The h Bar

rob

00:04

Oh, so you're saying it's analogous to someone trying to count the number of puppies running around on a given road from your house on the road where you can measure it, but, given their spryness, the puppies will try to run into your home to greet you, which would cause an inaccurate number of puppies on the road to be counted if some have retreated into your house? So your door must be kept closed (high resistance) rather than open (low resistance) to make sure you can get the right number? That was a bizarre analogy, but what came to mind first. — sangstar Feb 18 at 17:28

Secret

05:54

-1

Q: Tough Vectors try if you can solve PLEASE ☺

[1] [see this and answer the question below ][1] [2][!]tough question I dont know the answer and want a helping hand[2] Please somebody help me with this vector . It's really tough. It's better if you give me by this Thursday

vectors

I do not respect help vampires

invadingdingo

I'm about to pull my hair out with this homework lol

rob

@Secret I can appreciate that. But I think that linking to someone else's "we're not a homework help site" comment is probably not productive for communicating with a new user (especially one who has missed or ignored the warnings about asking homework questions). Probably better either to copy it out yourself or just leave it for someone else to do.

@invadingdingo I've got one up on you: I'm leaning back in my desk chair, and my kitten keeps leaping up and getting tangled in my braid. But perhaps we can help you.

invadingdingo

lol!

Help would be extremely appreciated haha

So I'm taking physics 2 now, and I get the concepts for the most part, but my teacher does not work examples, which, I think are pretty necessary in applied mathematics and physics, but oh well

Anyway, he has a habit of assigning homework with concepts that we vaguely touch on during lecture in great quantities.

rob

@invadingdingo Sounds good so far.

invadingdingo

So this problem has to do with a uniform charge across a plastic disk. I need to find the point on the central axis where the charge from the electric field is 1/6 of what it is at z=0.

At what distance along the central axis of a uniformly charged plastic disk of radius R = 0.577 m is the magnitude of the electric field equal to 1/6 times the magnitude of the field at the center of the surface of the disk?

That's the actual question ^

The worst part is that I only get 5 attempts at the problem, which makes me extremely worried to try out different methods for answers. As of now I only have 2/5 of those attempts remaining

rob

06:10

Well, how do you find the field at the surface of the disk?

invadingdingo

E = (sigma/(2 * epsilon sub naught) ) (1 - z/(sqrt(z^2 + R^2)) )

but since we know z = 0, then it's just sigma/2(epsilon sub naught) * 1

rob

@invadingdingo This is the general expression for the field on-axis from a uniform disk? It looks familiar.

invadingdingo

It's the magnitude of the electric field on the central axis for a uniformly charged disk.

rob

Okay. It's got a constant term σ/2ε, and a term that depends on z and R which is 1 when z=0?

invadingdingo

Right, but nothing really needs to be done with σ/2ε because when you set your Esub1 = to 1/6Esub2, the σ/2ε's cancel

So you're just left with (1 - (z/sqrt(z^2 + R^2)) = 1/6

rob

06:21

@invadingdingo I'm with you so far.

invadingdingo

Which seems like it'd be solvable for z and get the value, but when I solve for Z it's wrong

rob

@invadingdingo Okay, then, let's go through that algebra.

Sir Cumference

@Secret But he said PLEASE

invadingdingo

What I did first was set it up like -z/sqrt(z^2 + R^2) = 1/6 - 1

Sir Cumference

He emphasized politeness

invadingdingo

06:24

z/sqrt(z^2 + R^2) = 5/6 (removed negative from both sides)

z = 5/6 (sqrt(z^2 + R^2))

Now, square both sides

z^2 = 25/36 (z^2 + R^2)

z^2 - 25/36z^2 = 25/36R^2

z^2 (1 - 25/36) = 25/36R^2

z^2 = 25/36R^2 // 11/36

z = sqrt(25/36R^2 // 11/36)

and since R is .577m

z = 0.86986m

Does that look right?

rob

@invadingdingo I get the same.

invadingdingo

Alright, gonna try that out

rob

I keep the R out of the square root to make sure the units are the same on both sides, z = R * sqrt(25/11), but it's the same result. Different than yours from before?

invadingdingo

So that was right!

