Wikipedia:Reference desk/Archives/Science/2009 May 17

= May 17 =

what spider is this?
what species? i saw it in my back yard the picture is i posted it earlier but a picture of spiderman showed up since it had the same name as the commons picture, spidey.jpg. anyways any help identifying it would be awesome. —Preceding unsigned comment added by Spideygonnagetu99999 (talk • contribs) 03:47, 17 May 2009 (UTC)


 * I have never seen that before (I am in the UK), but I am assuming by the time stamp that you are more West from me, and therefore probably North America. Try this database. They have photos. Good luck.--KageTora - (영호 (影虎)) (talk) 04:37, 17 May 2009 (UTC)

My intuition tells me it's in Linyphiidae family, but I can't be sure. Sorry. --Dr Dima (talk) 07:30, 17 May 2009 (UTC)


 * I'd go for an Orb-web spider, possibly a Barn spider, it would help to have some idea of size. Mikenorton (talk) 12:02, 17 May 2009 (UTC)


 * Can you please describe the web of the spider you are asking to identify? That will help a lot. --Dr Dima (talk) 22:24, 17 May 2009 (UTC)


 * This spider is in my yard in Richmond, California its about 2-3 inches from front feet to back feet, the main part of its body being 1.5-2 inches, and its about 1-1.5 inches wide. the web is big and roughly hexagonical but more than six, like 18 to 20 ended shaped. —Preceding unsigned comment added by Spideygonnagetu99999 (talk • contribs) 23:38, 17 May 2009 (UTC)


 * This sounds a lot like an orb-weaver indeed. Orb-weaver webs are rather large (usually 1-3 feet across), near-vertical, and look something like this. Individual linyphiid webs, on the other hand, are small and sheet- or tent-like. Also, 1.5" size is about right for Araneus sp. (a typical orb-weaver), but huge for a linyphiid. --Dr Dima (talk) 04:40, 18 May 2009 (UTC)

Variable Frequency Sine Wave Generator
I need to create a sine wave generator circuit, whose frequency output f will be proportional to the input voltage/current. In a Wein-bridge oscillator, if in place of the resistance, a transistor is used, operating as an amplifier, with the gate connected to the frequency control terminal, will it suffice? —Preceding unsigned comment added by 218.248.80.114 (talk) 06:30, 17 May 2009 (UTC)
 * You are wanting a voltage controlled oscillator or VCO. You can buy ICs with this built in, or as you suggest a MOSFET or field effect transistor could be a resistance controlled by a voltage. If you want to you can use a capacitor controlled by a voltage, such as a reversed biased diode (varicap), this can very quickly change the frequency. Graeme Bartlett (talk) 08:08, 17 May 2009 (UTC)
 * You may consider designing an Arbitrary waveform generator to produce a sine wave output. The only active analog circuit this requires is a Digital-to-analog converter which is available as a single fully-specified component. It would receive from memory (RAM or ROM) at a relatively high clock rate samples of the wave to be generated. The memory would be loaded, or the clock rate controlled, by a digital controller that includes an  ADC for the input voltage/current. Cuddlyable3 (talk) 13:16, 17 May 2009 (UTC)
 * I've used a varac diode to tune the resonance of a voltage-controlled oscillator. This is conceptually the simplest VCO, because your applied voltage directly changes the resonant frequency of the LC-tank, but it is complicated by the nonlinear and uncooperative behavior of a varac.  Nimur (talk) 19:59, 17 May 2009 (UTC)

Human digestive system - top speed? ;-)
So I was curious how fast the human digestive system works, and read the article - apparently 40~50 hours? A lot longer than I'd have guessed. What I'd really like to know, though, is how fast something can run through you... I'm clearly thinking about really bad diarrhea here...

Surely I'm not the only one who has ever had really bad diarrhea and began to think back over what all they'd eaten trying to pinpoint the source?

