The h Bar

Sir Cumference

00:22

@Mostafa That's the...saddest name for a hero I've ever seen

That's like "She-woman"

@Blue Wait, are you...him?

Physics Meta

01:45

0

Q: The fate of a question decided by a Moderator?

I was wondering how could I improve the question here: non abelian string in QCD? It looks to me the question is not too broad, all sub-questions are related. But one of our moderators puts it on hold. Then I decided to remove the SUSY part. Also I am not saying any moderator can be a dictator....

Alfred Centauri

02:02

Just a thought out of the blue: Stimulated emission of down-votes.

Joshua Lin

https://www.youtube.com/watch?v=1IBR6mHtgqk

spreading the word of australians

Kristina L

02:18

Why is it possible to replace the radius in the area formula with the function you are rotating in a solid revolution (Disk method) problem?

Bernardo Meurer

@0celouvskyopoulo7 ^

Joshua Lin

You replace the radius with the function because the function is the radius of the disk of rotation

wow that sentence doesn't seem to have much meaning

1927 - That's When I Think Of You (1988)

►

0celouvskyopoulo7

@KristinaL It's called Cavalieri's principle.

The idea is that you're adding shells.

And integrals are like adding strips

So you just make strips that have the same area as the volume of the shell, and presto

Kristina L

02:36

Ah ok, thank you! I will be reading more into Cavalieri's principle, its making more sense after just skimming a paper on that.

@0celouvskyopoulo7

0celouvskyopoulo7

@KristinaL that's the general name for taking slices of something to compute its volume. Not sure if you'll find solids of rotation with it

But if you look it up on YouTube there are a billion videos

It's the hardest part of calc 2

Bernardo Meurer

Hardest part of Calc 2 is Stokes and Divergence theorems

That shit is evil

Also rotationals

Kristina L

I beg to differ

series kill me

Bernardo Meurer

That's Calc I

I hated that shit though

0celouvskyopoulo7

No, series are calc 2.

Stokes and divergence are calc 4, maybe calc 3

Bernardo Meurer

02:47

I did it all in 2 Analysis courses, lol

Next semester I'd have Complex analysis

BUT I'M JUMPING SHIP

Kristina L

That's Analysis 1 for @BernardoMeurer (AKA basically Calc 1 and 2, AKA hyper speed course)

You have a calc 4 @0celouvskyopoulo7? Dif eq?

Bernardo Meurer

Ah, right, it would be Complex Analysis & Differential Equations

0celouvskyopoulo7

@KristinaL there's a vector calculus course

How far you get in calc 3 depends on the TA, apparently

The honors course is like Bernardo's calc 2

Cryolune

Dunno what to say, this is awkward but at the same time I don't think anyone cares.

Hi xd

Kristina L

03:02

@BernardoMeurer HECK YEAH

@0celouvskyopoulo7 I've never heard of that course before, that's cool! We do a little bit of vector math in the beginning of calc 1, but I'm sure it's probably not the same thing.

@Cryolune Also, hello :) (But also goodnight because unfortunately I have to get some sleep).

Cryolune

@KristinaL night noight

@SirCumference Howdy.

Sir Cumference

@Cryolune Howdy :)

Cryolune

@SirCumference I've been working on digital drawing in photoshop for the last few weeks and I feel like I could probably design a decent banner this time for astronomy.se. Are you still interested in that?

:|

Sir Cumference

Sure :D

Cryolune

03:19

All the se sites have some kind of cool backdrop that's related to what the site is about but for some reason I can't think of any for astronomy.se except for stars. Do you have any ideas or should we just go with that like before (but with different star design and form)?

Avantgarde

03:35

Galaxy would be apt

probably_someone

Nebulae, perhaps, or galaxies, or planets, or... Seriously, astronomy is full of these things!

Avantgarde

A spiral galaxy

The thing is that they cannot add photographs (as far as I know). It must be some kind of an animated drawing.

Cryolune

thing is it can't look realistic so those things look kind of weird in icon form.