And looking at my previous work, I think I may have some how messed up the algebra, which is rare for me, haha

It shouldn't matter if the R is in the root or not, should it?

rob

@invadingdingo No, but I've learned to check at every step if my units are right. I catch errors much earlier that way.

invadingdingo

06:35

Ahh, I guess that is good practice

I'm going to attempt the next problem, so I'll probably be back shortly, haha!

Thanks for your help :)

rob

@invadingdingo A pleasure.

Avnish Kabaj

06:50

oh boy

its my first time using chat on pc

sooooo much better than mobile

John Rennie

07:05

@AvnishKabaj you've just got a laptop?

Secret

08:06

@SirCumference Too many of them abuse this word (see math chat examples) that the word becomes a bit pointless, though I agree it is better than those really rude ones

@rob good point. Will keep that in mind

Captain Bohemian

@AvnishKabaj wow!I just tried to use mobile phone to surf internet 1 or 2 month ago and was just successful the night before last night. That's the first time I used mobile phone to surf internet. I found it's less easy to type on mobile phone, but maybe it's because I haven't been used to typing there.

Slereah

09:10

rense.com//rodinaerodynamics.htm

Emilio Pisanty

09:21

@Chair some feedback on this edit

element names are never capitalized

i.e. you abbreviate helium as He with capital h but the noun itself is helium, not Helium.

ditto with unit names: you abbreviate N, A and W, but in text they're newton, ampere, watt.

if I remember correctly the SI brochure has the details

but it might also be in the NIST guide to the SI

Emilio Pisanty

12:13

@JohnRennie aha! caught you =P

data.stackexchange.com/physics/query/893922/…

John Rennie

@EmilioPisanty what's that query?

Emilio Pisanty

data.stackexchange.com/physics/query/893914/…

^ this one in particular

tag edits introducing a given tag into questions

particularly looking for extended re-tagging campaigns in the run-up to the award of a tag badge

I mean

not that I would do such a thing

John Rennie

To check if I'm deliberately tagging questions to boost my rep for that tag? :-)

Emilio Pisanty

well

John Rennie

I have the idea that doesn't work (or didn't work).

Emilio Pisanty

12:17

I was curious to see whether it's a common practice or not

@JohnRennie oh, no, it definitely works

John Rennie

I'm sure I asked about it and your tag score is based on the original tags not any retagging.

Emilio Pisanty

@JohnRennie you're probably confusing this with the dupehammer tag-edits requirements

John Rennie

Actually I didn't think I did much retagging apart from adding the homework tag

Emilio Pisanty

(namely, you can't edit in one of your gold-badge tags and then dupe-hammer the question.)

John Rennie

Yes, 500 homework tag edits and 30 GR tag edits :-)

Emilio Pisanty

12:20

heh

fair

John Rennie

Having said that, I do have a gold tag badge for the homework tag - possibly that is not something to boast about :-)

Emilio Pisanty

@JohnRennie yeah, I wouldn't boast about that =P

Secret

Last night dream will require the following background reading:

Scientists have created edible water

►

.
we are preparing to eat some food, which are contained in edible seaweed pods. This is also the first time we tried out these edible containers. After placing one big round one on the table, and breaking it open with the teeth, a blue trout like fish covered with ice was revealed. I then asked mum whether we are supposed to eat it raw. Later on, dad brought in a few more crates of pods. These much smaller pods contains water surrounded by an ice crystal, while other contained pure water. Sister and I then consumed some of these. I then found that the seaweed container had a plastic bitter…

....

So basically, last night dream expanded the concept so that you can put solid food like a whole trout inside one of these things
as well thicker membrane to make it more resilent to shipping (at the cost of harder to chew)
Rarely do dreams solves real world problems. Looks like the exposure to politics makes my problem solving machinary in the dreams more grounded so it actually came up with something that can be realised

Emilio Pisanty

(just sayin'.)

Secret

lol

thenakedscientists.com/articles/questions/…

also, because you are literally on drugs when dreaming

So I must be permanently on drugs then :P

user1732

12:28

@EmilioPisanty but they answer with a guarded "yes" at the beginning?

