The h Bar

12:02 AM

@rob, let me try my best to explain the purpose of the physics problem solving site. Currently, physics.SE is for conceptual questions only. The name of the new site is to solve more practical problems, some of which would normally be closed as homework here. The examples on the proposal page are some of what we'd have in mind. However, I might point out that while we would allow check your work type questions and other question types that are closed on physics.SE, we would expect effort.

@JohnRennie, @S007, I was thinking about the proposal, and I had some ideas. We already are allowing "more practical" type problems - what if we also allowed questions along the lines of experiment design and running? Not experimental physics, per se, but an extension of the problem solving to experimental physics.

DanielSank

@heather I am pretty sure that this new site is either going to fill up with terrible questions or will just wind up with the kind of "homework" questions we actually want here.

@heather Experimental design questions are perfectly welcome on this site.

I'm still waiting to see an explicit example of a complete question which would be ok on the new site but not ok on this site.

Anonymous

@DanielSank You know, if Brilliant.org can excel as a problem solving site, if Math SE can excel as a problem solving site, so would ours. BTW did you edit that question ?

DanielSank

I think the architects of the new site would do well to provide such an example.

Anonymous

Anyway it is useless to think of a new Physics site now, the mods just wont allow

DanielSank

@S007 Wait, what?

The mods are opposed?

Anonymous

12:08 AM

Yeah, I had mailed Robert , the mod on Area 51

Anonymous

He explained that SE has stopped making two sites on same subject from now on

Andras Deak

actually, Robert is the Community Growth Manager or something, he is the official stance

> the Director of Community Development for the Stack Exchange Network.

Anonymous

Like Computer Science has atleast 3 sites...yeah community manager

Anonymous

He is an employee there

DanielSank

@S007 That is very interesting. In fact, this is a great piece of data for our discussion about fixing up the homework policy in this site.

heather

12:10 AM

Yeah, but I kind of disagree with his position. I mean, I respect his decision, but I really don't think that the reason for his closing makes sense.

Anonymous

Well I asked him about that via mail...but that is not his desicion

Andras Deak

Sure, that doesn't automatically make him right. I'm just saying that it's not just "a mod", but the very person who (leads the people who) determine(s) which sites should go

Anonymous

but the team's decision

heather

@DanielSank, really? The ones on-topic on our site would be closed here because they are a. check my work or b. are "homework" (because even though they show work, they don't ask a conceptual question). So I think they'd be good questions, just not good here.

DanielSank

If a problem solving site is explicitly disallowed by SE, then we might have to make some changes here.

heather

12:11 AM

@AndrasDeak, no, I know. That's why I'd like to persuade him that it is a good idea.

DanielSank

@heather The policies are ours to mold. Nothing is written in stone.

Andras Deak

good luck:)

Anonymous

@DanielSank Not only problem solving, any new site related somehow to an existing site (even by name) will be closed from now on...so we got do make some adjustments right here in Physics SE

heather

@DanielSank, that will never happen. That's what I mean. People here are vehemently against there being homework-y questions on this site, and I don't blame them, and I appreciate their opinion. I think both sites would be the better for it if there was a separate "homework" site.

DanielSank

@heather No, they're not.

If that were true we wouldn't be having all these discussions in meta.

Anonymous

12:13 AM

@DanielSank Did you edit the question which I showed you earlier in the day? I really want to see if it can be reframed

DanielSank

@S007 I started to, and then I noticed that I think there's an error in the original post so I left a comment.

Please take a look and confirm/disprove my suspicion.

heather

@DanielSank, let me rephrase: there are enough people that are vehemently against it that it will never happen. Yes, there is discussion, but the current policy is against it, and the replacement discussion has been going on for years. On top of that, I believe that (most of) the current discussion is about clarifying the current policy, not changing it drastically.

Anonymous

@DanielSank Can you give the link again ?

DanielSank

@heather If you look at my (pretty well upvoted) recommendation about how to reword it, you'll see that the suggested changes explicitly do not mention "homework" at all.

@S007 Dude, it was your link!

I can go find it, maybe.

5

Q: Mutual $E$ force due to charged coaxial rings

I found the following question in a good physics book I was solving and although this is a computer problem, I wonder if it can be done without using computers. $Q.$ Find the force of attraction between $2$ coaxial rings of radius $R$ having charge $+Q$ and $-Q$. Assume that the common axis ...

heather

@DanielSank, no, but the same principles lie behind it. Questions there would be allowed on the proposed site.

DanielSank

12:16 AM

@heather If you can give me a single example of a good question for the new site which is not allowed on PSE, I would be most obliged.

