Wikipedia:Reference desk/Archives/Science/2011 February 27

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February 27

Why is blood iron test taken from finger?

When one donates blood, at least in the UK, they first test your blood to make sure you're not anaemic. To do this, they prick your finger and deposit a couple of drops of blood in cylinders of fluid - if a drop sinks, you have enough blood iron and you're okay to donate. Out of curiosity alone, this leads me to a couple of questions:

1. Can someone point me to a description of the actual mechanism of this test (the sinking blood bit) - what's in the cylinders, etc. Wikipedia's article about blood iron/haemoglobin tests seem to all be about fancier lab tests not this rough-and-ready one.
2. Why must they take blood from a finger (as Penfield's homunculus shows, one of the most nerve-ending-rich places in the body)? I asked the technician if they could take it from elsewhere, and he said they had to use the finger, but didn't know why.

Thanks. 91.125.171.235 (talk) 00:42, 27 February 2011 (UTC)

Re: 2; This is a fairly rough test for your Haemoglobin (Hb) level; sadly neither the WP article on Haemoglobin or blood test covers it in any detail, but fingerprick does mention that it's "typically quicker and less distressing than venipuncture".

The Hb level in blood from your fingertip won't be significantly different from that from anywhere else. The "technician" probably had no idea.

Probably the real answer is "fingertips are easy to get at without removing any clothing". Though fingertips are sensitive you'll hopefully have found that by the time you've given your blood donation, had your cup of tea and removed the sticking plaster from your fingertip the sharp prick of the sample is but a distant memory.

I've an appointment to give my 38^th unit next month - I'll be sure to ask about the above (particularly 1 above as I've always wondered too). If I get an answer I'll put it on my userpage. Tonywalton ^Talk 01:10, 27 February 2011 (UTC)

The blue solution is copper(II) sulfate (see this section), and here PMID 2646909 is a reliable source referring to the screening procedure. That paper is more than 20 years old, and I presume that since then the cutoff (i.e. the specific gravity of the solution used for the screening) has been adjusted to improve the test's performance. Entries here and here describe the procedure in more detail. -- Scray (talk) 01:34, 27 February 2011 (UTC)

Interesting that the specific reagent is available for sale here . -- Scray (talk) 01:37, 27 February 2011 (UTC)

(ec) I'd thought it looked like CuSO₄, however the system in the UK is that the blood drop goes into a blue (sulphate)- looking tube for males but a green (looks more like copper carbonate) tube for females. I'll be sure to read the refs above tomorrow (it's 1:44AM here, and I need some sleep!) Tonywalton ^Talk 01:47, 27 February 2011 (UTC)

It could just be that they add dye so people don't mix them up and they are the same thing just in different concentrations because the acceptable iron level is different for males and females (is it? If it isn't, that would invalidate my theory! I know women often have lower iron levels due to menstruation, but I can't see how that would make the safe level different). I've never seen this green liquid you describe, but then I've never donated blood as a woman! Are you sure they use different ones? --Tango (talk) 02:14, 27 February 2011 (UTC)

More recent, showing the cost-effectiveness of CuSO₄ screening: PMID 20808648 -- Scray (talk) 01:39, 27 February 2011 (UTC)

Additional detail on the performance and characteristics of the test here. -- Scray (talk) 01:42, 27 February 2011 (UTC)

Regarding site: this PMID 11274589 suggests that use of earstick (rather than fingerstick) tends to under-estimate anemia. -- Scray (talk) 01:46, 27 February 2011 (UTC)

Further, this page discusses "microcollection" (about 80% of the way down the page) and the issue of pain receptors in fingertips, stating that despite this problem it's preferred. -- Scray (talk) 01:50, 27 February 2011 (UTC)

Its easier to get a small drop of blood of the size used for this hematocrit test that can be dropped into the tube of solution from the fingertip with a stick from a micro lancet than if, say the forearm were stuck, but it should be possible to use alternate sites as is now done for diabetic blood sugar testing. Maybe they just like to keep using this early 20th century technology because it is cheap compared to some new meter and test strips which could measure iron in smaller droplets. But If the pain of the fingerstick is too much to bear, then the pain of the blood donation would be worse. Edison (talk) 14:23, 27 February 2011 (UTC)

Measuring iron concentration from a small volume of blood might be feasible, but it will not accurately assess degree of anemia. There are multiple carriers of iron in the blood other than hemoglobin. The question is not iron level - it's hemoglobin level. Iron deficiency is only the most common of many causes of anemia, and they don't want to take blood from any anemic volunteer. -- Scray (talk) 00:58, 28 February 2011 (UTC)

