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October 12

Eat stuff or convert light into energy

Is there a living being that can or could do both? Did some species lose the ability to do photosynthesis and started eating things? OsmanRF34 (talk) 02:42, 12 October 2013 (UTC)[reply]

Euglenozoa ancestrally have the ability to do both (autotrophic and heterotrophic) but kinetoplastids have lost the ability, including some very serious human etiologic agents. Wnt (talk) 03:08, 12 October 2013 (UTC)[reply]

Carnivorous plants do both. --Jayron32 03:16, 12 October 2013 (UTC)[reply]

There's also a certain Mediterranean wasp that can both eat stuff and do photosynthesis -- I forgot its name, though. 24.23.196.85 (talk) 20:16, 12 October 2013 (UTC)[reply]

The Oriental hornet has been featured in some articles for this. — Quondum 01:07, 13 October 2013 (UTC)[reply]

(edit conflict) There are many examples of living things that do both. Photosynthesis#Evolution might be a good place to start. Giant clams and upside-down jellyfish are a few examples. Saprophytic plants (no article?) such as some species of Triuridaceae are examples of species that have lost the ability to photosynthesize.--Wikimedes (talk) 20:19, 12 October 2013 (UTC)[reply]

File:Elysia chlorotica (1).jpg

Elysia chlorotica larva

There is Elysia chlorotica, a sea slug (gastropod) and other members of its genus. And I believe there are symbiotic flatworms as well, but I couldn't find a name or get an article. μηδείς (talk) 00:24, 13 October 2013 (UTC)[reply]

Here's a bit on photosynthetic flatworms [1]. Apparently they are considered to be salt water aquarium pests.--Wikimedes (talk) 02:08, 13 October 2013 (UTC)[reply]

Yeah, I saw that link. They don't give or link to a scientific name, and the pictures in the gallery look like different species. μηδείς (talk) 16:34, 13 October 2013 (UTC)[reply]

But, species that eat stuff and transform light into energy, just use the eaten stuff as nutrient, or as a source of energy too? And why would any species lose the ability to use light? Where is the advantage of having to hunt other living beings? OsmanRF34 (talk) 22:31, 13 October 2013 (UTC)[reply]

There will be no general rule about whether nutrients or energy are the primary benefit derived. Some species will use energy obtained from eaten stuff (e.g. the Oriental hornet), some probably not (such as the Venus flytrap, which probably primarily derives nitrogen-containing nutrients from the digested insects, which tend to be insufficiently supplied by the soils that it grows in). As a general rule, though, when a species occupies an ecological niche in which it gains no benefit from an adaptation, this adaptation will eventually disappear. A fish species that lives in lightless caves eventually loses its eyes after many generations; species that live where light or other necessary factors for using the energy in light is lacking will lose the ability to use the light in time. Even if a reliable alternative is present which makes an adaptation valueless, the adaptation will be lost. — Quondum 23:56, 13 October 2013 (UTC)[reply]

Einstein disproof of Friedmann's expanding universe calculations?

When Friedmann showed that the equation of General Relativity meant that the universe must be either expanding or contracting, did Einstein at first propose a proof that Friedmann was wrong (and later retract the proof)? Bubba73 ^{You talkin' to me?} 05:09, 12 October 2013 (UTC)[reply]

That would be Einstein's non-zero Cosmological constant which he later described as his "greatest blunder". Spinning Spark 21:31, 12 October 2013 (UTC)[reply]

That is what the statement sounds like to me, but it said that he had a proof "later retracted" that Friedmann was wrong about asserting that the original GR says that the universe must be either expanding or contracting - and that part I don't know about. Bubba73 ^{You talkin' to me?} 21:39, 12 October 2013 (UTC)[reply]

Searching Google Books, I found a reference to this in "The Inflationary Universe" by Alan Guth, p.45: [2]. -- BenRG (talk) 05:28, 13 October 2013 (UTC)[reply]

Google won't allow access to that page of the book for me, but here is part of Einstein's retraction I got with a differnt search for anyone else having the same problem;

My objection rested however — as Mr. Krutkoff in person and a letter from Mr. Friedmann convinced me — on a calculational error. I am convinced that Mr. Friedmann's results are both correct and clarifying. They show that in addition to the static solutions to the field equations there are time varying solutions with a spatially symmetric structure.

Guth also says "In spite of this comment, Einstein still believed that the Friedmann solutions were of no relevance, as he remained confident that the universe is static". A belief he held until Hubble's momentous discovery. Spinning Spark 09:44, 13 October 2013 (UTC)[reply]

What roads (non-mwy) in the UK have jct numbers?

I know the A55, the A14, and the A12.Is that all?Or is there more?I don't know. Pls if someone would give those in Ireland then help welcome. — Preceding unsigned comment added by Puntaalpo (talk • contribs) 09:29, 12 October 2013 (UTC)[reply]

This is more of a Misc Desk Q than Science. StuRat (talk) 10:54, 12 October 2013 (UTC)[reply]

It doesn't belong here, but I will note the A1 in England does (there is also an A1 in Northern Ireland and I can't speak for that one). --TammyMoet (talk) 12:59, 12 October 2013 (UTC)[reply]

You can download a pdf map from this Highways Agency page which seems to have the answers to your question. It is very detailed and takes a while to download and load. Spinning Spark 22:38, 12 October 2013 (UTC)[reply]

mwy = motorway and jct = junction? --NorwegianBlue^talk 17:28, 13 October 2013 (UTC)[reply]

That was my understanding when I answered. Fairly unambiguous I think. Spinning Spark 19:35, 13 October 2013 (UTC)[reply]

Peterborough is a hotbed of junction numbers[3][4] and, as you would expect, WP regards the matter as encyclopedic – Road transport in Peterborough#Numbered Junctions. Thincat (talk) 20:14, 13 October 2013 (UTC)[reply]

October 13

Floor tile material

I'm trying to determine the material some floor tiles are made of. At first, I thought they were vinyl, but after examining a few broken pieces, I now think it's something else. The tile pieces seem too heavy (dense) to be vinyl. They also feel cool to the touch, suggesting a higher thermal conductivity than familiar plastics. The tiles are about 1/8 inch thick (~ 3.2 mm). When I looked at the side of a broken piece, I noticed a rough texture that you're expect from broken earthenware. I also noticed a few sparkling (reflective) particles. What material could that be? Any educated guesses? --173.49.18.190 (talk) 03:06, 13 October 2013 (UTC)[reply]

Ceramic, just like you thought. Was that a bathroom tile? 24.23.196.85 (talk) 06:10, 13 October 2013 (UTC)[reply]

Ceramic is a possibility that I considered, but one thing I didn't mention was that the tiles seem at least somewhat flexible, as they seem to conform to and reveal the unevenness of the concrete floor below it. The location is not a bathroom. The tiles don't have any features that suggest they are designed for bathrooms. --173.49.18.190 (talk) 13:07, 13 October 2013 (UTC)[reply]

If it's from an older house, especially pre-1950's, it could be linoleum. Linoleum was made by mixing all manner of mineral powders, wood flour, etc, in a mixture of pine resin and linseed oil as a binder. The pine resin and linseed oil cured to a hard plastic, and the back side was often given a rough surface. Linoleum is extremely durable and you still sometimes find it. The fillers used, which probably account for >95% of the mass, do provide a higher thermal conductivity and considerably more thermal inertia that modern plastics. 121.221.33.239 (talk) 07:49, 13 October 2013 (UTC)[reply]

They could well be linoleum but a word of warning. Many vinyl floor tiles made between 1920 and 1990 contain asbestos and you should get professional advice before disturbing them see: here. Richerman (talk) 09:19, 13 October 2013 (UTC)[reply]

I don't know the vintage of the tiles, but they don't seem to have been laid during the period you mentioned (I think they're newer.) Do tiles that contain asbestos have markings or other telltale signs identifying them as such? --173.49.18.190 (talk) 13:07, 13 October 2013 (UTC)[reply]