And I don't mean the ones submitted on Area 51. I mean a complete explicit question.

heather

@DanielSank, okay, one moment...

DanielSank

Well, I'm going for now, but I will read your example later.

ciao

heather

@DanielSank, okay, found one

0

Q: Kinematic, free fall question. Newton Laws

A pebble is projected vertically upwards with speed 21ms^-1 from a point 32m above the ground. Find: a. the speed with which the pebble strikes the ground, b. the total time for which the pebble is more than 40m above the ground.

homework-and-exercises newtonian-mechanics classical-mechanics kinematics

if the OP showed work

that would be on-topic on the proposal site, off-topic here (with the work)

I'll find another better example

Anonymous

@DanielSank See this physics.stackexchange.com/questions/286508/…

Anonymous

An example of a closed question that should be allowed

heather

12:19 AM

0

Q: A train is moving up an incline of inclination $\pi/6$ with constant velocity. A bob is hung on the ceiling. The angle made by the bob with normal?

Mass of bob is 1 kilogram. Let the angle of Bob with the normal be $\phi$ the force acting on bob along the axis of motion is $$mg\cos(\phi)$$ Now as this train is having zero acceleration, bob must move with zero acceleration so the force acting on bob due to gravity ie $$mg\sin(\pi/6)$$ Ther...

homework-and-exercises classical-mechanics forces

@DanielSank, the above is a good one, I believe it shows sufficient work

Andras Deak

whoah what a one-box

heather

@AndrasDeak, ?

0celo7

@BernardMeurer get rekt someone complimented my shoes

Andras Deak

due to mathjax, that one-box of the latest question is huge

Anonymous

@heather No that question is ok for PSE...see the one I linked

Andras Deak

12:20 AM

@0celo7 met a mathematician?

Anonymous

that hasn't been closed @heather

0celo7

Mathematicians don't know what shoes are

Anonymous

1

Q: Question on time of collision and relative velocities (Answer verification)

I was doing this question which I found quiet alright, however after checking the answer, I wasn't very sure if my method was alright. The question is: A ship P has position $-11i+j$ km relative to a light house $O$. At the same time the position of a boat Q relative to the light house was $4i-1...

homework-and-exercises collision relative-motion

heather

@S007, yes, I agree with your example

Anonymous

This doesn't deserve to be closed

Anonymous

12:21 AM

@DanielSank

heather

Or how about this one:

0

Q: Tension question

A mountain climber, in the process of crossing between two cliffs by a rope, pauses to rest. She weighs 581 N. As the drawing shows, she is closer to the left cliff than to the right cliff, with the result that the tensions in the left and right sides of the rope are not the same. Find the tensio...

homework-and-exercises newtonian-mechanics

0celo7

Tension questionn

Anonymous

@heather yes good one

Andras Deak

Minor note: if you do set up a homework physics site, you'll probably need to watch for abuse pitfalls like on maths

Anonymous

@heather but that was a math error

heather

12:22 AM

@AndrasDeak, yes, that is a good point, thank you for bringing it up

@S007, so? Check my work questions would be on topic

Anonymous

trivial math errors are not allowed

heather

@S007, then where do you draw the line between trivial and significant? I think it is on topic.

Anonymous

check my work is allowed but trivial math errors will be closed

Anonymous

Any calculation error in maths

Anonymous

is trivial

Andras Deak

12:23 AM

On Stack Overflow debugging questions with proper effort and information are on topic, but there's a close reason for typos.

Typos, much like calculation errors, are not evident to OP a priori

heather

@AndrasDeak, hmm, yeah. @S007, I don't know, I guess that is something for debate. I would still be pro-check my work questions, though.

Andras Deak

My point is that these questions shouldn't affect the scope. They could be on topic, even if a posteriori they get closed as a brain-fart

Anonymous

@AndrasDeak Calculation errors will be pointed out maybe in comments and then closed

Andras Deak

yes, just like typos

but OP can make proper effort and not realize their mistake

heather

yes, there we go, I'm for that.

oh, how about this one:

0

Q: Prove that $\gamma^\mu \gamma_\mu = 4 $

I want to prove $\gamma^\mu \gamma_\mu = 4I $, but I get $\gamma^\mu \gamma_\mu = -4 I$, so I am making some mistake. Can someone provide a hint so that I learn what I am doing wrong? $$ \gamma^\mu \gamma_\mu = \frac{1}{2} \left( \gamma^\mu \gamma_\mu + \gamma^\mu \gamma_\mu \right) = \frac{1}{2}...