Destroy secret papers

I think I've been watching too many spy movies and so I've got this weird question in my head; here goes. Suppose I have a stack of (secret) papers in my super-secret attaché case that is airtight with a perfect vacuum inside and with the temperature of the contents maintained at -5 C. What would be a good chemical reaction (with reactants contained in some mechanism in the case) that could be used, if the case falls into enemy hands, to destroy all the contents of the case beyond possible recovery as quickly as possible, and that could be triggered by a button, remotely, or by an internal timer (or all three)? Assume this reaction might be needed at an arbitrarily long time into the future. 72.128.95.0 (talk) 03:48, 27 February 2011 (UTC)

I have a few ideas:

• Chloroform is pretty good at lifting ink off paper, as I found out when I accidently erased a paragraph in my laboratory manual.
• Concentrated sulfuric acid carbonise paper, it turns paper into a handful of carbon dust. (not that I'd encourage using your hands, for fear of acid burns). With enough paper, it may catch on fire, as the reaction can get very hot.

Plasmic Physics (talk) 05:51, 27 February 2011 (UTC)

Well you could try concentrated perchloric acid, but with these sort of destructive acids, you will not be allowed to carry it on board a plane. Graeme Bartlett (talk) 10:15, 27 February 2011 (UTC)

I believe codebooks carried by the navy in WW2 were designed to be destroyed by dumping them in a bucket of salt water. Dmcq (talk) 13:14, 27 February 2011 (UTC)

If all the documents were on flash paper (the nitrocellulose kind), you could possibly set them alight with an electrical spark, although it would probably best if the case wasn't under vaccuum (needs oxygen to burn completely). Alternatively, you could spray the papers with a mist of pyrophoric liquid, which would ignite upon contact with oxygen (although the enemy could just open the case in a nitrogen filled glove box, if they knew about your countermeasures). -- 174.31.194.183 (talk) 18:40, 27 February 2011 (UTC)

It seems to me that destroying papers is trivial, but doing so without any outward sign (like a flaming/exploding attache case) is more of a challenge. Using magnetic media instead of paper provides for a possibility of wiping them with a strong magnetic field, without any outward sign, if a Faraday cage is constructed inside the case, to keep the field from escaping (does the Faraday Cage work to keep a field inside, instead of outside ?). StuRat (talk) 20:55, 27 February 2011 (UTC)

There is a scene in Cryptonomicon, I vague recall, where various Wikileaks-style hackers had rigged up their office so that any media passing through the door would be subjected to a strong magnetic field, so that if they were ever raided, the police would find themselves with completely scrambled hard disks. --Mr.98 (talk) 17:45, 28 February 2011 (UTC)

I guess they have never heard of CDs, DVDs or flash drives then. Googlemeister (talk) 17:32, 1 March 2011 (UTC)

Have something that releases pure oxygen into the case, and make a spark. In pure oxygen, things burn very rapidly (for example the tragic example of the Apollo capsule interior that caught fire in pure oxygen). The oxygen could come from a small cylinder or a chemical reaction. To prevent the burnt remains of the papers being read, then you could have something such as large loose nuts and bolts in the case that would pulverise the ashes when you shook it. Or perhaps you could have "plastic" paper that would melt as well as burn. 92.24.189.108 (talk) 00:35, 28 February 2011 (UTC)

Ability to understand accents

I've always had a problem (more than most people) hearing what people are saying in a noisy environment(nightclub/factory/party). Tonight I was out with some friends and a bunch of their friends who were from various locations around the UK (so they had a variety of accents). Even in the quiet pubs I was constantly having to ask them to repeat what they'd said even though they could understand each other. Is there a term to describe why some people can't understand accents and is it connected to the ability to filter out background noise? --BlackberryPicking (talk) 04:03, 27 February 2011 (UTC)

Any fault in binaural hearing (such as a very slight blockage in one ear) will increase the difficulties in picking out individual voices from a noisy background. It is also possible that a slight under-development or slight damage to the Primary auditory cortex will have the same effect. I don't think that this causes specific difficulties with comprehension of unfamiliar accents, but it will increase the problems that we all have in converting sounds into words that we recognise, especially when the sounds don't follow the pattern we expect. (I have difficulties with the Glasgow accent, and I live less than a hundred miles away.) The more one hears of an accent, the easier it becomes to decipher the words, so perhaps it was just the noisy environment combined with unfamiliar accents, but you might wish to undergo a simple hearing test to rule out the possibility that one ear is less sensitive than the other (we can't give medical advice here). Dbfirs 10:08, 27 February 2011 (UTC)

