The h Bar - 2017-01-01 (page 1 of 2)

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12:27 AM

@heather Looks good. Maybe order it descending order in rep.

12:54 AM

@Sanya Here in the U.S. we're still living in the past

A year into the past, in fact

1:07 AM

I know this is a physics forum but interesting geography question (in response to Sanya saying the new year has already come). Trivia - which geographic islands/regions are West of the international date line but geographically East of the 180th meridian? (no cheating)

Bonus: what island chain/country has timezone UTC+14 (with international date line UTC+12)?

@user34445 Well, SE isn't a forum, it's a Q&A site...

Plus most of this chat is just math or random stuff :)

I know, but with a hat being given for starred chats plus/minus 6 hours from 1Jan2017 isn't it worth breaking the norms to be interesting?

@user34445 You just want the hat, huh? :P

and to be interesting ..

Ooooooh I got one :D

1:17 AM

;)

Someone must have found your comment worthy of a star ...

@DanielSank i have no idea

i'm jealous

1:35 AM

in theory salon, 10 mins ago, by vzn

code 2016 retrospective / Tmachine blog

2:00 AM

Help I'm drunk :(

3

2:22 AM

What time zone are you in? Either you're drunk too early, or you should be heading to bed ...

1 hour later…

3:40 AM

@Slereah You okay Slureah?

1 hour later…

5:00 AM

I hope Slureah is ok.

Happy New year from Eastern Standard

Pissed off layman

\o/

user228700

5:22 AM

Yello :-) Happy New Year!

Pissed off layman

user image

2

Happy new year from the U.S.! :D

user228700

@SirCumference :-) What time is it over there?

@Kaumudi.H 12:28am

You?

user228700

I see. It's 11 AM, 1 Jan, 2017 here.

5:45 AM

It's, ugh, 05:45 a.m. 1st Jan 2017 here!

Happy New Year all!

5:59 AM

Happy new year everybody !!!!!

user228700

@JohnRennie :-) Happy New Year!

Happy New Year Kaumudi :-)

And of course everyone else!

I had chole batore in my breakfast 😛

user228700

My first question of the year is boring. I'm attempting to derive the electric field intensity of a point at a perpendicular distance $r$ away from a line charge.

user228700

@SwapnilDas Yum!

user228700

6:07 AM

In trying to do so, there are some angles that I'm dealing with, whose "definitions" are not so clear to me...

An infinite line charge, or some finite length of line?

user228700

user image

user228700

For now, it's finite.

user228700

My textbook has defined $\theta_1$ and $\theta_2$ as:

user228700

> "The angle that the lines joining ends of the line charge distribution makes with the perpendicular line".

6:11 AM

OK that looks straightforward enough ...

user228700

Well, yes, I was able to derive the correct result. However, I faced a problem related to how these angles are defined when doing a subsequent problem.

@SwapnilDas Ooh, that looks good

user228700

user image

???

user228700

:-P Hang on...

user228700

6:15 AM

That ^ is a semi-infinite line charge.

user228700

And I've been asked to find the electric field intensity at point P. It seems straightforward and all, but what I don't understand is why $\theta_2$ defined the way it is. It looks like it is the explementary angle of the the one shown that should really be $\theta_2$. Am I making sense?

It does seem odd. Presumably the working will end up integrating from $\theta_2 = 2\pi$ to $\theta_2 = 1.5\pi$

user228700

Actually, no :-/ The formula is this:

user228700

$E_x = \{K\lambda/r\} (\sin \theta_1 + \sin \theta_2)$

user228700

$E_y = \{K\lambda/r\} (\cos \theta_2 - \cos \theta_1)$

6:22 AM

What is $\theta_1$? It looks as though it's just $\pi/2$.

user228700

Yes, in this case $\theta_1$ is just $\pi/2$.

user228700

But in the original diagram, they've given the value of the angle given by $2\pi -\theta_2$, which is 37 degrees.

user228700

So in the formula, they plugged in $\theta_1 = \pi/2$ and $\theta_2 = 2\pi-37$

user228700

But I don't understand why $\theta_2$ is $2\pi-37$ and not just $37$.

user228700

After all, isn't it defined as "The angle that the lines joining ends of the line charge distribution makes with the perpendicular line"?

user228700

6:28 AM

Ooh, also, in this diagram:

user228700

user image

user228700

While integrating, I had to use $-\theta_2$ and not just $\theta_2$.

user228700

I know what I did but dunno why I did it, which is worse than not knowing in the first place :-/

@Kaumudi.H Because one angle is clockwise and the other is anticlockwise?

user228700

Well, yeah, but...

user228700

6:30 AM

Okay, what about the other one..?