I did an archive search for "diarrhea speed" but only got lots of discussion about traveling at the speed of light, you sickos! ;-) —Preceding unsigned comment added by 61.189.63.198 (talk) 07:28, 17 May 2009 (UTC)
 * A patient preparing for a Colonoscopy must stay on a clear liquid diet for a day, followed by a day of purging using laxitives and lots more clear liquids. -Arch dude (talk) 09:04, 17 May 2009 (UTC)


 * Ever had a barium meal? It takes less than two hours for that stuff to come out and you need to make sure you are near a toilet when it starts!--KageTora - (영호 (影虎)) (talk) 09:36, 17 May 2009 (UTC)


 * I think some gastric infections (e.g. Norovirus) may affect other parts of the digestive system, in that something might stimulate strong peristalsis to get whatever is causing it out of the system. Digestive Transit (for which we don't have an article!) is the name given to the time taken for food to pass out of the system. It varies between people: if you don't drink enough and/or are constipated, things will take longer to leave you (if at all!). --TammyMoet (talk) 10:55, 17 May 2009 (UTC)


 * For estimating normal speed, eat some sweetcorn and time thje result. 78.146.103.200 (talk) 16:48, 17 May 2009 (UTC)


 * Also, the GI tract is not a FIFO queue. Different types of food have different characteristic time-scales.  It's possible for a faster item (like soup) to pass faster than a slow item, like steak.  By the time it's in the intestine, everything's pretty soupy anyway.  Nimur (talk) 17:04, 17 May 2009 (UTC)


 * A search for "diarrhea speed" gets stuff about the speed of light? Sounds like a joke from Spaceballs (Sure glad they called it "ludicrous speed" instead. :-) )Somebody or his brother (talk) 00:23, 18 May 2009 (UTC)

3 PHASE IN ELECTRIC POWER SYSTEM
I want to know why the electric powe generator is designed to be 3 phase not more than this? in other way why is the angle between Phase and the other must be 120 degree?~ thank u —Preceding unsigned comment added by 3ateka (talk • contribs) 10:13, 17 May 2009 (UTC)

The article for this is Three-phase electric power. This is an engineering decision. It isn't impossible to use fewer, or more, phases, but using three phases is most cost-effective. It is the solution that needs the least conductor material, and if you're going to build a power line across hundreds of miles, conductor material (copper) will be the decisive cost factor. --dab (𒁳) 10:34, 17 May 2009 (UTC)


 * Nikola Tesla used two phase power pretty effectively to run his induction motors. But when you add the third phase, starting the motor is easier and the flow of energy to the motor is steadier. There is little benefit to adding more phases, considering the higher cost of conductors, insulators, and structures, not to mention circuit breakers, transformers, etc. Edison (talk) 20:52, 17 May 2009 (UTC)
 * In the US, most residences only have one 60hz phase. We use both poles of one phase to get to "pushes" per cycle, so flourscent bulbs flicker at 120hz. the high-voltage is delivered to the transformers in the neighborhood via a single conductor, and each home is connected to teh transformer with two power conductore (the phase nad the anti-phase) plus a neutral. In industrial areas, the powe company delivers all three phases using three high-voltage wires that are 120deg out of phase with each other. I think motors can use both poles of each phase, for a total of six "pushes" per Hz. So in industrial areas, more capital is spent on the wires but less is needed for the big motors. for residential-type applications (motors below about 2hp), the cost differnce between the more expensive single-phase motors and the cheaper three-phase motors is small-to-nonexistent, so the whole thing makes economic sense. -Arch dude (talk) 23:18, 17 May 2009 (UTC)
 * With three phase, the torque is continuous. With single phase, (and maybe 2 phase, like Tesla's early installations) the torque pulses. Edison (talk) 19:02, 18 May 2009 (UTC)

Egg crystals?
I left a little bit of whisked egg in a bowl overnight in a warm, dry room, and it has formed what appear to be crystals, although they are a little cloudy (photo at http://www.benjamin-mills.com/photos/2009/egg-crystals.jpg). Is this really a crystalline solid, or just a kind of flaky amorphous solid?

