The h Bar

Anonymous

21:00

Yes, that ^

Anonymous

:P

Anonymous

Anyone here knows what short-circuit inductance means?

Anonymous

Short-circuit inductance

Short-circuit inductance is the inductance when one of the primary winding or the secondary winding of the transformer is short-circuited and measured from the other winding. This value is often called informally as leakage inductance of the transformer. But the term leakage inductance which is in the electromagnetic literature is defined as an inductance caused by a magnetic flux (leakage flux) which is interlinked with one winding of a primary winding or a secondary winding and does not interlink with the other winding. So a confusion is caused by called short-circuit inductance as leakag...

Anonymous

I can't make much sense out of this ^

Semiclassical

looking at the snopes page re: that urban legend, the conclusion is basically that: It wouldn't kill you, but it wouldn't exactly be a pleasant experience

peterh

21:03

@ACuriousMind Thanks. What if a newly discovered sockpuppet has a lot of worthy contributions, in this case isn't he merged into his original account (which may be then suspended)?

ACuriousMind

@peterh If the user requests the merge before being discovered, i.e. the sockpuppetry is accidental or not in bad faith, then that might happen, but generally sockpuppets are not merged. Note that deleting an account does not delete their worthy contributions, i.e. a standard account deletion does not delete any non-negatively scored posts.

Valter Moretti

@0ßelö7
Yes I noticed the question. I think the answer is positive if the spacetime is strongly causal.

ACuriousMind

Sockpuppetry is only bad if you use it to circumvent system restrictions - if you have two or more accounts that never vote for each other, don't accept each other's answers and don't do anything else that you couldn't technically do with a single account, then you are allowed to do that.

0ßelö7

@ValterMoretti Strong causality seems like a very strong condition considering I'm not asking for uniqueness.

peterh

@ACuriousMind Thanks. Can you say me, if he is gone for his own request?

Valter Moretti

21:08

@0ßelö7 indeed you have to impose and solve a Cauchy problem in a globally hyperbolic neighborhood of $p$.

0ßelö7

@ValterMoretti I have a sketched argument: Assume the point is in some globally hyperbolic neighborhood. On a spacelike surface $\Sigma$ through $p$, we can define a function with $f$ with $\nabla_\Sigma f$ specified. Then we have...

@ValterMoretti yes, that's exactly what I'm sketching here

Valter Moretti

@0ßelö7 so what is the probem?

0ßelö7

@ValterMoretti I feel like that's too involved: you have to solve forwards and backwards in time, then use an energy argument/hyperbolic regularity to conclude that everything is smooth

ACuriousMind

@peterh Yes, deletion by his own request. Any further information you would have to get from him directly, by whatever means.

peterh

@ACuriousMind Sad :-( Thanks anyways.

0ßelö7

21:11

@ValterMoretti If you evolve a little back in time you can use uniqueness/regularity to conclude the pasted together solution is smooth, I think.

But I'm not entirely sure if one can just run hyperbolic systems back in time

Valter Moretti

@0ßelö7 I think you have to decompose the vector into a tangent and normal component and fix in a neighborhood of $p$, over a Cauchy surface passing through $p$ two Cauchy conditions compatble with your conditions

@0ßelö7 every solution satisfying these conditions (and there are infinitely many) solves your problem

0ßelö7

@ValterMoretti Well I want to put a spacelike surface $\Sigma$ through $p$, then evolve forward in time to get $u^+$ and backwards to get $u^-$. If one specifies $\nabla u|\Sigma$ at $p$ and $\partial_t u$ at $p$, one gets the condition on the derivative

But then to check that $u^+$ and $u^-$ join smoothly on $\Sigma$ I would run $u^-$ back to some time $-\epsilon$, then go forward again to some time $+\epsilon$

By uniqueness it must be $u^-$ then $u^+$, and also smooth everywhere

Valter Moretti

@0ßelö7 $u^\pm$?

0ßelö7

Does that sound right?

1 min ago, by 0ßelö7

@ValterMoretti Well I want to put a spacelike surface $\Sigma$ through $p$, then evolve forward in time to get $u^+$ and backwards to get $u^-$. If one specifies $\nabla u|\Sigma$ at $p$ and $\partial_t u$ at $p$, one gets the condition on the derivative

Valter Moretti

@0ßelö7 I do not understand well

0ßelö7

21:14

@ValterMoretti I want the solution to exist in a neighborhood, so it needs to exist on both sides of the hypersurface

Valter Moretti

@0ßelö7 Indeed, if a solution of a Cauchy problem exists it exists both in the future and in the past of the Cauchy surface. There is no problem

0ßelö7

if I solve a Cauchy problem on both sides I then need to make sure it's smooth across the surface

Valter Moretti

It is smooth

0ßelö7

yes, by the argument above...