Are you obtaining a general expression for a point anywhere in space relative to the semi-infinite line?

user228700

No! (Sorry, I thought you were asking about the first question!)

OK, if the point is beyond the end of the charge, i.e. if the normal to the line is lower than the bottom end of the line, that makes the meaning of $\theta_2$ a bit tricky.

user228700

Yeah..?

@Kaumudi.H I'm now a bit mixed up about which question we're discussing ...

user228700

6:35 AM

We're discussing the second question, which is the one below the line thing...

And why $\theta_2$ is negative?

user228700

No, no, that was related to the first question.

user228700

I'm asking about the second one. This one:

user228700

user image

Ah

Ahhhhh

The point P is lower than the end of the line charge.

user228700

6:37 AM

:-P Sorry, I should've made it clear, what I was going on about...

user228700

Yeah...

So if you define $\theta_2$ as in the first question it doesn't work because, well, there is no charge below the normal

user228700

Okay, then will you please help me to understand how to define these angles correctly?

user228700

I don't really get that part of the integration...

Give me a mo and I'll draw a diagram ...

user228700

6:40 AM

Cool :-P

user image

user228700

Gimme one second...

In the first diagram our point $P$ is opposite a bit of the line charge so $\theta_2$ is well defined. We just put $\theta_1 = \pi/2$ into our formula and we get the right answer. Yes?

user228700

Yup.

In the middle diagram $\theta_2 = 0$, and that's OK we can still put that into our formula and get the right answer. Yes?

user228700

6:48 AM

Yeah...

user228700

Hang on.

user228700

What's the deal with defining this angle in the first place? Like, what's our objective?

user228700

(No, no, I know that we need to integrate all the charge elements...

user228700

And all of them exert different forces at that point depending on that angle.

user228700

Okay, never mind. Just go on and I think I'll understand...

6:51 AM

Well you started with the finite line charge and derived an equation. The semi infinite line should be described by the same equation in the limit of $\theta_1 \to \infty$.

So we're not doing the derivation again from scratch, just reusing the result from the first calculation. At least I assume that's the argument ...

user228700

Well yeah, I get that but $\theta_2$..?

OK, back to $\theta_2$ ...

The problem is how to define $\theta_2$ in the third diagram.

user228700

Yeah...

You could do this:

user image

And then in your equation replace $\theta_2$ by $-\theta_2$ ...

... because $\theta_2$ is now in the other direction.

user228700

But that's what we did the first time :-/

6:57 AM

@Kaumudi.H In the first question $theta_1$ is a clockwise angle and $\theta_2$ anticlockwise. Isn't that so?

user228700

Yeah...

In this question $\theta_1$ is a clockwise angle of $\pi/2$. The question is what you do with $\theta_2$.

user228700

Wait wait.

user228700

I'm super unclear about what the heck we're trying to do with these angles. Like, when we say "Integrate from this angle to this angle", that's sort of confusing, what with the clockwise and the anticlockwise and everything :-/ When we do that, are we like, accounting for all the charges below/above the angle or something? Some analogy like that would make it really simple to choose the angle...don't u think? Is there something like that..?

I haven't seen the working for the first question, but you don't need to use angles at all. It's just that in the integral you have ratios of distances that are naturally interpreted as the sine and cosine of an angle.

So it's convenient to express the final equation using the two angles.

user228700

7:02 AM

Right but how to choose the angles? >.<

Another diagram ...

user228700

Okay...

user image

user228700

Riight...

There are no angles in this diagram, and you could do the integration without using angles.

But $y/\sqrt(y^2 + x^2)$ is obviously $\sin\theta_1$

with the obvious definition of $\theta_1$.

user228700

7:09 AM

Yeah! ...

So it makes the algebra easier (or at least tidier) to replace $y/\sqrt(y^2 + x^2)$ by $\sin\theta_1$

I need to take a five minute break to make my mother her morning coffee. This is a vitally important mission!!!

user228700

Sure :-) I'll go eat some dark chocolate...

I'm back. Coffee safely made and Mum in a good mood for the day (I hope :-)

user228700

Oh, good :-) I'm back too, with some dark chocolate in my mouth. Mmm, soo good. Anyhoo, back to that angle?

Ooh, chocolate :-) 7 a.m. is probably a bit early for me to to eat chocolate though :-(

Anyway, back to the angle.

user228700

7:23 AM

Yes, probably :-( Yes, that angle...