I don't have a source for you, but in my work as an Audiologist, I often hear these two complaints together from people with even very mild high frequency hearing losses. Some people seem to have these problems with normal hearing thresholds on audiogram; no-one really fully understands why, though it is thought there may be some neurological/cortical involvement Si1965 (talk) 11:54, 27 February 2011 (UTC)

I think there is a natural difference. I've never really had much trouble understanding any English accents, as opposed to many of my friends, who sometimes have trouble understanding my accent. On the flip side, while I can understand people's accents, I often can't recognise them. Chipmunkdavis (talk) 12:15, 27 February 2011 (UTC)

In addition to the other causes listed, the McGurk Effect may come into play here. That is, you may be better able to understand somebody if you watch their mouth as they speak. If you are in a dark nightclub with flashing lights, it may be more difficult to get any visual cues. I've noticed this effect on TV news. When I watch the reporter speak, I understand what they say, but when I turn away, I often misunderstand what they say.

Also, context also plays a part in speech recognition. If somebody says "My favorite fruits are apples, orangutans, and bananas", you will likely hear "oranges". Those with accents may also use different words, or use the same words differently, and this could throw you off. For example, when somebody says they are going to "eat tea" or "wash their hands in the toilet", this doesn't make sense to Americans, so they might hear it differently. StuRat (talk) 20:41, 27 February 2011 (UTC)

Regarding the role of context: I'm not sure you'd hear oranges. You'd guess oranges. I have a friend who is very prone to malapropisms, such as saying "pacific" instead of "specific" and "bi-curiously" instead of "vicariously". These don't escape my attention at all, quite the reverse, they tend to distract me from whatever he's trying to say. 81.131.30.247 (talk) 00:45, 28 February 2011 (UTC)

The same thing applies to reading. The first time I read that, I got oranges. Googlemeister (talk) 14:30, 28 February 2011 (UTC)

Good point. StuRat (talk) 22:35, 28 February 2011 (UTC)

Wow, over 20 years in Audiology and I've never heard of the McGurk Effect. Thanks! Si1965 (talk) 23:31, 27 February 2011 (UTC)

You're welcome. I hope it helps with your job. StuRat (talk) 22:34, 28 February 2011 (UTC)

I can only offer anecdotal evidence: I share your inability to understand people over background noise, but I am quite good at understanding accents. I think the main thing that affects how well you understand an accent is how familiar you are with it. I am quite well-travelled, so I'm used to a wide variety of accents. That probably helps me. --Tango (talk) 00:57, 28 February 2011 (UTC)

I think that a lot of it has to do with traveling and being exposed to many different accents or ways of speaking a language. Mac Davis (talk) 14:16, 28 February 2011 (UTC)

(OP here) Thanks for all the responses, as always they are an education! I'd thought there might be a term to describe it but maybe it's too non-specific a problem. I kinda assumed it was probably a 'processing' problem rather than a physical problem (mainly because I'm 36 and have had the background noise problem since I was in my teens, maybe younger). Understanding accents wasn't really a problem until this night out so I think I will book a hearing test as well. --BlackberryPicking (talk) 18:45, 28 February 2011 (UTC)

Flavored condoms and infections

This page claims that flavored condoms can cause yeast infections if used during vaginal sex. Is there a source for this info? I'm a development worker in HIV prevention in the Philippines; if anyone can help me find an authoritative source linking flavored condoms to vaginal yeast infections, I'd appreciate it. You can email me at <email redacted>. Cheers, Casey.

I do not have a good source for you, but I work in HIV prevention in America and India. I have heard that flavored condoms can cause this problem but I have also seen condoms which specifically state that they are made for intercourse. On disreputable sites found by searching Google it seems that many people say that yeast infections can be caused by flavoring or chemicals in the condoms.

I recommend that you consult with a doctor in your area as condoms manufactured in different parts of the world may be made to different specifications. Also, I just sent a note about this post to your email address. Blue Rasberry (talk) 05:39, 27 February 2011 (UTC)

And is Blue Raspberry one of the flavors ? :-) StuRat (talk) 20:31, 27 February 2011 (UTC)

I suspect that the mechanism of infection is that flavored condoms are first used for oral sex, where they pick up yeast, then are used in vaginal intercourse, where they deposit the yeast. Much like Q-tips are supposedly "not for use in the ears", despite being on the end of a long stick so they fit inside ears, the same skepticism should be aimed at flavored condoms "not meant for oral sex". StuRat (talk) 20:31, 27 February 2011 (UTC)

And here I am, a day late and a dollar short. Flavoured condoms can indeed increase the risk of a yeast infection. The flavouring is designed to come off (pun not intended) with moisture and friction. If it didn't, the condom would be tasteless. When the becondomed penis rubs against the rugae of the vagina, the flavouring is deposited among the folds. Flavourings are usually acidic so they lower the pH level of the vagina, which favours the growth of yeast, and some flavours contain sugars, the perfect food for C. albicans.

