Wikipedia:Reference desk/Archives/Science/2008 May 21

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May 21

Evolution of nutritious veggies

What benefits does a plant gain from containing vitamins and other compounds that are nutritious to animals? I can see why a plant would want to make nutritious fruit so animals could eat it and disperse the seeds, but I'm wondering why there are nutrients in other parts such as leaves and roots which the plant presumably doesn't want eaten. Although I understand that animals have probably evolved to make use of the chemicals in plants rather than the plants evolving "for" the animals, I'd like to know why the plants contain these nutrients in the first place, what function they serve. 69.224.182.55 (talk) 01:56, 21 May 2008 (UTC)

Plants are biochemically complex too, thus the production of those chemicals for use in the metabolic pathways. —Preceding unsigned comment added by 199.76.188.69 (talk) 02:02, 21 May 2008 (UTC)

You could just as well ask why cows evolved to provide nutritions milk. That's not the purpose nature intended. --71.236.23.111 (talk) 02:12, 21 May 2008 (UTC)

uh....what? actually, 71.236, 'it is'. First off, it should be pointed out that the whole teleological notion of "nature intended" is intrinsically flawed. But as long as we're going to use that kind language, the 'intended purpose' of cow's milk is to supply nutrients to its calves. So yes, the nutritious quality of cow's milk is a direct result of evolution.

In response to the original question, though plants and animals look really different, the general biological strategies are often similar. For instance, take energy storage. Lipids (oils and fats) and sugars are general strategies used by plants and animals to store energy. Consequently, plants often store energy for their own purposes in areas that get eaten by animals; basically the animals are raiding energy from the plants. Also, plants and animals use vitamins and minerals in fairly similar ways (often as cofactors to enzymes, or precursors to other molecules). (In theory) Animals evolve dependency on nutrients from external sources when there is a sufficient quantity available to them in their diet that it is no longer energy efficient for them to synthesize the substance on their own . For instance, take vitamin c. It is incredibly important in a variety of metabolic pathways and consequently most organisms synthesize it on their own; humans are one of a (relatively) few exceptions. Other nutrients, such as minerals, simply cannot be synthesized and may just be present in sufficient quantities in the diet to be incorporated into the metabolism. Take iron. Iron is a metal that is an important component of many proteins, such as hemoglobin. As an element, it just can't be synthesized. Plants suck it up from the ground and we eat the plants (to be fair, we also get it from some water sources). --Shaggorama (talk) 05:17, 21 May 2008 (UTC)

As far as "that kind of language" goes, I can't help it, it's common English. (I could try a couple of others with a lot less ability and effect.) Plants having a strategy sounds only slightly less off to me than "nature intending" something. (Which I wasn't too happy with to begin with.) I agree that my post wasn't worded to mean what I intended to say if read by itself. What I was trying to imply was that cows produce milk for calves, not humans. While there are symbiotic relationships based on mutualism (both win). Lots are based on commensalism or parasitism. 71.236.23.111 (talk) 06:19, 21 May 2008 (UTC)

Well, on the one hand, humans are also mammals and produce milk as well. Our milk isn't that different from cows milk, so the nutritious qualitites of cows milk are nutirtious for us as well. BUT, this statement needs qualification. As we both agreed, cows milk is "intended" for calves, in the same way mother's milk is produced for the consumption of babies. As humans age, we lose an important enzyme for the digestion of milk, Lactase, causing Lactose intolerance in many people. Fact is, the evolution of the role of lactase in our bodies hasn't caught up to our consumption of cows milk. You are absolutely right: cows produce milk for calves, not humans. Likewise, cows milk didn't evolve a nutritious capacity for humans, and it makes many people feel sick. --Shaggorama (talk) 17:18, 21 May 2008 (UTC)

A lot of veggies are very different from their naturally occurring sources due to many years of plant breeding, selectively breeding more nutritious and easier to grow crops. Many veggies are Root vegetables which originally stored nutrition in their storage organs to allow themselves to be able to grow quickly when the correct season occurred. As you mention, some fruits have a different strategy, there flesh is deliberately attractive to animals so that the fruit and seeds are consumed and the indigestible seeds unwittingly deposited elsewhere (often with a healthly dose of compost!). GameKeeper (talk) 07:18, 21 May 2008 (UTC)

Most plants are not nutritious to humans or other animals. We tend to think of those that are as common only because we have evolved a long-term mutually symbiotic relationship with them, where we (like honey bees and flowers) aid the plant in its survival and reproduction for our own benefits. With things like agriculture it is a major conceptual error to forget that humans have been involved at an ecological level for tens of thousands of years. --98.217.8.46 (talk) 16:13, 21 May 2008 (UTC)