We define $\theta_1$ as the clockwise angle between the normal and the line to the upper end of the charge because it makes the equation simpler. That's the only reason.

Likewise for $\theta_2$.

So I'm not sure it helps to go looking for deep physical meanings to those angles.

user228700

Oh, riight, okay...

And once we've got an equation for the field in terms of $\theta_1$ and $\theta_2$ that should work regardless of the values of those angles.

So we can let $\theta_1 \to \pi/2$ for the semi-infinite line charge and our equation should still work.

user228700

$\infty$ ?

Oops, $\pi/2$ I mean.

user228700

7:27 AM

Don't u mean $\pi/2$?

user228700

Right :-)

user228700

Alright, so that and then there's $\theta_2$.

But the nice diagram with thise angles only makes intuitive sense when the normal from our point P hits the line of charge.

user228700

OK..? Exactly what dyou mean by "intuitive sense" tho?

Even with the finite line charge, if we move our point $P$ below the lower end of the line we have to start wondering what $\theta_2$ means.

Shall I draw a diagram or do you get what I mean by that?

user228700

7:30 AM

What we mean by what..?

Diagram incoming ...

user228700

No, no, wait.

user228700

Don't bother urself with even more diagrams! What we mean by what?!

user image

This is the finite line.

user228700

Right, yes...

7:34 AM

$\theta_1$ is still well defined because it's the clockwise angle from the normal to the upper end of the charge.

user228700

Riight...

But what about $\theta_2$?

$\theta_2$ is the anticlockwise angle from the normal to the lower end of the charge. But, but, but, $P$ is already below the lower end of the charge.

user228700

Huh? Okay..?

So what do we do now? Any suggestions?

i.e. what value do we giveto $\theta_2$?

user228700

Before that, why did u say "But, but, but, P is already below the lower end of the charge"?

7:37 AM

Well $\theta_2$ is the anticlockwise angle from the normal to the lower end of the charge. Yes?

user228700

Yes.

And if the normal is below the lower end of the charge, as in my diagram above, how can we draw this angle on thediagram?

user228700

Gimme a minute...

user image

user228700

What? Am I being unthinkably dumb right now? :-P I'm sorry, gimme a minute, please...

7:43 AM

No, I just added the diagram to illustrate the dilemma. It wasn't meant to hurry you along :-)

user228700

Idk, I'd probably just give up on the angles altogether and make do without them. I can still do it...

@Kaumudi.H yes, the angles were just a convenient way to simplify the equation and we don't have to use them. But actually the angles still work fine. Remember that the definition of $\theta_2$ is:

the anticlockwise angle from the normal to the lower end of the charge

And this definition still works.

user228700

But we have to subtract this angle from $2\pi$?

user image

That's the anticlockwise angle from the normal to the lower end of the charge

user228700

:-/ Awesome. So I was just no reading it carefully enough?

7:50 AM

Well it's kind of unintuitive.

We start with the angles making intuitive sense. But actually our maths is cleverer than us and the equation still works when th angles don't make intuitive sense.

user228700

Yeah, wow, wtf.

user228700

Sigh. Alright, okay enough of this angle nonsense. I don't wanna look at angles for 10 minutes now :-P But thanks so much for ur patience :-D

That's OK :-)

Sadly I have to work now for half an hour or so. Yes, even of New Years Day. Sigh ...

user228700

:-( On New Years Day? On a Sunday?!

user228700

Have fun working, I guess :-)

user228700

7:57 AM

Yaay, thanks to whoever it was that starred my "Happy New Year!". I just got a hat because of it! :-D

Ah, me too :-)

user228700

(Y) :-)

8:28 AM

Right that's work finished for the day :-)

@Kaumudi.H @JohnRennie Np :D

Shame the hats are going back in the box soon tho

user228700

@SirCumference Oh, it was you? Thanks! :-D

user228700

Yes, it really is :-(

Well, the hats wouldn't be special if they lasted all year.

@JohnRennie Eh, I beg to differ

They're basically a better version of badges

A better idea, imo, would be to let people keep their hats forever, but only collect hats during the winter season

Would make it more meaningful

8:40 AM

I certainly find the hats really liven up the site over Christmas. If they were around all year I don't think there would be as much fun getting the hats.

@JohnRennie I think if they could keep the hats forever, but only collect hats during the winter season, then people would be much more into it

Just my thoughts tho

Pissed off layman

How about various scholarly hats to be awarded like badges?

