The h Bar

@KyleKanos hey man, how are you? btw have you seen this post before? really funny :)
http://math.stackexchange.com/questions/820686/obvious-theorems-that-are-actually-false

Kyle Kanos

@Phonon @Phonon: I am doing well, yourself?

I have seen that post before

Half of it is lost on me, of course

Phonon

5:42 PM

@KyleKanos thanks not bad either, yeah same :),

noahnu

6:54 PM

Could anyone help me with a problem? Basically: A person falls off a ridge into a snow bank. Distance from ridge to snow bank is $3.6\,m$. Person descends into snow bank $0.80\,m$ before stopping. What is the acceleration in the snow bank?

I figured the first $3.6\,m$ follows the position function $y(t) = -1.635t^3$. Therefore, it takes $1.3\,s$ to fall from the ridge to the snow bank. $\vec{v_0}$ in the snow bank is then $-8.289\,m/s$. How do I figure out $\vec{a}$ in the snow if I don't have time?

Kyle Kanos

@noahnu Hint: It's one of the 4 kinematic equations

noahnu

Oh.. haha. Thanks.

soumyadeep

7:24 PM

@KyleKanos Hi You active?

Kyle Kanos

In and out

I did get your message the other day

And the answer is still, sadly, that I have not gotten to it

soumyadeep

Oh...I could make out from your conversations that you have been quite busy.But never mind -just pls read it when you got the time.

I have time till the end of this month.

After that i have a BIG exam on 2nd nov.

@Kyle Kanos I can actually post it as a question for the main site but that would mean saying the answers given were wrong...and i dont want to do it.May be i am misinterpreting somewhere-so i just wanted to consult you.

Kyle Kanos

If you think the answers are wrong, the best thing to do is offer a bounty

soumyadeep

@KyleKanos But that would not bring W.L's writing in highlight would it?Before i saw it i did not have the slightest confusion about the answer i.e your answer.

Kyle Kanos

I started skimming the document you gave me...I think I know what's confusing you

He says, on Page 3, that using E+ iR - L(di/dt) = Delta V is DEAD WRONG

That is what user3814483 was mentioning in the comments

That you either have to use Faraday's formalism, my equation 2, which always works or you have to introduce some "fudge factor" to account for the non-conservative nature of the field

soumyadeep

7:40 PM

@KyleKanos Yeah and in his lecture he says potential drop across an inductor is 0.

So what do we conclude -that KVL is not applied right?

Kyle Kanos

@soumyadeep For most cases in academic courses, it is correct because inductors generally aren't present.

When inductors are present in the circuit, you have to modify your equations

As I stated in my answer, Lewin's interpretation of Kirchoff's law is misleading

soumyadeep

@KyleKanos Umm,can you explain how?I am again lost..:(

And the issue that is hitting me again and again is that voltage drop across an inductor is 0.I simply cannot get it.

Kyle Kanos

So Lewin says that Kirchoff 2nd means the closed integral of the electric field is zero, $\oint \mathbf E\cdot d\mathbf s=0$

That's not really how it's taught in textbooks these days

It's generally given as $\oint\mathbf E\cdot d\mathbf s=-d\phi/dt$, which is Faraday's law

soumyadeep

@KyleKanos Really?so now i get the source of all this confusion.However in my text the first equation is written.

But that does not matterI think i have got it.

Except for the fact that voltage drop across inductors is 0.

On a side note-Cant we write proper Mathjax equations here?

And what is the time at your place?

Kyle Kanos

@soumyadeep There's a link in the top right, For MathJax use meta....

That will link you to a page to embed MathJax in chat

And it's 15:53 (or 3:53 pm to some people)

soumyadeep

7:57 PM

@KyleKanos ohh here its 01:26(1:26 a.m.) which is why i rarely find you in chat.

Kyle Kanos

Yeah, I'm an East-coast American

soumyadeep

@KyleKanos And i am an Indian

anyways i suppose we should go back to voltage drop across inductors.

@KyleKanos Lewin appears to have explained that voltage drop across inductors -at least in the way he concludes it.

Kyle Kanos

Yes, he explains that the measured value would be the inductor with the voltmeter

Again, this is him being misleading because every measurement with a voltmeter is that of the object and the voltmeter

soumyadeep

@KyleKanos Yes what is the problem with that? he says the voltmeter will give a reading and thats the time rate of change of flux through the voltmeter.

Kyle Kanos

8:13 PM

Oh, I see now

He's going back to the two ways of handling the inductors in a circuit

Way 1 involves Faraday's law and thus there can't be the potential drop across an inductor

Way 2 involves the "fudging" of saying there "is" a potential drop

He's using the voltmeter bit to say that Way 2 is the wrong way to think about it

soumyadeep

@KyleKanos Exactly.And i have understood all of Way 1 except for the last bit.