homework-and-exercises special-relativity metric-tensor conventions dirac-matrices

Anonymous

12:25 AM

@AndrasDeak that's why we have comments

Andras Deak

yes, what is your point?:D

I thought you were talking about scope

heather

or how about

-1

Q: Sphere rolling up frictionless slope

I'm working out a simple Newtonian mechanics problem. If we have a sphere of mass $m$ and radius $r$ rolling without slipping on a horizontal surface, then rolls up a ramp with velocity $v$, I'm trying to find the height that the ball will roll. So I'm just solving for conservation of energy. The...

homework-and-exercises newtonian-mechanics

(might end up being closed as trivial math mistake, but the sort of question that would be okay)

Anonymous

@AndrasDeak No. I am talking about questions having calculation errors. We cannot allow such questions. We should close them and point out the error in comments maybe

Anonymous

@heather yes. your examples are great

heather

@S007, we can't just close every question with calculation error. I think some of them should be okay...I think extremely minor type errors should be closed and then larger calculation errors are on-topic.

here's another one:

0

Q: A relative motion question

A plane wishes to reach a destination that is 700km[S15°W] of its starting position. The plane can travel at 400km/h in still air. There is a 40km/h wind blowing from [W47°S]. What direction should the plane fly in order to reach its destination? I know the vector equation should be something li...

homework-and-exercises relative-motion

that would be interesting: opening the site and then migrating some of these old questions over because they'd be on-topic.

Andras Deak

12:28 AM

@S007 Yes. But that doesn't make them off-topic a priori. If OP can't realize with reasonable work that they simply miscalculated, they should ask. Afterwards, it should get hastily closed. It's on topic, but it should not stay on the site. At least this is my notion of a posteriori off-topic questions.

heather

@S007, one other thought. I'm starting to wonder if we should have a chat room dedicated to this proposal.

Andras Deak

wait, I'll quote the "typo/brain fart" close reason for you:)

> This question was caused by a problem that can no longer be reproduced or a simple typographical error. While similar questions may be on-topic here, this one was resolved in a manner unlikely to help future readers. This can often be avoided by identifying and closely inspecting the shortest program necessary to reproduce the problem before posting.

Anonymous

@heather No. Think like this. We will just point out that there is a mathematical calculation error in the comments. and then the OP can use Math SE to rectify his errors. See, already the community gets pissed off my calculation errors. All we can do at the time being is negotiate...let's start with allowing check my work (excluding calculation errors)

Anonymous

The community isn't going to relax its rules too much....we have to negotiate between idealism and reality

Anonymous

and lets begin with allowing check-my-work (provided effort is shown)

heather

12:31 AM

@S007...but most check my work problems end up being calculation errors...I think significant calculation errors should be allowed, and smaller ones should be closed. That's all I'm saying. Whatever it ends up being I'm okay with, but right now we are debating what is on-topic and what is not, and that is simply my opinion.

Though I do see your point.

I guess this is also something to think about; these discussions are very instructive.

Anonymous

@heather No. Not at all. Did you see all your examples ? Only 2 of them were calculation errors!

heather

@S007, hmm...okay, let's go with that for now. But we can always debate this further later. But for now we will go with your view.

Anonymous

@heather See. This isn't calculation error

0celo7

@heather Compute $\sup\{q\in\Bbb Q\mid q^2<2\}$.

Anonymous

-1

Q: Sphere rolling up frictionless slope

I'm working out a simple Newtonian mechanics problem. If we have a sphere of mass $m$ and radius $r$ rolling without slipping on a horizontal surface, then rolls up a ramp with velocity $v$, I'm trying to find the height that the ball will roll. So I'm just solving for conservation of energy. The...

homework-and-exercises newtonian-mechanics

Anonymous

12:32 AM

@heather

Anonymous

Because the OP has made a conceptual error during the calculation

Anonymous

Not a mathematical error

Anonymous

or algebra error

Anonymous

Let's call mathematical error as arithmetic error/algebra error

Anonymous

that might be more helpful in differentiating

Andras Deak

12:34 AM

Shouldn't scope/on-topicness be something that can be determined by the asker?

Anonymous

@AndrasDeak new users usually don't know the scope

heather

@S007, let's just call it mathematical error...

Andras Deak

@S007 entirely irrelevant now

heather

@0celo7, wait, I'm confused...didn't we define this for sets that are subsets of $\mathbb{R}$?

