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2:16 AM
Another newbie question. All of the cold cathode and hot cathode gas discharge/vacuum tubes work by static electricity, right? So, somehow we can say if Thales of Miletus didn't discovered static electricity in the first place there will be no such things as gas discharge/vacuum tubes? But I thought in my opinion even without Thales, someone or some people will independently discover static electricity eventually.
 
 
3 hours later…
5:04 AM
@MohamedObeidallah I wouldn't say they operate using static electricity.
Typically electrons are produced by field emission or thermionic emission, then they are accelerated by a voltage applied between the electrodes. I don't see how static electricity is involved.
 
5:44 AM
ok I just did a quick search on Google that field emission theory was first proposed by Ralph H. Fowler and Lothar Wolfgang Nordheim while thermionic emission can be traced back to Edmond Becquerel, Frederick Guthrie and Thomas Edison. So, we can somehow say there will be no field emission and thermionic emission gas discharge/vacuum tubes without those people?

Field emission is emission of electrons induced by electrostatic fields, and since classical physics, it has been known that some materials, such as amber, attract lightweight particles after rubbing. Electrostatic phenomena arise f
 
No.
Suppose you have some chunk of metal. Electrons inside the metal have a lower energy than electrons outisde the metal. The difference in energy is called the work function.
So to get electrons to leave the metal you have to supply this energy.
If you apply a really, really strong electric field then this can supply enough energy to get the electrons out of the metal, and that is the basis of field emission.
 
@JohnRennie right. Thanks for your response. Who do you think is the first to demonstrate field emission from another metal to another metal? Ralph H. Fowler and Lothar Wolfgang Nordheim just make the theory only but I think they never demonstrated it.
 
I don't know. Sorry :-(
 
@JohnRennie oh nevermind, so it could be anyone who experimented with metals with an understanding of electrostatics somewhere around 1700 to 1800 and onwards. Because many people with engineering background said the inventor of this and that is not really the first person to create that something, as people always been copy each others idea and then attempted to improved it by trial and error. Thoughts?
 
I've never been very interested in the history of science I'm afraid.
 
6:37 AM
We have a whole stackexchange for that
if you need to
 
 
1 hour later…
7:40 AM
@Slereah thanks! Will bring this topic there later
 
 
4 hours later…
11:30 AM
"If the function is both a local diffeomorphism, as above, as well as an injective function then we call it an open embedding"
@bolbteppa Isn't a diffeomorphism already injective, since it's invertible?
Or is it because Schreiber uses a weird definition of diffeomorphism
are local diffeomorphisms not diffeomorphisms
 
12:08 PM
@Slereah Indeed, a local diffeomorphism is not a diffeomorphism, it's just a map such that every point has a neighbourhood on which the map is a diffeomorphism
it's a stronger variant of the "full rank" requirement for immersions and not Urs' invention, see en.wikipedia.org/wiki/Local_diffeomorphism
 
12:52 PM
how's the curfew working out for you? @Slereah
 
1:07 PM
@skullpatrol Not the clubbing type so I basically haven't noticed
I go home before dark
9
Q: A local diffeomorphism of Euclidean space that is not a diffeomorphism

El MoroCould someone give me an example of a local diffeomorphism from $\mathbb{R}^p$ to $\mathbb{R}^p$ (function of class say $C^k$ with an invertible differential map in each point) that is not a diffeomorphism.. in the real line (1 dim case) that would mean a function with a continuous non null deri...

 
 
1 hour later…
2:29 PM
"In the case at hand, several proofs of Hurwitz’s theorem are sketched on Wikipedia, with links to more details. Discovering that any one of these proofs is wrong would instantly hurtle anyone to the status of a mathematical superstar."
 
 
1 hour later…
4:00 PM
Why is Bayes' theorem so unintuitive ;(
 
Humans are notoriously bad with statistics
 
Can anyone point out if Full RISC $\subset$ CISC Systems, then why the hell, RISC $\not \subset$ i386 or any x86 architecture?
 
RISC isn't a subset of CISC
 
@Slereah Anecdotes are just so much more relatable.
 
Then what?
 
4:07 PM
@Slereah we're bad at most things, really
 
@JohnRennie So, RISC $\not \subset$ x386?
 
RISC and CISC are two separate approaches to designing a processor. Neither is a subset of the other.
 
the amazing thing is that we can learn not to suck that much
 
Also RISC and CISC aren't actual math objects
 
@JohnRennie So, which one is supports i386 or x86?
 
