^2 + R^2 - 2 z R \cos\theta}\right)}^{3/2} } \\ &= \frac{2 \pi R^2 \sigma \mathbf{e}_3}{4 \pi \epsilon_0} \int_{\theta = 0}^\pi \sin\theta d\theta \frac{ R( z/R - \cos\theta) } { (R^2)^{3/2} {\left({ (z/R)^2 + 1 - 2 (z/R) \cos\theta}\right)}^{3/2} } \\ &= \frac{\sigma \mathbf{e}_3}{2 \epsilon_0} \int_{u = -1}^{1} du \frac{ z/R - u} { {\left({1 + (z/R)^2 - 2 (z/R) u}\right)}^{3/2} }. \end{aligned}

Observe that all the azimuthal contributions get killed. We expect that due to the symmetry of the problem. We are left with an integral that submits to Mathematica, but doesn’t look fun to attempt manually. Specifically

$\int_{-1}^1 \frac{a-u}{{\left({1 + a^2 - 2 a u}\right)}^{3/2}} du = \frac{2}{a^2},$

if $a > 1$, and zero otherwise, so

$\boxed{ \mathbf{E} = \frac{\sigma (R/z)^2 \mathbf{e}_3}{\epsilon_0} }$

for $z > R$, and zero otherwise.

In the problem, it is pointed out to be careful of the sign when evaluating $\sqrt{ R^2 + z^2 - 2 R z }$, however, I don’t see where that is even useful?

# KaTex commentary

1. Conditional patterns, such as:
\left\{
\begin{array}{l l}
\frac{\sigma (R/z)^2 \mathbf{e}_3}{\epsilon_0}
& \quad \mbox{if $$z > R$$ } \\
0 & \quad \mbox{if $$z < R$$ }
\end{array}
\right.


messed up KaTex, resulting in render errors like:

2. The latex has to be all in one line, or else KaTex renders the newlines explicitly. Example:
Having to condense all my latex onto a single line is one of the reasons I switched from the default wordpress latex engine to mathjax. It was annoying enough that I started paying for my wordpress hosting, and stopped posting on my old free peeterjoot.wordpress.com blog. Using KaTex and having to go back to single line latex would suck!
3. The rendering looks like crap, unless you match your resolution to exactly those used to create the images. The mathjax rendering may be slower, but looks much better!
4. The Mathjax-Latex wordpress plugin has some support for equation labeling and references. I don’t see a way to do those with the WP-KaTex plugin.
5. I can have a large set of macros installed in my default.js matching a subset of what I have in my .sty files. I don’t see a way to do that with the WP-KaTex plugin, but perhaps there is just no documented mechanism. KaTex itself does have a macro mechanism.
6. Left justified display mode is hard to read. The mathjax rendered centered display mode looks much better.

## EDIT.

I’m not sure I was getting the katex plugin when I used the [ latex ] … [ /latex ] tags.  I see some comments that indicate that there is built in handling of these tags in the Jetpack plugin.  If I change frontend.php in the katex plugin to use [ katex ] … [ /katex ] tags instead, then I see much different results.

## Peeter Joot’s new blog: more to come…

After 611 blog posts on my old wordpress.com hosted blog, dating all the way back to 2009, I’ve decided to ante-up and pay for more flexible hosting.

My primary motivation for this was truly geeky.  I wanted the flexibility to be able to manage wordpress plugins (i.e. mathjax-latex and wolframcdf), and to also be able to put plain old html and arbitrary file content into the apache2 directory structure.  I’ve wanted plain html hosting for a while, but made do with google sites (i.e. crappy but free).  I’d also wanted to be able to use the wolfram CDF plugin on my blog, but also not enough to pay for it.  However, once I tried mathjax-latex, I was sold.  Compared to wp-latex, this “new way” completely kicks ass, and should save me a lot of time.

I tried out an amazon EC2 bitnami image for a while (amazon offers a free trial year to evaluate their offerings).  That’s a flexible setup and offers direct access to the Linux VM, which is very nice.  However, with an amazon EC2 image, I’m not really sure what I would end up paying.  The charts seem somewhat vague, depending on future usage of both machine and storage.  I would also have pay separately for a domain name, and pay separately for amazon hosting of the DNS entry.

I ended up deciding to use a go-daddy hosted wordpress instance, which is a flat rate service.  It is less flexible than a godaddy standalone web-hosting environment, but also cheaper ($12 for the first year, including the domain name, and ~$50/year after that).  It also looks like I can upgrade this to a more generic web hosting environment later if the cost of that seems justified.  I’ll see first if only having sftp access to htdocs is enough of a major inconvenience to pay that additional yearly fee.

Configuring a custom MathJax configuration was a bit of a pain with only sftp access, mostly because I had to copy the MathJax tree, which was very slow for so many small files.  I did that directory tree transfer with FileZilla since sftp ‘put –r’ appears to be busted.  This MathJax setup was way easier on the EC2 since the ssh shell allowed for wget and local unzip directly from the apache2 htdocs tree.  It’s a shame that the mathjax-latex plugin doesn’t allow the MathJax tree to be served from the default server (what the plugin settings calls the ‘MathJax CDN Service’).  Logically, I’d like to be able to use that CDN service, but have my configuration file hosted locally.  That config file (config/default.js) is a single small file, and is likely all that I’ll ever have to alter in that whole directory tree.

I haven’t decided whether or not I’ll keep my old peeterjoot.wordpress.com blog, or switch unconditionally to this new peeterjoot.com blog (which will be the new home for any of my mathematical or physics related posts).  This new blog has no blog-article content so far, and doesn’t yet have a theme template that I like.  What is here so far is:

• An enumeration of things I have written, including archives of all the individual pdfs that I have posted over the years along with my blog entries.  All these pdfs are now stored directly on the new site in the htdocs tree.  I will no longer be using any of my (three) old google sites pages as pdf stores.
• A chronological listing of all the Mathematica notebooks I have written.  The newest versions of these notebooks can still be found in my Mathematica github repository.  A snapshot of each of these is now also available on the new site, so if you have the CDF plugin installed, these can now be examined by clicking on the links directly.  Ironically, with chrome and my CDF installation, I’m able to view the .nb suffixed notebooks directly in the browser, but a click on any CDF (.cdf) notebook triggers a download?
• I’ve made a couple notes about my setup of the mathjax-latex plugin, and the differences in latex markup with that plugin compared to the wp-latex plugin (which is available by default on wordpress.com).  My future mathematical blogging should be way easier, probably won’t require any of my old tex2blog script, and will also look better!
• An About page, copied directly from the About page on my old blog.

More to come, … now that I’ve finally finished the Stokes theorem chapter in my Geometric Algebra compilation, I expect new posts to be more frequent.