Anonymous

@heather Well, if you can explain to the community what a mathematical error means and what it doesn't

heather

12:35 AM

@S007, or 'trivial calculation error'

that's better, actually, I think

Andras Deak

@heather you did:P

heather

@AndrasDeak, for a second I had no idea what you were talking about =P

Anonymous

@heather That's good. But we need to explain the community that what it means and what it doesn't. Maybe a new meta thread should do it.

Andras Deak

@heather directed replies are helpful, you know;)

heather

@S007, a hypothetical meta thread? The site's proposal is closed!

Anonymous

12:37 AM

@heather No here only on PSE

heather

@AndrasDeak, yeah, I figured it out when I scrolled my mouse over your comment =)

@S007, I don't think that'd be on-topic on physics meta...or, wait, what exactly are you suggesting? I thought this was about our proposal...

Andras Deak

@S007 promise I'll leave you alone in a moment, just one more round of explaining what I mean:)

Anonymous

Forget about our proposal...that's not gonna be allowed ever (according to the Stack Exchange Managers) @heather

heather

@AndrasDeak, actually, I personally find your comments very clarifying.

Anonymous

@heather

heather

12:39 AM

@S007, well, um. actually. I think we could persuade Robert. Especially if we generated enough support.

Andras Deak

scenario 1:
Asker: question about chemistry
Community: closing as off-topic, this is not physics
Asker: duh, I'm stupid

scenario 2:
Asker: why is physics broken?
Community: closing as off-topic, you made a trivial error on line 1
Asker: well how the hell should I have known?

heather

@AndrasDeak, huh...well, how do you handle that sort of thing on stack overflow?

(scenario 2, I mean)

Andras Deak

17 mins ago, by Andras Deak

On Stack Overflow debugging questions with proper effort and information are on topic, but there's a close reason for typos.

11 mins ago, by Andras Deak

> This question was caused by a problem that can no longer be reproduced or a simple typographical error. While similar questions may be on-topic here, this one was resolved in a manner unlikely to help future readers. This can often be avoided by identifying and closely inspecting the shortest program necessary to reproduce the problem before posting.

emphasis on "unlikely to help future readers"

it doesn't only apply to typos, but also trivial mistakes (often branded brain-farts)

So these questions get hammered and hopefully roombad (many people, myself included, exert peer-pressure to keep these trivial questions unanswered)

my point is only that these can't be deemed off-topic, even though they can and should be answered in a comment and closed

heather

@AndrasDeak, well that is what I would say. I would do the following close reason:

**Trivial Calculation Error**
This question was caused by a problem that can no longer be reproduced or a simple mathematical error. While similar questions may be on-topic here, this one was resolved in a manner unlikely to help future readers.

^why is the bold not showing up?

Andras Deak

Yes, exactly. But the scope doesn't exclude trivial miscalculation problems!

this is my only point

@heather multiline messages don't do formatting

heather

12:43 AM

@AndrasDeak, right, yeah.

Andras Deak

you could abuse mathjax here, I guess

heather

(::shock::) abuse mathjax!?

I'll just leave it.

@AndrasDeak, but yes, thank you. That is an important point I should've realized.

Andras Deak

no worries

Anonymous

Personally I feel very bad that such a good question was closed :-( physics.stackexchange.com/questions/289170/…

Anonymous

0

Q: Prove that $\gamma^\mu \gamma_\mu = 4 $

I want to prove $\gamma^\mu \gamma_\mu = 4I $, but I get $\gamma^\mu \gamma_\mu = -4 I$, so I am making some mistake. Can someone provide a hint so that I learn what I am doing wrong? $$ \gamma^\mu \gamma_\mu = \frac{1}{2} \left( \gamma^\mu \gamma_\mu + \gamma^\mu \gamma_\mu \right) = \frac{1}{2}...

homework-and-exercises special-relativity metric-tensor conventions dirac-matrices

Andras Deak

12:48 AM

Can't you vote to reopen? Or a mod hammer stays?

Anonymous

@AndrasDeak Once closed it will never open in most cases

Anonymous

I've never seen a closed question being opened

Anonymous

on PSE

Andras Deak

ah right, low-traffic site

dmckee

@AndrasDeak Sure you can vote to re-open. There are some moderator action users can't reverse and some it would be unwise to reverse, but for the most part that's a way of disputing the mod interpretation.

That said, I think the closure is in keeping with the current policy. But I have a question to ask of the assembled throng:

Andras Deak

12:50 AM

@dmckee thanks, I sometimes get confused. Mod deletions are binding, I know, so I often become unsure about closures:)

dmckee

Assuming this represents an erratum in the text (or that a similar question that represents an erratum in a major text comes up) would the question warrant an exception to the usual rule?

heather

@dmckee, I am a tad unclear on the context here. are you discussing the question that S007 just linked? Also, it isn't a textbook erratum...I thought that was confirmed in the comments. So I guess I'm not sure what you are asking, either.

