Wikipedia:Reference desk/Archives/Science/2009 April 29

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April 29

Image changing material

What is that material called that changes the image on a piece of plastic when viewed at different angles? -- penubag  (talk) 01:40, 29 April 2009 (UTC)

Polarizer? --Jayron32.talk.contribs 01:44, 29 April 2009 (UTC)

I'm not sure if that's it. This is a popular children's toy. It's a flat sheet of plastic with ridges and an image on it. The viewing angle affects what is seen on the plastic. -- penubag  (talk) 01:52, 29 April 2009 (UTC)

(EC) Are you referring perhaps to Lenticular printing images or hologramss? If so their individual articles describes how they work Nil Einne (talk) 02:09, 29 April 2009 (UTC)

Yes, lenticular printing is it. Thanks -- penubag  (talk) 02:40, 29 April 2009 (UTC)

Also if you wouldn't mind me asking, what does (EC) mean? I've seen it in multiple places and luckily I can ask on the RD.-- penubag  (talk) 03:10, 29 April 2009 (UTC)

EC = edit conflict (though this'll get confusing if a bunch of us try answering at the same time, and then get a notice that someone else has saved an edited version of the content since it was loaded = edit conflict). --Scray (talk) 03:22, 29 April 2009 (UTC)

It refers to edit conflict, which happens when two try to post at the same time in the same place. (Ironical enough this is a edit conflict) chandler ··· 03:23, 29 April 2009 (UTC)

Usually someone will mention an edit conflict because they haven't bothered to read the new replies (or if they have, haven't modified their reply) which could already address what they are discussing or may help clarify some uncertainty or whatever. In this specific case I started to reply but couldn't quite remember the name so had to search. I also got distracted by something else. By the time I found what I was looking for, you had replied but I didn't bother to consider your latest reply and modify you post hence I mentioned an edit conflict (if I did I probably wouldn't have bothered to mention holograms since I was a bit uncertain from your first post but your reply sounds precisely like lenticular printing) Nil Einne (talk) 23:40, 29 April 2009 (UTC)

:) thanks -- penubag  (talk) 03:37, 29 April 2009 (UTC)

analytical lab

if a small legal (chemical)analytical lab got a "DEA number" for handling chemicals of concern, would every one in the lab(exept for janitors,guards,etc.)also need a "DEA number" to handle the chemicals? —Preceding unsigned comment added by 66.237.50.35 (talk) 04:18, 29 April 2009 (UTC)

There is information about DEA chemical registration here that may help. --Scray (talk) 11:28, 29 April 2009 (UTC)

Fluorescent Mountain Dew

Youtube video: Take a bottle of Mountain Dew, pour out all but about one quarter inch, put a very small amount of baking soda in, pour a bit of hydrogen peroxide in and shake, and it glows brightly. What's going on here? Is this just fake? If not, how does it work? Does it have to be this soda? Or is it carbolic acid that's needed or soemthing else in soda?--70.19.69.27 (talk) 04:41, 29 April 2009 (UTC)

Give us a link to the video. Dauto (talk) 05:12, 29 April 2009 (UTC)

Well, 99% of YouTube 'science' videos are faked - but this one could just maybe be real. Our Hydrogen Peroxide article says:

"Hydrogen peroxide is used with phenyl oxalate ester and an appropriate dye in glow sticks as an oxidizing agent. It reacts with the ester to form an unstable CO2 dimer which excites the dye to an excited state; the dye emits a photon (light) when it spontaneously relaxes back to the ground state."

Is there any "phenyl oxalate ester" in Mountain Dew? The amount of dubious chemicals they put in that stuff...anything is possible! But perhaps something else in the soda has a similar effect and is creating an artificial "glow stick"?

Another possibility (assuming the video is indeed a fake) is that I believe that Hydrogen Peroxide glows nicely in UV light - so perhaps they have a UV lamp illuminating the 'set' when they do the experiment. The baking soda makes it froth up nicely, the Hydrogen Peroxide makes it glow and the bright food coloring in the Mountain Dew gives it the vibrant color. That's a guess though.

SteveBaker (talk) 05:54, 29 April 2009 (UTC)

There are a ton of youtube videos of this, all with lots of "it's a fake" comments. Either external light projected, which diffuses due to the cloudiness of the Dew or (more commonly suggested) another chemical is added (maybe glowstick material itself). DMacks (talk) 05:57, 29 April 2009 (UTC)

I'm not convinced that YouTube comments are any more reliable than YouTube videos... --Tango (talk) 12:51, 29 April 2009 (UTC)