I did galaxy before.

Avantgarde

Maybe a planet then. Like Saturn

A circle with a ring around it

I wonder how many people know about the physics logo

Cryolune

what do you mean by know about it?

Avantgarde

03:48

I don't know if people know what the physics logo is

in general, I mean

Cryolune

05:02

lol I can't really sleep so I made a rough version of how the astronomy site could look like @SirCumference feedback? I had stars and a comet too but tbh it looked too crowded and unclean.

:(

damn I wish I could draw a nice night sky painting with a moon and comet and stars and everything but it just wouldn't fit the stackexchange clean professional look. I don't really like working with these kinds of designs.

p.s. @SirCumference I didn't draw/sketch that moon, I created it with my mouse. that's why it looks so edgy (which fits this theme).

Cryolune

05:34

refined it a bit @SirCumference also gonna sleep now bye

David Z

Theming the astronomy site seems like a popular pastime

user228700

05:50

@JohnR: Morning! :-)

John Rennie

Morning :-)

Though for some reason it feels more like the middle of the night. I feel only half awake. Oh well. I'll just have to drink more coffee than usual :-)

user228700

Isn't the sun up yet?

John Rennie

Yes, the sun rises about 4:30 a.m. at this time of year.

The sun is up, and it's already bright daylight (at 06:52) but I still feel very sleepy.

user228700

Wtf, what time is it there?

user228700

@JohnRennie Ah, I see...

user228700

05:53

Didn't sleep well last night?

John Rennie

I keep forgetting whether the time difference is 4.5 or 5.5 hours.

@Kaumudi.H no. When the weather first gets hot (hot for the UK that is) I find it hard to sleep. After a bit you get used to it.

user228700

Oh :-( I see...

John Rennie

It's Ok though. Being sleepy isn't all that bad :-)

I don't have a lot I have to do today.

user228700

Ah, OK. More coffee, then! :-)

John Rennie

The trouble is that I find lots of coffee makes me fidgety (and I need to keep going to the toilet) without actually making me feel much more awake. But then I like drinking coffee so that's OK :-)

I think the solution is just to have a very chilled day

user228700

05:59

Ah, right :-)

John Rennie

I was feeling so sleepy yesterday I actually watched a film, which is something I do only a few times a year.

Hellboy

Amusing but rubbish :-)

user228700

Wow, lol, I see :-P

John Rennie

Perhaps I'll watch the sequel today ...

user228700

:-P Why?

John Rennie

Normally I spend my spare time reading, but when I'm really tired it's hard to concentrate on a book. The advantage of films is that they don't require many brain cells to process.

user228700

06:03

@Blue ...I don't go crazy listening to his songs. I like some of them, that's all. Hell, I haven't even listened to more 10 songs of his!

user228700

@Avantgarde That's certainly quite strange.

user228700

@JohnRennie Ah, I see...

user228700

Gchat?

John Rennie

@Kaumudi.H on it now ...

Sid

07:13

@blue I didn't notice yesterday(Technically today), you seem to have become... bluer.

Mithrandir24601

@JohnDoe If it's not pure, there are still things you can do (which are still unitary), but it's more complicated, yeah...

John Rennie

07:35

@Kaumudi.H: by chance I've just noticed this:

@RexYuan Your programming skills should be extremely valuable in grad school! I am in cognitive neuroscience, a related field, and find that the computer science courses I've taken make my life a lot easier. — neuranna 16 hours ago

Someone worth talking to? It sounds as if they have done a very similar course to the one you are considering.

user228700

Ohhh, wow.

user228700

@JohnRennie YES, YES! I'll definitely look into it, thanks so much :-)

user228700

BTW:

user228700

?

John Rennie

07:37

@Kaumudi.H I'm done now (temporarily) though I don't know a vast amount about Netflix.

user228700

Ahh, OK. I was asking my dad if I could subscribe for precisely one month then cancel (right before the first free month gets over) but he is too suspicious of the whole arrangement... :-/

John Rennie

I'm pretty certain you can subscribe for a month then cancel. All the big companies have this sort of offer.