Peanut allergy

Is it known what substance(s) in peanut is/are responsible for the severe allergies to the food that seem quite common these days? Have there been studies on possible changes in the nutrition/allergen content of peanut over the past several decades? --72.94.50.27 (talk) 02:46, 21 May 2008 (UTC)

I'd like to know something about this to. I never heard of peanut allergies back when I was a kid (less than two decades ago). Now it seems like it's this huge problem. Have humans suddenly become more sensitive to allergens, have peanuts become more noxious or has awareness just increased? --69.151.29.16 (talk) 04:22, 21 May 2008 (UTC)

See Peanut allergy for 72. As for 69's question. No, humans aren't more susceptible to allergies, it is because the environment that they live in has become cleaner (that's why allergies and asthma are more common in developed countries). Anyways a tidy environment has decreased the population of gut parasites in humans. This caused IgE, a part of our immune system that deals with parasites to become "bored" (due to lack of a better word) and tries to attack something else which usually the thing that the person becomes allergic to.--Lenticel ^(talk) 04:53, 21 May 2008 (UTC)

I have heard of the hygiene hypothesis too, but I'm not convinced yet that's the correct explanation, or that cleaner environment is the only or dominant cause of peanut allergy. The possibility that peanuts today are not the same as they were decades ago should at least be examined. The peanut allergy article does not mention any specific allergens in peanut; it doesn't seem to answer the original question. --72.78.102.134 (talk) 12:33, 21 May 2008 (UTC)

It's also likely that we just understand more about allergies now than we did, and communication has become more common (so it's better known). Many of the recent "epidemics" tend to come down to better diagnosis and more awareness of the allergies/conditions. -- — The Hand That Feeds You:^Bite 13:22, 21 May 2008 (UTC)

Fruits and vegetables

If you eat a good variety of common vegetables, do you need fruits? Clarityfiend (talk) 05:15, 21 May 2008 (UTC)

If you eat enough Big Macs, you don't need to eat anything else at all! --Shaggorama (talk) 05:44, 21 May 2008 (UTC)

That's funny. I can't see why not when veges have loads of benefits (weight loss, heart disease prevention, fibre etc). Currently there's a push to get people vege-wise here in Oz[1] in a ratio of 2 fruit, five veg. In countries where grains are the basis of a diet, I doubt fruit would get a look-in or would be a rare treat/seasonal thing.

But wait for it, there's the dark side of broad beans here[2] aka Favism. So maybe it's "common vegetables" plus all the other things (nuts, grains, etc) or is it just veges vs fruit? Julia Rossi (talk) 06:02, 21 May 2008 (UTC)

there's also a problem regarding the definition of fruits and vegetables.--Lenticel ^(talk) 06:15, 21 May 2008 (UTC)

In regards to my diet, I'd eat veggies over fruits. In my mind, however, a diet of fruits seems more attractive. Imagine Reason (talk) 00:37, 28 May 2008 (UTC)

Identify the flower

Does somebody know?

83.130.129.138 (talk) 05:42, 21 May 2008 (UTC)

Nice pic. There's Halesia as in Halesia tetraptera or Silverbell? Flowers around this time. Depends where you are and I'm assuming it's a tree (not shrub or vine). Anyone? Julia Rossi (talk) 05:55, 21 May 2008 (UTC)

Well, I'm from Israel. However, I took the picture in the Liberty Island :-) 83.130.129.138 (talk) 06:28, 21 May 2008 (UTC)

On closer inspection, it's an apple blossom, see the article Apple tree and scroll down to right side image "apple tree in blossom flower". I love a good mystery, : ) and now we know what grows on Liberty Island. Feel free to add your pic to the article to show blossoms in cluster formation. Julia Rossi (talk) 07:02, 21 May 2008 (UTC)

hiv

how long does it take to detect hiv after having sexual intercourse with an infected person ?

Thanx —Preceding unsigned comment added by 200.56.111.250 (talk) 07:39, 21 May 2008 (UTC)

HIV. Wisdom89 _{(T / ^C)} 07:41, 21 May 2008 (UTC)

People who become infected with HIV may have no symptoms for up to 10 years, but they can still pass the infection to others. After being exposed to the virus, blood tests results change from HIV negative to HIV positive usually within 3 months from[3] Julia Rossi (talk) 09:28, 21 May 2008 (UTC)

Is this a trick question? It is perfectly possible to have unprotected sex with an HIV infected person and not get HIV. However I would not recommend playing russian roulette with your healthy body. 122.107.199.2 (talk) 10:43, 21 May 2008 (UTC)

I think the OP means "reliably detect whether or not you have HIV", and Julia's answer looks correct to me. Of course, standard disclaimer: If you have any concerns regarding a medical issue, you should consult a doctor. --Tango (talk) 11:49, 21 May 2008 (UTC)