@Pissedofflayman then how would they be different from badges?

user228700

Uh, they'd be more fun?

@JohnRennie Well, badges are completely useless (and some are even worth not having, like deleting a post with -3 score). Hats are neat and fun.

8:46 AM

Yes, hats are fun. But they're fun because they only exist for a short time. If they were around all year they'd just be routine.

user228700

No, hats are fun because they look really cool! Badges are just...there.

@JohnRennie Sadly now, lots of people don't care enough to collect them :/

I mean, getting something and having it taken away isn't endearing to a lot of people

Even if it's fun collecting

user228700

^That.

Well I guess it comes down to personal opinion.

Yep.

user228700

8:48 AM

Speaking for myself, my avatar looks so much cooler with all these neat hats! :-D

Pissed off layman

One Christmas we used to have a sleigh bring in our avatars into the chatroom...

user228700

Oh, wow, that sounds really cool!

@Pissedofflayman Really?

How would you know that, unless this isn't your first account...?

Pissed off layman

...now that was fun \o/

user228700

@SirCumference Exactly what I was typing x'D

8:53 AM

ಠ_ಠ

Pissed off layman

-_-

<_<

>_>

I'll do the 0celo test on you

If he says "prove it" to anything I say, he's 0celo :P

Pissed off layman

:P

Should I watch star wars, btw?

Nah

Pissed off layman

8:57 AM

nah, was of time imo

really?

I haven't watched it

People look at me like I'm crazy when I tell them I haven't seen it

Pissed off layman

Ignore them :P

The original Star Wars trilogy are enjoyable action films and they're pretty gritty by modern sanitised standards. If you watch them I suspect you would enjoy them. Should you watch them? How can we answer that? What criteria would you apply to the decision?

Pissed off layman

9:03 AM

Entertainment value?

Peer pressure...

9:16 AM

hey, can someone help me with this problem? postimg.org/image/r1n9n9qkn

@MohamedZiad A better thing to do is ask about what you don't conceptually understand

That'll benefit you in the long run

yea, sry
at which point should the portion AB produce moment? isn't the vertical component of its tension is the only effective force? and the others faces reactions at B?

user223506

9:36 AM

hello!

10:49 AM

it's deaþly quiet here.

Wh..what was that...p or b..?

It's cool though, saw it for the first time :\

11:25 AM

hello physics people. i'd appreciate if someone could comment on where I'm stuck : problem 4 chapter 1 part b) of solid state physics by ashcroft and mermin. he gives us an expression for E the electric field in a metal and he asks us to show that this field satisfies maxwell equations. but he doesn't say a word on H the magnetic field

so div E = 0 is "easy" to show but not the other eqs because they involve H

i could calculate the curl of E to get a constnat times the partial derivate of H with respect to time I suppose, then integrate with respect to time to find out H ?

but im not sure i could do that. because it seems i need both E and H to show that the maxwell equations are satisfied

and im not 100% sure whether knowing only E is enough

any comment is appreciated ^^

Thorn or þorn (Þ, þ) is a letter in the Old English, Gothic, Old Norse and modern Icelandic alphabets, as well as some dialects of Middle English. It was also used in medieval Scandinavia, but was later replaced with the digraph th, except in Iceland, where it survives. The letter originated from the rune ᚦ in the Elder Fuþark and was called thorn in the Anglo-Saxon and thorn or thurs (a category of beings in Germanic paganism) in the Scandinavian rune poems. Its reconstructed Proto-Germanic name is Thurisaz. It is similar in appearance to the archaic Greek letter sho (ϸ), although both are...

11:42 AM

Inconsequential fact of the day: In the early days of the printing press the kit used in England came from the continent and had no letter þ. So the English printers substituted the letter y for þ. That's why you see ye instead of the. It was never pronounced with a y, just typeset that way.

user228700

@JohnRennie Oh, I see!

g'day

anyone seen passengers?

user228700

Ello :-) @JohnR: Is "Inconsequential fact of the day" a regular thing?

@Kaumudi.H I suspect it would rapidly get boring. Bernardo and ACuriousMind used to do a tell me something interesting every now and then.

user228700

@Sir: Are u the one who keeps starring random messages so that everybody can get the hat?

user228700

11:48 AM

@JohnRennie If u decide to keep going, please know that at least one person is interested in this :-P (a.k.a me)

will i get a hat?

user228700

@Kenshin I think so :-)

yipee

Happy new year @Kenshin @JohnRennie @Kaumudi.H

user228700

Happy New Year, @kool :-)

11:52 AM

ty man u 2

@koolman do you know Doraemon in real life?

