The h Bar

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12:48 AM

@Charlie I think in this case it really means tax free, as in "not subject to income tax". It's a special scheme to train people to teach in subjects in which there is a shortage of teachers. I don't know the details but I remember hearing about it a few years ago. £26k for maths/physics/computer science is pretty good, for other subjects it's much less.

It's similar to PhD studentships in the UK. Not sure if it's exactly the same, but for that it does not count towards income tax at all, if you have additional income you still have the tax-free allowance etc. The downside is that you don't get contributions for state pension.

(not subject to income tax or NIN contributions)

2 hours later…

vzn

3:03 AM

@DanielAdams welcome to thermo+fluid dynamics. its helpful to look into the history. re bolbteppas reponse: brownian motion was originally detected with random motion of pollen grains visible in microscopes. it is not commonly known but this was one of the origins of the atomic hypothesis: that invisible atoms/ molecules (water) bouncing around with heat energy proporptional to kinetic energy were "knocking" the grains. einstein calculated atomic parameters with some reasonable assumptions.

user586228

3:17 AM

@Buraian If it is possible can we get in touch about the previous problem soon?

We were supposed to discuss on tensors.

6 hours later…

Physics Meta

9:05 AM

Q: Why was my question from 4 years ago recently deleted?

I think it's a reasonable question which can be answered. Why was it deleted?

Q: Need help finding old deleted question

I wrote a question, and later provided my own answer. I spent a decent amount of time writing up that answer and I would like to have it for my notes, but it seems like it was deleted at some point. It was a question about how photons can be expressed in a cavity using the transmission function o...

support

2 hours later…

Charlie

11:00 AM

@fqq oh interesting ty

10 hours later…

Physics Meta

8:42 PM

Q: Why aren't "duplicate" questions allowed?

It implies that some random answer from the past is the definitive answer. And we all know that questions can be answered many different ways. And the duplicate question usually isn't an exact duplicate. And if someone answers my question now, then I can comment and ask a question, but the person...

discussion

2 hours later…

ACuriousMind

11:08 PM

Today's SMBC is very relevant to this chat's name

schn

what is meant by "properties that require two directions (second rank tensors), can be described by 9 numbers" as stated here on tensors? doitpoms.ac.uk/tlplib/tensors/….

Two directions...2*3 components=6.

ACuriousMind

@schn A "second rank tensor" is a matrix, hence 3^2 = 9 components

The idea is that it's an object $A$ with two indices for "directions", so it has components $A_{ij}$.

schn

@ACuriousMind I see. "...properties that require two directions..." just made me think about a vector space/plane, which is spanned by two vectors.

ACuriousMind

11:24 PM

sure, but the general tensor does not prohibit that the two vectors are parallel

so if you have $n$ basis vectors $v_i$, a basis of second rank tensors is given by $v_i\otimes v_j$, which are $n^2$ many different combinations

schn

Very basic question, but what are some "properties that require two directions"?

Physics related properties, that is.

ACuriousMind

I don't like that particular phrasing, but one example of a full second rank tensor in physics is the inertia tensor of a rigid body

schn

On a slightly related note, on slide 5 (sjsu.edu/faculty/beyersdorf/Archive/Phys208F07/…), what are some every-day phenomena that can have a response in a direction different than the driving force?

Maybe too basic to give a simple answer.

ACuriousMind

I'm not sure what that's supposed to mean, and the pdf you linked doesn't load for me

schn

11:45 PM

Basically it's about anisotropic media, where the dielectric tensor is introduced (since the medium under consideration now is anisotropic). Here is the slide before the slide I was referring to: