The h Bar

Yes. But in diffusion molecules don't on average stay in the same place. Diffusion involves movement of molecules on length scales that are large compared to molecular sizes.

user228700

Hm, OK...

In gases I guess the distinction is less clear since in gases the molecules do move large distances ...

user228700

Yeah, that's what.

user228700

6:06 AM

But alright.

Well in gases every property is ultimately described in terms of molecular motion.

user228700

Hm, yes.

That doesn't make diffusion and expansion in gases the same, just because both are ultimately explained in terms of molecular motion.

user228700

OK, I guess I understand it for the most part now.

user228700

OK, next question? This is turning out to be like a rapid-fire round, sorry :-P

user228700

6:12 AM

..? Shall I ask u..?

Yes, go ahead

Sorry, I was waiting for the next question :-)

user228700

Oh :-P Okay.

user116211

@Kaumudi Smart school.

user228700

@MAFIA36790 I have so many arguments against that but I haven't got the time, so OK.

user116211

._.

user228700

6:16 AM

(@JohnRennie: Checking to see if questions are worth asking...)

user228700

OK, so out of ALL this:

user116211

turned my head... pains...

user228700

user228700

6:24 AM

This is what I gathered, because I didn't understand most of what they are going on about:

user116211

@Kaumudi Which one did bother you?

user228700

user116211

pink...

user228700

Are my notes correct..? :/ (I hate my textbook, urgh

user116211

6:27 AM

What do you mean by size of the orbital?

user228700

@MAFIA36790 ..?

user116211

@Kaumudi I hardly knew such ink exists.

user228700

@MAFIA36790 Uh, I mean exactly what size of orbital sounds like..?

user228700

@MAFIA36790 I have a set of 10 differently colored pens, not a rainbow pen BTW.

user116211

@Kaumudi WTH...

user116211

6:29 AM

A wavefunction is decomposed into three parts in spherical coordinates; radial is one of them.

user228700

@MAFIA36790 It makes the book like livelier and also, makes me feel like revising these notes from time to time.

user116211

@Kaumudi Your book looks really dull though.

user228700

@MAFIA36790 Even this:

@Kaumudi yes, the notes look fine.

user228700

6:31 AM

? (@MAFIA36790 :-P)

user228700

@JohnRennie OMG really?! :O

Q: Is this the correct way to think about why static friction is directed radially during a turn on a level surface?

@Kaumudi were you worried some bits were wrong? If so, which bits?

@Kaumudi: look what someone has just asked:

0

So after much pondering of the fact that the net static friction force points in the center, perpendicular the tangential motion, I thought of this explanation. If we look at a car travelling around a circle, the wheels are always turned. There is a static friction force causing the tires to ro...

user228700

@JohnRennie Honestly, I was worried that everything was messed up, 'cause I understood nothing of what my textbook has given.

@Kaumudi I recall you saying you didn't understand polar coordinates - at least I think so.

The descriptions of the atomic orbitals are using polar coordinates so that would make them confusing.

user228700

@JohnRennie Oh, wow. OK, I'm gonna check that out (properly) as soon as these questions are over.

user228700

6:35 AM

@JohnRennie Yeah, I haven't learned about it yet.

Polar coordinates are really very simple. I suspect once you look into them you'll wonder why they ever seemed hard.

user228700

@JohnRennie No sir, it's not that they seem hard or anything. I just haven't had the necessity/time to learn it yet.

Atoms are spherically symmetric, and for any spherically symmetric system polar coordinates are the obvious choice.

user228700

@JohnRennie Hm, OK...

user116211

@Kaumudi They really make Schrödinger's Equation easy to solve for hydrogenic atoms.

user228700

6:40 AM

@MAFIA36790 Don't need to solve it :-P

user228700

@JohnRennie: Do u have to start work now?

I probably should start work now, but I'm happy to postpone it if you have an interesting question. After all, it is a Saturday :-)

user116211

The whole thing is separation of variables.

user116211

We generally start with Coulombic potential.

user228700

:-) All of my questions are very basic sir, so I dunno if they will be of interest to you but my next question is about relative angular velocity and I am just not at all understanding what the heck my book is telling me. So I was gonna post a pic...

user116211

6:47 AM

And spherical coordinate system is quite handy when you are working with Coulomic potential.

A: Trying to understand the necessity of orbital approximation or the independent electron model

@Kaumudi carry on ...

user116211

If yu want, @Kaumudi, check this related post:

user116211

6

Setting the stage Once the Born-Oppenheimer approximation is introduced, we have (first) to solve the so-called electronic Schrödinger equation that could be written as follows, \begin{equation} \newcommand{\el}{_{\mathrm{e}}} \newcommand{\elel}{_{\mathrm{ee}}} \newcommand{\elnuc}{_{...

user228700

@MAFIA36790 OK, thank you :-)

user116211

@0celo7, go back to bed. Have a sleep.

user228700

6:52 AM

Stupid phone, logging me out every now and then. Sorry.

user228700

@JohnRennie: Do u remember how I asked u about the definition of a reference line? I'd asked to make better sense of this only.