Yes - well, it wouldn't surprise me - as I said, there is this worrying trend that about 99% of so-called science videos on YouTube are faked. Most of the people who actually figure out the fake then go on to post fakes of their own instead of properly debunking the original video. Sad...very sad. I daren't ask how many kids are encouraged to play around with hydrogen peroxide in an effort to reproduce this. Anyway there are lots of videos where people try the exact same trick and it doesn't work - and just a couple which show someone adding contents of the inner tube from a glow-stick into the 'dew' and only then getting the same results. Since we know that hydrogen peroxide is one ingredient of a glow stick - then adding the 'missing' phenyl oxalate ester from a glow stick would indeed produce the effect we're seeing in the videos. So this is 'busted'. SteveBaker (talk) 12:51, 29 April 2009 (UTC)

See this discussion at snopes.com. Looie496 (talk) 15:50, 29 April 2009 (UTC)

Our article doesn't list wintergreen as an ingredient. If they found a bottle that had that then they might have gotten Triboluminescence [1].76.97.245.5 (talk) 21:08, 29 April 2009 (UTC)

Triboluminescence is a very dim light - and it requires physical force to be exerted. It happens when the candy breaks suddenly - there is a brief flash of light that you have to be in a totally dark room with dark-adapted eyes to see. What these videos proport to show is glowing that's visible in broad daylight - even after you stop shaking the bottle. Besides, explain how adding either baking soda or hydrogen peroxide would help that? No - it's not triboluminescence - it's an idiot with a video camera and too much time on his hands. SteveBaker (talk) 01:08, 30 April 2009 (UTC)

Hydrogen bonds

Chlorine is more electronegative than Nitrogen, so why doesn't it form hydrogen bonds? Is it because Cl has a larger atomic radius? Thanks. —Preceding unsigned comment added by 116.71.62.233 (talk) 07:43, 29 April 2009 (UTC)

Maybe I'm mis-understanding your question or the types of structures you're considering, but given that electronegativity means (casually) "holds its electrons tightly", it seems pretty intuitive that the more tightly an atom holds its valance electrons, the less likely/stable it would be to share them with an adjacent hydrogen atom. DMacks (talk) 07:49, 29 April 2009 (UTC)

No, my question is: Why doesn't Chlorine form hydrogen bonds? —Preceding unsigned comment added by 116.71.62.233 (talk) 08:40, 29 April 2009 (UTC)

And the answer from DMacks was: Because it is too electronegative to share an electron with a hydrogen. Have a look at electronegativity. The high electronegativity is one way to explain while certain elements "prefer" to make ionic bonds. TheMaster17 (talk) 08:47, 29 April 2009 (UTC)

I have to correct myself. I misunderstood what was a hydrogen bond in english. I thought it meant "covalent bond", but as a colleague of mine pointed out, it means the partially covalent binding that hydrogen can built to any electronegative atom (like the bond between the O in one water molecule to the H of another). I'm no chemist, but in my chemical training I was told that all partially negative atoms can form this kind of bound state with a hydrogen under the right conditions, so I would think that chlorine is perfectly fit to do so. Hmm, I'm suprised that DMacks as a chemist obviously also mixed up the terms. TheMaster17 (talk) 09:17, 29 April 2009 (UTC)

There are several inter-related effects involved, so depending on what you are comparing and unless you feel like really examining every detail, easiest to pick one that casually explains that one particular example. A more formal explanation (as always for (semi)covalent bonding) is whether your valence electrons are in n=2 or n>2, how many you have, and what your nuclear charge is, and these are observed as electronic effects such as polarizeability, electronegativity, bonding atomic-orbital overlap, atomic radius. They all correlate, but not really possible to pick any one as "the cause" of every example of any given effect. Electronegativity is a one of several good "rules of thumb" for patterns of hydrogen-bonding. Atomic radius as such is another that correlates with H-bonding, but seems poorer as a primary cause (per question as asked): electrons further out are less tightly held (further from the positive nucleus), so they should be more able to reach out and bond. It's virtually all really just back-rationalization of observations and the real reason is "because that's how it is", so we pick something that explains certain cases satisfactorily:( DMacks (talk) 13:36, 29 April 2009 (UTC)

Chlorine will bind with Hydrogen. It forms HCl, also known as hydrochloric acid. 65.121.141.34 (talk) 12:58, 29 April 2009 (UTC)

Yes, but that's not what hydrogen bonding means - a hydrogen bond is (usually) a temporary intramolecular bond formed between polar molecules. Our hydrogen bond article says that hydrogen bonding occurs in compounds in which hydrogen is bonded to oxygen, nitrogen or fluorine. These elements have electronegativiies of 3.04, 3.44 and 3.98 respectively on the Pauling scale. The electronegativity of chlorine is 3.16, in between nitrogen and oxygen - so asking why hydrogen bonding does not occur in chlorine compounds such as HCl is a good question (and one to which I don't know the answer). Gandalf61 (talk) 13:30, 29 April 2009 (UTC)