4:08 PM
Unlike say, a random access machine or a Turing machine
There's no definition of RISC
 
@Slereah RISC is just like a convention
 
@ACuriousMind including at being bad? hehe
 
@Azmuth x86 is a CISC instruction set
 
x86 is very much a CISC
 
Examples of a RISC instruction set would be ARM
 
4:09 PM
A mess, even
 
As far I know, ARM is compatibly with RISC, so, All of ARM must support RISC-V Compatible instruction set?
@JohnRennie Ohhhhh!
 
@JingleBells Yes, we're so bad at being bad that we invented whole traditions of knowledge to get worse at being bad!
 
who's bad?
 
I see, why I messed so bad
@skullpatrol CISC
 
@skullpatrol michael jackson
 
4:10 PM
RISC >>>>
Another question
 
CISC has its perks
 
@Azmuth RISC-V is also a RISC instruction set, but it is a different instruction set from ARM.
 
@ACuriousMind but if we're bad at being bad, aren't we good :D?
 
If you actually have to code assembly, CISC is much more pleasant
 
Neither of RISC-V and ARM are subsets of each other.
 
4:11 PM
RISC is only nice for people making processors
 
If RISC supports MMIX Instruction sets...
@Slereah RISC >>> Because designing algorithms is easy
 
@JingleBells that interpretation is a tad too optimistic for my tastes ;)
 
Also, Knuth's MMIX is supported in RISC
That's the greatest advantage
 
x86 has quite nice functions if you want to actually code assembly
Although to be fair, a lot of complex instructions are an illusion
 
@Slereah So, how much time does it takes to translate 100 lines RISC Instruction set module to CISC?
 
4:12 PM
Since it's just translated in microcode down the line
@Azmuth Depends on the code, I suppose
 
@Slereah I wouldn't call any coding in any assembly "pleasant" :P We invented high-level languages for a reason!
 
@Slereah I'm designing my own compiler and hence a high level language (roughly), so, to make it compactible on i386/x86 variant and RISC Variant, I've to make porting algorithms
 
@ACuriousMind All relative
 
ARM has the best support of all!
 
Object Orientation?
 
4:14 PM
If you're writing a compiler and not targetting LLVM as a backend you're not going with the times.
 
@Azmuth I wouldn't advise porting x86 to RISC
 
@ACuriousMind Try doing reverse engineering, you'll realize, how good haven is Assembly
 
Another compiler? Does the world need another compiler?
 
Unless it's a school project or something
 
@Slereah Not even for compiler design?
 
4:15 PM
The proper way would be to compile the high level language in both
 
@JohnRennie required in my engineering course, just a mini, small version to be tasted on virtual environment
@Slereah school project
 
If you translate twice, it's going to be fairly inefficient
 
@skullpatrol structured Oriented
 
hmm
 
If it's a school project ask your teacher maybe
If he tells you whatever just go ahead
 
4:16 PM
I understand the intuition behind Bayes' theorem and I understand how prior data affects a new hypothesis probability but I can't relate that to the equation. Should I try to make sense of the equation or? I don't know to what level it's possible to grasp this thing.
 
@Slereah The most painful thing is converting all of sighed datatypes into unsigned and vice versa, I keep making mistakes everytime
@Slereah Just about a 100 line MMIX Code in RISC-v, and we've to translate it to i386/x86 varient
I've yet to study CISC
Backlogs :(
@Slereah college project
actually...
uni project :P
@skullpatrol Have you completed all of MMIX from Knuth?
 
nope
 
So I guess translation must be easy for you :P XD
just kidding
 
The best of course is to compile in OISC
 
@Slereah nu.... RISC
@Slereah Professor gave a challenge to write a code that prints it's own code @JohnRennie
 
try try :) XD
 
@Azmuth That is called a quine
 
@Slereah never heard such creature till now!
 
Plenty of examples on the internet
 
@Azmuth that's a fairly standard problem.
 
4:19 PM
@Slereah lol, you are very smart!
 
Steady on
 
@JohnRennie c'mon it's just 1 week of class
here's my code:
 
If you're allowed to use the RAM containing the code itself, fairly easy
 
@JingleBells It's pretty straightforward: In the Bayesian interpretation, $P(A)$ is your prior belief for $A$ being true, and now you learn that $B$ is true. So you update your belief that $A$ is true to the conditional $P(A|B)$. The factor $P(B|A)/P(B)$ measures how "good" the evidence $B$ provides for $A$ is: If $B$ is completely independent from $A$, then this factor is $1$ and nothing changes - it tells you how much more likely $B$ is to be true if $A$ is true than otherwise.
 
