Wikipedia:Reference desk/Archives/Science/2015 August 27

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August 27

Average size of 12 year old

What is the average size of a 12 hear old? — Preceding unsigned comment added by 66.214.53.17 (talk) 00:29, 27 August 2015 (UTC)

African, or European? μηδείς (talk) 00:45, 27 August 2015 (UTC)

• Assuming you mean height that is a complex question. Human height can vary between male or female and between populations. It also depends on what data is collected and can vary between studies. If you can provide a little bit more information to your question we can try to get you data for the population you are looking for. --Stabila711 (talk) 00:53, 27 August 2015 (UTC)

Assuming she means human, too. Hope so, because the "average dog" question is much harder. InedibleHulk (talk) 01:01, 27 August 2015 (UTC)

Interestingly enough, male and female human children are about the same weight and height at 12 years old, according to this [1] source - roughly 90 pounds and 59 inches, with females being a bit taller and heavier than males at that age, on average. SemanticMantis (talk) 01:43, 27 August 2015 (UTC)

That would be 40.8 kg and 150 cm in proper units. Also, this would of course vary massively across the world. 131.251.254.154 (talk) 15:46, 27 August 2015 (UTC)

Mixing catechol with formaldehyde and HCl

I'm trying to obtain the condensation product, i.e. essentially using the formaldehyde as a protecting group for the catechol (forming a methylene bridge) but it appears that side reactions with the electron-rich ring might happen. (For example, reacting vanillin with a 250C solution containing sulfur dioxide yields vanillic acid). What happens if you reflux a reaction mixture (at 40-55C) containing 1M catechol with 1M formaldehyde and 0.5N HCl in cyclohexanol? Yanping Nora Soong (talk) 01:20, 27 August 2015 (UTC)

The OH groups are very electron donating, so I wonder if reacting with the formaldehyde wouldn't result in any number side reactions, including perhaps some sort of hetero Diels-Alder; especially if the extra electron density provided by the two -OH groups de-aromatizes the ring, making it more diene like; formaldehyde is a perfectly good dienophile for that purpose. Catchecol wouldn't be a good candidate for nucleophilic aromatic substitution because of the electron-donating nature of the -OH groups, but on the other hand, it WOULD be a good candidate for electrophilic aromatic substitution, with the HCl providing chlorination. Just some ideas. --Jayron32 16:12, 27 August 2015 (UTC)

I didn't see anything on the web about making MDMA out of alpha-methyldopamine, so it's probably not that straightforward. :) Wnt (talk) 18:23, 27 August 2015 (UTC)

decay/equilibrium constant for connecting tanks of unequal pressures

I wonder why the following problem is not posed in undergraduate chemistry problem sets:

10L of 1 bar nitrogen is held in Tank A. Tank B has 10L of 2 bar nitrogen. The tanks are connected by an airtight supply tube 50 mL in volume and 10 cm^2 in cross-sectional area. How long would it take for the pressures in both tanks to equalize (or what would the decay constant be) where one tank would be within 95% (or perhaps 99.7 %) of the pressure of the other? Yanping Nora Soong (talk) 01:25, 27 August 2015 (UTC)

I don't know, but I'm thinking that the exact shape of the "nozzle" from which the tube emanates is going to affect turbulence in the tube and that minor changes in the design will create major changes in outcome.

This may be liquid rather than gas, but I remember a certain darkroom with a large circular basin, think it was about a yard in diameter, with a drain in the middle. The sink would seem to clog up and fail to drain at any perceptible rate - you'd come back the next day and there would be dried developer/fixer/whatever all over the basin. However, you could take a piece of pipe and hold it around the stream of water produced by the sink - touching neither the tap NOR the last inch before the drain - and you could turn that stream on to any amount, full blast, enough to fill the whole sink in minutes, and it would all go straight down the drain. I marvelled about that for what must have been cumulative hours, never managed to figure it out. Wnt (talk) 01:41, 27 August 2015 (UTC)

Sounds like airflow to me. The sink needed a vent, but did not have one. Without it air trapped in a bend prevented any water from passing. But using the pipe entrains enough air to prevent the problem. Ariel. (talk) 03:29, 27 August 2015 (UTC)

This sounds like something you might study in an aerospace engineering class, or a fluid dynamics class, or an applied mathematics class on modeling with differential equations. Why isn't it posed to undergraduate students of chemistry? Well, it could be - but the math is really hard, and it's not really part of a standard chemistry curriculum! Insofar as you might study gas equations in physical chemistry classes, or you might study how multiphase flow works with different species of gas, there's merit to categorizing this type of problem in the arena of "chemistry" ... but it's not by-the-book what you'd probably find in the homework sets for a standard university-level chemistry class.