Anonymous

If someone is unable to understand a text, because the text has a major error then it should be allowed.

Andras Deak

I think the question is of a general conceptual nature

Anonymous

-1

Q: Two spheres arranged in a square, find force exerted on one of them

Four particles of the same mass $8.5kg$ are placed in a square configuration with side length $0.8m$. Find the magnitude and direction of the force exerted on one of the particles. I chose the top left particle, 1, to find the force of. 1 2 3 4 Attempt at Solution $F_{12} = \frac{G(2(8...

homework-and-exercises

Anonymous

12:57 AM

Does this deserve being closed ?

dmckee

@heather The question linked above, yes. On the face of it, that is a "check my work" problem and out not only in the current policy but I believe also be the evolving new policy. But if the confusion arises from an error in a common text, then the issue is likely to come up more than once and perhaps an exception should be made.

Anonymous

@dmckee Well we can close the second occurence as a duplicate in that case

Andras Deak

@dmckee I can easily see something like that edited to become a canonical reference of the issue

heather

@dmckee, hmm, an interesting perspective. I'd agree with AndrasDeak here.

Anonymous

Me too.

Andras Deak

1:00 AM

preferebly into something that contains "error in <source>" or the like for google

dmckee

I'm leaning that way, too. Out of simple expedience.

@AndrasDeak Nice. Make it part of the title because Google's search is context sensitive enough to understand that titles are important.

Andras Deak

I also suspect that the community will nicely price the relevance and severity of the error based on the importance of the source, and if it's marginal, then just close it down;)

@dmckee I would think so, yes

although "error in <source>" is unlikely to be searched by askers, that's usually not what you expect if you're doing life right:)

but anyway, it should only survive in such an informative form

note that I don't have any activity on physics.SE, I'm relying on my experience on another site

I should get a disclaimer to hang in my neck here:)

Anonymous

@heather Can we try this here

Anonymous

24

Q: Let's not close questions as homework except for blatantly obvious cases (at least for a while)

Overview There has been some concerns lately about whether we close too much. So we start giving leniency a chance. Scroll down to What we do to get to the fun part. Read the rest if you're interested. Why we do It's all about answers. Whatever we do here, whether it is commenting, asking, an...

discussion homework vote-to-close experiment

Anonymous

Can we have a one month experiment?

Anonymous

1:04 AM

@AndrasDeak

Anonymous

@dmckee

Andras Deak

hm??

1 min ago, by Andras Deak

note that I don't have any activity on physics.SE, I'm relying on my experience on another site

:D

Anonymous

doesn't matter

dmckee

I'd be against it, but I've always been a hardcase on that matter, so don't take my opinion as representative.

heather

@S007, hmm...I like it. But. I don't know if others do. Like dmckee, for instance, I think would be representative of others on the site.

Andras Deak

1:06 AM

No, it matters. This is exactly a matter of site internal affairs.

dmckee

I'm old enough to have been active on usenet in the pre-web days (back when gopher was a hot new technology), and I saw what can happen to help resources without some kind of barrier.

Andras Deak

oh, my personal opinion of homeworks is "let it burn" :D

I don't mind askers who made a genuine effort and got stuck and need pointers, but these all pave the way for help vampires.

Anonymous

1:26 AM

6

A: Homework Questions # Treatment

Physics.SE is not a homework help site, it is not meant to be a resource to teach people physics, and it is not a resource intended to help right conceptual misconceptions in students. When it was originally created, this site was meant to be a place where active physicists doing active research ...

Anonymous

"not a resource intended to help right conceptual misconceptions in students. "

Anonymous

^ can this thing be changed somehow? And should this notion be changed ?

Anonymous

@dmckee

Anonymous

and this "
Many homework-like questions and, in particular, check my work questions can and very well may be helpful to students. We don't care. Our goal is not to teach students or ensure that they learn; it is to help physicists. Check-my-work questions do not help physicists, they only waste the time of the answerer. The question may be able to be reformed such that it is asking about the concepts of a problem, and that could help a physicist in teaching a class or brushing up on their own understanding. Even if the mistake is a conceptual error, the question is still only specific to st…

Anonymous

I strongly oppose this in the current scenario ^

Andras Deak

1:29 AM

familiar stance:)

Anonymous

I think that we are just being too strict....

dmckee

@S007 If you don't like an answer you should vote it down. You can also offer your own alternative visions. I doubt that new answers on that question will attract much attention, however.