N, O and F are all period-2 elements; Cl is in period-3, and is thus much more massive than the other three, which causes inter-molecular forces to be unstable because the tiny hydrogen nucleus cannot hold a Cl atom in place (note this is just my ad-hoc explanation which may or may not reflect reality, the real reason the bond is not stable could be determined by calculating the energies of the quantum-mechanical wavefunction). Truthforitsownsake (talk) 13:41, 29 April 2009 (UTC)

That doesn't make sense. A heavier atom moves more slowly making it easier to be bond to. That's the oposite of what you said. I think the mass of the atom plays no role here, but the size of the atom (clorine is about twice as big as a fluorine, for instance) may be important. Dauto (talk) 16:28, 29 April 2009 (UTC)

I think I still didn't get it: Chlorine is really unable to form hydrogen bonds, though it is strongly electronegative? What happens in organic compounds were chlorine sits at the border of the molecule, in contact to water as solvent? Doesn't it form even weak hydrogen bonds with the solvent molecules (as N,O and F would do in this situation)? This would be a fact to remember for me. TheMaster17 (talk) 14:12, 29 April 2009 (UTC)

If you take a look at this chart: http://www.cem.msu.edu/~reusch/VirtualText/Images/hbondgph.gif, it looks like there is a slight stability increase in HCl compared to other diatomic hydrogen molecules, but it is nowhere near the magnitude that a "true" hydrogen bond causes. You might also be interested in the same question asked and a slightly different explanation given here: http://www.physicsforums.com/showthread.php?t=279991. Truthforitsownsake (talk) 14:37, 29 April 2009 (UTC)

I think I'll take a crack at it. Most of these are just educated guesses, but I think there are a few points to be made.

1.) The hydrogen itself must be bonded to an electronegative atom. This polarizes the H atom, giving it a partially negative charge. This partial negative charge helps create a temporary (transient) dipole-dipole moment with another electronegative atom.

2.) The chlorine (or whatever) atom itself is part of a molecule. Due to its electronegativity, it is polarized as well. In this case it's negative, however I believe it would not be as negative as a more electronegative element (like F) in the same position.

3.) In chemistry, there can be a lot of gray areas. Pretty much all molecular bonding is a mixture of covalent and ionic bonds, for example. The same can be true for hydrogen bonding. Indeed, the article lists a very large range of strengths (With molecular dynamics. I'm tempted to run some ab initio calculations of my own, although that would be OR). There is no hard line, and I imagine at the lower end, "slightly stronger van der Waals interactions" can be interpreted as hydrogen bonding, and vice versa.

4.) The hydrogen bonding article says that the H-bond has been shown to be partly covalent. If true, the better the overlap of orbitals, the stronger the covalent part of the bond. Hydrogen and fluorine overlap very well (1s-2p overlap). Hydrogen and chlorine overlap pretty well (1s-3p), but not as good as H-F (That's why HF is a "weak acid", but HCl is a strong acid - HF bond is so good, it doesn't want to dissociate). I think that this is a good explanation, but I'm lacking a bit of evidence at the moment.

Well those are my thoughts anyway. It's a good/tough question :) --Bennybp (talk) 04:47, 30 April 2009 (UTC)

So it all comes down to the point that HCl molecules should form hydrogen bonds but they dont. Why they dont form hydrogen bonds is a blur. Somebody contact the IUPAC. —Preceding unsigned comment added by 116.71.63.56 (talk) 07:28, 30 April 2009 (UTC)

Isn't this all related to whether HCl can "donate" hydrogen bonds or whether it can just receive them? Notice that HCl, HBr, HI, etc. all dissociate completely in solution and are all monoprotic acids. They all also interestingly, form gases at room temperature. H-F is a weak acid but is a liquid at room temperature. Sulfuric acid has two hydrogens, so still can donate hydrogen bonds after losing one, but are also weaker acids than the hydrogen halides (after H-F). Nitric acid boils at 83 C and has a pKa of -1.4, so maybe there's a trend here. I suppose the other idea is: how stable is an intermolecular HCl-HCl bond

About hydrogen bond donation -- are we talking about the hydrogen halide or the halide in general here? Interestingly, as a countertrend methylene fluoride has a way lower BP than methylene chloride, and carbon tetrachloride has a pretty high BP (highest of all the chloromethanes) despite having no hydrogen bonds to donate. However, BP of carbon tetrafluoride < methylene fluoride. John Riemann Soong (talk) 22:27, 30 April 2009 (UTC)

I believe that's muddying the water a bit. It's not fair to compare carbon tetrafluoride to methylene fluoride, since methylene fluoride has a permanent dipole-dipole moment and carbon tetrafluoride does not. And methylene chloride/fluoride is not capable of having "hydrogen bonds" - the fact that the chloride has a higher BP can be explained by increased van der Waals forces (Cl is bigger and has more electrons).