Ok, so now my question is on hold. and I allready edited once. In my point of view this should be within these limits; meta.physics.stackexchange.co… "For example, a question that proposes a new concept or paradigm, but asks for evaluation of that concept within the framework of current (mainstream) physics is OK."
So should I edit it (how?) or delete it?

user228700

@MAFIA36790 Didn't he say that his phone keeps logging him back in?

this; physics.stackexchange.com/questions/284717/…

6:58 AM

@Kaumudi OK. Incidentally the first statement on that page angular velocity is also always relative is not true.

user116211

@Kaumudi Maybe.

user228700

@JohnRennie Oh..?

user228700

"There is no such thing as absolute angular velocity"

There is no way to measure absolute velocity. However any rotation creates pseudoforces, so it is always possible to measure absolute rotation by measuring the associated pseudoforces.

user116211

@Kaumudi Check Newton's Bucket Experiment.

7:01 AM

But I suspect this is a distraction.

It is certainly possible to measure relative angular velocities.

user228700

@JohnRennie What is? The thing about pseudo forces?

I mean that if you're asking about relative angular velocity we probably don't need to worry about whether angular velocity can be absolutely defined.

user228700

@JohnRennie That doesn't make sense. Why not? If a quantity is absolute, there is no talking about relative, no?

Mass is absolute, but I can still say mass A is 1kg lighter than mass B.

user228700

Right. Alright then, let's skip that discussion.

7:04 AM

Probably wise

user228700

But I wasn't able to understand how they arrived at those formulae... :/

Probably because that's a really unhelpful way of describing it.

user228700

Also, is it OK to start/end sentences with the word "but"? I remember my English teacher in primary school told me it was incorrect but I've read this usage in books and all and I've always wondered if my teacher was correct that day, lo those many years ago :-P

@Kaumudi no, that's generally regarded as poor grammar, but on a chat room no-one cares.

user228700

OK :-)

user228700

7:07 AM

This:

Are you happy you understand how angular momentum is defined?

user228700

"Angular velocity of a particle 'A' with respect to the other moving particle 'B' is the rate at which position vector of 'A' wrt 'B' rotates at that instant" I cannot understand this AT ALL!

@JohnRennie Hi, You must have been the first to put this on hold. physics.stackexchange.com/questions/284717/… I am not interested to start to edit it x-times to try to find out if it will be aloud here. Can you be more specific?

@Kaumudi well that's a rather wordy definition. A more precise definition is $$\mathbf v = \mathbf r \times \omega$$

user228700

But they're talking about relative angular displacement, no?

user116211

7:11 AM

@Kaumudi $\mathbf r$ is relative.

user228700

So that's all there is to it? The displacement vector is swapped for the relative displacement vector?

@Kaumudi: when we define vectors like $\mathbf r$ and $\mathbf v$ we do so using some coordinate system that we've chosen.

Typically this is an inertial frame with ourselves at the origin

@Obliv Not just definitions, but also important features such as ideals and congruences. I have not included homomorphisms because they are not very important unless I need to compare between two algebraic structures

But we can choose a rotating coordinate system. In that rotating coordinate system $\mathbf r$ and $\mathbf v$ will have different values so the angular velocity we calculate will be different.

NB I copied all of this from wikipedia, because I found I always confuse the various types of ring generslisations

user228700

7:17 AM

@JohnRennie But won't these values be continually changing..?

user228700

Sir, I need to have lunch. I'm starving. You please carry on with ur work :-) I'll ask the rest of my questions another time.

@Kaumudi yes, choosing some random frame will generally result in the angular velocity being time dependent.

But that's not anything unusual.

It just means you're working in non-inertial frame.

user228700

@JohnRennie OK...

At undergrad level I generally advise students to be very careful about using non-inertial frames because your intuition may fail you and it's eaasy to make mistakes.

user228700

@JohnRennie But it so happens that I need to work with them every now and then because I have to solve complicated problems :/

user228700

7:22 AM

You generally advise students..? Were u a teacher at any point?

user116211

@Kaumudi He is teaching you.

user228700

@MAFIA36790 Of course he is :-) I was just wondering if he was ever a teacher IRL.

No, I've never been a teacher. I was speaking figuratively.

6 mins ago, by John Rennie

At undergrad level I generally advise students to be very careful about using non-inertial frames because your intuition may fail you and it's eaasy to make mistakes.

For starters, tons of fictitious forces to worry about

user116211

8:00 AM

Hallo @loong!

Loong

@MAFIA36790 hi

Q: If a system is deterministic, will it still be deterministic if time is reversed?