For example Amazon allow you to subscribe to Prime then cancel within a month for no charge.

user228700

Right, but he's suspicious only because I have got to give them my credit card/debit card details even before cancelling.

John Rennie

Actually I've done this several times to get free delivery on something :-)

user228700

@JohnRennie Haha, how were u able to do it multiple times?!

John Rennie

07:41

@Kaumudi.H that's normal. They won't debit the card until the first month is over, and if you cancel there will be no charge. Of course they are hoping that you'll like it so much you stay with them, or you just forget to cancel.

@Kaumudi.H I kept getting mails from Amazon saying would you like to try Prime for free even though I'd tried and cancelled several times already.

Which was fine by me :-)

Though when I look now I don't get the free trial option, so maybe it was a bug in their systems that they've now fixed.

user228700

@JohnRennie Right, but my dad doesn't trust that the details will be completely wiped from their server or whatever. How to convince him? :-(

user228700

@JohnRennie Ah, lol, I see...

John Rennie

@Kaumudi.H NetFlix are a big well known company and if they started abusing customers' credit cards they'd quickly get a bad name.

But you're going to struggle to persuade your dad because you have no evidence to offer him. And at the end of the day it is his credit card :-)

user228700

:'-( I'm bored!

user228700

...which is OK, really, but I do want to watch a few more movies.

John Rennie

07:46

I'm bored - the complaint of indolent youth throughout the ages :-)

Mithrandir24601

@Kaumudi.H Read something!

user228700

I'm not really bored.

Mithrandir24601

Read something anyway!

user228700

I want to watch a few more movies, that's all.

John Rennie

Learn GR - it's absolutely fascinating!

Mithrandir24601

07:47

Or that :)

user228700

@Mithrandir24601 I am. I finished The Ocean At The End of The Lane by Neil Gaiman yesterday and today I've started The Bell Jar by Sylvia Plath.

Mithrandir24601

('that' being 'learn GR')

user228700

Really, I dunno why I said "I'm bored" when I'm really not but my main complaint is that I want to watch a few movies.

John Rennie

Cycle to the beach

user228700

@JohnRennie I do that every morning! :-)

Mithrandir24601

07:48

@Kaumudi.H Neil Gaiman is brilliant :D Read some of his other stuff. Or Patrick Rothfus. That guy is a genius with words

John Rennie

@Mithrandir24601 she didn't like it - no taste that girl!

user228700

HEY, I liked Neverwhere! I loved it, in fact!

John Rennie

The trouble with Patrick Rothfuss is that Kvothe is a real twa .. err ... twit :-)

Frankly he deserves everything he gets.

user228700

The Bell Jar is promising...

John Rennie

Read American Gods. That's another great book.

user228700

07:50

@JohnRennie Don't have it :-/

user228700

I have Good Omens but eh, I don't want to read another novel by Gaiman just yet.

John Rennie

@Kaumudi.H that could be solved ...

user228700

:-) No, no, I think I'd better finish the few books I've bought first...

user228700

Thanks for offering though!

Mithrandir24601

@JohnRennie Have you ever read the Slow Regard of Silent Things?

John Rennie

07:51

@Mithrandir24601 yes, and I loved it :-)

Mostly because it wasn't about bloody Kvothe!!!

Mithrandir24601

@Kaumudi.H How can you not like anything by Gaiman? :o

@JohnRennie :D

It's the one (physical) book that I take everywhere with me. I have loads of others on Kindle, but the Slow Regard of Silent Things was unbelievable

user228700

@Mithrandir24601 I did like Neverwhere.

John Rennie

Rothfuss is a great writer, and I did like The Name of the Wind. But when I started The Wise Man's Fear I discovered that I'd just had enough of Kvothe. He's a typical petulant teenager and can be relied on to do the wrong thing.