Valter Moretti

You have only to prove that a solution exists, it is automatically smooth

0ßelö7

21:18

@ValterMoretti it will be one-sided smooth

Valter Moretti

Sure!

0ßelö7

i.e. $u\in C^\infty([0,T)\times \Sigma)$ and $u\in C^\infty((-T,0],\Sigma)$

but $u\in C^\infty((-T,T)\times\Sigma)$ is stronger

But what is true is that even if $u$ is only just $C^1$ across the boundary it will be a weak solution on say $[-T/2,T)\times \Sigma$, hence smooth there

Valter Moretti

The standard existence and uniqueness theorem for the hyperbolic Cauchy problem on Lorentzian manifolds, referred to a smooth Cauchy surface states the existence and uniqueness of the solution both in the past and in the future of the surface

0ßelö7

So it's smooth on $(-T,T)\times\Sigma$ after all

Valter Moretti

I mean a smooth solution also through the surface

yes

0ßelö7

21:21

So standard that I've never seen it...but I imagine the proof is what I just said, no?

It's a weak solution across the surface so everything is Gucci

Valter Moretti

The local proof is in Freidlander book

0ßelö7

I've been meaning to get that

Sadly my univ doesn't have a copy

Don't know why

Valter Moretti

the global proof can be found in the book by Baer Genoux and... I do not remember

0ßelö7

@ValterMoretti this one has been bugging me physics.stackexchange.com/questions/338815/…

Valter Moretti

However it is equivalent to the existence of the causal propagator

0ßelö7

21:24

@ValterMoretti that's a bit deeper into hyperbolic PDE than I might care to go.

Valter Moretti

arxiv.org/abs/0806.1036

have a look at that

0ßelö7

@ValterMoretti thanks

so speaking of strong causality

I saw you left a comment on that question

I never got it resolved and it's still bugging me

Valter Moretti

Yes, but I guess that causality is enough after a result by Bernal and Sanchez

0ßelö7

@ValterMoretti For? I think we're talking about different things

I'm talking about the convex normal neighborhoods question

Valter Moretti

I think that causal spacetime implies that locally the hyperbolic cauchy problem is well posed

Every point admits a geodesically convex neighborhood such that the double cone generated by any couple of points is included in the neighborhood and is compact.

0ßelö7

21:29

@ValterMoretti Right, that's a classic result of Penrose.

Valter Moretti

If the spacetime is causal, one should be able to restrict the neighborhood to obtain a globally hyperbolic (sub)spacetime

In general strongly causality is required but after Sanchez and Bernal causality is enough

0ßelö7

But that's not what I was looking for. I don't think their double cones have to be geodesically convex themselves.

Valter Moretti

No, they are not

0ßelö7

But Hawking-Ellis, Beem and Ehrlich, and some others claim it's possible to have sets that are causally convex AND geodesically convex at the same time

Valter Moretti

but the portion of spacetime is (strongly) causal and double cone are compact: the space is globally hyperbolic

0ßelö7

21:32

I's really hard to prove because causally convex sets give no information about spacelike geodesics

Valter Moretti

Regarding convexity of double cones the only result I know (and I think the only exists) is due to a n Italian collegue of mine

Let me some seconds to loom the paper

look for the paper

Convex neighborhoods for Lipschitz connections
and sprays
by E. Minguzzi Monatsh Math (2015) 177:569–625

0ßelö7

long paper

Lipschitz connections? Lol what did he need those for?

Valter Moretti

Yes he deal with very weak conditions regarding regularity of the metric

sorry I am writing with my phone and it is a bit difficult

he dealS

However, I do not think it is relevant for your original problem

You only need a globally hyperbolic neighborhood of $p$

0ßelö7

...I do? Are those automatically causally convex when you consider the whole spacetime?

Valter Moretti

"those"? Are you referring to double cones?

0ßelö7

21:40

The double cones should be causally convex.

Jasper

@0ßelö7 You changed your picture? Who is that now?

0ßelö7

@ValterMoretti I'm working right now, I'll read those papers and if I have more questions can I email you?

Valter Moretti

I see, you are referring to causal convexity, I was referring to geodesical convexity!

0ßelö7

@ValterMoretti I am referring to a set that is both causally convex and geodesically convex.

The claim is that a strongly causal spacetime always has sets that are both.

Valter Moretti

Ok, however I do not think it is relevant for you problem (that of the point $p$ and all that)

0ßelö7

21:42

This is a separate problem

@Jasper ethan klein

the mortal enemy of @BalarkaSen's avatar

Jasper

OMG, that is crazy.