May 2 '16 at 17:51, by Bernard Meurer

@ACuriousMind Tell me something I don't know

@Kenshin no

@Kenshin why do you think so

what makes you think that I think so?

Morning

Don't know

11:55 AM

@Kaumudi.H Nope, just woke up :P

user228700

@Kenshin Dude x'D

Tho 2 hours of sleep isn't really the best...

user228700

@SirCumference Wokay, good morning :-P

user228700

@SirCumference Dude. Seriously. Are u never gonna do something about this?

@Kaumudi.H Well I was up for 3 days until now

I think I should see a doctor

user228700

11:56 AM

^ That.

Maybe all the stress of exams is getting to me

Hello, all

user228700

U have exams right now? During the holidays?! Dear God...

@Kaumudi.H Nah, but I haven't gotten mine back yet

Still worrying

user228700

Oh, okay :-/

11:58 AM

Oh they go to school in the summer holidays up there in the north.

weird mob.

user228700

Wokay, bye!

u know sleep is required for memory

@Kenshin Yeah, among other things

but telling myself to go to sleep, or else I'll suffer for it, only makes me unable to sleep...

It's like a positive feedback loop

u know blue light keeps the brain awake

and blue light is emitted from phones, tablets and computers

therefore one should stay away from these things if one desires sleep

@Kenshin yeah but I've tried everything man

reading only keeps me up, telling myself to go to sleep only makes me more stressed

12:02 PM

have you tried counting sheep

@Kenshin actually, no

then u haven't tried everything

but that would probably only remind me that i better get sleep, which would make me stay up worrying

no because that would distract u from the count

u should try and get to 100,000 sheep

@Kenshin Uh, that would take 3 days

I think

12:03 PM

count faster

can you count 5 sheep a second?

@Kenshin I guess

it should take under 6 hours then to count that many sheep

But that doesn't seem very relaxing...

not relaxing

but it's boring and u will fall asleep

just don't think about other things

just get to 100,000 sheep

user223506

greetings!

12:05 PM

gday doc

@Doc 'morning

user223506

how are we all?

surprisingly energetic

gud u

we're running on adrenaline here

user223506

not too bad - got 2 answers now

12:06 PM

well, i think it's too late to get that 2017 hat now

It's 1207 UTC

user223506

lol - i have the 2017 hat

user223506

i am wearing it!

so also me, twice over

user223506

awesome - a hat in a hat....inception hats

12:08 PM

user image

Still, we see what happens overnight, it's 2208 jere.

hey guys, what is stack.imgr?

stack.imgur

@Kenshin it's when you stack up a bunch of imgur photos

who runs it?

stack exchange or independent

i guess imgur

12:09 PM

ok

user223506

@wendy.krieger its 2308 here - on the 1st

interesting

@Kenshin i think prefixes to ".imgur" can be made by anyone

stack exchange probably uses theirs when you upload something in the chat or in an answer/question

Yes, you must be in mexico or something @doc

39

Q: Any details about stack.imgur.com?

Since May 2011, images uploaded through SE sites are uploaded to stack.imgur.com. It seems that removing that stack part does not get one (the same) image, so it seems to be a standalone installation? Any details on that new domain? And: any new way to circumvent blocks of imgur.com? And this ni...

discussion images imgur

12:11 PM

ty John

user223506

@wendy.krieger Victoria, Australia

user223506

i went a bit crazy here physics.stackexchange.com/a/301415/140434

that's what i said: i'm up in qld.

user223506

@wendy.krieger aaaaaah got you

south of the border sort of thing.

user223506

12:12 PM

@wendy.krieger i used to be in qld

My sister is doing time down there,

ie lives there

user223506

i am moving to W.A. in a couple of weeks

Out to the sandgropers.

user223506

yup!

user223506

was in Cairns

12:14 PM

best of luck. they have a water issue out in wa

cairns is the only corner of the state i haven't lived in

user223506

@wendy.krieger its a craphole

home of the canetoad, i read

user223506

yes, and the ultimate canetoad - my ex!

i don't have an ex, why? don't know.

user223506

am struggling a bit on the site

12:19 PM

wtf Cairns is a nice place

user223506

not the part i lived in

user223506

living and working in Cairns was not particularly enjoyable

why bro

It's north of townsville. Is that enough?

user223506

high crime, dirty place

12:34 PM

Townsville doesn't seem a terrible place. Unless of course the Townsville tourist agency wrote that article :-)

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