As an aside, you do not normally "catch" a yeast infection, and especially not from the mouth, as all adult vaginas are always loaded with yeast. A yeast infection occurs when the naturally present yeast colonies overgrow, either because their competitors have been selectively killed off (as after a round of antibiotics) or because the vagina has been made especially hospitable for yeast growth (as by the use of food, flavoured condoms, etc. or even by high blood sugar levels secondary to diabetes). --NellieBlyMobile (talk) 01:42, 4 March 2011 (UTC)

polycarbonate

how can i tell the difference between polycarbonate, acrylic, and abs plastic ? — Preceding unsigned comment added by Wdk789 (talk • contribs) 09:43, 27 February 2011 (UTC)

Is destructive testing allowed? (i.e. break a piece off and run tests on it) Ariel. (talk) 10:37, 27 February 2011 (UTC)

no --Wdk789 (talk) 11:25, 27 February 2011 (UTC)

I don't know but I suspect it's quite difficult from visual inspection alone. Can you find some hints in our articles on polycarbonate, acrylic, and ABS plastic?--Shantavira|^{feed me} 16:37, 27 February 2011 (UTC)

The three plastics have different densities. ABS is 1.04, Acrylic 1.18, Polycarbonate 1.20. They also have different hardness levels and tensile modulus - if you have other plastics to compare it to you should be able to tell just from the feel. A pretty definitive test is their refractive index. You can try various colors of light and compare. (That's how they do it in recycling facilities BTW.) Ariel. (talk) 18:30, 27 February 2011 (UTC)

what do u mean by " refractive index You can try various colors of light and compare." — Preceding unsigned comment added by Wdk789 (talk • contribs) 03:52, 28 February 2011 (UTC)

Do you know what a refractive index is? It's a measure of how much light bends when it passes through a material. Different materials bend light by different amounts, and by comparing you can distinguish them. You can google for some methods of how to measure this. Additionally different plastics absorb different frequencies of light that is passed through it. See Spectrometer and spectroscopic analysis. Ariel. (talk) 04:13, 28 February 2011 (UTC)

how can i pass light thru if its a thick plastic and if its not clear? — Preceding unsigned comment added by Wdk789 (talk • contribs) 08:23, 28 February 2011 (UTC)

You'll need a sensitive spectrometer, but you can probably check the light that reflects from it. You've reached the limit of what I know on the subject though. Research how recycling stations do it - they have this exact same problem when dealing with mixed plastics. Ariel. (talk) 08:35, 28 February 2011 (UTC)

u mean how shiny it is? — Preceding unsigned comment added by Wdk789 (talk • contribs) 13:10, 28 February 2011 (UTC)

I don't mean that, but checking the shinnyness level may give you a clue of which plastic it is. What I mean is that different plastics reflect specific colors of light in varying amounts. Probably not visible to the eye, but if you measure it you can see a "notch" or "peak" in the reflection curve that is different for each plastic. Ariel. (talk) 03:55, 1 March 2011 (UTC)

Weak interaction: figures

I'm attempting to improve the referencing and clarity of the weak interaction page. The commonly quoted range is ~10^-18m; carrier mean lifetime ~3x10^-25s and W-boson mass ~80 GeV (80.4 GeV/c2). I'd like authoritative sources for these numbers if you know of them, and also which ones are calculated from which ones. The first two appear to be related by the maximum velocity of c (although not quite), but I don't know which one is actually measured. I did once see an equation linking mass to mean life for virtual particles, but I can't remember it and it doesn't seem to be on an obvious page here. If this does exist, then the same question applies. Thanks! Grandiose (me, talk, contribs) 14:07, 27 February 2011 (UTC)

The authoritative source for anything related to particle physics is the PDG page. Go to that page, click on "Summary Tables" and then click on "gauge and Higgs bosons". There you will find information about the W and Z masses MW = 80.399 ± 0.023 GeV, MZ = 91.1876 ± 0.0021 GeV. You will also find their full widths ΓW = 2.085 ± 0.042 GeV and ΓZ = 2.4952 ± 0.0023 GeV. The widths are related to the carries lifetimes through the formula Γ = ħ/τ as you can see on the page Resonance (particle physics) which unfortunately for you is still only a stub. Finally, as you pointed out, the range of the interaction is roughly given by multiplying the particle's lifetime by the speed of light. Dauto (talk) 15:51, 27 February 2011 (UTC)