Kyle Kanos

If $\oint\mathbf E\cdot d\mathbf s=0$ was right, then there would be a (conservative) electric field in the inductor

But since there isn't such a conservative field, there can't be something called a potential there

What is there is a time-rate of change of the magnetic flux, $d\phi/dt$

And that is what you'd measure.

soumyadeep

@KyleKanos Ohhh I forgot the basics of potential difference...

NOW i get it....

Kyle Kanos

Awesome sauce!

soumyadeep

However he claims in his lecture that electric field in an inductor is 0..

And i think it has something to do with that loop in the last bit of his document.

@Kyle Kanos I somehow related the potential part with the electric field being 0...I think Lewin does that too.But since you cleared that potential bit, this arises as a separate problem.

Kyle Kanos

8:24 PM

In the "one-loop inductor" there is nothing there to reduce or increase the electric field

So since the integral there is zero, then E must be zero as well

soumyadeep

@KyleKanos And Lewin opposes exactly this idea.

second last line of page 4.

Kyle Kanos

Hmm, I'm not sure then

soumyadeep

@KyleKanos So...I suppose we should end it here?

Kyle Kanos

That's the best I can do

soumyadeep

Thanks a lot though.I really do see things more clear now.

Bye.

Phonon

8:54 PM

@ACuriousMind hey man, there?

ACuriousMind

@Phonon Yup, I'm here :)

Phonon

@ACuriousMind can I ask one stupid question? :)

ACuriousMind

I've read you're quite busy at the moment?

Phonon

yeah :(

ACuriousMind

@Phonon Of course

Phonon

8:56 PM

@ACuriousMind isn't any superposition of Bell states necessarily an entangled state? (apart from all 0 cases)

@ACuriousMind asking in relation to this post, physics.stackexchange.com/questions/141143/…

ACuriousMind

I'm currently writing something on that question ;)

Phonon

aha :) cool

don't let me hold you then, go ahead ;)

ACuriousMind

And I don't think that all superpositons of Bell states are entangled, since they are a basis for the full space (so every vector is a superposition of Bell states), and there's a non-trivial subspace there that is not entangled

Phonon

okay, thanks for the clarification, then I guess my asnwer to that post, as it is, stands completely wrong, or?

ACuriousMind

Now that I've read it more closely, I think it is - all you conclude at the end (that I see) is that the vector is a superposition of basis vectors, which is proving nothing.

Phonon

9:06 PM

okay, I understand. But isn't the 3rd term alpha_1 beta_0 being 0 and all the rest non-zero, already enough to say that it has to be an entangled state?

ACuriousMind

Yes, it is, by the contradiction the OP already alludes to. But since I saw two explicit answers for this state, I thought I'd take a more general approach to thinking about entangled states for 2D Hilbert spaces

Phonon

k, right, good idea

ACuriousMind

9:28 PM

I think I've never before seen a question with four answers at zero score :D

On a different note: "This is classic Luke Skywalker stuff : ditch the variables and equations, instead hold true to the basic definitions. "

The things you see in the LQ queue...

Phonon

9:48 PM

haha yeah :)

Phonon

10:13 PM

@ACuriousMind hahaha you ll love this one xkcd.com/849 :))

ACuriousMind

@Phonon Hehe :D You haven't really lived until you stood at a blackboard and said that ;)

Phonon

;))

@ACuriousMind xkcd.com/171 :)

ACuriousMind

That one's just mean (but funny ;P) (I will refrain from commenting on its veracity until I know more about string theory)

ACuriousMind

10:36 PM

And now I must rest, the nights aren't that long now that my semester has finally started

user54412

11:32 PM

So, there are 58 questions tagged neutron-stars and a similar number about white dwarfs, but there is no tag for the latter. Should this be remedied?

user54412

Actually 13 + 5 more + 1 more white dwarf questions

user54412

(side note - does anyone know how to use or in searches for phrases along with is:question?)

David Z

11:47 PM

Here's the search info on meta:

188

Q: A new search engine for Stack Exchange

After the performance problems we have run into with Lucene.NET we've decided to make a change, we're moving the network on to elasticsearch. Here's where to get started: http://stackoverflow.com/search What works: All search operators should be in Many changes below from the old search behav...

I actually don't see anything about an OR operator

And yes, I would favor adding a tag for white dwarfs. Seems like a pretty clear case to me.

user54412

This suggests or only works between tags

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