No. In my country there are laboratories that will examine a sample and for a small fee tell you if there is asbestos. I guess there will be similar labs in other Western counries. 121.221.33.239 (talk) 15:08, 13 October 2013 (UTC)[reply]

No - if you look at the website I mentioned above it says "Vinyl flooring and wallpaper that contain asbestos cannot be recognized on sight" but it does tell you how to deal with them. However, it's not for me to say whether you should follow that advice or not - you must decide that for yourself. Richerman (talk) 19:21, 13 October 2013 (UTC)[reply]

It sounds like vinyl composition tile (VCT), which is a mixture of clays and other minerals in a vinyl resin binder. It's the ubiquitous 12" x 12" tile material seen in schools, large grocery and discount stores (Walmart uses square miles of the stuff), and institutional buildings all over North America. It replaced the more-durable vinyl asbestos tile (VAT), which used asbestos fibers as part of the composition, in the 1970s. It's not always obvious which is which, though VAT isn't especially dangerous unless it's been abraded or pulverized. Usually (but not always) VAT is more brittle than VCT, which will bend a little before breaking, and tends to come in 8" x 8" or 9" x 9" configurations as well as 12" x 12". There is no specific marking for asbestos tiles, since at the time they were being made and installed, asbestos was considered a good thing and was incorporated into a wide range of building products. Here's one manufacturer [5]. Acroterion (talk) 19:05, 14 October 2013 (UTC)[reply]

Vegetable identification

Hi guys. What is the vegetable behind the Apple? Thanks. Ben-Natan (talk) 05:39, 13 October 2013 (UTC)[reply]

It looks like a gourd. Plasmic Physics (talk) 06:26, 13 October 2013 (UTC)[reply]

Some variety of Cucurbita pepo, perhaps a yellow crookneck squash. μηδείς (talk) 20:32, 13 October 2013 (UTC)[reply]

Solution book of University Physics with modern Physics 13th edition

While solving problems of Sears and Zemansky's University Physics (by Hugh D. Young and Roger A. Freedman) 13th edition, I face two problems. First, answers of only odd-numbered problems (not the even-numbered problems) are given in the end of book. Whenever I solve even-numbered problems, I am not able to check whether my answers are right or wrong. Second, some problems are really very tough. Also, neither my teachers nor my friends use that book, and they often make excuses whenever I ask my unsolved problems to them. Therefore, I need a link to a website or any solution book. Thanks for helping me. Scientist456 (talk) 07:49, 13 October 2013 (UTC)[reply]

You need this booklet with answers to even numbered questions which should have come with the book. You can buy second hand copies from Amazon Marketplace (sorry, I can't post a link, the site seems to be on the blacklist) but most of them do not specify which edition they are for and I don't know how different the questions are between editions. Spinning Spark 10:41, 13 October 2013 (UTC)[reply]

What is a prepalate?

I am contributing to the article on Kitab al-'Ayn, the first dictionary for the Arabic language. The author arranged the letters according to the source of pronounciation instead of alphabetically.
"Oh, that's a question for the language section of the reference desk."
No, ominous voice. I don't need to know about linguistics, I just need to know what a prepalate is. Is that another word for the hard or soft palate, for example? MezzoMezzo (talk) 10:33, 13 October 2013 (UTC)[reply]

This diagram comes from our article Places of articulation. The prepalate is at 6, between the very back of the alveolar ridge and the hard palate. The hard palate is at 7 and the soft palate further back still, at 8. It's a small area and often seems to be subsumed into the areas surrounding it for linguistics purposes. A Google search reveals that oral surgeons do often refer to it as a discrete area in the context of describing the extent of cleft palate. - Karenjc (talk) 12:10, 13 October 2013 (UTC)[reply]

What is the scientific explanation for this !

on link http://www.youtube.com/watch?v=jxOyST6gM_A — Preceding unsigned comment added by 37.237.196.193 (talk) 11:03, 13 October 2013 (UTC)[reply]

Very amatuerish camera trickery, eg stop motion shooting/editing. For arms shown projecting thru solid objects, chromakeying is the proffessional way to fake it, but this video is sufficiently rough and blurry that they could simply have made a hole in a sheet of fibreboard or similar. 121.221.33.239 (talk) 12:53, 13 October 2013 (UTC)[reply]

This is not amateurish camera trickery. The video quality isn't as good as one might expect under the circumstances, but the person involved is Criss Angel, a famous magician, and the video is apparently a (somewhat low quality) copy of an episode of his TV show Criss Angel Mindfreak, which presumably was broadcast with a much higher video quality. Criss Angel is easily rich enough to afford a high-quality custom built trick gate. Red Act (talk) 03:28, 14 October 2013 (UTC)[reply]

Your search keywords are "revealed" and "exposed". There are hundreds of videos trying to explain these magics tricks. Cacycle (talk) 17:47, 14 October 2013 (UTC)[reply]

A very rich dude acting as a boofhead and using professional staff and equipment to do work with a very amatuerish look then. 121.215.39.252 (talk) 23:51, 14 October 2013 (UTC)[reply]

Well, yeah, that's kind of his shtick. His genre is guerrilla magic, so it fits the style he's aiming for to look like an unpolished amateur "average joe" that you might happen to see walking down the street on an average day, except that he's doing something astonishing. I think the idea is that the "magic" is supposed to seem more amazing if it takes place under ordinary circumstances involving an ordinary guy, instead of taking place on a stage under the auspices of a guy in a fancy outfit, under circumstances that are clearly highly controlled. Red Act (talk) 01:28, 15 October 2013 (UTC)[reply]

Is there any picture of the mixed flow compressor in the pratt and whitney pw 600f?

hey guys do you know if any picture of the mixed flow compressor in the Pratt and whitney pw 600f exists? cause pratt and whitney mentioned this themselves and the wikipedia artickle says it but there is only one patent picture buzt it downt shows rhe mixed flow compressor..Saludacymbals (talk) 17:12, 13 October 2013 (UTC)[reply]

You've seen our Pratt & Whitney Canada PW600 article I take it? Spinning Spark 21:33, 13 October 2013 (UTC)[reply]

yes i saw the article but there is only the picture of the fan and not the compressor. so is there any picture of it?

amazing spider web bridge line (which type of spider?

http://www.youtube.com/watch?v=zubftSpCg2E in this video you can see an amazingly thick bridgeline of a spider web but there is a photo in the description of this type of spider. dguys do you know wich type it is? maybe darvins bark? Saludacymbals (talk) 18:36, 13 October 2013 (UTC)[reply]

Male and female variation, height and puberty

It is often said that men are taller than women; however, some women are taller than some men.

It is often said that girls begin puberty somewhat earlier than boys; is it also true then that some boys begin puberty earlier than some girls?--Whatdeanerwastalkingabout (talk) 23:57, 13 October 2013 (UTC)[reply]

While the logical link you are suggesting is fallacious, the answer is yes, some boys begin puberty at an earlier age than some girls. Puberty onset depends on a variety of factors, including sex, genes, nutrition and social circumstances. So boys with favorable factors for early puberty will experience it earlier than girls with unfavorable such factors. Effovex (talk) 01:56, 14 October 2013 (UTC)[reply]

It is often said that women are more fallacious than men. But some men are more fallacious than women. μηδείς (talk) 02:18, 14 October 2013 (UTC)[reply]

The trouble with women is that they all make sweeping generalisations :) Richerman (talk) 09:09, 14 October 2013 (UTC)[reply]

People say men are cunning linguists. However, some women are more cunning linguists then men. — Crisco 1492 (talk) 09:15, 14 October 2013 (UTC)[reply]

Hmmm... I was going to say something about women being more fellatious but thought better of it. Richerman (talk) 09:24, 14 October 2013 (UTC)[reply]

What in the world did you think I was punning about? μηδείς (talk) 18:56, 14 October 2013 (UTC)[reply]