Thanks, does one find the width first and use it to calculate the lifetime, or vice versa? Grandiose (me, talk, contribs) 16:48, 27 February 2011 (UTC)

Either thing can in principle be measured but in practice the width is often easier to measure. Dauto (talk) 16:57, 27 February 2011 (UTC)

Actually, rethinking what I said above I realized that the distance obtained through the formula I gave : d=cħ/Γ is the average distance traveled by carrier particles after they are produced in collisions. In a interaction virtual particles carry the interaction and the average distance traveled by those is given by d=cħ/M which is about two orders of magnitude shorter for the weak interaction. See for instance the first paragraph of Yukawa potential. Dauto (talk) 16:49, 27 February 2011 (UTC)

Sorry. There is still a mistake above. I forgot a time dilation factor in the expression for the average distance traveled by carrier particles after they are produced in collisions. Dauto (talk) 03:44, 28 February 2011 (UTC)

Thanks. I have most of the key factual information now reasonably referenced. I was attempting to get some grasp of the strength of the weak interaction force, but have suddenly realised I have no idea of what a force means in this context [a mechanism]. (I should point out I'm in above my head and thus treading carefully.) Possibly the force preventing decay? In any case here has a couple of useful comparisons, but do we have anything more detailed/a better source? Some sort of formula or graph, for instance? I might be missing some facet or other, so do bear with me! Grandiose (me, talk, contribs) 18:02, 27 February 2011 (UTC)

In a fundamental level the strength of an interaction is given by the coupling constant. I'm busy now so I won't be able to give you a better explanation. I will try to post again later today. Dauto (talk) 19:04, 27 February 2011 (UTC)

cloud formation

I watched a video about global warming they say that cosmic rays forming with water droplets is what creates clouds and they say when the sun is more active fewer particle get through to make clouds so this is why it is colder. Is this true do cosmic rays create clouds — Preceding unsigned comment added by Lufc88 (talk • contribs) 14:34, 27 February 2011 (UTC)

The main cause of cloud formation is Cloud condensation nuclei, but it is true that a more active sun could alter the stage and altitude at which clouds form (see Solar variation). The exact process and the interpretation of the observed correlations has not been agreed on, but the effect is probably small compared with other causes of cloud formation. Did you mean more particles from a more active sun? Dbfirs 17:26, 27 February 2011 (UTC)

No, it isn't. That's a load of bull, that's what that is. Dauto (talk) 17:12, 27 February 2011 (UTC)

Then please change our articles and add appropriate references (though I agree that the effect is disputed and probably small). Dbfirs 17:27, 27 February 2011 (UTC)

What articles? Dauto (talk) 03:18, 28 February 2011 (UTC)

Well they do only suggest a possible effect. I've just re-read the OP's post about what the video claims, and I agree with you that cosmic rays are not "what creates clouds", so I'm with you on the bull call there. Dbfirs 10:08, 28 February 2011 (UTC)

no it said when the sun is more active fewer rays get through so there are less clouds so things warm up quicker. But my intial question was so cosmic rays form with water to make clouds and if so how? — Preceding unsigned comment added by Lufc88 (talk • contribs) 17:32, 27 February 2011 (UTC)

One possible process is that increased solar activity interacts with the earth's magnetic field to deflect more of the galactic cosmic rays that are the main cause of ionisation in the atmosphere. Whether this ionisation affects cloud condensation nuclei to any significant extent is not currently known. Dbfirs 18:07, 27 February 2011 (UTC)

You're very quick to say that Dauto. Henrik Svensmark is the primary researcher of cosmic rays' effect on atmospheric nucleation. It's new research, and it could turn out well--we don't know yet. Mac Davis (talk) 14:11, 28 February 2011 (UTC)

I'll eat my hat if that turns out to be an important factor. Dauto (talk) 15:17, 28 February 2011 (UTC)

Is there any special reason you're driven to make this pledge, or is it just a hunch? 213.122.11.147 (talk) 22:25, 28 February 2011 (UTC)

Clouds form when relative humidity reaches 100% Nucleation is not necessary or effective. Dauto (talk) 04:20, 1 March 2011 (UTC)

Well the lead to our article on Cloud condensation nuclei disagrees with your opinion (though I'm less sure about the exact effects of cosmic rays because the process seems to be complex and not fully understood). Dbfirs 08:20, 1 March 2011 (UTC)