When you ask how stable the intermolecular HCl-HCl bond is, that is exactly the point. This is not at all about 'true' (covalent or ionic) bonding, but about intermolecular forces where there exists 1.) A hydrogen bonded to N, O, or F. AND 2.) An N, O, or F on another molecule that is free to interact with the aforementioned hydrogen. And we've given it that distinction because they seem to be much stronger than other interactions.

So does the hydrogen in HCl interact with the Cl on another molecule? Yes. However, we've chosen not to call that a hydrogen bond since it would be relatively weak. The reasons I've outlined above are my best guesses as to why it would be so much weaker than say, HF, despite Cl still being fairly electronegative. It's obvious electronegativity is not the real explanation.

Sorry, I was comparing C-Cl4 with C-F4. That was my intention. I meant to compare carbon tetrachloride and carbon tetrafluoride. John Riemann Soong (talk) 00:30, 1 May 2009 (UTC)

And just to point something out (and I believe SteveBaker likes to point this out sometimes): The concept of a hydrogen bond is a purely human concept. And in fact, especially in organic chemistry, all the explanations you hear are just rationalizations, and generally don't have any bearing on what actually happens (which is all governed by quantum mechanics). Nature doesn't care if our feeble minds can properly label everything and sort them neatly into boxes - it just does what it does. --Bennybp (talk) 23:47, 30 April 2009 (UTC)

Swine influenza

Are people, who dont eat, work, sleep etc., with pigs at risk of catching the disease? —Preceding unsigned comment added by 116.71.62.233 (talk) 08:46, 29 April 2009 (UTC)

Yes. But the risk is low at the moment (but greater for those in Mexico than those in North Korea). Kittybrewster ☎ 08:49, 29 April 2009 (UTC)

(EC) This depends on your definition of "at risk". It seems the virus is able to be transmitted from human to human, so you can catch it without getting anywhere near pigs. But the prerequisite for this is that the other person is infected, and at the moment, the outbreaks are local and rather small in all countries except mexico (and the situation there is far from clear because a normal influenza is also circulating). TheMaster17 (talk) 08:54, 29 April 2009 (UTC)

Another important fact is that most people (outside of mexico at least) seem to get better rather quick. Why there seems to be such a contrast between mexico and the rest of the affected countries is a mystery, at least to me, at the moment. TheMaster17 (talk) 09:02, 29 April 2009 (UTC)

I don't find this puzzling at all. I have high confidence in the Mexican government's ability to count the bodies of the dead (that's the numerator). The denominator (the number of people in Mexico who've had this flu) is another matter entirely - Mexico city is absolutely huge, and the health care system is overwhelmed. As a result, one would expect an under-estimate of the denominator, giving a falsely high death rate. --Scray (talk) 11:22, 29 April 2009 (UTC)

And as the mexican administration had to correct even the number of deaths from the new H1N1 type A strain today, it seems they are unable to get any number correct. All calculations done so far are false. Now the new mystery (and also the real mystery in the past) to me is the complete naivness of the WHO. They have sparked significant panics all over the civilized world (you should hear what they are teaching here in germany at the schools: according to one teacher, the "new flu" is worse than the black death from medieval times...), because they obiously did not expect that the mass media would twist every word and number they release. But scientifically speaking, there is nothing near an EPIdemic ongoing at the moment, with the exception that the situation in mexico is unclear. So to use the word "possible PANdemic" in this connection is unfounded: Every year we have a "possible pandemic of flu", if the new yearly strains should acquire the required characteristics. TheMaster17 (talk) 12:04, 29 April 2009 (UTC)

There are really three versions of this kind of disease:

1. Swine Flu that only pigs get that can't be passed to humans - which has probably been around in various strains for millions of years - just like bird flu, cat flu and flu varieties for most other species on the planet.
2. Swine Flu that mutated so that humans could catch it from pigs but could not pass it on to other humans - perhaps it's been around for a while - it's hard to tell because people don't usually live in very close proximity to pigs, so not many people catch it.
3. Swine Flu that's mutated AGAIN so it can be passed between humans.

All of the concern with Bird Flu a few years ago was because that virus had just taken the step from (1) to (2) in countries where people habitually allow their chickens and ducks to wander through their houses. At stage (2), the disease is easily controlled - don't go near birds and you won't catch it - but the concern is that it's now only one mutation step away from the super-dangerous stage (3) when it can pass like wildfire from person to person.

So the huge concern is that Swine Flu appears to be in stage (3) right now. Worse still, many new diseases are much more lethal than older strains. That's because the virus cannot spread from a dead victim, hence, over time, diseases evolve to be less lethal so that they can spread more easily. A disease that's so mild that you don't even take a day off work spreads VERY easily. Hence there is significant evolutionary pressure on diseases that spread by coughs and sneezes to become gentler. Brand new diseases such as Swine Flu have not yet evolved that gentler nature - so for the first year or so, they can be very dangerous.