Ben (talk) 16:39, 17 May 2009 (UTC)
 * Looks like a beautiful set of radial cracks caused by the drying out of the egg, like dessication cracks in mud. The radial symmetry comes from the shape of the bowl and the thickness of the layer. On a completely flat plate the cracks would have been roughly hexagonal, like in the basalt of the Giant's Causeway (though of course they're a result of cooling rather than drying). Mikenorton (talk) 17:27, 17 May 2009 (UTC)


 * I agree - not all geometric patterns are due to crystal lattice structures. This appears to be uniform fracturing due to dessication.  Nimur (talk) 19:56, 17 May 2009 (UTC)

OK, thanks guys.


 * Ben (talk) 22:58, 17 May 2009 (UTC)
 * This also looks like the basis for a really good science-fair project for elementary school kids to demonstrate dessication cracks. -Arch dude (talk) 23:24, 17 May 2009 (UTC)

Little gas poker
What do you call the hand-held wand used for lighting candles and gas stoves? It has a liquid gas container and a piezo ignition. I thought it was a gas poker, but Googling suggests that that is a more heavy-duty device for starting fires. I want to find one on line but I don't know what they're called (in the UK.)--86.25.195.233 (talk) 17:55, 17 May 2009 (UTC)


 * Piezoelectric gas lighter or static electricity generator. Axl  ¤  [Talk]  19:03, 17 May 2009 (UTC)


 * (e/c) Could it simply be known as a lighter? Lakeland (that nirvana of kitchen goods) currently has on sale something called a Handy Lighter which performs that function, although it doesn't mention what ignition method is used. |countryGB Kitchen lighter also produces some relevant results on Google. -- Kateshort forbob  19:09, 17 May 2009 (UTC)

What is the name of this bird?
Does anyone know the name of this bird, photographed in Austria? Thanks a lot... --Edcolins (talk) 18:48, 12 April 2009 (UTC)


 * It looks like a Common Blackbird. Axl  ¤  [Talk]  19:17, 17 May 2009 (UTC)


 * From the size of the eye, its posture and the rather fluffy plumage at the base of the breast I'd go for an Alpine Chough. Mikenorton (talk) 19:29, 17 May 2009 (UTC)


 * Yes, you're right, Mike. The colour of the legs also fits. Axl  ¤  [Talk]  19:32, 17 May 2009 (UTC)
 * Many thanks! Great! --Edcolins (talk) 19:44, 17 May 2009 (UTC)

HIV and preeclampsia
I have been trying to get a dissertation topic for Part II FMCOG relating to HIV and preeclampsia since 80% of antenatal patients I attend to are HIV positive. Please, could you suggest some topics.41.219.251.119 (talk) 20:51, 17 May 2009 (UTC)

LOL
I've just been watching a comedy show on TV. I notice that at the funny bits all the audience laughs at the same time, and rarely at other points. Are funny things just funny, or do we accept cues from other people that encourage us to laugh at the same time out of some kind of evolutionary advantage. When I watch comedies alone I often find that I never laugh througout the whole programme, but appreciate which situations are humourous. Also, I have heard that laughing benefits us in bearing our teeth, which is an aggressive response to a situation as seen in many primates. Your thoughts, please, russ (talk) 20:58, 17 May 2009 (UTC)
 * Most TV shows have a "laugh track" with recorded laughter, or a "live studio audience" including the writers and friends of the cast who laugh predictably at the appropriate places. In 1939s when NBC in the U.S. started their experimental broadcasting, there was a concern that theatrical movies would not play well on TV if comedies, because in the theater, as in live theater, there is a range of latencies for people to figure out the joke and laugh, and the movie actors would pause after a gag (as on stage) to allow for that before the action moved on. Laughing out loud may be something of a group phenomenon. The concern was that the solo home viewer would be put off by the resulting slow pace, but it would be less of a problem with group viewing. The "laugh track" can act as a filler to avoid the latency problem. It also helps the clueless understand that something funny happened. The humorist Mark Twain had a standard joke he used at the beginning of an appearance to measure an audience. Their latency in getting it and laughing told him whether to dumb down his material and use broad humor for a dullard audience, or to use his more demanding material. Edison (talk) 21:25, 17 May 2009 (UTC)