No it doesn't. As you can see by yourself that article doesn't mention the need of cosmic rays in order for the condensation nuclei to work. As I said, Cosmic rays are neither necessary nor effective. Nucleation happens when humidity reaches 100% because the cloud condensation nuclei are already present and quite effective by themselves without any need of cosmic rays or anything else. Dauto (talk) 18:45, 1 March 2011 (UTC)

Are we reading the same article? The way I read it, nucleation is both both necessary and effective (and opinion differs on the importance of cosmic rays). Dbfirs 23:48, 1 March 2011 (UTC)

Probably the best thing to do is let the OP make his own mind up on whether or not comic-rays might have a noticeable effect on the climate. As CERN points out, there is satellite evident that which shows a possible correlation between cosmic-ray intensity and the amount of low cloud cover. A study of the link between cosmic rays and clouds with a cloud chamber at the cern ps--Aspro (talk) 19:38, 1 March 2011 (UTC)

Thanks for that paper. I may have to eat my hat after all... :). Dauto (talk) 20:39, 1 March 2011 (UTC)

There's apparently continuing debate about it,^[1] and the experiment proposed there apparently hadn't been done as of 2009, and some say the effect should be 100 times too small. But the introductory figure with the tight correlation between sunspot cycle length and "temperature anomaly" seemed very intriguing. Either someone found a way to fudge the numbers to look awesome, or it's a heck of a coincidence, or else there's something to it. My bets are on #1, but I don't know the field well enough to spot the magic.

What interests me if the model is true, though, is the question of what a magnetic field reversal does to cosmic rays and therefore to cloud formation and global temperature. After all, the Earth's magnetic field does much more to keep out cosmic rays. It's easy to find popularized sites claiming such dire causation,^[2] but I'm having much more trouble finding anything that looks professional. In fact, what I found there was a model by which Ice Ages cause geomagnetic reversals!^[3] At this point I'm still confused, but hopefully someone in the field will shed more light on the topic... Wnt (talk) 17:46, 3 March 2011 (UTC)

Wow. I'm also finding that IceCube and a similar project have been mapping cosmic ray sources in the sky, and finding considerable variation.^[4] I think this means that the Earth should, over time, due to distant supernovae or its motion within the galaxy, should encounter varying amounts of cosmic rays. But what I didn't find, which I'm curious about, is whether the pattern of where cosmic rays strike the atmosphere can be correlated with anything interesting (coldest spots, deserts, etc.) Wnt (talk) 18:01, 3 March 2011 (UTC)

mass & motion of earth

Is there any effect on the mass and motion of the earth from taking mass from the earth (mostly satellites and space probes that never come back to earth)? and also, the fact that more and more of the earth's mass (airplanes ships cars trains) moves around where before (1000 years ago) it was all stationary in the earths crust? How much does meteorites counter this effect? —Preceding unsigned comment added by 98.221.254.154 (talk) 16:22, 27 February 2011 (UTC)

The mass will change but by a very small amount. It could be offset by meteorites hitting the earth or add to the loss in mass we get as the atmosphere drifts off (slowly) into space. The force of gravity between ourselves and the moon (and the sun etc) is directly proportional to our (and their) mass, but you need to look at the mass of the earth and see where the decimal point is, compared with satellites that might be of the order of 1000 kg. (See Wikipedia for facts on mass) Victuallers (talk) 16:37, 27 February 2011 (UTC)

Our article on Micrometeoroids suggests that the mass of the earth might be increasing by around ten million kg per year from the inflow of Cosmic dust alone, so a few satellites are not going to make a measurable difference. Moving vehicles that don't go beyond the atmosphere will have an effect that normally cancels out to zero ( though if the whole human population with all their transportation converged on one island, there would be be a measurable effect on mass distribution and the orbit of the moon would change). The eruption of a large volcano can change the length of the day by a very tiny but measurable amount, so having many aeroplanes high in the sky, and many satellites in orbit might also slow the rotation very slightly (because angular momentum is conserved), but the effect will be very small because the earth "weighs" nearly six million million million million kg. Dbfirs 17:54, 27 February 2011 (UTC)

The entire Earth's crust (which is the only part we can have any impact on) makes up less than 1% of the mass of the Earth. There is no way anything we do could have a noticeable effect on the Earth's mass. The total mass of everything launched into space is perhaps a few tens of thousands of tonnes (that's a guess, but it's probably about right - it's more if you include fuel, but that's mostly burnt early in the launch, so the resulting CO2 and water probably remains part of the Earth). The total mass of the Earth is 6 thousand billion billion tonnes. As you can see, our impact is minimal. --Tango (talk) 01:09, 28 February 2011 (UTC)