Avnish Kabaj

@JohnRennie nope

I just don't use it

That much

Smartphones are more accessible

@CaptainBohemian o.O

Novalium Company

12:43

Hehmm... How exactly the spinning of the electrons in an iron atom will produce the magnetic domain?

Captain Bohemian

@AvnishKabaj For me, computer is more accessible. There are free computers in library or computer center for everyone to use, so even far before I had money to buy a computer, I used computer commonly. There is no free smartphone put anywhere for everyone to use, so you must have money to buy one to use it.

Novalium Company

I mean, wood for example also has electrons and I suppose some of them spin, why don't magnetic domains form? Maybe because in a wood atom (lol, no such thing but you get the idea) there are more electrons which cancel out and in the iron atom there are more than half of them which align and that's why a magnetic domain forms?

The spinning of one electron will cause one magnetic domain in the specific direction, right?

Secret

REALITY DOESN'T EXIST HERE... | Museum of Simulation Technology

►

user351417

@EmilioPisanty Oops sorry I was negligent... I don't often check the conventions and grammar when they don't blatantly harm readability (I didn't capitalize that stuff though: that was what the OP had written). But you're right, I should start correcting everything I see, rather than just the blatantly noisy/illegible stuff.

Novalium Company

Also how do we know in which direction does the moving/spinning/jiggling electron create the magnetic field/domain?

Also the magnetic domain is basically a small magnetic field created by the electrons in a very small area of a ferromagnetic material (e.g iron..)?

Novalium Company

13:24

Also how can an electron create a magnetic field when it's just one electron, how there can be a dipole because we know the magnetic field requires north and south side, how can a single electron do that?

Novalium Company

13:40

Also the magnetic field around a wire doesn't look like a dipole to me at all?

I mean, I don't see the north nor south pole in the ilustration.

Please forget about the last one, I got it.

Semiclassical

14:43

@NovaliumCompany note that, even in a ferromagnet, you don't necessarily have magnetic domains. For instance, if you heat up a ferromagnetic to a sufficient temperature (the Curie temperature) then none of the electron spins will line up locally

as a consequence, you won't have magnetic domains

Emilio Pisanty

MAGNETS: How Do They Work?

►

Semiclassical

if you then cool the ferromagnet below the curie temperature, though, then the local interactions between the electron spins will be more significant than their random thermal motion

as such, you'll have those spins locally aligning to get magnetic domains.

the lower the temperature, the larger those domains will be

(To make this quantitative, the Curie temperature of iron is at 1043 K. That's well above room temperature, but still short of the melting point at 1800 K.)

Emilio Pisanty

@ACuriousMind I'll race you to the rep cap

no wait, too late

Semiclassical

lol

Emilio Pisanty

I'm finally at the halfway point, though

so

only 75 rep-cap days to go!

how hard can it be?

ACuriousMind

14:55

@EmilioPisanty :P

John Rennie

@EmilioPisanty you're only semi-legendary? :-)

Emilio Pisanty

@JohnRennie yeah, I know

Semiclassical

I tend to find it easier to write up answers on MSE than Physics SE if I'm honest

Emilio Pisanty

I'm semi-disappointed by that fact

Semiclassical

probably b/c MSE questions tend to be more about the actual computations than on the concepts

(Not that there aren't concepts in math or computations in physics, but the emphasis is different)

vzn

15:08

re dreams there are some other major (nonscientific but deep/ crosscultural) "theories/ explanations" about their nature en.wikipedia.org/wiki/Dreamtime en.wikipedia.org/wiki/Ayahuasca#Effects

Semiclassical

I mostly find myself just jealous of how creative my dreams are.

Secret

I think politics is actually not all bad for me, it makes my dreams more likely to come up real life solutions (c.f. the edible water bottle stuff above)

vzn

15:31

re dreams ran across this other remarkable connection/ history/ account in a old book from my to-read pile by Talbot, other major books on subj, although it would seem few physicists are aware of it (maybe like Newton + his alchemy/ metaphysics?): Pauli-Jung brainpickings.org/2017/03/09/atom-and-archetype-pauli-jung which also reminds me of en.wikipedia.org/wiki/Not_even_wrong

John Rennie

@ACuriousMind thanks, I was going to suggest the comments to that primordial fluctuations answer were removed. The OP has become annoyingly persistent.