Gas flow-rate through a nozzle is hard - it is literally actually rocket science. Don't underestimate how difficult it is to solve this problem with any degree of accuracy!

Here's a graduate-level textbook, Fluid Mechanics for Chemical Engineers. Every aspiring rocket engineer should buy this book, or one of many equivalents! Chapter 2 covers the tank-filling equation, developed from the Bernouilli equation.

Nimur (talk) 02:10, 27 August 2015 (UTC)

Basically that's engineering, not chemistry. You need to know about first order resonant systems, and the flow resistance of pipes. I have a sneaking suspicion that this is slightly more complex than a spring mass damper system, but I'm not going to work it out. A numerical simulation would be a good back check. Greglocock (talk) 03:41, 27 August 2015 (UTC)

No one has mentioned it, but for gases you also have the problem that they have different densities when stored at different pressures which will also affect the flow rate in somewhat complicated ways. In the way of spherical cows let's assume the tube has negligible resistance and length. Let's further assume that the rate is sufficiently slow that we can treat the problem as isothermal (gases will in general change temperature when they change pressure). Via Bernoulli's Law, and assuming no resistance, the flow rate from tank 2 to tank 1 could be estimated as ${1 \over 2}\rho _{2}(t)v(t)^{2}=P_{2}(t)-P_{1}(t)$. In the isothermal limit ${P(t) \over \rho (t)}={P(0) \over \rho (0)}$, where P is pressure and ρ is mass density. So:

$v(t)^{2}=2\left({P_{2}(0) \over \rho _{2}(0)}-{P_{1}(0) \over \rho _{1}(0)}{\rho _{1}(t) \over \rho _{2}(t)}\right)$

However, since total mass is conserved, $V\rho _{1}(t)+V\rho _{2}(t)=V\rho _{1}(0)+V\rho _{2}(0)\rightarrow \rho _{1}(t)=\rho _{1}(0)+\rho _{2}(0)-\rho _{2}(t)$

This leads to: $v(t)^{2}=2\left({P_{2}(0) \over \rho _{2}(0)}-{P_{1}(0) \over \rho _{1}(0)}{\rho _{1}(0)+\rho _{2}(0)-\rho _{2}(t) \over \rho _{2}(t)}\right)$

By conservation of mass, the total mass M in a tank must change as ${dM \over dt}=V{d\rho (t) \over dt}=A\rho (t)v(t)$.

So ${d\rho _{2}(t) \over dt}={A \over V}\rho _{2}(t){\sqrt {2\left({P_{2}(0) \over \rho _{2}(0)}-{P_{1}(0) \over \rho _{1}(0)}{\rho _{1}(0)+\rho _{2}(0)-\rho _{2}(t) \over \rho _{2}(t)}\right)}}$.

Believe it or not, that equation has an analytical solution, but it is complicated enough that I am not going to try and write it down. The above discussion should be enough to show that the issue is somewhat complicated, even after liberally making approximations. One could make further approximations and say that the initial time constant looks something like:

$\tau \approx {V \over A}{\sqrt {{1 \over 2}{{\rho _{2}(0)} \over {P_{2}(0)-P_{1}(0)}}}}$, but that only works at early times.

In short, this is not a calculation that is really suitable for a chemistry class. Dragons flight (talk) 10:07, 27 August 2015 (UTC)

Thanks. I'm a chemistry graduate; dabbled in engineering (took two MSE classes) and studied a bit about Fick's laws of diffusion but having both worked in McDonald's and on the lab bench I've often thought about diffusion processes. They never mentioned "avoid turbulence" in organic chemistry lab (or when you were connecting the CO2 to the soda fountains in McDonald's) so I didn't think turbulent flow would be even a problem! (I have solved boundary layer equations in Widely Applied Physics and got an A, but I don't remember most of what I solved. If flow is so essential to chemistry why isn't it studied in chemistry? Yanping Nora Soong (talk) 10:51, 27 August 2015 (UTC)

It is studied in some areas of chemistry but for many applied problems the best answer is either A) avoid situations where uncontrolled mixing is important or B) make measurements to determine what is happening rather than trying to figure it out analytically. For example, when mixing two tanks of gas, one would almost certainly use pressure gauges to know when they had equilibrated rather than trying to predict it. Dragons flight (talk) 11:07, 27 August 2015 (UTC)

Have we got an article on ...