 * Most shows with live audiences employ applause signs to tell the audience when to applaud and when to laugh. The audience also never goes in "cold" - before the action starts the stage manager or someone similar will always stand in front of the live studio audience and give them directions on when it's appropriate to laugh, etc. Frequently for comedy shows, (especially the late night shows like the Tonight Show) they will also have a warm-up comedian* to get the studio audience in a good mood. (* Amazing - Wikipedia doesn't seem to have an article on warm-up act) -- 128.104.112.117 (talk) 21:52, 17 May 2009 (UTC)

We do laugh more openly when there are others to hear it. And as for the way primates bare their teeth, it is apparently more to do with a reassurance that you mean no harm. A fear grin.Popcorn II (talk) 22:40, 17 May 2009 (UTC)


 * I think we take cues from others, but I have some admittedly personal observation that shows that there are always those who are slower and faster, even if not warmed up. The warm-up guys are there mostly, I think, to get the slower ones up to speed. Otherwise, you'll have a situation like what happened with a friend. He's the stoic kind who always tries to keep his cool and usually only grins or chuckles. Well, we were all laughing at one of those silly B movies and making fun of it and he sits there reall quiet for about 20 minutes, then suddenly bursts into a laughing fit that's like all his laughter came out at once. He kept getting the cues, but something in his personality was holding him back, till all those cues caught up with him.Somebody or his brother (talk) 00:34, 18 May 2009 (UTC)


 * yes, there are laugh tracks and prepared audiences, but also laughter is strongly contagious. Looie496 (talk) 02:28, 18 May 2009 (UTC)


 * After a performance has been recorded with a live audience the sound track can be "sweetened" by adding laughter or applause to the genuine reactions. It is hard to detect when that is done skilfully but on occasions audience reactions seem suspiciously mistimed. Cuddlyable3 (talk) 13:11, 18 May 2009 (UTC)


 * You know that this laughter is totally faked in at least some shows because it's added to shows where no live audience is present - and it sounds exactly the same. I recall, for example, that when we watched M*A*S*H (TV series) in the UK, it was broadcast without the canned laughter.  I couldn't believe it when I first came to the US and watched the exact same shows with canned laughter...it totally changed the show from something rather pointedly anti-war to just another sit-com.  Weird. SteveBaker (talk) 13:33, 18 May 2009 (UTC)

Magic and illusion
I have several questions about magic: 1- What's the difference between magic and illusion?

2- Is their a true magic? I mean is it true what the media says and what is sometimes posted on wikipedia about magicians such as David Copperfield and the Liberty Statue? If so how can we explain it physically?

Thanks awfully..--Email4mobile (talk) 21:00, 17 May 2009 (UTC)