But we aren't going to nuke Jim's post (he is as entitled to his opinion as you are to yours) and can't change the acceptance status.

Anonymous

I am not talking about liking or not. I was talking about...can we bring a change in the current notion about what PSE is about? I think we need some change

dmckee

@S007 You can try. Historically the active user base has been consistently opposed to a notion that we are here to help with homework or primarily as a resource for learning physics.

Now, the truth is that we answer a lot of fairly basic question and do serve as a significant learning platform for the kinds of students who are either sufficiently autodidactic or far enough into their formal education to ask questions more or less like a physicist, but that was not the original purpose.

heather

@dmckee, see, I just think that a new site would be better; but the proposal has been closed.

If only I could somehow persuade Robert Cantaino...

Bernard Meurer

1:37 AM

@dmckee I will have to hold back on getting that M68000 :/

dmckee

@heather I personally doubt that you would get the kind of expert user base for a homework help site that you want, but Mathematics has exactly that so I could be wrong.

Andras Deak

good night

dmckee

@BernardMeurer Sorry to hear that. There are emulators (e.g. easy68k.com) if you just want to fool around.

heather

@dmckee, I honestly think there would be enough people to make it work.

@AndrasDeak, good night, and thank you for your help!

Bernard Meurer

@dmckee I need to pay for my applications for transferring schools :p

@dmckee I might start fooling around with an emu, good idea :)

Anonymous

1:39 AM

@AndrasDeak good night. it's morning at my place :-P

dmckee

Ouch.

Andras Deak

@S007 half past 2 AM here, but only because we're back to standard time;)

Anonymous

@AndrasDeak okay...good night now !

Bernard Meurer

Yeah, it will be a while until I can buy things again hahahaha

heather

@dmckee, do you think there's any chance of the proposal getting reopened?

Bernard Meurer

1:48 AM

@dmckee I'm almost finished writing my blog post on compiling the Linux kernel for calculators, would you mind giving me feedback on it once I'm done?

dmckee

@heather I'm simply not active enough on area51 to have a way to guess. But I have to say that Robert's post looks like he's reporting a policy that has been discussed in some detail. Such policies have been reversed in the past but only after the accumulation of experience and some intensively data-filled discussions.

@BernardMeurer I don't really know anything about the kernel, but I can read it for style and such.

Anonymous

@heather maybe it can be opened...just see the amount of resistance astro got initially, but later it was allowed

Anonymous

61

Q: Why wouldn't this just be part of the existing physics Stack Exchange site?

Related: http://meta.physics.stackexchange.com/questions/4223 We tried an astronomy Stack Exchange site, and it ended up being merged with Physics after the beta ended unsuccessfully due to lack of traffic. What makes us think this proposal wouldn't have a similar fate? What has changed sinc...

science

Bernard Meurer

@dmckee I don't talk much about the kernel, in a next post I will a little more, in particular about how to make a configuration file for compiling it. I want more of a feedback on writing and formatting and those things :)

John Rennie

5:15 AM

@heather @S007 I fear we aren't going to win this one. What Robert Cartaino says makes sense for the SE. You have to remember that the SE are paying for this and they hope to recover the cost through advertising. They want a few big sites not lots of small ones.

From their perspective the homework issue is just some internal squabble for the physics community to deal with and not their problem. They see the proposal as more work (and therefore expense) for them and a dilution of potential advertising revenue.

user228700

5:45 AM

@JohnRennie: Morning :-)

John Rennie

Hi Kaumudi

Physics on a Sunday? :-)

Anonymous

@JohnRennie Maybe you are right. But can you suggest an alternative solution instead of creation of a new site ?

user228700

Yes :-) Modern physics so that's a relief.

Anonymous

(I respect Robert's decision)

John Rennie

@S007 Basically we're back at square one. There is no way we're going to allow the exercise sort of question here, and to be honest I support that policy.

Anonymous

5:53 AM

@JohnRennie Well, I am just going LOL now....:-D....maybe just leave it up to Time

user228700

@JohnRennie: I come with a basket of questions! (:-P) Do u have any time?

John Rennie

Yes, it's a Sunday morning and none of my servers died overnight. So I'm just drinking coffee and chilling :-)

user228700

Congratulations on the continued existence of ur servers! (:-P)

John Rennie

To be fair we don't get many server deaths. Just as well really or people would be asking why I wasn't doing my job properly :-)

user228700

Hm, yes :-)

user228700

5:58 AM

OK, my first question is about intensity of light, when speaking of the photoelectric effect.