You could always try phallacious. -- Jack of Oz ^{[pleasantries]} 11:06, 14 October 2013 (UTC) [reply]

Note being one to engage in such puns and frivolity, let me try to give a straight answer. Things such as you describe generally follow a bell-shaped curve. I'm limited to what I can draw here, using ASCII text, but here's a rough idea of what it looks like:

  ^       _
  |      / \
N |     /   \
  |    / MEN \
  +------------>
       Height

That graph shows the number of men at each height. Note that there are more men of average height than very short or very tall men. If we did the same graph for women, we would find a similar pattern. although the average would be less than for the men:

  ^    _
  |   / \
N |  /   \
  | /WOMEN\
  +---------->
    Height

If we then graph both together, we get lots of overlap:

  ^    _  _
  |   / \/ \
N |  /  /\  \
  | /W /  \ M\
  +------------>
      Height

So, this shows that the average woman is shorter than the average man, but there are some women taller than some men. It's also likely that the tallest man will be taller than the tallest woman, and the shortest woman will be shorter than the shortest man, although not always, as the bell-shaped curve tends to become more variable at the ends. Now, you will sometimes find a pair of bell-shaped curves which don't overlap, like the weight of adult mice versus the weight of adult cats:

  ^    _        _
  |   / \      / \
N |  /   \    /   \
  | /Mice \  /Cats \
  +------------------->
         Weight

In that case, we could say that all adults cats are heavier than all adult mice. StuRat (talk) 13:05, 14 October 2013 (UTC)[reply]

Any two bell-shaped curves overlap by definition, a normal distribution does not have upper or lower limits. It's just that the overlap in some cases is negligible, so the chances of finding a cat smaller than a mouse are very small (assuming that their sizes really follow the bell-curve). - Lindert (talk) 14:06, 14 October 2013 (UTC)[reply]

Which of course brings us to the point of contention that most readily occurred to me, because they aren't necessarily likely to follow a bell curve at all. Stu's initial framing premise that "things such as the [the OP describes] generally follow a bell-shaped curve" is not necessarily at all true. It would depend greatly on the feature being considered (with considerable variation even amongst the obvious and superficial phenotypical features). Snow (talk) 22:06, 14 October 2013 (UTC)[reply]

Hippety Hopper. DMacks (talk) 14:10, 14 October 2013 (UTC)[reply]

Well no, it may not always be the best model, but I think he did a good job of showing how overlapping distributions lead to plenty of cases that happen differently than the average case. And because of the Central limit theorem, bell curves pop up all over the place in nature. In the case of height or start of puberty, it makes sense to expect a roughly bell-shaped curve, even though it obviously won't go off to infinity like an ideal one, and may have other deviations explainable by the properties of the things being measured. Katie R (talk) 14:46, 15 October 2013 (UTC)[reply]

Yes, it's not possible to have an infinite height or a zero/negative height. For puberty, I suppose it's possible for somebody to have some medical condition which causes them to live their entire life without hitting puberty. Puberty could also begin at, or even before, birth, due to some medical condition, but obviously can't start before conception. I have no idea whether such medical conditions actually exist, but they are theoretically possible. StuRat (talk) 17:48, 15 October 2013 (UTC)[reply]

Delayed puberty has many possible causes, some of which may "delay" it indefinitely. DMacks (talk) 03:40, 17 October 2013 (UTC)[reply]

October 14

Do fruit flies spontaneously generate?

I have seen and raised the maggots of Drosophila melanogaster in genetics lab in college. But although I have had briefly self-sustaining colonies of whatever teensy fruit fly is endemic to the NE US come into the house with fresh produce, why have I never seen one of their babies? (And please don't post pictures.) Thanks μηδείς (talk) 02:15, 14 October 2013 (UTC)[reply]

For the same reason you never see a baby pigeon - you're not looking in the right place! Richerman (talk) 09:19, 14 October 2013 (UTC)[reply]

Perhaps the best place to look is where fruit or their skins have been allowed to acidify, as might be suggested by the first answer in this link. While they may fly inside to lay their eggs on fresh fruit, they might be developing into maggots and then flies outside in the discarded compost and garbage. — Quondum 15:52, 14 October 2013 (UTC)[reply]

History of Animals, although it is not the latest scientific research on the matter, supports the theory of spontaneous generation. Thincat (talk) 09:59, 14 October 2013 (UTC)[reply]

Aristotle may have been a great philosopher, but as a scientist he was a bust. ←Baseball Bugs ^{What's up, Doc?} carrots→ 02:38, 15 October 2013 (UTC)[reply]

A great white marble bust. Plasmic Physics (talk) 02:42, 15 October 2013 (UTC)[reply]

As a serious note, Aristotle's science was excellent. He tried to induce principles based on observation. The problem with his science is that for the next millennium and a half people repeated his conclusions as dogma without using his method or testing his results. μηδείς (talk) 23:06, 15 October 2013 (UTC)[reply]

Fruit flies, humans and everything else we have here did in fact spontaneously generate from a cloud of Helium and Hydrogen gas. Count Iblis (talk) 02:57, 15 October 2013 (UTC)[reply]

'Everything' requires more than hydrogen and helium, what about the other elements? Plasmic Physics (talk) 03:18, 15 October 2013 (UTC)[reply]

The other elements can be spontaneously generated from hydrogen. Count Iblis (talk) 14:07, 15 October 2013 (UTC)[reply]

Spectral radiance/Planck's law

The article on spectral radiance, which I was linked to from the Planck's law article, defines spectral radiance as "the quantity of radiation that passes through or is emitted from a surface and falls within a given solid angle in a specified direction".

The Planck's law article states that the spectral radiance of a black-body, $B(\nu ,T)$ , is given by

$B(\nu ,T)={\frac {2h\nu ^{3}}{c^{2}(e^{\frac {h\nu }{kT}}-1)}}.$

I'm trying to relate this law to the definition of spectral radiance above. I'm guessing the surface referenced to in the definition refers to the surface of the black-body, correct? But what solid angle are we talking about? Solid angles are defined with respect to a point (ie the origin of a coordinate system), but no such point is identified in the Planck's law article.

The reason I'm asking this question is that I'm confused about the derivation of the Stefan-Boltzmann law from Planck's law. I would have thought that, to use the notation from the article,

$j^{\star }=4\pi \int _{0}^{\infty }B(\nu ,T)d\nu$ ,

where the factor of 4π comes from the fact that there are 4π steradians in a sphere. But apparently,

$j^{\star }=\pi \int _{0}^{\infty }B(\nu ,T)d\nu$ ,

and I don't really understand why.

Finally, is the radiation emitted by a black-body at a particular point on its surface travelling perpendicular to the surface at that point? — Preceding unsigned comment added by 74.15.138.165 (talk) 02:36, 14 October 2013 (UTC)[reply]

A black body radiates in all directions from any part of its surface. A black body also absorbs radiation coming from any direction inpinging on any part of its surface. The law of reciprocity applies, and you can easily verify that all directions apply by looking at a hot enough black body surface from any angle. It does not appear dark at any off-normal angle. 121.215.39.252 (talk) 04:35, 14 October 2013 (UTC)[reply]

You can derive the Stefan-Boltzmann law from Planck's law as follows. Consider a box filled with thermal radiation in thermal equilibrium. This is described by Planck's law it is isotropic. The energy density as a function of frequency is some function u(nu) (so u(nu) dnu is the amount of energy per unit volume in the frequency interval between nu and n + dnu ). Then because the photons move at speed of c and are isotropic, the flux of photons with frequencies between nu and nu + dnu coming from a small solid angle dOmega is c u(nu)dnu dOmega/(4 pi). Now imagine a small black sphere of radius r inside this isotropic photon gas.

