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napstablook
napstablook
03:10
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it is asking the energy of confined particle
my attempt here is $$E=hf=\frac{hv}{\lambda}$$ and since $\frac{h}{mv}=\lambda$
i replaced v in the original equation to give $$E=\frac{h^2}{m\lambda^2}$$
since for a standing wave $\lambda=\frac{2L}{n}$
$$E=\frac{h^2 n^2}{4mL^2}$$
but the answer is half of it. and according to actual quantum physics also the answer is half of this. can anyone explain what I am doing wrong?
 
1 hour later…
antimony
antimony
04:42
sorry for what is probably the stupidest question in the history of this place.
but i'm a bit confused by sub-metre area calculations
eg. (3*3)*(1E-3) = 9E-3 aka 9mm^2...makes sense
but (3E-3*3E-3) = 9E-6 aka 9um^2...makes sense mathematically, but physically i'm confused?
i thought it should be ok to multiply in native units (ie. xE-3), but it seems to create garbage area results??
 
1 hour later…
John Rennie
John Rennie
06:03
@antimony are you trying to calculate the area of a square 3mm by 3mm? Or 3 microns by 3 microns?
 
1 hour later…
ACuriousMind
ACuriousMind
07:28
@napstablook $E=hf$ is for photons or free particles if you use the deBroglie wavelength/frequency, not for bound or confined particles.
ACuriousMind
ACuriousMind
07:59
@Charlie all the maps in physics are always exactly as smooth as they need to be ;)
 
9 hours later…
BioPhysicist
BioPhysicist
16:51
How some of my conversations with biologists go:

Biologist: "What if you changed the model so that (insert system property here) behaves like this, which is a more accurate description of what is happening in the system."

Me: "I could put that into the model, but I don't see what we would learn from it. I don't think we have enough data to say how the more complicated assumptions would give us a different output than the simpler assumptions."
PM 2Ring
PM 2Ring
@antimony There are 1,000 millimetres in a metre, but 1,000,000 square millimetres in a square metre.
@JohnRennie I'm so tempted here to comment: "I wouldn't send a knight out in a dog like this!" :)
Qmechanic
Qmechanic
17:14
Latest Veritasium video May 22, 2021: This is Math's Fatal Flaw.
3
John Rennie
John Rennie
17:30
@PM2Ring :-)
PM 2Ring
PM 2Ring
17:43
@Qmechanic "Special thanks to Prof. Asaf Karagila for consultation on set theory and specific rewrites" He's a mod on math.SE & mathoverflow math.stackexchange.com/users/622/asaf-karagila
satan 29
satan 29
18:00
what are physicists' general attitude towards veritasium and other physics/science yt channels?
ACuriousMind
ACuriousMind
I don't think there's a "general attitude". I have very much a love-hate relationship with much of pop-sci (and I suspect that I like it more the less I'm able to actually judge its quality because it's farther from my field of expertise :P)
satan 29
satan 29
I think 3B1B is excellent if we are talking mathematics: and mostly all people (including some professional /semi professional mathematicians) seem to appreciate his work
but I dont think he has a physics counterpart
Mithrandir24601
Mithrandir24601
18:28
I think the last Youtube physics popsci video I saw was one of those with a relatively clickbaity titles making a technically valid but also irrelevant point
So wasn't impressed with that
On the other hand, here's an actual physics youtube channel: youtube.com/channel/UCn31-2HxFWpcKnbdFi-wkdg
(about ZX-calculus)
@ACuriousMind Quantum Computing is very pop-sci-like, because it's all about getting tons of money to actually make one at the minute. Very frustrating because you have nonsense claims about how quantum computers are only a few years away, while in the past few years we've gone from a handful to ~50 rubbish qubits (i.e. an order of magnitude, but loosing quality as well) and we need ~10^6 fault tolerant qubits. That's not a 'few years' of work
It's all very feasible, but claiming we're a few years away now will harm us in 20 years time, when we're still 20 years away
I know that's very specific, but stuff like that why I rarely like pop-sci. When I was in school, my teachers got me actual physics books to read
Let's take this example: This is Math's Fatal Flaw. It's about Godel's incompleteness theorem. Stupid title. Not awful content on the history of maths
bolbteppa
bolbteppa
18:46
A math video with over 3 million views in a few days is something
Tangoed
Tangoed
Hello everyone! I'm trying to interpret $(\vec{U}\cdot \nabla)\vec{u}(\vec{x_A})$
But shouldn't it be $\vec{U}\cdot (\nabla \vec{u}(\vec{x_A}))$?
$\vec{U} = \frac{\vec{u}}{|\vec{u}|}$ btw
ACuriousMind
ACuriousMind
@Tangoed $(\vec v \cdot \nabla) = \sum_i v_i \partial_i$
Tangoed
Tangoed
Oh ok $\nabla$ is just the operator
Sorry if the question is too basic, I didn't know where to ask
ACuriousMind
ACuriousMind
note that $(\nabla \vec u)$ doesn't mean anything, you can't take a gradient of a vector field
Tangoed
Tangoed
@ACuriousMind I thought I could, giving me a tensor
ACuriousMind
ACuriousMind
19:00
forming the tensor $\partial_i v_j$ is a possible operation, but it isn't usually written as $\nabla \vec v$
(it is likely not a popular thing to invent notation for because it is not a real tensor if your space is curved)
Tangoed
Tangoed
Ohhh, ok!! Thanks @ACuriousMind
 
3 hours later…
Sir Cumference
Sir Cumference
21:59
@Mithrandir24601 Yeah, I remember Veritasium making a video explaining why he needs clickbait titles
Though that one in particular bugs me

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