Mithrandir24601

@Kaumudi.H Well, that's something :P

@JohnRennie That's... Part of the point, surely

John Rennie

Right, but I'm in my late fifties now, and petulant teenagers are just a pain these days :-)

user228700

07:55

@Mithrandir24601 And I've only read one more of his many books.

John Rennie

I meet enough of them in real life and I don't want to also read about them in my spare time :-)

Mithrandir24601

@JohnRennie I suppose you have then. Commiserations

@Kaumudi.H Read the Name of the Wind!

user228700

@JohnRennie Aren't u just sitting in ur house everyday? :-P You have ur niece and u have a bunch of people at The h Bar, which isn't really real life but still!

John Rennie

Oh no - my secret revealed. I have no life!

Amazingly I do have friends, and given our ages they tend to have teenage children :-)

Mithrandir24601

@Kaumudi.H Says the teenager sitting at home, bored, with nothing to do and importantly, chatting on hbar to said person :P

user228700

07:59

@JohnRennie :-P Ah, that makes sense :-)

John Rennie

Teenagers are rubbish really

Present company excepted of course

:: John crosses his fingers behind his back ::

user228700

@Mithrandir24601 Twice I clarified that I am not bored, I have tons to do and now I'm chatting here because I'm quite tired from already have done lots this morning (like standing in line for an hour to apply for my learner's permit for car).

user228700

@JohnRennie Bye -_-

Mithrandir24601

It's nice not being a teenager and so, not having to interact with other teenagers any more :)

John Rennie

> have done lots this morning (like standing in line for an hour to apply for my learner's permit for car)

Mithrandir24601

08:01

@Kaumudi.H It was 'exaggeration for comic effect' :P

John Rennie

see, only a teenager would define that as lots :-)

user228700

@JohnRennie :'-( I'm going.

John Rennie

Teenagers are also notorious for storming off in a sulk :-)

user228700

For the record, I spent ~9 hours roaming the streets of Chennai yesterday!

user228700

And that's certainly not nothing!

user228700

08:07

Have a look at this delicious dosa I had on my little adventure:

John Rennie

Ooh food!

user228700

John Rennie

I always fast on Mondays, so I'm a tad peckish right now.

Raita on the left?

user228700

I wanted to take a picture of the filter kaapi I had but it looked too good so I gulped it all down in one breath.

user228700

@JohnRennie No, coconut chutney.

John Rennie

08:09

Ah, I've never eaten coconut chutney.

Tamarind and chilli. That sounds as if it's going to be quite a sharp taste.

user228700

I also had two sandwiches: grilled mayo pasta sandwich and grilled chocolate brownie sandwich. Both were exquisite! I was quite hungry by lunch though, which is why I didn't pause to photograph them...

user228700

@JohnRennie It's actually not too sharp at all. Quite bland, I'd say...

Mithrandir24601

'grilled chocolate brownie sandwich'... That's a new one :)

user228700

I also went to Anna Centenary Library and read for a while.

user228700

I discovered that they've got a whole section devoted to cognitive science! :-o

user228700

08:12

...none of the books I found were for beginners though :-(

John Rennie

@Kaumudi.H my recollection of tasting tamarind pulp is that it's very astringent ...

user228700

I went to lots of places, actually! Ticking places off my "to-visit before I leave for college" list, essentially...

user228700

@JohnRennie Who adds Tamarind pulp to coconut chutney?!

John Rennie

Coconut chutney

Coconut chutney is a South Indian chutney-side-dish and condiment, a common in South Indian states. The condiment is made with coconut pulp ground with other ingredients such as tamarind and green chillies. It is served with Idli, Dosai and vadai. == See also == Kalathappam Kinnathappam Chammanthi podi == References... ==

user228700

Ah, that's only the tiniest little pinch!

user228700

08:14

@Mithrandir24601 It was great! :-)

Abhishekstudent

Here is the question

Then he took a strip by going in y direction

Now, why is he taking d as dy. d is the distance between two capacitors, but here there are no capacitors in y and y+dy. I am not understanding how d is coming out to be dy.