Otherwise it's trickier
 
4:21 PM
@Slereah here's the answer....
main(){char*p="main(){char*p=%c%s%c;(void)printf(p,34,p,34,10);}%c";(void)printf(p,34,p,34,10);}
 
Code Golf is full of various quines and related challenges
 
@Slereah Try :)
@Slereah So you are a physicist and a BTech Engineer too!? Insane!
 
Am I?
 
@ACuriousMind interesting
 
Most physicists are secretly computer nerds.
 
4:22 PM
@Slereah How do you know all those virtualisation and computer assembly?
 
@JohnRennie but he is smart, sir :-)
 
I kinda hate computers, actually
@Azmuth School, mostly
 
@JohnRennie I would dispute the "secretly" part ;P
 
:-)
 
@JohnRennie Do you have a computer degree?
 
4:23 PM
0
Q: I need to destroy the Earth. Is this what it might look like?

Crash GordonThis is a small but critical part of my story; I need to kill off most of humanity (leaving a 5- or 6- digit number of survivors) and render the planet uninhabitable long-term, but survivable for a short time (as short as a few hours is OK). So I've been thinking about this scenario: A large com...

I have days like that too.
 
@Slereah so they taught you Assembly and compiler design in school?
@skullpatrol yep :)
 
@ACuriousMind your secret is out
 
@Azmuth Just assembly
 
@ACuriousMind Well, one might counter argue, how you are a pro Programmer and you didn't tell us anything except Physics part :P
 
4:24 PM
Of course, if you want a more GR way to destroy the earth, thunderbolt singularities are the way to go
 
> This is not a guide for wusses whose aim is merely to wipe out humanity
 
@Slereah Reverse Engineering, Compiler Design, Virtual Machine, simulation, algorithms?
 
Nah
 
@Azmuth Uh...I've actually talked about programming and/or my job in this chat plenty of times
 
@JohnRennie Thanks for the info, I'm interested :P
 
4:25 PM
I have a software engineering degree, so it wasn't very focused on assembly
 
@ACuriousMind I just heard it 4 days back....
@Slereah I too have a soft eng (in progress) degree
hehehe XD
Now, I mostly spend time with AI and coolio stuff... :)
 
AI is boring
Boo
 
Machine Learning and AI (Unsupervised, AGI, RL, Deep RL) are boring!???!! okay, i trust u
 
Just don't get me started on laptops.
Amazingly I have finally started selling off my immense pile of old laptops on ebay.
 
(I work in AI)
 
4:27 PM
@JohnRennie Wooowooowooow!
 
@Slereah Data Science to be accurate
that's not AI! :P
 
Didn't know you knew my job better than me!
 
lol, Yep, me too!
XD :)
 
You're lucky that I'm too bored by it to discuss it further
 
4:29 PM
Y'all soo lucky, you have to have multiple degrees in multiple subjects
 
Am I
 
I'm just BE -> in CSE
yep
u r :)
 
I have multiple degrees because I couldn't find a job
 
@JohnRennie did you run out of shelf space or what's caused this sudden change of heart?
 
I hated those degrees
 
4:30 PM
@Slereah LMAO, same condition... jobs are very very difficult here
 
Hell part of why I originally went into theory is because I hated coding
 
@ACuriousMind I literally had no more room for any more laptops :-)
 
I didn't want to do more bloody simulation
 
I've started searching for unemployment schemes rather than a job
@JohnRennie If I were you, I'd have sold my whole house just to save few laptops for me
@Slereah Oh okay, yes, But, I like CODING!
 
Boo says I
 
4:32 PM
YOO says I
 
@Azmuth I tried to give them away to Indian students, but the Indian government prohibits imports of laptops even as gifts.
I managed to get one laptop through customs after two weeks of arguing with the customs office.
 
@JohnRennie Yes, all electronic items, you can although give them personally....
 
@Azmuth nope, even as gifts the customs block them.
The only way is to mislabel them as "books" or something that is allowed, but then if the customs spot tem they get seized and you lose the laptop.
 
@JohnRennie Ummm!? You can get your laptop inside, actually.
Why they would block tourists bringing Laptops and Phones?
 
If you visit India you're allowed to bring your own property with you, but even then if the customs suspect you're bringing stuff in to sell they will seize it.
 