ACuriousMind

@JohnRennie No problem.

John Rennie

Now there's an unusual problem with a PhD

ACuriousMind

@JohnRennie I saw that question and am still amazed at the kind of situations people turn to the internet for help with.

Abhas Kumar Sinha

lamo

vzn

15:39

lol +1 sounds like a headline on dailymail :P ... ps internet anonymity is sometimes a feature, sometimes a bug...

Semiclassical

daaaaang

Abhas Kumar Sinha

lamo XD, read the last answer

Semiclassical

"tell her to p*** off"

Abhas Kumar Sinha

Lamo funniest part

Semiclassical

that whole Q&A is so full of wtf

Abhas Kumar Sinha

15:42

Anyone here completed SS Krotov's First Exercise?

Need help in solving the mechanics

vzn

the situation reminds me of how corporations/ universities have (increasingly detailed) codes of conduct to try to regulate "outside worker relationships" eg requiring "disclosure to HR"... some similarity...

John Rennie

@AbhasKumarSinha doesn't Krotov include solutions?

Abhas Kumar Sinha

@JohnRennie My publications is not the original version of the Original Version, it only has questions and answers

Futuresciencetist

Why you guys abducted me?

Abhas Kumar Sinha

@Futuresciencetist What?

John Rennie

15:53

@AbhasKumarSinha which question are you trying to do?

Futuresciencetist

@AbhasKumarSinha not to you

To the mods

John Rennie

@Futuresciencetist Are you asking why your question was closed?

ACuriousMind

@Futuresciencetist The moderator message to your earlier account was specific enough in its reasons for your suspension. Creating another account during your suspension and using it on our site is not acceptable, and will lead to even longer suspensions if you persist. I will now delete this account, do not create another.

Abhas Kumar Sinha

@ACuriousMind My account was suspended? I don't think so. was it?

ACuriousMind

@AbhasKumarSinha Sorry, I replied to the wrong message :P

Futuresciencetist

15:57

@JohnRennie I am also the one who was suspended

For no reason

Emilio Pisanty

@ACuriousMind what happened here? i.e. what does "Deleted by Community" mean? User was removed and negative-score questions went with it?

Semiclassical

I have a sudden urge to make popcorn

ACuriousMind

@EmilioPisanty Yes, user deletion deletes all negatively-scored posts w/o answers automatically.

Abhas Kumar Sinha

@JohnRennie A string passing over a light frictionless pulley carries at its end 2 variable unequal masses where sum of the masses (m) is constant. If breaking section of the string is 15/32 of the weight of the sum of masses (a) find it's the least acceleration (b) find the least value of greater mass

John Rennie

What number question is that?

Abhas Kumar Sinha

16:00

@JohnRennie Not sure about question number, was given as Homework by the teacher

Emilio Pisanty

hmmmmm

our current roster of suspended users seems to be rather light on users whose suspension runs out before 2024

Abhas Kumar Sinha

@JohnRennie Also need hints for Krotov's question number - 1.58

ACuriousMind

@EmilioPisanty How exactly are you viewing the roster of suspended users? I don't think I've seen a "suspended" tab on the "users" page :P

Emilio Pisanty

Are one-week, one-month and one-year suspensions less common now, statistically speaking, than they were, say, during the first half of this site's existence?

@ACuriousMind data.stackexchange.com/physics/query/632466/suspended-users

not perfect

but a reasonable try

ACuriousMind

I can tell you that there's a bunch of currently suspended users not on that list.

Emilio Pisanty

16:04

yeah, figures

can I ask what kind of pathways produce that kind of situation?

users with deleted posts primarily?

(CC @KyleKanos)

ACuriousMind

@EmilioPisanty There's probably various factors, off the top of my head 1-week suspensions might be to short to reliably end up in the data.SE cache, LQ-suspensions often concern users who didn't get more than 1 rep to begin with, and it's relatively common that users try to get around a suspension by deleting and recreating their account - which doesn't work, but the newly created user has none of the previous posts associated to it.