How much radiation will this sphere absorb per unit time? We don't need to calculate any integrals to find out, if we look at photons coming from any particular direction, then the intercepted flux by the sphere is the same as what a disk perpendicular to that direction of radius r would absorb. So, the sphere absorbs an energy per unit time of pi r^2 c u(nu)dnu dOmega/(4 pi) from the incident photons coming from photons from any solid angle range dOmega. Sice both the radiation and the sphere is isotropic, this doesn't depend on Omega, so we can integrate over all solid angles by multiplying by 4 pi. The total flux through the sphere from the photons from any direction is thus pi r^2 c u(nu)dnu = A/4 c u(nu)dnu where A is the surface area of the sphere. Since a black object at temperature T will emit as much radiation per unit time and frequency as it would absorb when placed inside a photon gas of temperature T, this means that a sphere will emit thermal radiation per unit frequency and unit area of c u(nu)dnu/4. This is then also valid for a black object of any arbitrary shape, because each emitted photon comes from some point on the surface and how much radiation each surface element emits per unit time, doesn't depend on the orientation of the other surface elements, as the emission process is a local process. Count Iblis (talk) 14:58, 14 October 2013 (UTC)[reply]

Another way to think about it: A piece of hot surface emits only into a half-space, so maybe the factor should be 2 π instead of 4 π. But if it really would be 2 &pi then it would have to emit the same energy into every direction of that half-space, and so if you start with a hot glowing sheet of some material being perpendicular to your line of sight, and then you tilt it, you should still receive the same amount of light from the surface, despite the fact that the surface's image on your retina is now smaller. I.e. the glowing sheet should actually look brighter if you tilt it. This obviously isn't true; the correction factor is equal to the cosine of the angle. Thus, we need an integral not only over the frequencies, but also over the angles:

$j^{\star }=\int _{\Omega }cos(\theta )\int _{0}^{\infty }B(\nu ,T)d\nu d\Omega$

Written out in full:

$j^{\star }=\int _{0}^{\frac {\pi }{2}}\int _{0}^{2\pi }cos(\theta )\int _{0}^{\infty }B(\nu ,T)d\nu sin(\theta )d\phi d\theta$

Thanks, I understand now. 74.15.138.165 (talk) 17:26, 15 October 2013 (UTC)[reply]

Icek (talk) 09:17, 15 October 2013 (UTC)[reply]

Are neutron star collision energies as large as supernova energies?

I've read that the nature of the energy emitted is mainly in gamma rays, and that unusual magnetic fields are created, so it's somewhat different than a supernova, but is the "pow" comparable in energy?76.218.104.120 (talk) 04:18, 14 October 2013 (UTC)[reply]

Well, a supernova certainly gives off more energy at once, when it explodes, but it's quite possible that the total energy given off from a neutron star, or even a normal star, over it's life, might be more. StuRat (talk) 12:41, 14 October 2013 (UTC)[reply]

I imagine that the OP was referring to the energy emitted at the time of a collision between two neutron stars, as might happen when they spiral into each other. This is one of the speculative sources of gamma-ray bursts. — Quondum 18:00, 14 October 2013 (UTC)[reply]

Cytomegalovirus and HIV

This letter to Nature, dated 1990, suggests a relationship between cytomegalovirus (CMV) and Human Immunodeficiency Virus (HIV) infection where CMV increases susceptibility to HIV infection. I can't, for lack of google-fu, find any more recent discussion of this relationship between these two viruses. I don't really expect an answer to the question, but I'm looking for instruction that my google-fu might become stronger and am expecting that the masters at the reference desk will know how to find such things. Specifically, it'd be nice to have an actual paper that discusses this link, rather than just a letter. 71.231.186.92 (talk) 04:39, 14 October 2013 (UTC)[reply]

Have you searched Google Scholar, http://scholar.google.com/schhp?hl=en, rather than plain old Google? μηδείς (talk) 04:56, 14 October 2013 (UTC)[reply]

Yeah, a couple of ways. 71.231.186.92 (talk) 05:24, 14 October 2013 (UTC)[reply]

I have read the same, that there is a strong relation. Electron9 (talk) 07:30, 14 October 2013 (UTC)[reply]

Three things - firstly, you might not be aware that a "letter" in Nature is an actual, proper, peer-reviewed academic paper in it's own right, for presumably historical reasons Nature calls it's shorter papers "letters" and it's longer papers "articles". The difference is explained in more detail here [6]. Secondly you might want to have a look through these results [7], which are (most of) the papers which cite your paper (I got to this be searching for the title of your paper in google scholar and clicking the "Cited by" link below it). Google scholar doesn't have quite as good coverage as the proprietary databases that also do this, but those are really expensive. I find this search approach is normally the best for looking for more recent discussion on the topic of a paper. Finally you should keep in mind that the paper claims to have found a link between CMV and HIV infection in human fibroblasts, grown in vitro. This may or may not be in any way reflective of the situation in vivo. You should also bear in mind that fibroblasts are not the primary site of infection in HIV, although they can become infected. As far as I can tell from my quick read of the abstract this paper suggests one of the mechanisms by which this can occur, and would probably have no bearing on the primary site of infection which is the CD4 T cell (it would not affect these cells in the same way as the virus utilises the CD4 receptor to gain entry to these cells, rather than an Fc receptor, which is what is induced by the CMV in the McKeating et al. system). Equisetum (talk | contributions) 12:31, 15 October 2013 (UTC)[reply]

Is light from light-emitting diode emitted with same phase?

Does light from light-emitting diodes have the same phase ..? Electron9 (talk) 07:29, 14 October 2013 (UTC)[reply]

No, a normal LED is an incoherent source. Spinning Spark 07:34, 14 October 2013 (UTC)[reply]

Not with a normal LED, but a laser diode is a closely related device which does produce coherent light. Red Act (talk) 15:24, 14 October 2013 (UTC)[reply]

What is the opposite of Wikipedia's List of mental disorders?

Is there a list of things that are unique to a healthy, able, sane mind? What is the study of ideal brains called? — Preceding unsigned comment added by 174.65.23.49 (talk) 11:42, 14 October 2013 (UTC)[reply]

Mental health and positive psychology study that sort of thing. Unique though, hmm, what is unique to a healthy animal? it can't be six legs like an insect or a trunk like an elephant. Dmcq (talk) 13:29, 14 October 2013 (UTC)[reply]

Yes, I would have said "things that are common to a ...", instead of "unique". One example might be having all parts of the brain be operational, such as can be shown on a blood-glucose utilization scan. However, some people have been amazingly functional with even large portions of their brains disabled or absent. StuRat (talk) 13:42, 14 October 2013 (UTC)[reply]

Thank you. — Preceding unsigned comment added by 174.65.23.49 (talk) 16:13, 14 October 2013 (UTC)[reply]

My brother-in-law wrote a book about that and titled it Optimal Human Being. It seems to get the idea across. Looie496 (talk) 20:23, 14 October 2013 (UTC)[reply]

Can axon tracts travel around the internal capsule?

Can axon tracts travel *around* the internal capsule, or must all the axon tracts travel *through* the internal capsule thereby making the internal capsule a gateway canal-like thing between the cortices and rest of the body? I am having trouble visualizing the bigness of the internal capsule in 3D in my head, based on 2D myelin-stain representations. (Hey, it isn't easy, particularly if this brain sliced in coronally at one or a few locations!) 164.107.102.151 (talk) 17:59, 14 October 2013 (UTC)[reply]

If you are asking whether there are any direct pathways between the cerebral cortex and spinal cord that don't travel through the internal capsule, the answer as far as I know is no. But there are a number of indirect pathways that go by completely different routes. Our internal capsule article ought to be helpful to you for visualizing it. Looie496 (talk) 20:17, 14 October 2013 (UTC)[reply]

October 15

Overview of chemistry - book?