The actual virus is transferred at the moment of intercourse - but there is so little of it (compared to the volume of your blood) and HIV reproduces so slowly that it may take months for the virus to replicate enough to be detectable in a small blood sample - so (as Julia said), three or four months before it's gonna show up in a blood test...and that same slow growth rate explains why it may be years before any symptoms show up. However, if you know you had unprotected sex with an infected person, you shouldn't wait to see a doctor because there may be pro-active treatment options that can be done even before a test can be performed. 71.155.164.147 (talk) 14:20, 22 May 2008 (UTC)

black holes

I want to know if light can,t escape black hole, then how gravitons are able to leave it and affect the near by objects by gravitational force? —Preceding unsigned comment added by Pkbansal (talk • contribs) 12:13, 21 May 2008 (UTC)

Gravitons are entirely theoretical and, as I understand it, not generally acknowledged to be likely or necessary. Gravitation as warped space-time absolves the need for such particles. Further developments in quantum mechanics (and other fields of physics) are needed to fully explain or refute the graviton. — Lomn 12:42, 21 May 2008 (UTC)

But black holes can be electrically charged too, and photons certainly exist—how do they get out? I agree with your answer, though. This is a case where the virtual-particle picture of fields is misleading, and I'm not certain it works at all. There's more to quantum field theory than Feynman diagrams.

There's clearly some level at which gravitons make sense. You'd expect a theory of quantum gravity to predict quantization of gravitational wave amplitudes, i.e. real gravitons. And weak-field general relativity can be quantized against a fixed background, leading to Feynman diagrams with virtual gravitons ([4]). But I don't think virtual gravitons make sense in a background-independent theory. -- BenRG (talk) 19:40, 21 May 2008 (UTC)

Anyway, I think the answer is that gravitons have to be massless, yes? Or else they'd be affected by gravity as well. --98.217.8.46 (talk) 14:46, 21 May 2008 (UTC)

Photons are massless, they're still affected by gravity. --Tango (talk) 15:21, 21 May 2008 (UTC)

Of course photons have mass, namely $h\nu /c^{2}$. What they lack is rest mass.

Well, yes, but gravitons contain energy and therefore mass as well, don't they? --Tango (talk) 18:42, 21 May 2008 (UTC)

Hi. I heard that gravitons can't have mass otherwise the physics wouldn't work. Also, perhaps the gravitons are more clustered near the black hole, so that don't actually leave it? Also, would gravitons actually approach each other? Probably not, because even photons don't do that and gravitons are always travelling away from each other. Also, black holes are more like a deep tunnel in spacetime, deep but not very wide, so an object parsecs away likely won't be very much affected by an average black hole, and an object takes forever to be ingested into a black hole because the centrifugal force is almost as strong as the gravitational attraction. They might send gravitational waves if they collide. Also, light doesn't actually escape a black hole, it spirals around it before being stretched and swallowed, although some say radiation might shoot out of some black holes' polar axes. I've also heard that the only laws of physics and forces black holes don't almost always break are gravity, rotational force, and electromagnetic force. Thanks. ~AH1^(TCU) 20:02, 21 May 2008 (UTC)

I think when people say gravitons have to be massless, they mean rest mass. --Tango (talk) 20:25, 21 May 2008 (UTC)

There is an anser to this question from the sci.astro newsgroup FAQ here. Long story short, it says that the Coulomb force is mediated by the exchange of virtual photons which can escape from inside the event hroizon, so a black hole can carry a charge. A consistent quantum theory of gravity (if we ever develop one) would presumably involve virtual gravitons which would behave in the same way. What you can't do is send either electromagnetic or gravitational waves from inside the event horizon to the outside, as this would require the escape of real photons/gravitons. Gandalf61 (talk) 08:31, 22 May 2008 (UTC)

Though I have a lot of respect for Steve Carlip, this FAQ answer seems completely wrong. The electromagnetic field of a charged black hole does not, in any sense, come from inside the hole. Virtual particles can't get through the event horizon in any meaningful sense. Nothing that happens inside the event horizon affects the electromagnetic field outside. That's as true in quantum gravity (as it's presently understood) as in classical physics. -- BenRG (talk) 15:18, 25 May 2008 (UTC)

Chemistry

What are the gases that dissolve in water? Why do some gases dissolve and some don't? —Preceding unsigned comment added by 124.43.204.248 (talk) 15:53, 21 May 2008 (UTC)

Are gases really different than solids or liquids, or is "Like dissolves like" (a polar or ionic compound will dissolve in water) not affected by the physical state of the compound at room temperature? Remember that many things are "a little soluble": there is no sharp distinction between "soluble" and "insoluble", so you have to decide how soluble you want. If it weren't for oxygen dissolved in water (concentration of 7.6 mg/L at 20 °C), all the fish would die, but that sure looks like a low solubility (carbon dioxide is 1.7 g/L at that temperature). DMacks (talk) 16:35, 21 May 2008 (UTC)