While humans are not able to significantly change the mass of the earth, we are able to shift its distribution a bit, thus changing its moment of inertia. This NASA article mentions that filling the Three Gorges Dam reservoir (which can hold 40 cubic kilometers of water, at a maximum of 175 metres above sea level) could increase the length of a day by 0.06 microseconds. In comparison, the 2004 Indian Ocean earthquake was calculated to effect a 2.68 microsecond decrease in the length of the day. To what precision are astronomers able to measure our rate of rotation? Sidereal day#Exact duration and its variation would suggest that such measurements are made to the fraction of a nanosecond. -- ToE^T 02:37, 28 February 2011 (UTC)

Bond angle of water

At Water (data page), I read this:

Bond angle 104.4776° (equilibrium)

Not being ignorant of geometry, I put that into the cosine function on a calculator and got −0.250001484. So arccos(−1/4) ≈ 104.477512186 and that got rounded. If it's not exactly arccos(−1/4) then the three consecutive 0s are an odd coincidence, and Occam's razor favors such a simple thing. So I edited it to say this:

Bond angle 104.4776° = arccos(−1/4) (equilibrium)

I find various web pages explaining why the bond angle is not 109.47°, which is the arccosine of −1/3, and is what would result from a certain kind of tetrahedral symmetry. Instead of just explaining physical reasons why it should be smaller than arccos(−1/3) ≈ 109.47°, shouldn't it also explain why it's the angle whose cosine is such a simple fraction?

Can someone here explain that? Michael Hardy (talk) 17:13, 27 February 2011 (UTC)

A google search for "cosine of the bond angle of water" gets 0 hits! Michael Hardy (talk) 17:19, 27 February 2011 (UTC)

that's just a funny coincidence. You should revert your edit. Dauto (talk) 17:57, 27 February 2011 (UTC)

If it's a "funny coincidence", then why does it agree to seven decimal places? Getting the same decimal digit seven times in a row happens once in 10 million trials if it's "just a coincidence". Michael Hardy (talk) 18:03, 27 February 2011 (UTC)

....and how do they actually measure it so accurately that they get seven digits? Could it be that they concluded it's arccos(−1/4) and then computed that to seven places, rather than actually doing a physical measurement that's so phenomenally accurate? Michael Hardy (talk) 18:05, 27 February 2011 (UTC)

Think what you will. It is a coincidence. Besides, the second page you linked has the figure 104.45 degrees. Dauto (talk) 18:07, 27 February 2011 (UTC)

The page Properties of water also has the figure 104.45 degrees. Dauto (talk) 18:08, 27 February 2011 (UTC)

(ec)The lack of Google results probably means that the correspondence to the arccos might be a coincidence. The reason why arccos(-1/3) is the bond angle of, say, methane, is because of the tetrahedral symmetry of the molecule. Water doesn't have that symmetry, or any other symmetry which would constrain the bond angle. You have a plane of reflection that splits it down the middle, you have another in the plane of the three atoms, and you have a 2-fold axis splitting the angle in the plane of the three atoms. There is absolutely no geometric reason why the angle would be exactly arccos(−1/4). Indeed, anyone rounding arccos(−1/4) ≈ 104.477512186 to 104.4776° is off (you would round it down to 5, not up to 6). 104.47759732 (which also rounds to 104.4776) is not equal to arccos(−1/4), no matter how hard you squint at it. I would *not* put the correspondence in there unless you were able to cite a reference for it (no original research and all that). Frankly, there is a heck of a lot of precision in that angle measure, and I am slightly uncomfortable with the absence of a citation mentioning how it was derived (from experimental measurements - how precise were they? From quantum chemical calculations - what level of theory? From geometric considerations - what was actually calculated?). Looking at the history [5], it looks like the reference was for (Hoy 1979), which was then removed [6] with the edit summary of "fix references". I'm not sure what paper was meant by (Hoy 1979) (no further specification was given), but the edit summary mentions the CCCDBD, which currently lists the experimental equilibrium bond angle as 104.48 [7]. -- 174.31.194.183 (talk) 18:25, 27 February 2011 (UTC)

The figure we give would appear to come from this paper, assuming this is the Hoy (1979) that was meant in the edit [8]:

A. R. Hoy and P. R. Bunker, A precise solution of the rotation bending Schrödinger equation for a triatomic molecule with application to the water molecule, Journal of Molecular Spectroscopy 74(1), 1--8. (January 1979). doi:10.1016/0022-2852(79)90019-5

The abstract gives a figure of 104.48, and says it was the result of least-squares fitting a model with 7 parameters (one of which was the equilibrium bond angle) to 375 spectroscopically observed vibration mode energies.