John Rennie

@AbhasKumarSinha I can't find that question in Krotov ...

Abhas Kumar Sinha

@JohnRennie which one? first or second?

@JohnRennie leave the first question for now, can you try question number - 1.58

John Rennie

The first one - the masses on a pulley.

@AbhasKumarSinha is that the rod suspended by two threads question?

Abhas Kumar Sinha

Okay, I see, I'll try to find the first one myself, but the second one is from Krotov, 1.58

John Rennie

16:10

That one?

Abhas Kumar Sinha

yes, exactly

that one

1.58. A horizontal weightless rod of length
3/ is suspended on two vertical strings. Two
loads of mass m1 and m2 are in equilibrium
at equal distances from each other and from
the ends of the strings (Fig. 29

John Rennie

I'm afraid it's too late in the day for me to want to do JEE questions. Catch me tomorrow from around 05:00 UK time. It's probably best to ask in the Problem Solving Room so you don't annoy people here.

Abhas Kumar Sinha

Okay, no problem

ACuriousMind

@JohnRennie 5am is a time when you want to answer JEE questions?

John Rennie

@ACuriousMind :-)

ACuriousMind

16:14

The cases in which I'm awake at 5am are usually not ones in which I'm able to answer any question coherently :P

John Rennie

I start work at 5 a.m. and assuming no servers have died overnight I don't have a lot to do for a couple of hours.

Abhas Kumar Sinha

when you go to sleep?

John Rennie

So it's actually quite a good time to bug me for answers to JEE questions.

@AbhasKumarSinha about 9 p.m. So I get the regulation 8 hours sleep.

ACuriousMind

Heh, Wine's output is occasionally amusing: "The GLSL shader has been disabled. You get to keep all the pieces if it breaks"

Abhas Kumar Sinha

That's actually great, i've a bad habit of sleeping at 2 am and getting to school for 6

Emilio Pisanty

16:17

@ACuriousMind ok, thx for the breakdown.

ACuriousMind

::punts::

Emilio Pisanty

::Barbara Streisand::

Avantgarde

16:29

@JohnRennie Have you been solving JEE questions for a while now? Might as well take the exam itself.

danielunderwood

16:42

@JohnRennie are you a server babysitter during the day?

Yashas

@Chair why do they have their own SE site?! I want one for my college >.< (joking)

Anonymous

17:20

It's not even a college course. More like a high school intro to CS :3

Sir Cumference

You know what would be cool? I writing prompts SE site

Yeah it'd be a ripoff of r/WritingPrompts but it could work

Anonymous

17:37

Stack Exchange teaming up with EdX and Coursera for the CS courses, would be good idea, if they're planning to take that path

Anonymous

24

A: What is CS50 and why can't I access it?

We're not really ready to announce this quite yet, but you might surmise from the title that this is a site "For Students of HarvardX CS50x: Introduction to Computer Science." The Area 51 link is just following a template that, in this case, doesn't apply.

Anonymous

BTW the CS50 site looks like an "Experiment" on their part

Anonymous

The previous beta site based on a similar theme was apparently closed due to lack of activity

danielunderwood

18:00

That would be interesting. I've always wanted to participate in the edx/coursera course forums, but it always seems a bit overwhelming. So many conversations going on and usually a handful of people who have no idea what they're talking about

Emilio Pisanty

19:57

@JohnRennie @ACuriousMind meanwhile, this is also on the HNQ list, which never ceases to amaze me

4

Q: Vector space objects in schemes - confusion

Let $R$ be the ring $\mathbf{C}\times\mathbf{C}$, and consider the affine line $\mathbf{A}^1_R$. $\mathbf{A}^1_R$ can be given the structure of additive group scheme over $R$, denoted $(\mathbf{G}_a)_R$. $(\mathbf{G}_a)_R$ carries a functorial multiplicative action of $R$ making it into an $R$...

ag.algebraic-geometry ac.commutative-algebra algebraic-groups arithmetic-geometry etale-cohomology

I imagine that math gobbledygook gets way more gobbledygooker than that, but that's already way past the point where I fall off the train

Hiya @KyleKanos

Kyle Kanos

@EmilioPisanty hah, I looked at just the title of that query and said to myself, "I think I made something like this"

Emilio Pisanty

@KyleKanos ;-)

Kyle Kanos

Hi.