And also, there's a solubility table in regards to water. --Wirbelwindヴィルヴェルヴィント (talk) 16:40, 21 May 2008 (UTC)

I believe the solubility of a solid or liquid in water generally increases with temperature, whereas the solubility of a gas decreases with temperature. That's the only major distinction I can recall. --Prestidigitator (talk) 20:58, 21 May 2008 (UTC)

One very soluble gas is ammonia. You can easily get a 30% solution. This is polar. nitrogen and oxygen only dissolve a little, but what does is very important to life. Graeme Bartlett (talk) 21:38, 21 May 2008 (UTC)

You might try reading our article on Henry's Law. shoy 16:31, 22 May 2008 (UTC)

Physics

We feel pain when we touch a heated object or boiling water. What is exactly the cause for this? —Preceding unsigned comment added by 124.43.204.248 (talk) 15:57, 21 May 2008 (UTC)

"Pain" is how the brain interprets the nerve impulses generated by sensory neurons. DMacks (talk) 16:14, 21 May 2008 (UTC)

See Pain.-Arch dude (talk) 16:48, 21 May 2008 (UTC)

If you want a less biologically oriented answer (as implied by your title), you might be interested in heat transfer. --Prestidigitator (talk) 21:00, 21 May 2008 (UTC)

Full wave rectifier

All the diagrams of a full wave rectifier I've seen incorporate a diode bridge with a capacitor to smooth the output. However, this appears to be basically equivalent to a single diode with a capacitor. Plugging it into this simulator doesn't yield any obvious reason that this isn't used – so, since I'm assuming there's a reason, I'm curious as to why this simpler rectifier isn't used? Angus Lepper^{(T, C, D)} 16:47, 21 May 2008 (UTC)

File:Simple Full Wave Rectifier.svg

What's rectifying the full wave? Only one diode means that half of your input waveform is entirely clipped out. See rectifier and you'll see your circuit, sans smoothing capacitor, as a half-wave rectifier. — Lomn 16:58, 21 May 2008 (UTC)

Ah, I was being daft! Of course half of the input waveform is clipped out; this was being hidden by the capacitor (so it sort of works, but requires a larger capacitance to make up for the 'lost' wave). I somehow managed to convince myself that the capacitor was doing some magic. Whoops! Cheers, Angus Lepper^{(T, C, D)} 17:13, 21 May 2008 (UTC)

The half wave rectifier was used when diodes cost a lot, back in the hollow state thermionic valve era. Nowdays the rectifier costs perhaps 1% what they used to but the transformer still costs plenty, so we get more diodes and transisters in the power supplies. Graeme Bartlett (talk) 21:43, 21 May 2008 (UTC)

One important reason that a full wave rectifier is used is that a half-wave rectifier imposes a DC load on the AC source. That is, with a half-wave rectifer, the load current, averaged over time, is flowing in just one direction. For many AC sources such as transformers, this is a very bad thing. In the case of a transformer, the DC current can saturate the transformer's iron core, leading to extreme amounts of current flowing in the primary of the transformer. By comparison, a full-wave rectifier draws AC current from the source (with essentially no DC component) so the transformer operates normally.

Atlant (talk) 13:23, 22 May 2008 (UTC)

Ah, I see. Thanks!! Angus Lepper^{(T, C, D)} 14:08, 22 May 2008 (UTC)

The diagram label is incorrect. It is a half wave rectifier. -- Alan Liefting (talk) - 09:51, 27 May 2008 (UTC)

Aspartame

He discovered its sweet taste serendipitously when he licked his finger, which had accidentally become contaminated with aspartame.

— http://en.wikipedia.org/wiki/Aspartame#Discovery_and_approval

There are electronic tongues which taste, for example, toxicity in different substances, since this would be unethically dangerous for humans to do. Do they use any sort of this technology to test new and existing chemicals for taste, such as sweetness?

Same question, but bitterness?

Same question, but sourness?

Etc.

Etc.

Thank you.68.148.164.166 (talk) 17:24, 21 May 2008 (UTC)

I'm sure the more careful scientist would test it on animals first. Of course some molecules are more dangerous than others and you may know it from its chemical make-up. Imagine Reason (talk) 00:41, 28 May 2008 (UTC)

Stink bomb cure

I noticed on the side of a box of stink bombs "if swallowed, take vinegar or juice of citrus fruits, followed by vegetable oil, and seek medical advice". What does the odd combination of acid and oil do to the stink bomb chemicals? Why do they recommend this strange combination of foods? Laïka 17:49, 21 May 2008 (UTC)