However, as others have pointed out - we know how many people have died (around 100 so far) - but we have no idea how many were infected and survived - possibly without any symptoms at all. If only 200 people were infected but 100 died - then with a 50% mortality rate, this would be a deeply scary disease. If a million people were infected but 100 died - then this is not much worse than a typical flu season and there is really no reason to panic. We still don't know where we are along that scale - so we're taking reasonable precautions until we know for sure.

This disease was certainly passed from a pig to a human sometime in the past when it was in stage (2) - but now that it's mutated into stage (3) - which must have happened inside a human - it's just a regular human disease and you're far more likely to catch it from another person than from a pig because the pigs can only have type (1) and (2) of the virus. To get type (3) they'd have to catch it from an infected human - and that's really unlikely! You need to be much more concerned about catching it from another person than catching it from a pig.

So in areas where the disease is rampant - avoid crowds - stay away from sick people - wash hands regularly, keep your hands away from your eyes, nose and mouth where possible (unless you've just washed them carefully). If you get flu-like symptoms, stay away from other people and call a doctor. However, those measures are far from 100% effective - the virus can hang around for quite a while on non-biological surfaces and people show no symptoms until they've had the disease in their bodies for many days...so these safety measures are only improving your odds.

SteveBaker (talk) 12:28, 29 April 2009 (UTC)

So the chances of a person living in a muslim country getting infected is even lower because muslims dont eat pork, right? —Preceding unsigned comment added by 116.71.39.119 (talk) 12:43, 29 April 2009 (UTC)

No lower than for a person in a non-Muslim country that doesn't go near pigs. I would expect that cooking pork makes it safe to eat - it does with most diseases and I haven't seen any health warnings telling people not to eat pork. That this flu originally came from pigs is pretty much irrelevant at this point. It can pass from human to human - that's going to be by far the most common way people catch it from now on. --Tango (talk) 12:50, 29 April 2009 (UTC)

I think you've misunderstood. The disease came from pigs originally but now it's carried by humans.

For now any country that's not Mexico is pretty safe. However, it's entirely possible for someone infected to get on a plane and fly to any other country of the world and start a new outbreak. He would not have to bring a pig with him.

I'm not sure we can say how likely that is, we're not experts, and even the experts are still working on it. APL (talk) 12:53, 29 April 2009 (UTC)

According to CNN, you can not get the disease from eating pork. (Unless you are eating it from a trough with pigs around.) 65.121.141.34 (talk) 12:56, 29 April 2009 (UTC)

Yes - that's well worth emphasizing - we have enough economic and societal problems as it is without people suddenly abandoning eating pork for absolutely no good reason! Flu is spread through respiratory processes - coughing and sneezing. Dead pigs do neither - they are perfectly safe. But as I said before - you're never going to catch THIS strain of swine flu from a pig anyway - even in the exceedingly unlikely event that it's coughing an sneezing all over you. The evolutionary step that enabled this disease to spread from human to human happened inside a human - not inside a pig. If a pig ever did catch this strain, they'd be getting it from the farmer! Really: right now, the only things to avoid is other people with symptoms - and (arguably) people who visited Mexico during the last couple of weeks. SteveBaker (talk) 13:17, 29 April 2009 (UTC)

Since this is the reference desk, please allow me to recommend the following article to provide you with information that you may find useful in regards to your question: Swine influenza. You may also want to check out this as well: 2009 swine flu outbreak cheers, 10draftsdeep (talk) 20:10, 29 April 2009 (UTC)

Forget the "swine" part of it. Once it goes human-to-human the original vector does not matter. It's just "a flu", one that humans at the moment have no native resistance to. --140.247.251.93 (talk) 20:16, 29 April 2009 (UTC)

Of course they have "native resistance". While few people have had the opportunity to develop immunity, most human bodies are well equipped to fight (resist) such an illness. We can see this by the fact that the overwhelming majority of people who have had the illness have recovered. You may find it useful to read the World Health Organisation's FAQ, with particular note on the safety of pork products, and the fact that most infected people have merely displayed standrad flu symptoms and have recovered without medical treatment. Gwinva (talk) 22:33, 29 April 2009 (UTC)

What warning level does the WHO have to raise the alert to (already at 5 out of 6) for it not to appear rude to wear a mask when dealing with the public all day? Edison (talk) 23:24, 29 April 2009 (UTC)

Generally speaking, at level 5, i would think it's ok to wear a mask where ever u are in the world. I live in london and I am going to start wearing a mask in busy public places. —Preceding unsigned comment added by 80.47.226.203 (talk) 00:09, 30 April 2009 (UTC)

If you are not suffering from flu yourself, or are not actually caring for someone with flu then it's almost certainly pointless wearing a mask, and not particularly helpful. "The available scientific evidence does not suggest that this is an effective preventative measure". The influenza virus is not airborne, but passed by droplets (that is, from coughs, sneezes, runny noses) which requires contact. So, avoid direct contact with ill people and wash your hands frequently (along with other basic hygiene). Further info here Gwinva (talk) 01:05, 30 April 2009 (UTC)