0

If you were to drop a ball, it would be easy to calculate when it will hit the ground and how much energy will be absorbed by the ground (let's assume there's no air resistance and the ball does not bounce). If you were to then reverse time, would energy from the ground gather rapidly toward the...

determinism time-reversal-symmetry

Well it has to be, otherwise time reversal symmetry will be broken

But even then it is not the main point, what will it mean for a system that evolve backwards to be nonrandom nondeterminstic and what happens when such is interacted with a system that is deterministic in forward evolution?

Danu

8:30 AM

Almost feel bad for this guy:

physics.stackexchange.com/a/278877/24653

Q: Can Planck's constant be defined through the speed of light

I think this should be ok now;

-5

This is the question; Could it be defined through the speed of light using only known mathematics?

quantum-electrodynamics

@Danu The future may be already there, except there might be never any way to know...

@JohnRennie Hi, You vote the first close for my question. -Is it now ok?

@innisfree Ok?

@bosoneando Ok?

Q: Can I copy a whole section from a book as an answer?

8:51 AM

2011 stuff: I have always wondered about the thought experiment of an entangled pair where one pair is relatively boosted wrt another

user228700

@JohnRennie Oh, OK :-)

Physics Meta

9:38 AM

0

We have a question that has been unresolved for a few months. I have now found a section of a book (1-2 pages) which is excellently written and I wish to share as a possible answer. Am I allowed to just copy the whole section out and reference the source. I fear any attempt by me to paraphrase it...

discussion

10:02 AM

@Danu "Instead, one could assume a spaceshit traveling at 99,99% c." -spaceshit? :-) I thought the maximum speed of spaceshit was something like 99.97% c. but maybe the shit is different in n-dimensions, and my shit in 4-dimensional space just stinks.

physics.stackexchange.com/questions/278870/…

Danu

10:43 AM

@JokelaTurbine lol

anyone currectly not afking can help me on a monoid concept question?

Ramanujan

Hi nerds

user228700

11:07 AM

Hi, fellow nerd :-)

11:28 AM

physics.stackexchange.com/questions/277183/… what an impressive first post :O

user116211

@Secret What question?

user116211

@Sanya Thank you Sanya! Is it you who gave me a bronze badge? thank you for that as well in that case! BrouH's question is a very important one, and I've never seen it formulated as neatly and simply as he/she did. I wish BrouH would send it to pedagogical journals like American Journal of Physics or European Journal of Physics and see what answer it'd receive there... — pglpm 8 mins ago

@MAFIA36790 ?

user116211

@Sanya Thank you Sanya! Is it you who gave me a bronze badge? thank you for that as well in that case!

"A subset S of M is said to be a generator of M if M is the smallest set containing S that is closed under the monoid operation, or equivalently M is the result of applying the finitary closure operator to S"

@MAFIA36790 Is it saying that $S$ closed under the monoid operation of $M$, or only $M$ is and applying the monoid operations on $S$ gives $M$?

11:38 AM

@MAFIA36790 you mean I should reply to that or sth?

I'm unsure of what point you are making

user116211

@Sanya No; he looks happy ;)

well, that's something I'd be happy about; I was really happy reading the post

it came up in the late answers review; otherwise I'd probably have missed it

user116211

@Secret Monoid is defined to be a semi-group which itself is a magma and thus closed.

user116211

But I'm reading still the upper para...

Qmechanic

@EmilioPisanty : Just logged in. Did any mod help you?

user116211

11:46 AM

Hmm, I've not come to the concept of generator but let me understand the part...

David Z

@Qmechanic I left a comment on the answer

innisfree

I want to understand some trivial things about baryogenesis and statistical mechanics. Anyone know about Sakharov's 3rd law?

user116211

@Secret The operation $M$ must be closed for $S$ to be the generator of $M\,.$

But is S does not need to be closed under the operation, as otherwise, I am not able to see how one can make M by applying the operation on all elements of S?

That is, if the operation always maps any element in S back to S, then it cannot give elements in M-S?

user116211

Basically, One of the conditions for $S$ to be the generator of the algebraic structure $(M, \circ )$ is that $\forall x,y \in S: x\circ y \in M\,.$

12:03 PM

ok that makes sense, since based on what I understood, a generator should act like for monoids as basis sets to vector space

innisfree

In Sakharov's 3rd, does thermal equilibrium <-> Gibbs' canonical ensemble?

Meanwhile, I am on the group article. I am currently trying to understand the interesting properties of each algebraic structure that are derived form their axioms

e.g. ideals in rings, set of idempotents in semigroups, symmetry in groups etc.

user116211

I have not still encountered ideal.

There's a notion of ideals in semigroups too. But ideals are most important in rings

An ideal $I\subset S$ is such that $\forall r\in I, \forall a \in S, ar\in I \text{ or } ra\in I$

user116211

@Secret okay.

12:10 PM

ACM said that ideals are generalisation of the notion of even and odd numbers

For me, its more like a generalisation of an absorber, because anything multiplied by an element in an ideal get sent into the ideal and thus there is no escape