The h Bar - 2021-02-27

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12:14 AM

The meaning of terms pressure volume in enthalpy iw bit unclear to me

Can somebody give an example of how we have to do work to establish the system by creating room for the system

4 hours later…

4:43 AM

@PrateekMourya ?

2 hours later…

6:18 AM

0

Q: i know the answer but the stupidity of the designers of the site prevent me from answering because i have no reputation. what am i to do?

Why does the bathroom become hot after a bath? i know the answer to this question being a veterinarian when we bathe body cools down only temporarily. since we are warm blooded animals our body generates heat to regain our temperature. this causes heating. if ventilation in bathroom is poor the h...

4 hours later…

10:03 AM

hello

ahoi

11:06 AM

@ZeroTheHero data.stackexchange.com/physics/query/1379110/…

forked from a previous query that misplaced the PostTypeId for questions into answers

obviously the top layer when sorted by average answer score needs to be taken with a grain of salt, as it contains several users with relatively few answers and one or two huge hits (HNQ or otherwise)

data.stackexchange.com/physics/query/393837/… shows the (log-scale) answer score distribution per user

12:14 PM

0

Q: Difference in "edited posts" in editor stats and user profile

I noticed that there is an inconsistency in the number of edited posts between the "editor stats" (which is the dialog that you can see in post revisions) and the user profile. For example, according to the editor stats, I have had 102 edit suggestions approved: while on my user profile, the num...

bug editing suggested-edits

2 hours later…

2:22 PM

0

Q: Conditions for on-site preservation of an answer

I answered a question made by someone who has not been seen on the site since April 29, 2016. It's my impression that "accepted" answers (answers accepted by the participant who had formulated the question) will be retained, but that others may be deleted. As my answer has evolved on the basis ...

@EmilioPisanty Thanks. It still gives a perspective (really different) from your previous query. I eliminated the "noise" by increasing the minimum number of answers to 100.

3:24 PM

@ZeroTheHero indeed it does.

still, I think a query like that makes it too easy to overvalue average rep

particularly for users (disclaimer: includes me) with a long tail of answers with one or two upvotes, which don't necessarily provide a large amount of lasting value but which were helpful to OP at the time

i.e. it's a lens which tends to devalue the aspect of the site where we give help people who ask for it (as opposed to the aspect where we provide a corpus of knowledge for the future)

Whats a background potential...

researchgate.net/publication/…

In the Kinetic equation an author describes one of the potentials as a background potential, what are these ?

3:59 PM

@EmilioPisanty agreed. The user community is sufficiently diverse to sustain valuable users with different interests and style, including users with extremely informative but so popular answers. In fact for me this last category is the real virtue of this site.

@ACuriousMind your alma mater is in the news

which news bro

didnt know you were keeping track of german news

Ektara

Good night

I know y'all have missed me a lot. But it's late now...

I'm going to sleep...

4:15 PM

@RyanUnger haven't seen anything, what's it about?

4:26 PM

Hm, I just opened a job application form, and they are asking some strange and maybe even illegal questions.

What is your gender?
Are you currently undertaking, or considering, a gender reassignment process?
Are you married or in a civil partnership?
What is your ethnic origin?
What is your religion or belief?
What is your sexual orientation?

(The list of options for the ethnic origin question makes it even worse.)

wtf

On the other hand, they seem to be unable to ask about my date of birth or age, which would be the most normal thing in a job application form.

user image

Wildly inappropriate

Oh well

yeah

Does there exist an exterior derivative analog for functionals? We quite often write $\delta S$ when working with actions. It's fine if $S$ is just a function then $\mathrm dS$ is just the exterior derivative, but $S$ is a functional. Is this just physics notation?

It's usually called "varying" the functional

4:36 PM

You can treat it as a differential form on the tangent space to whatever infinite dimensional manifold your functional $S$ is on

@FadedGiant is there a "Prefer not to say" option on all the rest of the questions?

I actually have no idea how you formulate functionals geometrically

I know in field theories the state spaces have to be infinite dimensional (at least I've heard), but I haven't really gone much further into it

@user85795 yes

Yeah, it's busywork

1 hour later…

5:50 PM

@Charlie The mathematical buzzword there is "calculus of variations", geometrizing infinite-dimensional manifolds rigorous is actually pretty difficult/subtle since there is no one clear unique definition of what an "infinite-dimensional manifold" should be - similar to how the naive infinite-dimensional vector spaces aren't as nice as the finite-dimensional ones, and the ones you want if you want to do analogous things to the finite-dimensional case are Banach or Hilbert spaces

6:20 PM

ok ty, I will avoid opening that can of worms in that case

3 hours later…

9:14 PM

Consider equation 5.8.3 here for the Doppler effect.

phys.libretexts.org/Bookshelves/University_Physics/…

Then consider equation 6 here.

wias-berlin.de/workshops/nusod05/ThPD1.pdf

i.e. the fraction is expressed in terms of the refractive index $\frac{1-nv/c}{1+nv/c}$

Are these equations equivalent? How is the latter derived?