Can you recommend me a book that is an overview of chemistry? An overview as in it would have a short description of numerous sub-fields and some of their main results and references. --81.175.225.92 (talk) 00:22, 15 October 2013 (UTC)[reply]

Any high school or introductory college text will probably suffice. The Central Science by Brown, LeMay, and Bursten, or General Chemistry by Kotz and Purcell are two that I have used before. --Jayron32 00:54, 15 October 2013 (UTC)[reply]

Jayron, do you mean Chemistry and Chemical Reactivity by Kotz & Purcell, or General Chemistry by Kotz, Treichel, & Weaver? 121.215.39.252 (talk) 03:25, 15 October 2013 (UTC)[reply]

Both, actually. I've use the two of them, and was trying to recall by memory, and conflated the two. --Jayron32 10:45, 15 October 2013 (UTC)[reply]

ray of light theory of eyes

one of our articles says " The first theory, the emission theory, was supported by such thinkers as Euclid and Ptolemy, who believed that sight worked by the eye emitting rays of light."

Let's try to understand how the Greeks could have thought so. Why don't people see in a dark room? (cave etc)? Why does a 'source' of light need to be seen in this case, if eyes cast their own light? Most perplexingly - if eyes cast their own light, why can't we see the 'eyes' of other people glowing in the dark, for example? Could you explain a little the very loose kind of thinking that made this almost kind of make sense in a childlike way? 212.96.61.236 (talk) 00:30, 15 October 2013 (UTC)[reply]

Well, cat's eyes and some other animals reflect light, and that could be mistaken for emitting light. Perhaps they just thought human eyes emitted "invisible light" (what we might call infrared or ultraviolet), which changed to visible light under the right conditions. StuRat (talk) 00:49, 15 October 2013 (UTC)[reply]

But we clearly don't see in the dark... moreover, put someone in a darkish place and then put a whole chorus full of people in front of it, their eyes all shining into it: that person can't see any better. so....? I mean it just seems so unworkable... 212.96.61.236 (talk) 01:11, 15 October 2013 (UTC)[reply]

I see in the dark, and furthermore my eyes are sensitive to near IR (down to 800 nm) and near UV (up to 320 nm) -- the IR looks a dark reddish-brown, and the UV looks gray. 24.23.196.85 (talk) 05:04, 15 October 2013 (UTC)[reply]

That seems extremely unlikely, unless you are a bird, and even then you wouldn't be able to see IR. The visible spectrum is 390 to 700nm, no reference I've seen shows the outliers being anywhere close to 320-800. That's much further outside the realms of believable than human biology would permit. I would find some sources of true IR and UV light to really test yourself in a double-blind manner (you can start with the LED at the end of a remote control, with someone who isn't you pressing a button in such a way that you can't tell if they're pressing it). If real, go present your self to the Guinness record-keepers. — Sam 63.138.152.139 (talk) 14:24, 15 October 2013 (UTC)[reply]

People (like my mother) who had cataract surgery before the latest generation of implantable lenses became available are able to see a little way into the ultra-violet. The idea that you could see into the infra-red or in total darkness is ridiculous. If User:24.23.196.85 truly has these super-powers then (s)he should go find a reputable laboratory where these capabilities may be investigated. (...or WP:NOR...either way). SteveBaker (talk) 15:16, 15 October 2013 (UTC)[reply]

Of course it's not possible to fully reconcile reality with the emission idea, which we know to be wrong. People have speculated on the reasons why this idea was so popular -- and there is even discussion on the question of why many people continue to believe it today. That comes down to thinking of ways that the emission theory has intuitive appeal. StuRat mentioned one reason (shiny eyes of animals). Another possibility is the subjective experience of heat or palpable pressure when someone is watching you; this matches the social understanding that a person who's staring at you is doing something to you, rather than you to them. As for the question about the loss of sight in darkness, that was explained either by some interaction of the sunlight with your eye's light, or by the idea that the eye doesn't create the light, but gathers it in and then sends it out again at whatever you're looking at. Here is a discussion of some of the past arguments for the emission idea: [8]. And a paper [9] and short article [10] discussing the question from the point of view of science education. --Amble (talk) 02:15, 15 October 2013 (UTC)[reply]

Are you looking for a source, or chat partners to tell you you are right? We can't tell you how correct you are, but we can recommend you read visual perception, emission theory, and intromission theory. μηδείς (talk) 02:19, 15 October 2013 (UTC)[reply]

I am offering some answers that have been given to the original question, and was slow in adding the sources from which I drew the information. --Amble (talk) 02:45, 15 October 2013 (UTC)[reply]

I am not the boss here, feel comfortable adding what you think appropriate. μηδείς (talk) 02:48, 15 October 2013 (UTC)[reply]

Superman's eyes emit light. But he's a strange visitor from another planet, where apparently things work differently. ←Baseball Bugs ^{What's up, Doc?} carrots→ 02:33, 15 October 2013 (UTC)[reply]

Resolved

binder related to textile.

Hi, I want to know that which chemical can decrease the strength or stickiness or can make it completely useless to work.. but I want to know a chemical name, addition of water do decreases its strength but I after a lot of research I havnt got any chemical which can help me out in decreasing the strength of binder.. Pleas help me out. — Preceding unsigned comment added by 182.180.45.104 (talk) 09:44, 15 October 2013 (UTC)[reply]

Can you be more specific? There are many different kinds of binder and many different kinds of textile. Water is a chemical, and it can be used to dilute many binders, as can many solvents.--Shantavira|^{feed me} 11:54, 15 October 2013 (UTC)[reply]

I have a hard time following your Q, but if you're trying to dissolve something, then most things which can be dissolved are water-soluble, oil-soluble, or alcohol-soluble. So, one of those will probably work. Peppermint oil, for example, can dissolve lots of adhesives. If none of those work, then a strong acid or a strong base might work. StuRat (talk) 17:38, 15 October 2013 (UTC)[reply]

i am using binder nameing UD BINDER of BASF for textile printing. i have discusses it with many chemist but non of them helped me out.. i had used a lot of acids to decreases its strength but non of them work out even though i had used strong base also but not succeeded but when I add strong base in the paste which is use in printing after 2 days it's make it like rubber but i want some thing like when i add that powder based chemical in that printing paste it become useless.. its strange but i want to make the printing paste totally unworkable and it happens only when binder losses its strength.. so I want powder based chemical which make binder totally use less..

It might help if you explained what you are trying to achieve. Why do you want to make the binder useless? Wouldn't that be the same as just not using a binder? Spinning Spark 22:25, 15 October 2013 (UTC)[reply]

I think the OP must have got some printing paste on his/her clothes, and is trying to find some chemical that will remove it. What I don't know is why he/she is using strong acids and bases, despite the danger of ruining the clothes in question altogether. 24.23.196.85 (talk) 23:27, 15 October 2013 (UTC)[reply]

Thermal radiation

I have read the articles on Thermal radiation and Black-body radiation, and I am still struggling to understand the actual mechanism that causes radiation to be emitted from energetic atoms. The explanation in Thermal radiation just says:

These atoms and molecules are composed of charged particles, i.e., protons and electrons, and kinetic interactions among matter particles result in charge-acceleration and dipole-oscillation. This results in the electrodynamic generation of coupled electric and magnetic fields, resulting in the emission of photons, radiating energy away from the body through its surface boundary

I have two conflicting models in my head, from forgotten Physics classes. One or both may be completely incorrect. They are

If you take a dipole magnet and vibrate it, you will produce an EM wave. If you were able to vibrate it really really really fast, the frequency of that EM wave would be that of visible light, so it would emit visible light. In a warm body, each atom is like a tiny dipole magnet that vibrates, producing EM waves.
In a warm body atoms are colliding with each other, occasionally causing electrons to jump energy levels. When they return, they may emit photons. Thermal radiation is caused by these emitted photons.

Is either explanation close to correct? — Sam 63.138.152.139 (talk) 14:08, 15 October 2013 (UTC)[reply]

The second statement is basically the quantum mechanical version of the first, but then specialized to electrons. The first picture is a classical picture of vibrating charges, but then if you describe this more precisely using quantum mechanics, each such vibrating charge is an (approximate) harmonic potential and there are then energy levels here too. So, it also emits radiation due to the system falling back to a lower energy level.