 * 1. "Magic" generally refers to one of two things: illusionism, which is essentially the art of fooling people into thinking that they're seeing something that appears to be impossible or supernatural, whereas in reality it's all done with trickery and clever misdirection, or paranormal magic, which actually is supernatural -- that is to say, it actually violates the known laws of reality. The latter, it should be stressed, is something that a lot of superstitious people certainly believe in, but of which there exists no actual evidence. The former is illusion and the latter is delusion, you could say. (It's particularly important to understand that there's no evidence of someone actually performing supernatural acts of magic, but there are many, many instances of people claiming to have magical powers and being exposed as liars and frauds. That in itself doesn't prove that there's no supernatural magic, of course, but when there's no evidence for something and a lot of the people who claim that there is are exposed as liars, it doesn't exactly make an argument for its existence.)
 * 2. I don't know what you mean when you refer to what the media says and what is sometimes posted on Wikipedia. I can tell you, however, that David Copperfield did not actually make the Statue of Liberty disappear, he just made it look like it did. That's an illusion. We don't know exactly how he did it, though, because like most magicians, Copperfield likes to keep his tricks to himself, but William Poundstone's explanation for how it was done (explained in general terms in the article I just linked to) seems fairly sensible. If that's how he did it, it's a classic example of misdirection, an illusionist's most important tool: the idea that Copperfield would actually move the audience instead of the statue without anyone noticing is just so audacious that most people would never even consider it! But it could certainly be done.
 * So no, I don't believe that there's any magic in the supernatural sense. There certainly isn't any hard evidence of magic -- just stories people like to tell, but as the Copperfield example above illustrates, even if people really believe in something, that doesn't mean it actually happened. It's easy to fool people when you know how, and in many cases, the more outrageous the trick, the more willing people are to believe it. -- Captain Disdain (talk) 21:38, 17 May 2009 (UTC)
 * Some think there's a difference, some don't. - Running On  Brains  00:41, 18 May 2009 (UTC)


 * I think that one of the most convincing reason to disbelieve these 'magic' acts is that if the magician could truly perform them, he'd be able to do so in much clearer, testable situations. The industrial value of being able to make objects weighing hundreds of tons simply 'vanish' - or to have people who could fly or create coins out of thin air would be extensive and revealing the techniques involved would earn the performer billions of dollars.  The fact that they restrain themselves to a simple stage presentation is very telling.  All of this quite aside from the violations of scientific laws of all kinds that would be required.  So we can be VERY confident in saying that (without exception) magic tricks are tricks - and most magicians are only too happy to admit that. SteveBaker (talk) 00:43, 18 May 2009 (UTC)


 * The method David Copperfield used to make Lady Liberty disapear is a known, if rather elaborate conquerors' trick. This Article explains it, if you really want to know. APL (talk) 02:26, 18 May 2009 (UTC)


 * I refer to the same method above (except in our own article), but our article says that it's speculation, not actual proof of how he did it. I agree that it's probably how it was done, but in the interests of the accuracy of our article -- is it actually sure that this is how it was done? -- Captain Disdain (talk) 07:41, 19 May 2009 (UTC)

Thank you very much, gentlemen for this valuable emphasizing. --Email4mobile (talk) 06:53, 18 May 2009 (UTC)
 * Magicians generally refer to large stage effects (tricks) using big boxes and large props as illusions, although some smaller tricks can also be called illusions. - Mgm|(talk) 11:30, 18 May 2009 (UTC)


 * As the aforementioned Gob puts it... "It's an illusion, Michael. A trick is something a whore does, for money." -- Captain Disdain (talk) 14:05, 18 May 2009 (UTC)