I don't have access to the full paper, so I can't confirm whether or not it contains the 4 d.p. number inside. You're right to be suspicious that this is so close to such a round number as arccos(−1/4), and it's not impossible that there might be some hidden symmetry underlying this (cf the virial theorem etc). It's also possible that somebody has said "Ooh, that looks close to arccos(−1/4)", and simply made up the extra figures. But there doesn't seem to be any immediately obvious reason to expect it.

We give the bond angle for ammonia as 107.8°, which would be about arccos(-0.306); but that doesn't seem to reflect any numerology -- unless there's something you can see there too? Jheald (talk) 18:41, 27 February 2011 (UTC)

I do have access to the full paper: the value given is indeed 104.4776(19) degrees. Buddy431 (talk) 21:07, 27 February 2011 (UTC)

This is clearly a coincidence. Look at for instance the angle of the Hydrogen sulfide molecule which has a structure very similar to water. Dauto (talk) 18:45, 27 February 2011 (UTC)

FWIW, ab initio calculation (as of 2005) gives 104.50°; the discrepancy between that and the observed value is said to be accountable for, by various factors not included in the basic calculation. [9] Jheald (talk) 19:13, 27 February 2011 (UTC)

It may also be worth noting that the current value for the bond length is 95.777 pm, not 95.84 -- this should be updated. I'd change the [:File:H2O 2D labelled.svg SVG] but someone's changed all the text elements to drawing paths, so somebody else can have the fun of working out what the fonts and sizes are meant to be.

It's a bit alarming that (1) we were showing out-dated information; and (2) that when somebody updated it, they only updated one data-element not the other; that they may have added spurious digits; and that we didn't spot any of this till Mike got suspicious of the numerology. Jheald (talk) 19:45, 27 February 2011 (UTC)

The quoted value is indeed the one given in the paper (they didn't add spurious digits), though there's a not insignificant amount of uncertainty in the final two digits. Wikipedia, and indeed any reference source, will always have out of date information. There's not much we can do about that (although I am suspicious of the article cited, seeing that it's over 30 years old). Buddy431 (talk) 21:07, 27 February 2011 (UTC)

Someone has now changed it to say 104.48 degrees, citing the paper by Hoy. That's more credible since it's not so many decimal places. When I put 104.48 into a calculator and hit the cosine key I get −0.250042041528. It still seems suspiciously close to −1/4, since it has two 0s immediately after −0.25. Michael Hardy (talk) 22:13, 27 February 2011 (UTC)

It is just a coincidence. Dauto (talk) 03:27, 28 February 2011 (UTC)

Decay products (further questions)

Further to my question above, I have a couple of questions about the data given in this document. As I read it something like 10% of W- bosons should decay into e- and electron antineutrinos. However, explanations of beta minus decay always gives this outcome – is there a reason why? I'm also a little bemused by the numbers given. The first four (all leptons) given are ~10%, hadrons are listed at 67% and 67+40>100. My reading of the numbers below this suggests they don't total to 67% either. Can you assist? Grandiose (me, talk, contribs) 20:17, 27 February 2011 (UTC)

β⁻ decay can't produce heavy particles because there isn't enough energy available (the nuclear energy only decreases slightly), but that energy is available in spades when a "free" W boson decays. There are three leptonic decay modes (eν, μν and τν) and those together with the hadronic modes add to 100% within the margin of error. The "ℓν" decay on the first line must come from a different measurement or calculation that didn't distinguish the lepton generations. Likewise, the lines below "hadrons" are limited by the experimental data that happens to be available. It's a bit confusing because this summary document doesn't cite its sources. -- BenRG (talk) 20:58, 27 February 2011 (UTC)

Thanks, that's that pretty much cleared up. Grandiose (me, talk, contribs) 21:01, 27 February 2011 (UTC)

Circuit of Unique Equivalent Resistance

Hello. Is it possible to design a circuit that has a unique equivalent resistance regardless of the order in which resistors are connected? Thanks in advance. --Mayfare (talk) 23:17, 27 February 2011 (UTC)

Please clarify. Surely you are asking for something harder than the fact that the equivalent resistance of a string of series resistors is independent of the order in which they are connected. Similarly the sum of a series of numbers does not depend on their order (commutative property). Edison (talk) 23:47, 27 February 2011 (UTC)

I'm trying to identify with one ohmmeter reading the 2-Ω resistor given 10 resistors, nine of which are one ohm each. --Mayfare (talk)