4:36 PM
@JohnRennie oh okay... didn't know about it, actually
 
Trust me on this, I've been there :-(
 
haha :) yes, ofc :-)
 
@JohnRennie is there a reason why electrons with parallel spin experience lesser repulsion ?
I read this fact in my chemistry book and I don't think it's correct .
 
@Ankit Magnetic term, probably
Since it's the main one where the spin matters
 
@Slereah @skullpatrol @ACuriousMind @JohnRennie write something about me: secret20.com/1913029 :)
 
4:43 PM
@Slereah does a spinning electron produce electric current ?
@Slereah aslo can you please elaborate on your statements ?
 
@Ankit erm ... um ... is that due to the exchange interaction?
As I recall the maths behind it gets a bit scary.
 
@JohnRennie what is that ?
 
I bet Wikipedia has a page on it ...
 
@JohnRennie on what ?
 
It's why if you have a p³ configuration the spins align parallel.
e.g. in nitrogen, 1s² 2s² 2p³ the three p electrons have their spins parallel
Hund's rule? Something like that anyway.
 
4:50 PM
@JohnRennie I read the above written question under Hund's rule btw.
 
Aha! I cleared out my old laptops just in time. I have to have this!
 
niiice
 
@JohnRennie the mathematics is not clear to me as I am a high schooler . So can you give me the physical reasoning behind the fact taht electrons in two orbital with parallel spin experience lesser repulsion than if they had opposite spin . But why is this ?
Should I ask this question on the main site ?
 
@JohnRennie You wrote that? (I guess?)
or @skullpatrol ....?
 
@Azmuth yes
 
4:58 PM
@JohnRennie Yeah!!!! I knew it! :)_
 
@Ankit I'd have to go away and think about it.
 
cya
 
@Charlie what is exchange energy ?
 
5:13 PM
A quantum mechanical effect between identical particles
I wouldn't worry too much about it
 
@Charlie was that enough ?
 
Was what enough?
 
@Charlie your definition ?
 
My definition wasn't rigorous no
Don't worry too much about it, it's a bit complicated if you're only just being introduced to Hund's rule
 
@Charlie Yes I am knew to it but I really don't know why the repulsion is reduced ?
 
5:16 PM
Like I said don't worry about it for now, just accept that the partial filling minimises the energy
 
Sorry but it's aginst my rules :(.
Against*
@Charlie Okay let's move ahead.
Can you tell me why every overlapping of two atomic orbitals causes a bonding as well as a anti bonding orbital ?
Like why can't there be two bonding orbital ?
In place of an anti bonding orbital ?
@Charlie are you here ?
Can someone else help me here ?
 
The anti-bonding orbital is just the higher energy state of the two-electron molecular bond
 
Why is it higher ?
 
You will go endlessly down the rabbit hole if you always ask why in chemistry, your best bet is to either get comfortable accepting a lot of things on trust or pick up a quantum mechanic textbook, there is unfortunately precious little middleground
I'm not sure how to answer that
Hello qmechanic
 
5:33 PM
@Charlie : Hello.
 
:)
 
5:45 PM
@JohnRennie Hey, I've checked now, you can import laptops and phones to India... infact, my laptop was purchased from USA. But, there is a separate Visa and process for that.
 
@Azmuth There might be a difference between buying a laptop for import and trying to gift one. There's probably some sort of customs or something when you buy it internationally and ship it to India; but if you gift one they might think you actually bought it and are trying to get around any customs fees.
 
Gift Tax Act, 1958 was an Act of the Parliament of India which was enacted to impose tax on the act of giving or receiving gifts under certain circumstances as specified by the Act. == Charges of Gift Tax == As per the Gift Tax Act, the gifts that exchanged hands post 1 April 1958 were subject to taxes as mention in Schedule I. Post 1 April 1987, this was amended to the rate of 30% (Thirty percent) and included gifts that were made in that particular Assessment Year. On 1 October 1998 however, the applicability of Gift Tax ceased to exist.As per the Gift Tax Act 1958, gift (in the form of cash...
 
 
5 hours later…
10:37 PM
wait wut
The gravitational force exerted by a symmetric sphere of mass M on a particle external to itself is exactly the same as if the sphere were replaced by a particle of mass M located at the centre.
how is this true lol
non uniform density
so by symmetric sphere does it means each layer is uniform or doesn't necessarily need to be uniform?
looks like everyone is offline
 
10:57 PM
@Stupidquestioninc I'm quite sure spherically symmetric includes that the layers have to have equal density. So density can vary with radius, but the entire shell at the same radius would need to have the same density for it to be really symmetric.
 

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