John Rennie

Ah OK. So the coconut chutney is kind of like a cream then?

user228700

@JohnRennie Little bit, yeah.

Abhishekstudent

This is the pic of the question

user228700

@Abhishekstudent Ah, that's the guy who shouts a lot, isn't it? :-P

Abhishekstudent

08:16

This is the pic of the solution

Here is the question
Here is my doubt
Now, why is he taking d as dy. d is the distance between two capacitors, but here there are no capacitors in y and y+dy. I am not understanding how d is coming out to be dy.

@Kaumudi.H no

he is the other one @Kaumudi.H

user228700

The one in the second picture, yeah?

Abhishekstudent

He shouts a little

can you help me out

looks like i am stuck for awhile

Kenshin

yo peeps

Abhishekstudent

peeps?

Kenshin

people

@Abhishekstudent I can explain your doubt

Abhishekstudent

08:28

go ahead

@kenshin

@Kenshin

Kenshin

im eating pizza

Abhishekstudent

ok

Kenshin

maybe dy means small y rather than d times y?

Abhishekstudent

i know that it means smally\

i mean small y

Kenshin

so d = dy

this means we are considering a slither

Abhishekstudent

08:32

why

Kenshin

does this resolve the doubt?

Abhishekstudent

there arent any capacitors

from y to y+dy

d= distance between plates of capacitors right

?

Kenshin

no capacitors

but there is capacitance

a small amount of it given by dC

Abhishekstudent

how could you consider a distance then

Kenshin

are you asking why capacitance depends on distance within the capicitor?

Abhishekstudent

08:37

yes

Kenshin

I don't know why

perhaps he is considering many very small capacitors all in series

with each capacitor having a length of dy

Abhishekstudent

maybe

Anonymous

10:08

@Sid Yeah, I'm metamorphosing ;)

Anonymous

@Abhishekstudent 'd' just stands for the thickness of each capacitor. At a distance 'y' from one plate you take an infinitesimal capacitor of thickness 'dy'. So the infinitesimal capacitance of that capacitor comes out to be $$dC=\dfrac{KA\epsilon_o}{dy}$$.

Anonymous

@Kaumudi.H Okay, okay :P

ACuriousMind

12:50

"By a simple scaling argument we find that the eigenvalues of these Fourier modes scale with $T^{-1}$" - if it's so simple, then why didn't you write it down? Argh.

Balarka Sen

13:12

Because it's easy. You can do it.

Left as an exercise bruh

Jaime Gallego

The margin was obviously not big enough

Cryolune

I always wake up at 7, brush my teeth and then go back to sleep until 8. idk why I do that. it's like there's an invisible force drawing me to my bed.

maybe there are really 5 forces of nature. xd

0celouvskyopoulo7

@BalarkaSen $R,S$, $r$, or $\mathrm{scal}$ for the scalar curvature?

John Jack

@ACuriousMind Hi

Secret

Incredible team skipping challenge - Guinness World Records

►

Felix.C

13:22

I have some trouble understanding BJT's in active mode. Suppose a NPN-BJT in active mode with $v_{CB} > 0$ so everything should be fine. Now I understand that the great currents we achieve despite having our collector-base diode reverse-biased is due to minority carriers.

Now collector breakdown voltages of BJT's are quite high, so we can assume that this is not the case. According to Sedra&Smith the condition for a diode being reverse-biased is that our diffusion current stays positive e.g. the current through the diode stays smaller than the saturation current, assuming there is no leakage. So I assume the minority carriers swapped into the collector are not considered diffusion current, so how can we find the diffusion current of a BJT in active mode?

Anonymous

@Cryolune 8 am or 8 pm ? :D

Secret

But also, notice how the skip envelop does not change, that will actually help

Balarka Sen

@0celouvskyopoulo7 scal but too much to write

0celouvskyopoulo7

@BalarkaSen I agree but $\mathrm{scal}g$ looks strange

I could add a space $\mathrm{scal}\,g$

but that looks like I mean scalar curvature of g

I want scalar curvature times g

Anonymous

@Felix.C I think your question is more suited for Electrical SE. BTW this might help you.