Does the box also have a list of ingredients? Judging by our article on them, I don't think stink bombs all contain the same chemicals. Drinking acid suggests the contents are a strong alkali that needs to be neutralised - I'm not sure about the oil... it might form some kind of barrier against the contents, perhaps? --Tango (talk) 18:38, 21 May 2008 (UTC)

The only ingredient it mentions is Ammonium hydrosulphide, which is indeed an alkali. I guess that explains the acid, although probably, having hydrogen sulphide and ammonium citrate in your body is at least as bad. Besides which, stomach acid is many times stronger than lemon juice, so the contribution made by the juice would be tiny. Laïka 19:08, 21 May 2008 (UTC)

Yeah, it doesn't quite make sense, does it? It's my only guess, though. --Tango (talk) 20:22, 21 May 2008 (UTC)

It isn't always necessarily about strength, but also about the amount of available reactants. Probably if there's a base that is of concern, ingesting plenty of acid that you know your body can handle without harm wouldn't be a bad plan, but that's just an idea to ponder regarding a possible explanation; definitely not advice. Don't know about the vegetable oil. Could it be acting as a more effective solvent or something? --Prestidigitator (talk) 21:11, 21 May 2008 (UTC)

I would have thought not dissolving would be better - more likely to pass straight through. A less effective solvent, maybe? --Tango (talk) 21:23, 21 May 2008 (UTC)

Prognostic marker - practical implications of (non-)independence

If I have a prognostic marker of cancer, and some multivariate analysis of some description shows that it's not an independent marker, am I right in thinking that it's still a useful marker if it's easier to measure than those other markers and that the only drawback is that it doesn't offer anything in addition to those aforementioned markers? ----Seans Potato Business 18:13, 21 May 2008 (UTC)

I suppose so if you are going to not be bothered checking out those other harder to measure markers. And you have to consider if you want a prognosis or not. For example I don't care if the cockroaches in my kitchen have cancer or not, even if there is an excellent marker for it. Graeme Bartlett (talk) 21:49, 21 May 2008 (UTC)

POLY MERASE CHAIN REACTION (Regarding Factors Limiting Speed)

Straightforward question!

Okay, for DNA replication an RNA primer is needed. Does this mean that for PCR the amount of DNA being replicated each cycle is LIMITED by the number of RNA primers included in the sample?

Thanks! —Preceding unsigned comment added by 83.141.77.130 (talk) 18:20, 21 May 2008 (UTC)

In standard PCR, almost universally one uses DNA primers in their reaction. Secondly, yes absolutely, the primers become incorporated into the amplicon as its number grows exponentially. Therefore, the primers become depleted as the reaction proceeds with time. This can definitely affect the outcome. Wisdom89 _{(T / ^C)} 19:49, 21 May 2008 (UTC)

---True, although in practice it is rarely an issue since usually the primers are in excess. If one uses a pair of primers at one micromolar, which is common, that would be enough for up to one micromolar product, which is an extremely large amount (100 microliter reaction, 250 bp product: 16.7 micrograms]. The nucleotides probably run out first. Woodlore (talk) 01:36, 22 May 2008 (UTC)

I have always been curious about how external samples are removed before you start replicating. For example how do you remove bacteria and stuff from a rape victim sample? Bastard Soap (talk) 19:04, 26 May 2008 (UTC)

I don't know, but I'm not sure it would be necessary to remove "externals" since the primers should be specific enough to ignore non-human templates. Woodlore (talk) 00:47, 27 May 2008 (UTC)

is this true????

chicory

“ use of chicory as a coffee substitute (Root chicory - Cichorium intybus var. sativum) has been shown to damage human retinal tissue, with dimming of vision over time and other long term effects. Although small amounts of root chicory consumed medicinally or as a seasoning can be healthy and/or harmless, root chicory contains volatile oils that can be metabolized in the liver and digestive tract into toxic by-products that damage retinal nerve cells and cause dimming of vision if regularly consumed in large quantities as a coffee substitute ”

How can that possibly be true?? Why is it legal then, or -- I just looked carefully at the box -- there is no warning at all whatsoever :(

I would guess for the same reasons that coffee, alcohol and cigarettes are all legal, even though consuming large quantities of any of them can be detrimental to your health. The quote does say "Although small amounts ... can be healthy and/or harmless," so the question is just "what constitutes a safe, daily amount?" — Sam 18:54, 21 May 2008 (UTC) —Preceding unsigned comment added by 63.138.152.238 (talk)

I've noticed this as a common fallacy, that "x can't be harmful otherwise it would be illegal / have a warning", I guess it could be an appeal to authority. Some common examples of x are burned meat, aspartame and sodium benzoate. (also alcohol as previously mentioned). --Mark PEA (talk) 22:08, 21 May 2008 (UTC)

chicory is Generally Recognized as Safe. This abstract in PubMed supports that. I actually skimmed through the whole paper; it finds chicory to be safe in rats at the doses tested, and seems to imply in the introduction that it is not reported as toxic for humans. So there is probably no reason to worry. Still, if you find a refereed journal citation that reports chicory toxicity in humans, please post it. Kindest wishes, --Dr Dima (talk) 04:31, 22 May 2008 (UTC)