$n=c/v$ surely.

In the first link, it is $f=f_0\sqrt{\frac{1-v/c}{1+v/c}}$. In the second it is $f=f_0\frac{1-nv/c}{1+nv/c}$.

9:39 PM

@Charlie I think that every time I'vee seen it used, it could be easily fixed by replacing it with $\delta S/\delta \varphi$ and fixing the expression accordingly

@schn The first is the relativistic Doppler effect in vacuum, the second is the non-relativistic Doppler effect in a medium

they aren't equivalent, these are genuinely different phenomena

@ACuriousMind I see

How does the second one follow though if $n=c/v$; the fraction is just $0$ then?

@schn I'm not sure what you mean by $n = c/v$ - the $v$ there is the speed of the observer relative to the source, not some speed of some wave

this speed surely has nothing to do with the refractive index

You're right

@ACuriousMind How did you recognize it as the non-relativistic doppler effect?

because it doesn't have the square root

if you know both it's easy to remember that the relativistic one has a square root but the non-relativistic one doesn't

(of course I googled both before responding to make sure I didn't have them the wrong way around :P)

9:58 PM

I trust you. However, I'm a little suspicious about the first one since there is no reference in the paper, it is merely stated (maybe because it is very straighfroward...)

@schn the first one states as a section heading "The Relativistic Doppler Effect", what more do you want? :P

I'm tired :) I meant the latter

if you're interested in the derivations, just look at the respective Wikipedia pages for relativistic Doppler effect and Doppler effect

@schn the second one talks about Doppler in a (meta)material, so it's clearly not talking about the relativistic Doppler shift that is independent of a medium

Right

since it's a paper and the Doppler effect is a basic element of physics far from the edge of research, I'm not surprised it doesn't see a need to give a citation for it

10:03 PM

Then I will trust the paper :)

But for the purpose of learning

it's always nice to see a full derivation.

well. the paper might still be wrong in how it applies it or whatever, i'm not vouching for that as I haven't read it

10:20 PM

@ACuriousMind On Wikipedia (en.wikipedia.org/wiki/Doppler_effect), $c$ is not the speed of light but in the paper it is, so using $n=c/v_m$, where $v_m$ is the light speed in the medium, one ends up with the fraction $v/v_m$ in the numerator. And on Wikipedia, $c=v_m$ (it says "propagation speed of waves"...is there a difference between light and sound waves in this case?), so I get $\frac{1-v/v_m}{1+v/v_s}$ where $v_s$ is the speed of the source, and in the paper apparently $v_s=v$...

To summarize:

@schn I really don't know anything about the paper you're reading there - it looks like for some reason they're assuming that the source and the target move with the same velocity $v$ w.r.t. the medium?

Right. I get $v/v_m$ in the numerator and denominator when using $n=c/v$, where $v_m$ is the speed of light in the medium and $v$ is the speed of the source. So the fraction in equation 6 in the second paper becomes $\frac{1-v/v_m}{1+v/v_m}$. According to Wikipedia then, $v$ is both the velocity of the source and receiver.

it's 2 pages, how much insight do you expect it to convey - short papers like this are often heavily condensed because someone wanted them to fit into two pages

Yeah

100 page arXiv preprints might be a bit more annoying to read, but I think I've never really learned anything from these short papers - they're more like pointers: "Hey, here's this interesting thing, search for the buzzwords from this paper or longer stuff from the same authors to find what it's really about"

11:15 PM

@ACuriousMind What effects would an inverse Doppler shift have?

...since we were chatting on the topic.

Also, isn't red and blue shifts associated with the relativistic doppler effect and not the classical one?

@schn red/blue just means shifts towards teh respective end of the spectrum of wavelengths of light

it's usually relativistic since the Doppler effect for light is mostly relevant in astronomy where the light obviously travels through vacuum and at relativistically relevant distances, but there's nothing inherently relativistic about the terminology

I'm not sure what an "inverse Doppler shift" is supposed to be

Sorry, reverse Doppler shift, so that the frequency increases the further away the source moves...kind of unintuitive

11:30 PM

sure, but these metamaterials they're talking about are unintuitve to begin with

as they say, they don't exist in nature, so why would you expect to have intuition for them ;)

yeah, I guess this has not really been observed either except in a handful of experiments maybe

just because you can observe something that doesn't mean it's intuitive ;P

the Casimir effect is real, I would never call it intuitive

that's true

11:46 PM

@ACuriousMind not very important maybe, but I was deleting the extension .pdf and landed here (wias-berlin.de/workshops/nusod05). I also can't find the paper on arXiv, so what kind of paper is it do you think?

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