Valter Moretti

OK, contact me via email if you need. Please notice (a) I am terribly busy (b) I should refresh my mind about these issues

Since I am presently working with QFT and quanternionic QM

0ßelö7

@ValterMoretti I've talked to my advisor about this but he's rusty too. We're doing GR on hypersurfaces so we don't need any hyperbolic stuff or causality :)

Valter Moretti

Maybe there is a much more easy approach to solve your problem...but I do not know.

0ßelö7

@ValterMoretti Like I said, it's claimed to be true in Hawking and Ellis, but when I read Hawking's original paper he uses a different argument

Valter Moretti

21:46

My impression is that we are trying to kill a fly with a gun

4

0ßelö7

@ValterMoretti That's what G. Todorova (maybe you have heard of her) said, but she didn't want to discuss details with me until after her upcoming conference

Semiclassical

The Flaming Lips - Waitin' For a Superman (lyrics)

►

Valter Moretti

OK, now I must stop :)

0ßelö7

thanks, cheers

Valter Moretti

@0ßelö7 cheers!

0ßelö7

21:51

@EmilioPisanty "mean curvature flow has applications to the renormalization group flow in string theory" :'D

@Semiclassical pls tell me you are a fluid mechanics wizard

Semiclassical

nope

lılostafa

@0ßelö7 Conformal maps are used a lot in electromagnetic problems too...people usually use tables for those

0ßelö7

no, I'm done with that

I have two fluids classes

Anonymous

Kyle is a fluid mechanics guy iirc. :P

0ßelö7

why the :p

Anonymous

21:57

@0ßelö7 He doesn't come around much

0ßelö7

you know who was a fluids guy?

Chris White

@BernardoMeurer remembers

Anonymous

I've heard of him a lot in this chat :D

0ßelö7

he was the best of us

Anonymous

Wasn't he an astrophysicist ?

0ßelö7

full GR black hole MHD

lılostafa

21:59

@0ßelö7 I remember him. why did he leave?

Anonymous

astro.berkeley.edu/researcher-profile/3343634-chris-white

Anonymous

"Black hole accretion, astrophysical fluid dynamics, the numerics of general-relativistic magnetohydrodynamics, high-performance computing"

Anonymous

Lots of cool stuff

0ßelö7

@lılostafa he was removed as room owner because a troll got a rise out of him

I don't know exactly what happened and we get in trouble for speculating so I'll leave it at that

I have talked to him since then, fwiw

Anonymous

Good to have contacts with knowledgeable people. I need to find someone who specializes in circuits and stuff.

Anonymous

22:03

The EE chat is always so empty

0ßelö7

@Blue for sure, but email is terrible compared to a whiteboard

lılostafa

@0ßelö7 This room was so different back then. DavidZ was quite active. The two main users in theis room where ManishEarth and CrazyBuddy. Chris White, tpg, and others were active too.

No ACM, No JR (in the chat)

0ßelö7

what?

Chris White was here a year ago

ACM was more active back then

Manish has been gone since I've been here (3 years)

tpg was more active, yes

lılostafa

I mean 3 ~4 years ago

0ßelö7

many of the current users weren't even born then

Anonymous

22:07

@0ßelö7 Agreed. But that's the second best option when it's not possible to be physically present.

Anonymous

BTW ManishEarth has dumped physics and moved to CS now :P

0ßelö7

@Blue Especially with (let's face it) older individuals, text conversation can be challenging

@Blue good, physics is a dead discipline

Anonymous

@0ßelö7 Says the mathematician who wants to increase GDP

0ßelö7

I think math is dying too

lılostafa

@0ßelö7 I clearly remember when ChrisWhite hit 10k rep. and when Qmechanics's rep was well below 20k :)

0ßelö7

22:09

the golden era was the 70s

I was born in the wrong generation

Anonymous

You wouldn't have had laptops then

lılostafa

@Blue Why is he still a mod here?!

0ßelö7

@lılostafa historical inertia

Anonymous

@lılostafa I think mods are permanent

Anonymous

(As long as they perform their bare minimum duties)

0ßelö7

22:11

@Blue if I were a mod we'd be able to test that theory >:3

> 14C air

oh come on

they're making me interpolate the properties

Anonymous

@lılostafa I need your help

lılostafa

Dec 10 '13 at 7:20, by user54412

One day the prof did some hand waving with representations (basically a Clebsch-Gordan transformation, I later realized). I asked how he got the result, and all he said was, "That's just group theory. Moving on..." At this point I had two solid years of abstract algebra under my belt, and nothing he was doing resembled group theory in any way, either because it wasn't or because the notation was obfuscated.