Indeed, if there is no citation in the article, and you have a counter-citation, the offending paragraph should be removed... — Sam 14:44, 23 May 2008 (UTC) —Preceding unsigned comment added by 63.138.152.238 (talk)

Connecting together small generators

Hi all,

If I had a bunch of small generators (wind turbines, or even solar panels), each of which might be producing 0-5 volts, what is the best way to collect all that power together? Can I just wire them up in series? Will the fact that they're not all producing the same amount of power be a problem?

Thanks! — Sam 18:52, 21 May 2008 (UTC)

When you say "power" are you talking in the technical meaning of that word or are you concerned about different voltages from different sources? The (DC) voltages don't matter. You can (for example) connect up a 1.2 volt battery to a 9 volt battery and make a 10.2 volt source and it'll work just fine. HOWEVER: Imagine you took a 2 Gigawatt nuclear power station and had it produce a 9 volt DC output - and connected THAT up in series with your 1.2 volt AAA battery and tried to use the resulting 10.2 volt source to power a small city. The battery would (of course) vaporize in the first gajillionth of a second! So you have to bear in mind that each device is conducting the current for the entire series of devices. Also, if your devices are (for example) small AC generators being turned by hamster wheels - then their voltage oscillations would not be in sync because some hamsters run faster than others. The result would be that your final output voltage would be bouncing around all over the place. Not good! 71.155.164.147 (talk) 21:22, 21 May 2008 (UTC)

You are only going to be able to produce the current that the lowest current device produces. So if your solar panel can make on amp, but your wind generator 2 amps, you will only get half its power. It will be better to get a special circuit in a box to combine the power by doing DC-DC conversion, and that can draw appropriate current from each source. If you have AC generators, you will have even more troubles, as you have to match the phase produced, else one will cancel out the other! Graeme Bartlett (talk) 21:56, 21 May 2008 (UTC)

I'll also point out that at least some generators are also perfectly happy acting as motors, so you could find yourself in the situation where the turbines located in the (momentarily) windier places are acting as generators and the turbines located in the {momentarily) calmer places are acting as fans. Designing a practical power-combiner is actually a pretty-tricky exercise. One practical alternative is to provide each wind turbine with its own inverter so that they all produce mains voltage. An inverter that is specifically designed to feed power back to the mains will also be willing to follow the frequency and phase of the external power line and will simply contribute as much power to the power line as its turbine is currently generating. At a much simpler level, diodes can steer the power from a group of DC generators connected in parallel, but at any given moment, one turbine (the one tending towards the highest developed voltage) will probably be supplying the bulk of the electrical power.

Atlant (talk) 12:17, 22 May 2008 (UTC)

Hmmm... I don't quite understand the use of the inverter, but what happens if each turbine (which produces AC) had its own smoothed rectifier? Then they'd all be producing (roughly) DC current. Couldn't I just put all those together in series? — Sam 13:33, 22 May 2008 (UTC) —Preceding unsigned comment added by 63.138.152.238 (talk)

I'm having trouble imagining a "smoothed rectifier" arrangement that won't cause one stopped turbine from impeding current flow in the whole series string. Perhaps you have a different idea in mind than I have?

Atlant (talk) 13:54, 22 May 2008 (UTC)

As an example, someone might be able to obtain several old bicycle lighting alternators fors little or no money, since many now use LED lights which are battery powered. These could conceivably all be connected to wheel of a stationary bike to act as a human powered generator. The alternators will naturally be out of phase with one another. A solution would be to attach each to a rectifier bridge to produce DC. Then each could be connected in series or in parallel to produce greater voltage or greater current output, respectively. Capacitive filtering could smooth out the ripple. Edison (talk) 19:31, 22 May 2008 (UTC)

You can't connect rectifier bridges in series unless they're all powered; an un-powered rectifier bridge blocks the flow of current through the rest. Parallel connections work, but then you have the "load sharing" problem that I mentioned.

Atlant (talk) 14:02, 23 May 2008 (UTC)

Really? I made a set of rectifying circuits in the circuit simulator, put them in series, and then swapped one of the AC generators for a resistor (a load). Barely any current seemed to go through the load at all (it preferred going through the low-resistance diodes). Did I wire them up wrong? (See circuit at image shack) — Sam 14:40, 23 May 2008 (UTC)

No, my mistake. I should have drawn more of the schematic for myself. ;-).