John Rennie

OK, PE effect is easy even at 6 a.m. on a Sunday

user228700

It's defined as the total energy of the wave divided by the number of photons carrying that energy, yeah?

user228700

@JohnRennie I googled it and my my, u have answered a lot of questions about it!

John Rennie

Well that's the number of photons per second, if that's what you meant

user228700

Hm, yes, okay.

user228700

6:01 AM

Okay, hang on for just one second as I check something to make sure my question is not entirely stupid...

user228700

The formula $E=h\nu$ gives the energy of one photon in the wave, yeah?

John Rennie

Yes

user228700

So intensity...what is it a measure of?

John Rennie

Intensity is energy per second per unit area. i.e. Take 1 square metre of illuminated surface and the energy falling on that square metre in one second is the intensity.

user228700

But what about that first definition?

user228700

6:07 AM

7 mins ago, by Kaumudi

It's defined as the total energy of the wave divided by the number of photons carrying that energy, yeah?

user228700

@JohnRennie You speak of the luminous intensity, yes? Is there no difference? How do the number of electrons factor in, then?

John Rennie

On average every photon ejects $x$ electrons, where $x$ is called the quantum yield. usually the quantum yield for the photoelectric effect is very low. You get one electron for every $10^5$ to $10^6$ photons i.e. $x=10^{-5}$ to $10^{-6}$

user228700

Um, OK..?

John Rennie

You seem doubtful?

Anonymous

@Kaumudi If I am not interfering, the definition "It's defined as the total energy of the wave divided by the number of photons carrying that energy" is wrong for intensity...that is the energy of one photon in the wave

Anonymous

6:14 AM

(no of photons)*(energy of one photon)=energy of wave

John Rennie

@S007 Oops, yes, @Kaumudi I've just realised I misread your original question. That may have caused some confusion!

user228700

Alright, this makes sense but what then, is the relationship b/w the number of photons emitted and the intensity of the wave?

John Rennie

no. photons emitted per second times energy per photon = energy per second = intensity

All per unit area

DanielSank

Hi, everybody.

user228700

Ah, this ^ @JohnRennie: U corrected my orginial wording by using the phrase per second (as opposed to the total number of photons constituting the wave) so what I said before, that was correct, with that bit of correction, yeah?

user228700

6:19 AM

@DanielSank: Hello :-) How's it going?

DanielSank

@Kaumudi it is going well.

Tomorrow I will prepare strawberry shortcake with a new friend.

So I have something to look forward to.

user228700

@DanielSank Sounds delightful ^.^

DanielSank

Indeed.

John Rennie

You asked:

21 mins ago, by Kaumudi

It's defined as the total energy of the wave divided by the number of photons carrying that energy, yeah?

user228700

Yes, but u corrected it:

John Rennie

6:21 AM

That gives you the energy per photon.

user228700

20 mins ago, by John Rennie

Well that's the number of photons per second, if that's what you meant

John Rennie

^ that was wrong, I misread your question

user228700

But how? That's basically what u've written here:

user228700

4 mins ago, by John Rennie

no. photons emitted per second times energy per photon = energy per second = intensity

John Rennie

To be honest I'm not sure what you are asking. Maybe we need to back up a step ...

user228700

6:22 AM

OK, one second.

John Rennie

Meatballs in Morrocan sauce!

Since food was mentioned ... :-)

user228700

@DanielSank: Ooh, that reminds me! My mother prepared about 30 Gulab Jamuns two days ago. Half have disappeared. I will post a picture after we are done discussing the PE effect.

John Rennie

Oooh I love Gulab Jamuns

user228700

:-D Me too.

John Rennie

But they're such a pain to make.

user228700

6:26 AM

Not at all! I will tell u how after we are done with physics!

John Rennie

My mother used to make them starting with dried milk powder rather than taking fresh milk and boiling it down.

user228700

See, my mom doesn't do all that ^

John Rennie

Yes?

user228700

:-P

John Rennie

6:29 AM

Aha! It is possible to buy them in the UK, but you have to go to a specialist Indian food shop and that makes them expensive.

user228700

Oh, I see :-(

user228700

If u bought it here, it would cost u about 2 pounds, I reckon.

John Rennie

amazon.co.uk/Mazza-Premium-Gulab-approx-pieces/dp/B010VIYKU6

user228700

:-P I dunno how good they will be, sir...