Not all authorities seems to quite agree with that, for example the CDC [2] "You are well and do not expect to be in close contact with a sick person but need to be in a crowded place. Limit the amount of time you spend in these crowded places and wear a facemask while you are there." and [3]. It seems to me that after dealing with the public, if you wear a mask and carefully discard it each time (perhaps with gloves) and then wash your hands properly it may be somewhat effective if it's difficult to stop touching your face completely Nil Einne (talk) 12:18, 30 April 2009 (UTC)

IMHO, we would to well to follow current US government practices and call this the "2009 H1N1" flu - because calling it "Swine flu" is clearly confusing the general public - and since pigs and pork products are entirely unrelated to the problem - all we're doing is handing a bunch of farmers a problem they REALLY don't need right now! SteveBaker (talk) 13:32, 30 April 2009 (UTC)

Why in the world would we follow the US government? It's a worldwide issue, commonly known as Swine flu, it would be more confusing calling it "H1N1", no one has any associations to that name. It's not we who're causing problems, its not our problem that people are ignorant (and this they can't eat pork, because its widely reported that you can't get it from pork). Swine flu is the obvious Common name, I can only speak for the two languages I know and it's known as Swine flu (and translation) in both. chandler ··· 13:40, 30 April 2009 (UTC)

I think Steve meant "we" as in people in general, rather than Wikipedia. Wikipedia's policies are clear - we use the most commonly used name. --Tango (talk) 14:21, 30 April 2009 (UTC)

firing an SCR

Hi, I am doing a project on controlled rectifiers. At some point, I have the output of an op-amp (+11v) to fire an scr. The gate current required to fire it is nearly 100uA. I believe the opamp isnt able to provide that much of current. Any idea what device to use to increase the opamp output current ? 218.248.80.114 (talk) 20:45, 29 April 2009 (UTC)

Calling all electronics design experts! Steve Baker? 100 microamps sounds like a minimal demand on an op amp. Most of the old ones I used could carry tens of milliamps. Which op amp? Which SCR? I have some in the basement if I could find them, but using data sheets is more of an approved "engineering" approach. [4] has a test circuit to make sure the device has not been accidentally fried (all too common in labs). Edison (talk) 23:16, 29 April 2009 (UTC)

(Sorry - I'm a digital guy - if it's not 1's and 0's, I'm relatively clueless) SteveBaker (talk) 13:27, 30 April 2009 (UTC)

You don't say whether you actually need the +11 V to fire the SCR, or whether a lower voltage would do. Assuming that you do need 11 V, you could use a CMOS device like the CD4050B. This device will output a voltage as high as the supply voltage that you give it, up to a maximum of 20 V, and it can source around 8 mA. The maximum input current is 1 uA, usually much less, so your puny op-amp could probably just about drive it. Like all CMOS-input devices, you have to be careful not to zap it with static. --Heron (talk)

1918 Spanish flu mortality

Can someone make sense of the following figures for me:

An estimated one third of the world's population (or ≈500 million persons) were infected and had clinically apparent illnesses during the 1918–1919 influenza pandemic. The disease was exceptionally severe. Case-fatality rates were >2.5%, compared to <0.1% in other influenza pandemics. Total deaths were estimated at ≈50 million and were arguably as high as 100 million.

— 1918 Influenza: the Mother of All Pandemics
Jeffery K. Taubenberger, David M. Morens

Wouldn't 500 million peeople infected, and 50-100 million dead imply a case fatality rate of 10-20% ? Is >2.5% just a (oft-quoted) conservative figure ? Abecedare (talk) 20:45, 29 April 2009 (UTC)

Perhaps the numbers are including people killed by things like famine caused by the Spanish flu that might kill people who were not infected? 65.121.141.34 (talk) 20:54, 29 April 2009 (UTC)

That would be a reasonable hypothesis, but

Johnson NPAS, Mueller J. Updating the accounts: global mortality of the 1918–1920 "Spanish" influenza pandemic. Bull Hist Med. 2002;76:105–15

which I read, suggests that it is not true. The 50-100 million figure does include deaths due to influenza, pneumonia, bronchitis, phthisis cased by the infection, but not secondary effects like famine etc. Anyone have easy access to the sources cited for the >2.5% estimate ? Abecedare (talk) 21:10, 29 April 2009 (UTC)

People surviving once and being re-infected again later - possibly mutiple times - before dying of it, may skew the results slightly. Vimescarrot (talk) 21:24, 29 April 2009 (UTC)

True, but each person would have to be infected 4-8 times, in order to make the numbers work. Seems unlikely, especially since infection by the (common) flu virus is supposed to provide temporary immunity. Abecedare (talk) 22:10, 29 April 2009 (UTC)