Lye in a swimming pool
(Caution: do not try this at home.) On CSI Miami, in Dissolved (CSI: Miami episode) someone added enough sodium hydroxide to a swimming pool to make the pH a bit above 12.3 (per the CSI Ph meter). How many kilograms of sodium hydroxide would have to be added to a home swimming pool of say 100,000 liter capacity to achieve that alkalinity? How many liters of supermarket vinegar (say 8% acetic acid) would it take to get the pH back down to 7.0? It did not look like they poured very many gallons of vinegar to neutralize the lyewater. Would the high alkalinity solution instantly break a glass beaker if poured into it as in the episode? Would concentrated lye water have a strong smell? Edison (talk) 21:16, 17 May 2009 (UTC)
 * NaOH is awfully strong, but dilution in 100,000 liters is no small effect. If my calculations are right, to make a pH of 12.3, you only need a molarity of 0.02, or about 0.8 grams of NaOH per liter of water.  In a 100,000 liter swimming pool, that gives you about 80 kilograms of NaOH.  More to the point, how did somebody add the NaOH?  It's much more likely that it was not added as a pure solid, but added as a more concentrated liquid (like Lye).  Since lye is only about a pH of 13, you would basically need to fill most of the swimming pool with it to reach a pH of 12.3.  Nimur (talk) 21:51, 17 May 2009 (UTC)
 * Glass, especially chemical lab beakers made of pyrex, does not react with sodium hydroxide. Lye does have a distinctive smell, and if the pool were full of it, it ought to be obvious.  Nimur (talk) 21:54, 17 May 2009 (UTC)
 * And, while we are debating the validity of the scientific events in a TV show whose extent of scientific reality falls somewhere between "just making shit up" and "faking it", droping that much NaOH into a swimming pool will likely be exothermic enough to boil the water, and spatter everyone in the area with a nice boiling, skin dissolving solution of sodium hydroxide. Yum, yum.  --Jayron32. talk . contribs  23:04, 17 May 2009 (UTC)
 * Nimur, the paper-thin things I've accidentally left in base-bath overnight or flasks etched with melted NaOH would disagree. Now of course that's not how the typical person would use such a material:) DMacks (talk) 02:33, 18 May 2009 (UTC)
 * That analysis only works if the pool water is not buffered. If the owner has added a pH buffer to the pool water, or simply lives in a region where the tap water naturally contains buffering agents, you'll need to add additional NaOH to overcome the buffering effect. How much extra is hard to say, as water buffering capacity is usually measured by alkalinity, which assumes that you're adding acid, not base. -- 128.104.112.117 (talk) 18:16, 18 May 2009 (UTC)
 * Why would they use such a weak acid as vinegar to neutralise it? You can get "Muriatic Acid" (weak Hydrochoric acid) at pool supply stores - it would be much better at the job than vinegar! SteveBaker (talk) 00:19, 18 May 2009 (UTC)
 * Red Devil is nearly pure crystalline sodium hydroxide, you can buy it in any grocery store, and it's cheap -- on the order of a dollar a pound to the best of my recollection. So based on the numbers above you could probably do it for around $500 or less. Looie496 (talk) 02:43, 18 May 2009 (UTC)

Saturated NaOH will etch glass over time, but does not react violently. And making a pH 13 solution of NaOH by dumping pure solid into water is nowhere near exothermic enough to cause boiling (unlike sulphuric acid!), possible just because it takes to long to dissolve. A higher concentration of a weak acid probably would be just as good for such a neutralisation, as the act of neutralising would pull the equilibrium to one side, so the lower dissociation constant in water is not a factor. Was the pool being used to kill people, or just dispose of bodies? Pure NaOH is corrosive, but can still be picked up without gloves (not recomended though!), so i'd guess wouldn't kill people quickly enough.YobMod 08:00, 18 May 2009 (UTC)
 * 100,000 liters is more than even some large aquariums. It's not a reasonable size for a home swimming pool. - Mgm|(talk) 11:27, 18 May 2009 (UTC)
 * One cubic meter is 1000 liters, so a pool 10mx10mx1m would have 100,000l, as would one 10mx6mx1.5m (well, close enough). Large, but not impossible for a home pool.--Stephan Schulz (talk) 12:14, 18 May 2009 (UTC)
 * Yeah - our home pool is 26,000 gallons - that's 98,000 liters. It's not by any means a huge pool - if you had a part that was deep enough for diving (which many of my neighbours do) then it could easily double that. SteveBaker (talk) 13:21, 18 May 2009 (UTC)
 * On the show, they neutralized the lye with a few gallons of vinegar. But a lab beaker shattered when the lye solution was poured in it and a live human was "boiled" to death. Edison (talk) 18:59, 18 May 2009 (UTC)
 * Meh. I use 3 M NaOH on a daily basis, which is 15 times as concentrated as here. Spilling it on the skin has no effect, except the usual soapy feeling from bases (not even peeling the next day, like most acids), so i very much doubt it could kill someone unless they are held in it for a while. And the 3 M solution i use is poured from Schott bottles to measuring cylinders to beakers - none of them ever break or even crack slightly. Overall seems a poor murder method indeed!YobMod 07:38, 19 May 2009 (UTC)