Ramanujan

Mouth is watering😍 did you eat gulag making @JohnRennie?

user228700

6:31 AM

Anyhoo, shall we please come back to physics?

user228700

@Ramanujan Wait till I post a picture later :-P

John Rennie

@Ramanujan My mother used to make Gulab Jamun. I've never made them myself. Anyway BACK TO PHYSICS!

user228700

:-P Yes, back to good 'ol physics!

user228700

So. Intensity. Hang on, lemme make sure Ik what I'm asking/talking about...

user228700

OK, exactly what is the relationship b/w intensity and power? Maybe I've gone and confused these two terms..?

Anonymous

6:36 AM

Power/ Area = Intensity

user228700

Ik that the number of electrons does factor into this, somehow, but how?!

John Rennie

People tend to be vague talking about power and intensity. It often isn't clear whether they mean energy per unit area or just total energy. However if we're being precise then intensity is energy per unit area per second. Power tends to mean total energy per second. So power = intensity times area.

Anonymous

energy of wave = (energy of one photon) * (no of photons) = h \nu. And (energy)/(area)/(time)=intensity

John Rennie

The definition of intensity and power applies to light incident on some surface regardless of whether we are talking about the photoelectric effect or not.

You say Ik that the number of electrons does factor into this but I don't see how number of electrons is relevant to how we define intensity

If we start with an intensity we can convert this to number of photons per second per unit area, call this $N$, by dividing by the photon energy $h\nu$.

Then the number of photoelectrons per second per unit area is $Nx$ where $x$ is the quantum yield.

user228700

@S007 No, the energy of one photon is given by $h\nu$.

user228700

6:43 AM

See JR's answer above ^

Anonymous

yes

Anonymous

sorry

Anonymous

wrote in a hurry

Anonymous

@Kaumudi

user228700

@JohnRennie Hm, yes, this is what I was initially trying to understand...

user228700

6:45 AM

@S007 OK :-)

user228700

OK, I think I understand this fairly well now.

user228700

I was just confused about the definition of intensity and also which energy goes into the formula; which, in retrospect, seems kind of obvious.

Anonymous

energy of wave = (energy of one photon) * (no of photons) = h \nu * (number of photons). And (energy)/(area)/(time)=intensity

Anonymous

now it is correct

Anonymous

@Kaumudi

user228700

6:48 AM

Yes.

user228700

Now back to PE...

Anonymous

btw happy diwali to all the indians out here:-P

user228700

How is the term saturation current defined? I've read up but I haven't found anything particularly useful.

user228700

@S007 Happy Diwali :-) Was it yesterday/today?

Anonymous

both days we celebrate

Anonymous

6:51 AM

:-D

user228700

Oh, OK :-D

John Rennie

Suppose you have light with some intensity I, then it will produce $n$ photoelectrons per second, where $n$ is a function of intensity, wavelength and quantum yield. So far so good?

@Kaumudi no, quantum yield is the (average) number of photoelectrons produced per photon.

Anonymous

@heather @JohnRennie I have some good news "If you are interested in starting this site immediately outside the SE network (no need to gather a certain amount of questions or followers), you can mail to admin@physicsoverflow.org to discuss things with our system developer."

user228700

I misread, sorry.

user228700

Yes, go on...

Anonymous

6:53 AM

If you are interested in starting this site immediately outside the SE network (no need to gather a certain amount of questions or followers), you can mail to admin@physicsoverflow.org to discuss things with our system developer. We already did help the in private beta closed OpenScience community in the same way. Graduate level + questions of the type Physics SE calls homework, such as for example QFT calculations, are already on-topic on PhysicsOverflow. — Dilaton 21 mins ago

Anonymous

maybe I can try it @heather

John Rennie

OK in our experiment we apply a voltage to try and collect these photoelectrons. But if we use a low voltage we won't get all the photolectrons. Some will fly off in diffrent directions and we'll miss them.

user228700

Yes, OK...

John Rennie

Suppose we manage to collect some fraction $f \le 1$ of those $n$ photoelectrons per second, then we collect $fn$ electrons per second and if $e$ is the electron charge that gives us a current of $I=efn$. OK so far?

user228700

Yes.

John Rennie

6:58 AM

The fration of the electrons we collect increases as we increase the voltage, so the current we get increases as we increase the voltage. But $f$ can't be bigger than 1 because $f=1$ means we are collecting all the photoelectrons.

That means the current we measure can't get bigger than $I = en$.

And this is what we call the saturation current. It means that in our experiment the current increases as we turn up the voltage (because $f$ is increasing) but at some point the current levels out and stops increasing.

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