Anonymous

13:36

@Felix.C Oh, and iirc diffusion current stands for both electron diffusion current and hole diffusion current. Total current is a sum of both of them.

Anonymous

Diffusion current

Diffusion current is a current in a semiconductor caused by the diffusion of charge carriers (holes and/or electrons). The drift current, by contrast, is due to the motion of charge carriers due to the force exerted on them by an electric field. Diffusion current can be in the same or opposite direction of a drift current. The diffusion current and drift current together are described by the drift–diffusion equation. It is necessary to consider the part of diffusion current when describing many semiconductor devices. For example, the current near the depletion region of a p–n junction is dominated...

Cryolune

@Blue am

Secret

If I recall, this music is what first taught me what "towering evil" means

Tetrisphere - T Grey

►

ACuriousMind

@JohnJack Hi

Balarka Sen

@Avantgarde I listened to the Caravan song you reco'd me by the way.

Avantgarde

13:48

@BalarkaSen And did you like it?

I'm listening to the album right now

Balarka Sen

It's interesting, but not my taste :)

Too 70's, maybe?

Avantgarde

Ok. You like 60s?

Cryolune

What kind of music do you like @BalarkaSen?

John Jack

@ACuriousMind If you have time please have a look at this post, no one is responding with bounty on it...even if you just glance with peripherals at my proposed answer...

Avantgarde

He likes progressive dance pop

Balarka Sen

13:49

I ended up listening to the metal album of Monkey3, The Fifth Sun, in full. I like it a lot now.

I have a strange taste, @Cryolune. Avantgarde pretty much sums up a lot of it.

Avantgarde

Ah yes, Monkey3 is a pretty nice band. They play stoner rock

ACuriousMind

@JohnJack The reason why unitary/orthogonal matrices have orthonormal column vectors is literally just writing out what the matrix multiplication $ U^\dagger U$ resp. $O^T O$ does in terms of the column vectors.

All you need to do is observe that the $ij$-th entry of $U^\dagger U$ is the product of the $i$-th with the $j$-th column vector.

Felix.C

@Blue Ok thx

John Jack

@ACuriousMind Is my proposed answer correct in this particular case where I am considering sub-blocks?

ACuriousMind

I do not understand why you are talking about "sub-blocks" at all

Balarka Sen

13:53

@Avantgarde Yeah. I haven't actually looked a lot into psychedelic stoner stuff, but I should. Greenleaf is an interesting band, on that note.

John Jack

@ACuriousMind I'm not trying to prove the general result, I'm trying prove for that specific case.

ACuriousMind

As in, you could leave out any mention of the $P_{kl}$ entirely and the equation $[U^\dagger U]_{ij} = U^\ast_i U_j = \delta_{ij}$ would still be correct.

Avantgarde

@BalarkaSen Lots of good stoner rock. I haven't heard Greenleaf yet. Will look them up. I think you will like this given that you enjoyed Monkey3- youtu.be/DGPyHQ2FSUQ

ACuriousMind

@JohnJack But you don't need anything from that specific case! It doesn't make anything easier, in fact, it just confuses the issue. As I said, if you erased any mention of the $P$ and left the rest as it is, the proof would still be correct, so I do not understand what you actually want to know

John Jack

@ACuriousMind Just want to know if it's correct, you seem to be saying that it's correct but it contains needless info?

ACuriousMind

13:58

Yes.

The Dark Side

Anyone willing to talk some classical mechanics?

Avantgarde

Sure

Balarka Sen

@Avantgarde Preparing to blast it into the headphones. If you ever look at Greenleaf, look at Trails & Passes

that's the one album

John Jack

@ACuriousMind Okay thanks. The context of why I chose the specific case I didn't include, but I understand you can prove generally without referring to sub-blocks.

ACuriousMind

@TheDarkSide Just ask your question :P

Balarka Sen

13:59

BTW, where from India are you?

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