"had clinically apparent illnesses" - perhaps the mortality rate is computed using a figure for all infected, not just cases of "clinically apparent illness". But I don't know how they would estimate that. Rmhermen (talk) 21:58, 29 April 2009 (UTC)

The world population in 1918 was around 1.5Bn. So even assuming that everyone was "infected", 50-100million deaths would imply 3.3-6.7% mortality. Still significantly higher 2.5% ! Abecedare (talk) 22:10, 29 April 2009 (UTC)

our article indicates that morbidity and mortality rate estimates vary greatly. Not surprising, when there's mathematical disagreement within one source as quoted above! Gwinva (talk) 23:02, 29 April 2009 (UTC)

What is meant by the term "Case-fatality" in the OP's question? Might it mean that 2.5% of people who were under the care of a doctor (looking after their "case") died? Then it might not be surprising if 10% of the general population died if the vast majority could not get to a doctor. Perhaps the 2.5% refers to patients who were treated and their cases followed carefully. The remainder might be very hard to pin down to definite causes - hence the 10% to 20% uncertainty. But that's speculation. SteveBaker (talk) 01:01, 30 April 2009 (UTC)

Reinfection of some individuals during the multiple waves of the flu might have an effect as well. Rmhermen (talk) 23:21, 30 April 2009 (UTC)

I think the OP has identified a widely-quoted discrepancy that does need correction. The trick here is the ">2.5%". The "actual figures" are ten to twenty percent, which is indeed greater than two and a half percent. my guess is that basically all major countries has percentages greater than 2.5%, but some populous countries such as India and China had percentages that were MUCH greater than 2.5%. Our current knowledge of the 1918 epidemic is filtered through the perspective of the American and European reportage of the 1918 era, which focused primarily on American and European countries where the case fatality rates were closer to 2.5%. -Arch dude (talk) 08:15, 2 May 2009 (UTC)

effects of caffeine

Hopefully this question doesn't overstep the mark and creep into asking medical advice - which I know cannot be issued by wikipedians. If this question is unacceptable I am very sorry. The article on caffeine says its half life is approx 4.9 hrs in healthy adults. I am a healthy adult and my question is why don't the effects last that long? - as presumably you should still be feeling half of 'buzz' you originally got 4.9 hours after you ingested it. After a less than 2 hours I seem to be back to normal. This is the case for many other people I know as well. Is there any reason why people stop feeling the effects before the caffeine has even had a half life? I don't consume caffeine excessively, 2 cups of tea a day at most so I wouldn't say I have a high tolerance to caffeine. Many thanks and I hope someone can shed some light on the subject! —Preceding unsigned comment added by 92.16.11.107 (talk) 21:33, 29 April 2009 (UTC)

I would expect that the half-life is not necessarily a reliable indicator of the amount of time it will 'noticeably' have an affect on the body when asking a person. For instance many people will wake up the morning after a night of drinking and feel perfectly fine, but if tested they may well still be over the limit. What they 'feel' is 'normal' but what they are is still showing the effects of alcohol consumption. Hopefully someone will be able to clarify for you though, would be interesting to know ny156uk (talk) 22:36, 29 April 2009 (UTC)

The "half-life" probably refers to the detectable amount in your system. That isn't the same as the amount bound to receptors and due to weakening action potential that again doesn't equate the same effect. 76.97.245.5 (talk) 23:41, 29 April 2009 (UTC)

It's of note that my doctor told me that since I have sleep-related problems I should really stop drinking caffeine around 2pm, because otherwise there will still be appreciable amounts kicking around my body come 11pm, leading to some of the effects albeit in obviously weaker form. Enough to keep you from falling asleep? Maybe if you are sensitive to it. (I don't think I'm actually all that sensitive to it, and that my sleep issues are unrelated to it, but anyway, it makes some sense, and I have cut back.) --98.217.14.211 (talk) 02:09, 30 April 2009 (UTC)

Whether ones "sleep issues" are due to caffeine or not, caffeine will still most probably exacerbate them. Read up on adenosine, adenosine receptors and Caffeine#Mechanism of action. --Mark PEA (talk) 19:16, 30 April 2009 (UTC)

Body size

The Vitruvian Man (c. 1485) Accademia, Venice

Is there a website where it shows a picture of a body, either boy or girl, and beside it, they have the measurement of the body? —Preceding unsigned comment added by 99.226.95.73 (talk) 23:06, 29 April 2009 (UTC)

Alive or dead? Which measurements? Edison (talk) 23:10, 29 April 2009 (UTC)

I suspect OP is looking for some clip art? Just search Google Images for "human height". You'll find plenty.--86.25.194.171 (talk) 07:04, 30 April 2009 (UTC)

You mean like this for bodybuilding or maybe these videos on how tailors take measurements? Clarityfiend (talk) 23:00, 30 April 2009 (UTC)