The relevant processes are spontaneous emission when making a transition to a lower energy level, excitation when absorbing a photon, and stimulated emission. Count Iblis (talk) 14:19, 15 October 2013 (UTC)[reply]

Hmmm, you say the second statement is a QM version of the first, but there seems to be a very critical difference: in my "vibrating magnet" explanation I can make my magnet produce *any* frequency by vibrating it faster or slower. In my QM explanation, my iron magnet could only produce those few precise frequencies defined by the energy differences in the electron shells of iron atoms (the "quantum" of quantum mechanics), right? — Sam 63.138.152.139 (talk) 14:35, 15 October 2013 (UTC)[reply]

If you look more precisely at e.g. the rotation of molecules mentioned by Gandalf below, then there are energy levels there too, but they are so densely packed that it looks like a continuum. Also, you have to take into account the interactions between the different molecules, an N particle system will have energy levels that for large N will be extremely densely packed. So, physically you putting a magnet in your hand and letting it vibrate at seemingly an arbitrary chosen frequency, or an atom emitting a photon are not distinct physical processes. The former involves many more particles and has a far larger number of degrees of freedom, so the emitted photons can have many more possible frequencies. But it is ultimateley the same quantum theory that explains everything (classical mechanics is only an approximation to quantum mechanics; unlike classical mechanics, quantum mechanics is always valid). Count Iblis (talk) 15:23, 15 October 2013 (UTC)[reply]

Both explanations are (more or less) correct, depending on the wavelength of the radiation. Infrared radiation and microwave radiation are caused by vibrations and rotations of molecules. Shorter wavelengths, such as visible light and ultraviolet radiation, are caused by transitions of electrons between energy levels within atoms/molecules. There is a table at Electromagnetic spectrum#Rationale. Gandalf61 (talk) 14:36, 15 October 2013 (UTC)[reply]

Wow. Thank you for that table -- my jaw just dropped. I had simply no idea that the continuum of the EM spectrum was caused by separate distinct processes, rather than a continuum of one process (like vibrating atom faster and faster). Thanks! — Sam 63.138.152.139 (talk) 14:41, 15 October 2013 (UTC)[reply]

Follow-up question, after seeing the table that Gandalf61 linked to. (please excuse me: my brain is trying to crush together two mental models that have always happily lived in separate bins and now need to be entangled together):

For the "vibrating dipole" model, my physics teacher always likened it to a whip: if your bar magnet is sitting on a table, there is a magnetic field line coming straight out of the North pole. Move the magnet and that field line shifts, but the displacement moves away from the magnet like a wave down a whip, traveling at the speed of light. Vibrate it back and forth and you get an EM sine wave. This was my model for thermal radiation, and a key point of this is that the wave has an amplitude in *physical space*, and that amplitude is the displacement of the magnet (or atomic dipole). That is, in magical-theoretical-world, if you put out metal filings and viewed the magnetic field lines like kids do in school, and your filings were absolutely weightless and frictionless etc etc., wiggling the magnet side to side would produce a sine wave of filings that you could even photograph (ignoring the speed of light).

In the QM model, an electron drops to an lower energy level and the energy is lost to a photon that is emitted with a specific frequency based on its energy. But this "frequency" has always seemed to me to be almost metaphorical -- the photon is a packet of energy with an associated wavelength, but it's not like a sine wave wiggling through space with an actual physical amplitude.

Now I find that both models are kind-of correct for different frequencies. Are both photons of exactly the same "kind"? Is the vibrating magnet actually producing a sine wave with an amplitude in physical space? How about the photon emitted by the electron? The two explanations seem so different, yet it seems they both produce the exact same thing. — Sam 63.138.152.139 (talk) 15:06, 15 October 2013 (UTC)[reply]

You need to note a couple of things. Firstly, merely vibrating a magnet DOES NOT produce electromagnetic radiation. If you mechanically rotate a bar magnet, you will get a rotating magnetic field, that is all, no matter how fast or slow you rotate or move it. To get EM radiation, you must have both a varying magnetic field AND a similarly varying electric field in the proper phase relationship. This is easily demonstrated as it is easy to produce very intense varying magnetic fields by passing large varying currents through a wire coil. However, by other means you can produce radio waves that embody significant energy yet the magnetic filed component may be quite small compared to that produced by a current in a coil.

Secondly, yes a warm body DOES radiate EM radiation (with a continuous spectrum albiet peaked at a given frequency). However, this does not require, and mostly does not involve, collisions between atoms or molecules (or collisions between any sort of particle. Collisions cannot occur in a solid or in a pure crystal. But all non-trasparent substances radiate. For instance, carbon, a solid, is a near perfect black body radiator at all temperatures in which it can exist as a solid, including up to the sublimation point, ~3900K, at which it will glow yellowish-white.

Collisions are relevant when considering gasses. Collisions transfer energy from one molecule to another, by the impact changing the translational and rotational velocity of the molecules concerned. Atoms and molecules must distribute their energy between translation, rotation, and electron orbital configuration. Only the spontaeous changes in orbital configuration contribute to black body radiation. The division of energy between translation & rotation, and electron orbitals, is governed by the emission laws.

[Special:Contributions/121.215.39.252|121.215.39.252]] (talk) 15:12, 15 October 2013 (UTC)[reply]

If you vibrate a magnet you will have a time dependent magnetic field and hence an electric field. Count Iblis (talk) 15:26, 15 October 2013 (UTC)[reply]

So, please Count Iblis, explain then why a radio transmitter needs an antenna, and does not radiate significant energy from its tank coil. After all, considerable energy goes into the tank coil, every half cycle of the carrier frequency, far more (typically 10 to several hundred times) than what leaves the antenna into space. That energy does not leave the coil by going off as EM radiation, it gets passed back and forth to and from that tank capacitor. And as I alluded to ealier, the magnetic component of the EM leaving the antenna my be considerably weaker than the magnetic field near the tank coil. Do not be confused by the fact that any time-varying magnetic field will induce a voltage in nearby conductors, and the electric field thereby created may result in some EM radiation as a secondary effect.

Lastly, consider this: A sinusoidaly varying current in an ideal coil absorbs no energy (as does a sinusoidal current in a capacitor) as the current is 90 degrees out of phase with the voltage. However, EM radiation contains/carries energy, lost to space, which is why an antenna presents an electrical resistance at its terminals (practical antennas may display reactance as well, but resiatnce is always present). Since an ideal coil, which of course does produce a sinusoidal magnetic field, absorbs no energy, there can be no EM radiation. Vibrating a magnet, and any other rythmic mechnical thing you can do to a magnet is not essentially diffrent to driving a periodic current through a coil. In vibrating a mass, you exchange kinetic energy between the mass and the driving device, twice each cycle. In vibrating a magnet in free space, some of the energy gets stored twice each cycle in the magnetic field but it always returns to the driving device, also twice each cycle.121.215.39.252 (talk) 15:45, 15 October 2013 (UTC)[reply]

Let's stick to one well defined example, let's consider the vibrating magnet modeled as an exact dipole magnet and work out this example from first principles in full detail. Here you can't a priori assume that a freely vibrating magnet will execute an exact harmonic motion and will therefore not radiate any energy, as you would then assume what you want to prove. An outline of this is is as follows. What you need to do is solve the Maxwell equations (taking e.g. the case of a frced harmonic motion of the magnet) which leads to an expression for the electromagnetic fields which are given in terms of the retarded potential, so the magnet at position r' and time t contributes to the field at position r at time t + |r-r'|/c, this time lag is going to lead to an 1/r contribution to the asymptotic behavior of the fields. If you ignore this time lag, then there is no 1/r behavior. Then the energy flux is proportional to the square of the fields which behaves as 1/r^2, therefore energy will leak away to infinity (the energy flux through a sphere of radius r is the proportional to r^2*1/r^2 = 1, so this stays finite in the r to infinity limit).