Atlant (talk) 16:50, 23 May 2008 (UTC)

Power factor correction may help. You want to ensure all the generators are in phase, or else they will deconstructively interfere with each other, reducing power. Nimur (talk) 16:09, 23 May 2008 (UTC)

I can't ensure that the generators will be in phase, but if they are all rectified before being connected in series, wouldn't all their interference be additive? — Sam 16:21, 23 May 2008 (UTC) —Preceding unsigned comment added by 63.138.152.238 (talk)

Snails and their homing instinct

Snails visit my garden and eat my plants. Sometimes I remove them and take them to a field 50 metres from my home. I'm sure they come back! Has there been any work done on the homing instinct of snails and the distances involved? Any links or information would be appreciated. Thanks S Snailsaregreat (talk) 19:46, 21 May 2008 (UTC)

How do you know these are the same snails? Start marking their shells with an X using a permenant marker - then you'll know if they are "homing" or whether there are just a lot of snails around. 71.155.164.147 (talk) 19:49, 21 May 2008 (UTC)

Thanks - will try this but still curious to know if there is any research on snails homing instinct? Snailsaregreat (talk) 19:52, 21 May 2008 (UTC)

A quick scan of a Google Scholar search looks like there is some research suggesting a degree of 'homing'. Angus Lepper^{(T, C, D)} 20:15, 21 May 2008 (UTC)

To get a better idea of how home-loving your snails are, add a splash of coloured nail varnish to their backs before releasing them. Xn4 02:32, 23 May 2008 (UTC)

Scientific control

A positive control is a procedure that is very similar to the actual experimental test, but which is known from previous experience to give a positive result. A negative control is known to give a negative result. The positive control confirms that the basic conditions of the experiment were able to produce a positive result, even if none of the actual experimental samples produce a positive result. The negative control demonstrates the base-line result obtained when a test does not produce a measurable positive result; often the value of the negative control is treated as a "background" value to be subtracted from the test sample results.

What does "positive", "negative", "positive result", and "negative result" mean?68.148.164.166 (talk) 21:34, 21 May 2008 (UTC)

See truth value and null result. A positive control should be a sample which is known to produce a true result for a given proposition. A negative control should be one that is known to produce a false result. These can be used to test the validity of the procedure and/or possibly for calibration of results. For example, if you are testing whether samples are over 5 grams of mass (a ridiculous though concrete example), a positive control might be a bowling ball and a negative control might be a penny (~1g). I guess that is just a restatement, but I'm not sure what else you are looking for. --Prestidigitator (talk) 22:18, 21 May 2008 (UTC)

A "positive result" is "whatever data would indicate something is present or worked" and a "negative result" is "whatever data would indicate that thing is absent or didn't work"; it's all in relation to a specific experimental subject or design or detector. Think of an experiment to examine radioactivity in various samples using a Geiger counter. A "positive control" might be something that is known to be radioactive and a "negative control" would be something that is known to be non-radioactive. You test each one and make sure you get the expected readings: a "positive" detection of radioactivity for something that is and a "negative" (non-) detection of radioactivity for something that is not. You can thus calibrate instruments and data analysis for when you test things whose radioactivity is not known. DMacks (talk) 22:20, 21 May 2008 (UTC)

Can BritaTM Containters Leach BPA?

Can BritaTM Containters Leach Bisphenol A? Are they made of polycarbonate? Are they made of a plastic that can leach BPA?68.148.164.166 (talk) 21:40, 21 May 2008 (UTC)

Can you find the plastic number marking on the container? 2 and 3 are safe, I think. Imagine Reason (talk) 00:43, 28 May 2008 (UTC)

Ask the manufacturer. Coincidentally, my brother forwarded this email from P&G: "PUR dispenser bodies are manufactured from an acrylic-based polymer classified as recycling code #7. PUR dispenser lids are manufactured from polystyrene, code #6. PUR dispenser filters are made from polypropylene, code #5, and also contain no BPA." I still won't use it, however, because it no longer filters flouride or even chlorine. Imagine Reason (talk) 02:38, 28 May 2008 (UTC)

Egg drop

I am doing the Egg Drop in science class, but I can't figure anything out! I only have straws, paperclips, and masking tape. What am I supposed to do???!!! —Preceding unsigned comment added by 66.92.160.148 (talk) 21:43, 21 May 2008 (UTC)

Do you have an egg? Is this a competition to see how far you can drop an egg without breaking? Graeme Bartlett (talk) 21:59, 21 May 2008 (UTC)

I assume (because it's a science class) it's this type of Egg drop competition as opposed to some sort of cooking class contest:) The whole point is that you figure out what to do. Be creative! What are some ways you can keep the egg from falling so fast? What are some ways you can keep the egg from into the ground so hard? What are some ways you can keep the egg from breaking when it hits the ground. You actually won't learn a thing if you just do what someone else tells you. DMacks (talk) 22:13, 21 May 2008 (UTC)