If you want to know the body measurements of children, sellers of sewing patterns such as Simplicity and Butterick have patterns for childrens' clothing with typical body dimensions. Edison (talk) 00:31, 1 May 2009 (UTC)

I added a fairly well-known one. -Arch dude (talk) 00:35, 2 May 2009 (UTC)

Question

Hello, I need help. I would like to know how to solve the following question, what Geometry/Physics you need to know for it. A satellite is launched into a circular orbit around a planet with negligible radius in a direction tangent to the orbit at a distance R, and another satellite is launched with half the velocity of the first one. What is the minimum distance between the planet and the satellite during the orbit, in terms of R? —Preceding unsigned comment added by 173.66.142.5 (talk) 23:17, 29 April 2009 (UTC)

Escape velocity or maybe inverse square law would be some starting points. --Jayron32.talk.contribs 03:06, 30 April 2009 (UTC)

Circular motion should help too. Martlet1215 (talk) 07:17, 30 April 2009 (UTC)

Hedge plants

I haven't found this in our article, but what causes a tree/shrub to be suitable as a plant for a hedge. a} I had read before that trees that you trim the top off die. Why isn't that happening when I trim e.g. a birch to be a hedge. b) Would fig trees work and could you still get figs from them if it were trimmed to be a hedge? 76.97.245.5 (talk) 23:47, 29 April 2009 (UTC)

There are several factors that are involved when deciding how suitable a plant is to be a hedge. Of some importance would be the plants ability to be trimmed on a regular basis. most plants can survive this as long as it is started when the plant is relatively young. Some plants, for instance some pines, will not reshoot if cut back, especially if they are old plants. On the other hand there are many plants, lonicera, privet, hawthorn which will shoot back when trimmed severely, allowing radical shaping and control. Some plant will never look good as hedges, fig for instance with its large leaves and irregular stems is hard to imagine as an attractive fence (but there’s no accounting for taste). Finally it would be best if the plant did not sucker and spread like bamboo or some species of sumach a predictable, controllable hedge is probably preferable. Birch trees are really not dense enough nor vigorous enough to make a satisfactory hedge. A fig might work but my experience with figs is that to get reasonable fruit requires watering and this leads to excessive leafy growth unless the roots are restricted. That is why they are often grown in a large pot or tub.Richard Avery (talk) 08:07, 30 April 2009 (UTC)

When trimming becomes difficult and the hedge is getting too high, it is common practice to "lay" the hedge by partly cutting through stems and bending them to a horizontal position, thus stimulating new leafy growth at a lower level. It is still true that some trees are not suitable for this process because they do not readily produce new shoots from old trunks. Dbfirs 08:20, 30 April 2009 (UTC)

The key to trimmability is whether apical meristem tissue remains in the untrimmed part. If you cut off the top of a mature tree, for most species it just stays that way. But some species have lots of meristem tissue staying quiescent (because auxin hormones tell them to) and when they realize they are now at the top/outside of the plant, they begin the process of tissue growth (sprouting). That's what makes a good hedge plant. Younger sprouts are good candidates for developing a hedge, because they have lots of meristem tissue - so if you clip off a very young branch, you will often get two more in its place. Good candidates for hedges (in cold climates anyway) are cedar and alpine currant, because they have that property of multiplying when trimmed. Franamax (talk) 19:54, 30 April 2009 (UTC)

Thanks. I'll see where I can get with the shrubs/trees we have available. (BTW. I meant beech instead of birch, keep mixing those up unfortunately.) I was thinking it would be silly to rip out the wild shoots that keep cropping up all over our yard and then going out spending oodles of money on plants for a hedge from a nursery. On the other hand I didn't want to invest all the work in digging just to have the things shrivel and die. The info on apical meristem tissue was what I was missing. (We're in Atlante and the general rule on gardening here seems to be that everything you don't want grows copiously whereas intentionally planted stuff gets crowded out and dies.;-) 76.97.245.5 (talk) 06:15, 1 May 2009 (UTC)

The apical meristem bit is the main thing. There is another point though – all trees and shrubs have apical meristem in the buds of shoots, but some also have it under the bark, as epicormic buds. These allow the plant to grow again even when cut back beyond all foliage – for example when pollarded or coppiced. That's why many broadleaved shrubs make better hedging plants than, say cypress or pine, as you can't kill them by cutting them back too far. Beech (or at any rate our species, European Beech) is an excellent hedging plant – it has excellent epicormic growth and forms a dense hedge down to the ground. Also, any leaves within three or four metres of the base of the stem stay on through the winter, thus making a beech hedge an attractive screen when many other plants would be bare. American Beech seems to do this too (see File:Fagus grandifolia CT3.jpg). Incidentally, I believe figs fruit better if hacked about a bit. Richard New Forest (talk) 13:12, 5 May 2009 (UTC)