Another way to approach this, which is however not so practical for calculations, is to consider the problem of the self-force in electromagentism. If you consider the freely oscillating magnet, then it will oscillate according to a damped harmonic oscilator. But where does the damping force come from? This is, of course due to the emitted radiation, but the source of that is the magnet itself. How to properly deal with this was only solved recently. Count Iblis (talk) 17:56, 15 October 2013 (UTC)[reply]

Count Iblis, you cannot just ignore a logical argument and go off somewhere else in gibber-land. You need to show why my discussion above is wrong - and you haven't done that, because I have merely recited facts well known to graduates in electrical and electronic engineering world-wide. There is no difference between a magnetic field established by a current carrying coil (or a straight conductor for that matter) and a magnetic field established by a simple dipole magnet. In free space, an ideal coil or conductor absorbs no energy, and no EM radiation occurs. (In practice, of course, while we can have superconductors, we cannot have completely free space. There is always other (imperfect) conductors somewhere with closed loops. Current will be induced in the closed loops, setting up their own varying magnetic fields, which will do the same to the originating coil. By Lenz's Law (for which the proof is the impossible existence of perpetual motion machines) the induced voltage in the originating coil will always be in a direction/phase that will oppose the originating current, thus synthesising an electrical resistance.) I discussed sinusoidal exitation above to simplify it for the OP, however my argument does apply to any varying exitation, as any engineer will know (think in s-plane). 120.145.145.144 (talk) 00:41, 16 October 2013 (UTC)[reply]

Those text books will ignore the effects leading to radiation being emitted, but they will only tell that much later when they actually treat the subject of electromagnetic radiation, because they have not yet introduced the complete Maxwell equations before that.

Thing is that even an uncharged conducting metal sphere rotating in a perfect vacuum will emit electromagnetic radiation and slow down as a result of that. But this is due to quantum electrodynamical effects. Count Iblis (talk) 01:21, 16 October 2013 (UTC)[reply]

I'll take that as your subtle admission, Count Iblis, that any such EM radiation, if it in fact occurs, coming from a vibrating magnet or a wire/coil carrying a perioic current, is negligible. It has to be, or said textbooks, including what electrical/electronic undergrads have to study on Maxwells' equations (we got Maxwell in 3rd year of a 4-year course), would not have ignored it. Nor could practicing engineers get away with ignoring any such effects, which they universally do. Nor could they ignore it in the design/engineering of mechanical filter resonators, many of which achieve extremely high Q-factors (20,000 and better, which means the knietic energy is >20,000 times what is lost each cycle), well beyond what is practical in LC resonating circuits. They are carefully sized pieces of vibrating metal. And it is something that can be ignored with respect to the OP's questions too. His teacher was wrong; vibrating dipoles are not the source of black body radiation - electron orbital drops are. 120.145.145.144 (talk) 05:40, 16 October 2013 (UTC)[reply]

Infrared and microwave radiation is emitted and absorbed by changes in rotational and vibrational modes of polar molecules - see our articles infrared spectroscopy, vibronic spectroscopy, rotational spectroscopy and rotational-vibrational spectroscopy. Gandalf61 (talk) 08:13, 16 October 2013 (UTC)[reply]

True, but that is not black body radiation, and each applies to specific phases - eg the last 2 you mentioned apply only to the gas phase. The radiation and absorbance in these cases does not conform to the black body emission laws, and black body radiation applies to solids and liquids, and in theory, to gasses. In fact, the known atomic structure is not even required to derive the ideal black body emission curve. 120.145.145.144 (talk) 12:28, 16 October 2013 (UTC)[reply]

The OP asked about the "actual mechanism that causes radiation to be emitted from energetic atoms". The black body model is a theoretical abstraction based on thermodynamic principles. It does not posit a particular emission mechanism, and so it is something of a red herring in answering the OP's question. The OP's teacher was not wrong, although they may have given an oversimplified explanation. Gandalf61 (talk) 13:03, 16 October 2013 (UTC)[reply]

The OP was specifically asking about black body radiation - his/her first sentence is I have read the articles on Thermal radiation and Black-body radiation. And later in his/her question, he/she uses the terms "warm body" and "thermal radiation". So all this nonsense about dipoles and electric fields created by vibrating magnets is a side track. If we are talking about factors affecting the thermodynamic efficiency of gasoline engines, do we concern ourselves about oil drawn past the pistons, just because its calorific value has a tiny tiny theoretical impact? Yes, black body theory itself does not posit a particular emission mechanism - I said that myself. But that only means bodies must radiate with a tell-tale spectrum - we still need to understand what the actual mechanism is. The OP's teacher was wrong, and wrong in the same sense that you and I would be wrong by saying the energy of a gasoline engine comes from the lube oil burnt. Only more so. 124.178.48.59 (talk) 14:34, 16 October 2013 (UTC)[reply]

Surgical Caps and Shoe Covers

Hello. How do you visually distinguish between surgical caps and shoe covers? They look very similar. Thanks in advance. --Mayfare (talk) 16:13, 15 October 2013 (UTC)[reply]

Er, well, I'm not sure how to answer this. If you were physically presented with them, the difference would be obvious. If you're looking at a photo, it's hard to come up with solid criteria, because shoe covers are often crumpled and folded in a way that makes their shape hard to recognize. Basically hair covers are round, about a foot in diameter, and relatively thin; shoe covers are foot-shaped with the opening on one end, and pretty robustly constructed. Looie496 (talk) 16:44, 15 October 2013 (UTC)[reply]

It may depend on which part of the world you are living in (you don't give your county of origin) but I would say that visually 'shoe covers' are just large enough to encapsulate the foot and 'caps' are larger enough to cover the head, hair (and Tin foil hats for those quacks that feel they need ware need them). --Aspro (talk) 16:52, 15 October 2013 (UTC)[reply]

Head covers and shoe covers are generic medical supplies, used in vast quantities (along with gloves, masks, and gowns), and I think they're probably the same shape all over the world. In the veterinary facilities where I've worked, the most obvious difference was that the shoe covers were bright blue and the head covers were white. Looie496 (talk) 17:04, 15 October 2013 (UTC)[reply]

If you do a Google Image search for both items you will see there are big differences between the two. The shape to begin with, round for the head and narrow for shoes.Shoe Cover Surgical CapHope this helps! Mike (talk) 16:57, 15 October 2013 (UTC)[reply]

As a practical issue, there's no need to distinguish between them, as they come in labelled boxes. - Nunh-huh 02:39, 16 October 2013 (UTC)[reply]

Domestic waste solder

I have a handful of bits of solder, mostly crap sucked up with my desoldering tool and bits that ran off during tinning my soldering iron. I live in Edinburgh, Scotland. Should I make an effort to dispose of this in a special way or is that just for commercial enterprises producing large quantities of waste? --2.97.26.56 (talk) 20:12, 15 October 2013 (UTC)[reply]

Around here, (in California, in the United States), you would get in touch with your local county's Household Hazardous Waste program and determine the best way to dispose of those types of materials. If you're actually in Edinburgh, here's the website for your city government waste service. Nimur (talk) 20:39, 15 October 2013 (UTC)[reply]

It's WEEE, something that ideally you wouldn't put in the landfill waste stream. They do accept WEEE at Edinburgh's community recycling centres (it goes in the "small electrical"). But obviously a wee freezer bag full of WEEE (ahem) isn't worth driving out to e.g. Sighthill. Personally I keep a ziploc freezer bags of the little nasty stuff that they don't collect at the kerb (batteries, CF bulbs, WEEE, paint, etc.) and take it to the recycling place only when I'm taking something large. -- Finlay McWalterჷTalk 20:40, 15 October 2013 (UTC)[reply]

Okay, thanks. I'll hold onto it until my next visit to the recycling centre. 2.97.26.56 (talk) 21:22, 15 October 2013 (UTC)[reply]

