This blog is for comments and reports of errors on my physicspages.com site. I will also post notifications of new articles published on the main site using this blog, so if you want to be kept up to date, check here regularly, or else subscribe to it for email notifications. I realize this may look like a roundabout way of organizing things, but I don’t trust WordPress with anything substantial any more.

If you’re reporting an error or want clarification of an equation in an article, PLEASE include:

- The URL of the page – just copy this from the browser’s address bar at the top.
- The equation number containing the error.
- And of course, what you think is wrong with it.

Thanks, and I hope you find the new site as useful as the old one.

The original post date of each article on the physicspages blog is given at the top of that article.

For articles posted after September 2017, there will be a separate post on the comments blog (this blog), which you can locate in the ‘Archives’ list at the right. You can leave a comment on that particular article from its corresponding entry in the comments blog.

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Your site is a five star resource! thank you 😀

I was curious about whether or not you’ve considered opening your TeX source.

It may seem a bit idiosyncratic but I have been transitioning my note-taking flow to markdown. It’s all part of an interest in linked-data a la https://obsidian.md/ .

The goals are 1) friction-less conversion to alternate file formats and 2) facilitate development of highly reconfigurable knowledge graphs. A network of notes could be relatively easily converted into TeX, rendered into PDF, or even plopped into a static website. Your site is about as close to this concept as I’ve seen.

The highest impediment is conversion from pdf to plaintext (LateX or otherwise). Even with https://mathpix.com/ it is a process. Using raw source and https://pandoc.org/demos.html would be twice as fast.

There are other challenges to this idea, like the development of sensible bibliographic schemas and balancing interoperability and portability of the captured data. Once I tease that out I would send a pull-request with the restructured content.

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I’m not really sure what you’re asking. If your suggestion is that I make physicspages open source in a similar

manner to, say, Wikipedia, then, no, definitely not! It would be far too much work to check that modifications to the site were accurate and correct.

At present, all the Latex source is written in files created by the Latex editor Lyx, and Lyx handles the conversion to PDF by some internal magic that I have no control over. Lyx makes writing Latex files about as easy as using an ordinary word processor to create an ordinary text document, so if someone wanted to create a Latex file for their own notes, my advice would be to use Lyx.

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Disregard my earlier question, I see how it was done now.

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https://physicspages.com/pdf/Griffiths%20QM/Griffiths%20Problems%2006.29.pdf In your solution for the PERTURBATION DUE TO FINITE SIZE OF THE PROTON IN HYDROGEN, Equation six shows the result of your exponential expansion, but I am not getting a similar result when I expand. Can you explain your expansion here?

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I saw your message on Physics Pages that you had moved it to a new hosting company. Then, I noticed that the link (https://physicspages.com/shankar.html) of the solutions to quantum mechanics by Shankar didn’t work. Please check this link and fix it. Thank you very much.

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Try reloading physicspages.com and then try the link to Shankar’s book again. It should work now. If not, you’ll need to clear your browser’s cache, then try again.

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Regarding homework problem P12.7 in Thomas Moore’s A General Relativity Workbook.

You draw a graph of Sin (critical emission angle) for both (+) and (-) and claim in Equation (36) that it is well defined for all r > 0. The quantity in brackets [ ] in the case of (-) is 0 when r = 2GM. But, we need to take [ ] ^ -1. The total expression is 0/0 which is undefined. I agree with you that we should be using the (+) form of the emission formula for r < 3GM. In which case, we can avoid this problem since the + form is well defined at r = 2GM, to w.i.t. 0 for the emission angle as you correctly compute. But, your blue graph should have an undefined point at r = 2GM. It shows an answer of about 0.7, which I'm unable to understand how you calculated, and I do not think is correct.

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Thanks for the comment. I’ll try to have a look. In the meantime I’ve added your comment to the original post.

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In your solution to Shankar’s Quantum Mechanics, problem 12.03.01, equation 9 and the subsequent ones, you’re missing a factor of rho in the integrand. (The rho is also written in the question, I think it’s a typo)

Also, I think the paragraph after equation 11 does not necessarily prove equation 12.

Thanks for your detailed solutions!

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Fixed now. Thanks.

As for equation 12, in the textbook it says to show that “it is enough if obeys” Shankar’s equation 12.3.6 (my equation 12), which is certainly true, since then the value of in equation 11 is zero.

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