Anonymous

I can't understand what short-circuit inductance is

lılostafa

I was in the room when Chris White posted this^ :)

0ßelö7

@Blue ::turns off GPU fan::

wonder if this will actually warm my feet

Anonymous

22:15

You need to explain it to me as if I am a newborn

Anonymous

I understand nothing of this

Anonymous

Short-circuit inductance

Short-circuit inductance is the inductance when one of the primary winding or the secondary winding of the transformer is short-circuited and measured from the other winding. This value is often called informally as leakage inductance of the transformer. But the term leakage inductance which is in the electromagnetic literature is defined as an inductance caused by a magnetic flux (leakage flux) which is interlinked with one winding of a primary winding or a secondary winding and does not interlink with the other winding. So a confusion is caused by called short-circuit inductance as leakag...

Anonymous

1

Q: What do they mean by "short-circuited" here?

"Short-circuit inductance is the inductance when one of the primary winding or the secondary winding of the transformer is short-circuited and measured from the other winding." I can't understand the part of the sentence in italic. What do they mean by "short-circuited" in this context? Even the...

Anonymous

The answers there make no sense

lılostafa

@Blue WHat is the problem with the definition here?

Anonymous

22:20

@lılostafa What does the definition even mean? If there is a short circuit across the inductor the ends of the inductor should be at the same voltage, isn't it? What does "short-circuit inductance" stand for?

Anonymous

@0ßelö7 Unless they are geeky like JR ;)

Anonymous

Well, even voice communication with old professor's is sometimes terribly difficult

Anonymous

Our (old) physics professor knows his stuff...but is terrible at communicating

0ßelö7

yeah there's nothing worse than being 15 minutes into a meeting and then realizing the other guy understood something completely different

lılostafa

@Blue Have you studied the concept of impedance?

Anonymous

22:28

@lılostafa Yes

lılostafa

What is important here is the impedance of that part of the circuit (which is not zero, it has a non-zero imaginary part)

Anonymous

@lılostafa Which part of circuit?

lılostafa

The short circuited part

Anonymous

Anonymous

I can't see any short-circuit in the wikipedia diagram

Anonymous

22:31

There's no connecting wire across any inductor

vzn

@Valter hi again, fyi our guest speaker sessions remain popular, you were requested as a guest, plz continue to keep in mind, understand you are busy, but yet you do seem to have time for cyberspace :) physics.meta.stackexchange.com/questions/7783/…

lılostafa

@Blue You were asking what happens if we short circuit the secondary of a transformer, right?

Anonymous

@lılostafa Not really. I'm trying to understand what the wikipedia page means by $L_{sc1}$ and $L_{sc2}$

Anonymous

(In that diagram)

Anonymous

They say those are short-circuit inductances

Anonymous

22:33

I don't see how

Anonymous

There's no short-circuiting in the diagram in the first place

Anonymous

BTW what does the dotted inductor in that diagram represent?

DanielSank

Hi, everybody.

Ooooh, circuits! I know those.

lılostafa

@Blue It took me a while to refresh my memory about it (from 5 years ago!); I think that circuit is just the real model of the transfomers taking $L_{sc}$ into account. For measurement you need to just short-circuit it (as you said)

Anonymous

@DanielSank Aaaaahh...I need you :'D

Anonymous

22:47

Could you explain that thingy?

0ßelö7

Circuits are just topology.

DanielSank

@0ßelö7 False.

0ßelö7

@DanielSank Well, and Yang-Mills theory too.

DanielSank

@0ßelö7 Correct.

Ok @Blue what do you want to know.

The first sentence of the Wiki article is quite clear.

lılostafa

@Blue The diagram is the "equivalent circuit", as it says itself

0ßelö7

22:49

Yang-Mills is really catchy. What's something popular named after someone but isn't a really catchy name?

Anonymous

@DanielSank I don't get it. There is no short-circuiting in the diagram they have given in the Wikipedia page. What is the physical significance of $L_{sc}$ ?

DanielSank

Well first of all, did you notice the text in the diagram that says "short" and "open"?

Anonymous

@DanielSank I noticed. I don't know what that means. How can it be both open and short?

DanielSank

It can't. They're saying to consider the case where it's open or short.

Anonymous

Alright. Then?

0ßelö7

22:53

@Blue Schroedinger's circuit

3

Anonymous

@0ßelö7 klelk

0ßelö7

is that a stoke victim laughing?