By the way, you should no longer be using lead containing solder unless it is for maintenance of equipment that predates the ROHS directive. Spinning Spark 22:17, 15 October 2013 (UTC)[reply]

While it may come in scope of local regulations, I would not be concerned about a mere handfull. You need to keep things in perspective and understand the partly political motivation for the European Lead-Free Directive. Lead is ubiquitous in the environment. Those that frame laws and regulations seem not to understand how and why. Lead was used in all manner of things, including paint. That contributes to lead dust everywhere. Another source is the used of lead sheathing in power and telephone cables for about 80 years, until satisfactory plastic sheaths were developed in the 1970's. I was involved in the installation and testing of lead sheathed cables in the 1960's and 1970's. The sheath was about 3 mm thick and the cable pressurised with air, so as to enable detection of sheath damage and keep out moisture. Those cables still in use or just abandonned and left in the ground (which is most of them) have become porous, constantly leaking air. In many cases the lead has, over the intervening 40 to 80 years, become paper-thin. Where has the lead gone? Leached into the soil generally of course - where it can be further distributed whenever someone disturbs the soil for building construction or whatever. Authorities became concerned about the lead levels in the blood of childen 30 or so years ago. They thought that lead in gasoline was the problem, so various countries around the World banned lead in gasoline. That improved things a bit in the USA because of their high population densities, considerable use of private cars, and low use of diesel engines in busses and light trucks. But Australia and Europe, which have always used diesel engines in any sort of truck, didn't see much change in blood levels. So Europe decided to ban the use of lead altogether - at least that will mean lead levels don't get any worse, and help countries like Australia where some of the environmental contamination comes from dust released in the mining, processing, and transport of lead.

In any case, the lead in solder is pretty much trapped with the tin and rendered harmless. There has never been much concern about electronics technicians and electronics factory staff being affected by lead from solder - though it has always been standard to caution workers to wash hands before eating.

120.145.145.144 (talk) 01:07, 16 October 2013 (UTC)[reply]

Human mortality question

Based on known age-specific mortality rates, what is the expected time between successive deaths of the world's oldest inhabitant?→31.54.112.70 (talk) 22:42, 15 October 2013 (UTC)[reply]

A little over one year. You don't have to estimate it, since we know the true answer: World's oldest person#Chronological list of the verified oldest living person since 1955. Someguy1221 (talk) 22:48, 15 October 2013 (UTC)[reply]

You would have good luck asking this question at the Math desk. It's a basic stats question and the Oldest person page is a large enough dataset to get a good estimate. I don't know how to do the math for you offhand, but they will. I'm guessing a poisson distribution would be a good start. Shadowjams (talk) 02:29, 16 October 2013 (UTC)[reply]

October 16

Trackside thing

File:Trackside thing.jpg

What is this?

Over the past few years, thousands of these things have appeared alongside UK railways. What are they? They are about five feet tall and in groups of maybe 30-60 spaced about 20 feet apart. This one was quickly snapped on my phone near Milton Keynes.--Shantavira|^{feed me} 08:02, 16 October 2013 (UTC)[reply]

They are trackside lights for night inspections. Source Mike (talk) 13:53, 16 October 2013 (UTC)[reply]

Children crying because they can't do something?

Is it common for children to cry because they find something difficult? I recall crying about age 10 because I found it too hard to join up my writing (my cursive is now pretty neat IMO) which seems a strange thing to get upset about. --129.215.47.59 (talk) 10:43, 16 October 2013 (UTC)[reply]

The Wikipedia article titled Crying states "Crying is believed to be an outlet or a result of a burst of intense emotional sensations". The inability to complete a task can bring on intense stress, which would be an "intense emotional sensation". --Jayron32 10:48, 16 October 2013 (UTC)[reply]

If people around you are doing something effortlessly, and being praised for it, but you can't do it yourself, that's pretty frustrating. Happens to adults too. I'm in my sixties and still can't do joined up writing, not legibly anyway. Fortunately one develops a sense of perspective with time.--Shantavira|^{feed me} 11:06, 16 October 2013 (UTC)[reply]

Power used by mobile phone and radio towers

When a load is connected to the secondary of a transformer, the power drawn by the primary from the source increases because of the magnetic coupling. Will something similiar happen when a mobile phone is switched on? Suppose a thousand phones get switched on (and hence get 'connected' to the tower), will the tower use more power? The same question applies to radio transmission towers. Does a radio tower consume a more power when radios get tuned to it's frequency (and hence gets 'coupled')? - WikiCheng | Talk 10:53, 16 October 2013 (UTC)[reply]

From first principles of physics, we know that a transmitting antenna's effective impedance does change due to the presence of a receiving antenna, even if the receiver is many miles away. But that effect is tiny - you can do the math to verify. In the case of ordinary telecommunications, radio antennas operate in the far field (as opposed to near field). Definitionally, this means that the effects of the receiving antenna are too far away to matter.

A much more prominent effect is that modern digital telephones use a bidirectional protocol. Telephone transmitters are not broadcast towers: they are nodes in a many-node, asymmetric full-duplex communication. The transmitter has more work to do when multiple devices are attached. Perhaps the easiest protocol to intuitively understand is time-division multiplexing; adding more telephones would require a higher duty cycle; in other words, the transmitter is on for a longer part of each time interval, and therefore uses a higher average power.

Depending on where you are, and which company runs your mobile telephones, time-division multiplexing might be supplanted by more advanced digital communication protocols; but in principle, whichever scheme they choose will have the same general relationship between number of users and total transmitter power usage. (Thanks to the rule-of-thumb about circuit design, the gain-bandwidth product, we can relate the engineering tradeoffs between time- and frequency-multiplexing of the transmitter design back to first principles of physics, and the conservation of energy, and so on). Whether you spend the power over a wider frequency-spectrum during a short interval (typically, using complex digital codings); or if you use a narrow spectrum for a longer interval (using time-division scheduling); the same power-bandwidth product gives the same signal-to-noise ratio. In actual designs, practical details may shift the optimal choice in one direction or the other. So, this gives the engineers who design radio protocols a little room for flexibility, and lets them pick the best-available scheme that is implementable in today's electronics technology.

In closing, I should mention that the concepts of base load and variable load also apply to transmitters; it is plausible that for a large transmitter, the base load is so close to the variable-load that the transmitter's power supply cannot reasonably switch modes, or otherwise deliver a variable quantity of power. Such large power-supplies are difficult to design efficiently, and this is an active area for new engineering research and development. Now that software can switch transmitters on and off as fast as, say, once per millisecond (!), power supplies need to be designed that can toggle between peak and idle at rates very close to those software latencies. This seems trivial to the engineers with backgrounds in software and digital systems, but as the power supply designers need to build capacitors and inductors and so forth, they are constrained by device size and switching time. So, while controlling a digital signal at two gigahertz is very easy using today's computers, swinging a couple hundred kilowatts on and off at even one kilohertz is quite difficult. Compound this difficulty by the fact that your cellular tower is sometimes in a remote area that might not connect to a utility electric grid: it might have its own diesel engine or gas turbine... Nimur (talk) 13:37, 16 October 2013 (UTC)[reply]

A geometrical analyze of camera capture blur?

If one takes a picture with a camera like the Canon EOS 5D Mark III which has a image sensor with a size of 36 x 24 mm and 5760 x 3840 pixels. That captures an object that moves 0.178 meter sideways during the exposure time at a distance of 60 meters from the photographer. How many mm or pixels will the light from the object traverse during the image exposure? I suspect the distance from the middle of the lens to the sensor plays role here but don't find any data to calculate with. Electron9 (talk) 12:15, 16 October 2013 (UTC)[reply]

You have not supplied one vital piece of information. You need the focal length of the lens in use. The pixels traversed will obviously be greater if the lense is set for a higher zoom-in. 120.145.145.144 (talk) 12:43, 16 October 2013 (UTC)[reply]