I ran out of close votes but there's like 4 or 5 more posts I see that need a vote... There should be some sort of a roll-over for close votes

Emilio Pisanty

@KyleKanos I have close votes

(just saying)

Kyle Kanos

I'm on mobile & the app swapping to copy & paste is too much effort. I'll just come back in 4 hours and vote then

Emilio Pisanty

20:03

¯\ _(ツ)_/¯

in more serious news though

Kyle Kanos

You got a job!?

Married!?

Fatherhood!?

Emilio Pisanty

@BernardoMeurer I'm extremely sorry to hear about this. I imagine there's a large number of people upset by it.

9

National Museum of Brazil consumed by fire

absolute *******ing tragedy.

Kyle Kanos

That is very sad

Emilio Pisanty

I don't think you can even begin to quantify just how bad it is.

gateprep

20:18

@EmilioPisanty

Pls help me I am reposting thr question once again

I am having problems in Legendre transformation in thermodynamicws
There is a question where du=Tds-pdv and a Legendre transformation giving a function f(T,v) is asked to be found out.
here d(TS) is assumed and the whole differential is written and finally d(TS-u) is found to give something on the right hand side and TS-u is assumed to be a function.As mucha s I know Legendre transformation is used to go from one function to other only because of the fact that we do not have the same independent variable with us that we require..In this context I do not get the intuition behind d(TS) appear…

Emilio Pisanty

@gateprep sorry, I don't do blatant do-my-homework-for-me questions.

gateprep

Not really I tried it my self

I seem completely lost

I am even telling u the first step where I am getting stuck

I just need help

Emilio Pisanty

For clarity, comments like this are not OK. Posting in this chatroom is OK, but explicitly pinging other users to get them to respond to your questions isn't particularly appropriate.

@gateprep I'm not interested. Hopefully someone else will be.

gateprep

:) Allright

ACuriousMind

@gateprep You have been repeatedly told - at various times, in various chatrooms - to stop pinging people with your demands for help. Since neither that nor a brief suspension seem to have changed your behaviour, here is a longer one. If you want to participate in our chats you need to learn to listen when people tell you that your behaviour is inappropriate.

3

Bernardo Meurer

20:33

@EmilioPisanty Thank you for your thoughts :(

It really is a tragedy

I used to go there all the time when I was a kid

Emilio Pisanty

21:09

@BernardoMeurer =/

Bernardo Meurer

21:25

Yeah, just really sad

vzn

21:38

Gravitational-constant mystery deepens with new precision measurements / physicsworld

Avantgarde

The website says "Two extremely precise measurements of the gravitational constant G have yielded significantly different values .."

While the Nature article's abstract says "These values have the smallest uncertainties reported until now .."

vzn

22:02

@Avantgarde it all has to be placed in context. its not a simple area compared to some other physical constants. the physicsworld article cites over 200 experiments. a key question is whether there is agreement within reported experimental error and that does not seem to be the case. a graph of results/ errors across experiments would help to understand this. undergraduate physics majors would encounter/ be somewhat familiar with similar concepts in their laboratory exercises.

rob

22:16

@Avantgarde Both of those statements are correct. The Nature paper is two measurements by the same group using different techniques. The uncertainty on each measurement is about 11 ppm, and both measurements agree with the most recent CODATA evaluation (which has uncertainty 50 ppm), but the two measurements disagree with each other.

This is Figure 3 from the paper: historical measurements in chronological order. The two new measurements are at the bottom in blue, and the most recent CODATA just above them. It looks like there were measurements in 2000 and 2006 with comparable uncertainties and values, but the situation is clearly a big mess.

Avantgarde

23:01

Yeah haha, I realized what I wrote the second after I hit enter :P

Secret

23:42

That cruel fate of nature. I was planning to roast gateprep for their question behaviour. Well it seems they are improving, hopefully this ban will help them to get better at not getting help at a demanding way

Transcript for