Anonymous

It's a superposition of lel and kek

Anonymous

Schrodinger's lol

lılostafa

@Blue the point is that the ideal transformer is supposed to have only a coupling constant, but in this (closer to reality model) each side has a self-inductance too (i.e., all of the flux of the other side doesn't link this side)

physical significance of $L_{sc}$ ^

0ßelö7

22:57

@Blue turning off the fan increased the temp by 25C...that has a significant effect on the heat of the room

had to turn on the AC

Leaky Nun

the axiom of choice?

lılostafa

Is it clear now? @Blue

0ßelö7

that AC is always off @LeakyNun

Leaky Nun

@0ßelö7 lol

Anonymous

@lılostafa I'm trying to understand. Give me a couple of minutes

Anonymous

22:58

Thanks for the help though

0ßelö7

@LeakyNun if I never use AC I never have to check if my functions are measurable

it's the perfect mathematical plan

Leaky Nun

lol

lılostafa

@Blue and it seems that dotted-line inductor is just $L_{sc}$ and shows the equivalent circuit of the non-ideal transformer when we short-circuit the secondary of it. The solid line shows the equivalent circuit when we open-circuit it.

Anonymous

@lılostafa You mean for an ideal transformer coil $\phi= Mi$ only? ($i$ is the current in the other coil)

Anonymous

$M$ is the coefficient of mutual inductance

lılostafa

23:05

Yes

0ßelö7

the surface area of an open cylinder is $\pi d\ell$, right?

Anonymous

So they are separating the real inductor coil into ideal inductor coil + short-circuit inductor coil to take into account the reality?

lılostafa

yeah exactly

Anonymous

Gotcha. Pheeeeeew

Anonymous

I owe you something

0ßelö7

23:07

hellooo

lılostafa

see the second part (leakage current) here: en.wikipedia.org/wiki/Transformer#Real_transformer

0ßelö7

need geometry help

lılostafa

@0ßelö7 yes true (assuming d is the diameter)

Anonymous

@0ßelö7 Who are you, kid? And why did you hack the ~~great~~ nerdy mathematician 0celo's account?

0ßelö7

thx

lılostafa

23:09

@Blue np

0ßelö7

@Blue I am him

lılostafa

@0ßelö7 np

0ßelö7

I just don't know any geometry

Anonymous

@0ßelö7 Not believable :P

0ßelö7

@Blue I'm analysis now

Anonymous

23:11

@lılostafa I'll gift you a 100 mangoes if you ever visit India

Anonymous

:D

0ßelö7

but really this problem about heat conduction on a cylinder is taking too long

Anonymous

@0ßelö7 Is it a physics problem or a math one?

0ßelö7

physics

Anonymous

Then you can ask here....John is good at thermo

0ßelö7

23:14

a hot cylinder is in a breeze, calculate the initial $\partial T/\partial t$

Anonymous

Sounds like a question on convection

0ßelö7

yes, but it is not quick

Anonymous

For heat loss by convection $\frac{dQ}{dt}=hA\Delta T$, but I guess you already know that

0ßelö7

yes, hence why I needed to check the $A$

and I think $\dot Q=mc_v \partial T/\partial t$

Anonymous

You need to consider both inner and outer surfaces I think

0ßelö7

23:17

it's solid

Anonymous

Oh

0ßelö7

I need to find the density and $c_v$ now, but the problem doesn't say anything beyond steel

wonderful

the TA is gonna love this one

Anonymous

You don't need necessarily need $c_v$. It's will be just $mc\Delta{T}$ where $c$ is the specific heat for the process.

0ßelö7

$c$ or $c_v$, doesn't matter for incompressible solids

the table has $c_p$

I just need some number

Anonymous

"An equivalent statement of the Dulong–Petit law in modern terms is that, regardless of the nature of the substance, the specific heat capacity c of a solid element (measured in joule per kelvin per kilogram) is equal to 3R/M, where R is the gas constant (measured in joule per kelvin per mole) and M is the molar mass (measured in kilogram per mole). Thus, the heat capacity per mole of many elements is 3R."

Anonymous

23:21

Use this ^

0ßelö7

moles? what kind of engineer are you

I'm just going with standard carbon steel

yolo

Anonymous

Just use a calculator to convert from moles :P

0ßelö7

why do that when I can read off the metals table in the book

Anonymous

@0ßelö7 That's another proof that I'm not an engineer XD

Anonymous

I'd happily be a chemist...but physics attracts me more

0ßelö7

23:26

.5C/sec

seems reasonable!

ok, time to typeset other fluids homework

David Z

23:55

@vzn if you want to keep discussing that, it might be better to use the backup room because I'm only able to get online sporadically these days

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