I'm looking for an article that explains the way that any "freefall" through a straight tunnel through the earth always takes the same time (around 40 minutes). I don't know what it's called, so I can't search for it. -- SGBailey (talk) 12:00, 27 August 2015 (UTC)

See Gravity train. - Lindert (talk) 12:08, 27 August 2015 (UTC)

Cosmos

If the atoms resulting from the Big Bang. How massed or gathered to be the first stars if atoms were traveling at similar speed of light and the distance between them fixed because it is going the same speed. What thing that stopped by and make them assembled together to be the first stars. If it were of gravity where it came from and what its source? — Preceding unsigned comment added by Abed23455890 (talk • contribs) 13:19, 27 August 2015 (UTC)

Wikipedia has articles you can read on the early time after the formation of the Universe. You can find this information at articles titled Chronology of the universe, Big Bang, Planck epoch, Inflationary epoch, and Baryogenesis. --Jayron32 13:29, 27 August 2015 (UTC)

Anapoles, Majorana fermions, dark matter, invisible silicon disks, quantum computing, the non-radiation from electrons in atoms etc.

I just read [2]. Its new result seems to be that the Cartesian formula for radiation and the "vector spherical harmonics used in a Mie basis set" description were inconsistent - and the Cartesian version was wrong?! I'm not sure what of the rest is new and what is known, but it says that you can make an "anapole", a toroidal EM field that emits no radiation at a distance, where "The absence of radiation is the result of the current being divided between two different components, a conventional electric dipole and a toroidal dipole (associated with poloidal current configuration), which produce identical fields at a distance." It seems to suggest this configuration is the reason why electrons in atoms don't radiate electromagnetic energy (I'd always heard it was quantization, Pauli exclusion principle etc., but who knows?). It says that people actually made silicon wafers with fields like this in them, which were therefore "invisible" in some sense. A figure caption mentions dark matter, and that's when I went back and looked at stuff like [3] and Majorana particle. Apparently neutral leptons that have no charge for some reason are expected to have this electromagnetic field (why? Can you basically pinch off a little smoke ring of EM field and leave it out, stable, contained, as a brand new baby dark matter particle?) Anyway, I am in dire need of a clue here, any clue. :) Wnt (talk) 22:13, 27 August 2015 (UTC)

You can read a paper on the topic at http://wwwrsphysse.anu.edu.au/nonlinear/papers/pdf/SREP_2015_5_9574.pdf. I will comment further when I have looked at it! It looks as if his area of research is nanotech.[4] Graeme Bartlett (talk) 23:51, 27 August 2015 (UTC)

OK that paper is not really on the topic, but it is trying to make cylinders invisible. But it only works at one wavelength and polarization. Note that there are currents involved with something that carries the current, so it is not just an electromagnetic only phenomenon. It looks as if here geometric theory of diffraction was applied. The formula in the paper is very cut down to just decomposing the incident radiation into cylindrical harmonics, with the actual geometric theory of diffraction scattering formula taking up half a page. Graeme Bartlett (talk) 00:23, 28 August 2015 (UTC)

The term "Mie basis set" seems to be an invention of the press release people. Spherical harmonics#Orthogonality and normalization could be the actual basis set described here though. Other known stuff is that the neutral lepton is a neutrino which is a dark matter particle and which may or may not be a Majorana particle. Electron electric dipole moment is close to zero if not zero. Neutrino dipole is likely zero too.[5] Graeme Bartlett (talk) 00:44, 28 August 2015 (UTC)

"Mie basis" probably refers to Mie scattering. -- BenRG (talk) 01:28, 28 August 2015 (UTC)

The thing you read is a press release from ANU, seemingly quoted verbatim. It was written by a journalist who's paid to aggrandize the university, and who probably doesn't know any physics, so it's probably a bit overhyped. The paper that prompted it seems to be "Nonradiating anapole modes in dielectric nanoparticles" by Miroshnichenko et al (doi:10.1038/ncomms9069). I don't know anything about nonradiating configurations of classical charges, but Wikipedia has an article about it, nonradiation condition. Research on that subject goes back to 1910 if not earlier, and the word "anapole" dates to 1957, so this is definitely not the "new theory" that ANU's PR department says it is. My uneducated guess is that this particular paper is not very important, given that it was published in an unprestigious author-pays journal and there are some grammatical oddities in the abstract suggesting that the journal editors didn't look at it very closely (e.g. "attract" instead of "have attracted"). -- BenRG (talk) 01:28, 28 August 2015 (UTC)

That article nonradiation condition is definitely interesting. At the moment I don't have the foggiest notion of how a model for a radiation-free motion of charges can get us to Planck's constant, though! It has other maintenance tags up on it, and is pretty short - I'd love it if someone reading this would take some time to improve and expand it. Wnt (talk) 12:25, 28 August 2015 (UTC)