Well, we aren't allowed to do your homework for you. But here are some things that might help: You're trying to stop the egg from breaking when it's dropped - right? Think about the kinds of things that we do to stop people from breaking when they are hurled violently at a block of concrete. What happens in a car crash for example? The person is held inside the car by straps - and the car is designed to crumple to absorb the energy of the crash. Could you build something around the egg that would crumple on impact? Could you restrain the egg inside that thing to stop it flying forwards into the ground? How can you join straws together to make a structure? Suppose you wanted some straws to be strong and others to be more easily crumpled? How might you do that with the things you have? 71.155.164.147 (talk) 22:11, 21 May 2008 (UTC)

Algebraist suggest that our article, egg drop competition might be useful. 71.155.164.147 (talk) 22:14, 21 May 2008 (UTC)

It's also worth noting that some shapes (such as spheres and eggs) are enormously strong when a uniform load is applied to them but break rather easily if the load is concentrated at one point or in a smallish area of the sphere/egg. A common demonstration of this principle in science classes is to ask someone to close their hand around and egg and crush it. If they don't cheat and put all the force into one of their fingers, it's remarkably hard to do (and, of course, very entertaining if they finally succeed). NB: A smock is usually worn by the person attempting the crushing.

Atlant (talk) 12:06, 22 May 2008 (UTC)

Having them squeeze it end-to-end is less susceptible to having one overly-strong finger crack through the side in a full grasp. DMacks (talk) 16:15, 22 May 2008 (UTC)

In high-school, my friend and I did a rather complex way of catching the egg. We had our plastic box and lined the bottom with a stiff spongy foam. On top of that was a softer foam layer. On top of that, a layer of gelatin. On top of that was a layer of packing peanuts. The idea being that each successive layer would absorb some of the momentum and slow the egg down enough that, when it got to the bottom layer, it would safely rest on the harder foam. Our first drop worked perfectly! Our second drop... went right through the hole in the gelatin the first egg made. D'oh. — The Hand That Feeds You:^Bite 23:05, 22 May 2008 (UTC)

BPA In Camelbaks?

Clicking on SPORTS-RECREATION then HYDRATION PACKS then VIEW ALL then 100 OZ (2.95 L) then STOAWAY 100 OZ on [5] will get you the StoAway 100 oz. Does the StoAway 100 oz use BPA or polycarbonate or any other plastic that leaches Bisphenol A? Thanks!68.148.164.166 (talk) 21:54, 21 May 2008 (UTC)

Our article, egg drop competition, might be useful. Algebraist 21:57, 21 May 2008 (UTC)

Probably not. Which is a shame because the previous questioner might have found it interesting. 71.155.164.147 (talk) 22:13, 21 May 2008 (UTC)

FWIW, I think someone deleting an earlier section while I was typing was responsible. Algebraist 22:18, 21 May 2008 (UTC)

Why don't you try contacting the manufacturer? --Shaggorama (talk) 04:38, 22 May 2008 (UTC)

Alternatively, you can just read the website. Franamax (talk) 08:45, 22 May 2008 (UTC)

What is completely collapsible personal transportation (fits flat in briefcase)

I'd like to shorten my very early morning walk to the commuter rail, which I currently walk to. Is there anything that is, like, I don't know, a rollerskate that goes over your shoes, for example, or a collapsible skateboard or longboard, or unicycle or bicycle (the very small, trick kind). I just want to shorten my walk by a little bit, the main requirement is I be able to collapse it, preferably flat, and put it away/out of sight. No one should know I have it. I'm inspired by these kids I see who have trick sneakers that magically contain a wheel too, you know what I mean? —Preceding unsigned comment added by 87.88.122.226 (talk) 22:34, 21 May 2008 (UTC)

p.s. if I can avoid it I wouldn't like to use a full-sized bike, or even a bulky half-way collapsible one I've seen some people take on the commuter rail.

p.p.s is this any faster than walking?? It seems because you can't "coast" at all, and there are no gears, there should be no leverage, ie no different from applying feet directly to the ground by walking. But maybe I'm missing something. So would someone on an "ultimate wheel" be able to ride it faster than the same person walking with the same 'intensity'? Thank you. —Preceding unsigned comment added by 87.88.122.226 (talk) 22:53, 21 May 2008 (UTC)

Magic carpet? --hydnjo talk 22:55, 21 May 2008 (UTC)

We have an article on the subject of that link: Ultimate wheel. I don't believe such a device would shorten your walk. You can view videos of it on YouTube; it's difficult to ride and not fast. You want something that can coast, but not the even more difficult impossible wheel! A skateboard with fold-out wheels is an interesting idea although I am not aware of one that's being marketed. The closest thing I can think of is the Wave skateboard, which is a two-wheel tandem design that might fold compact, although I'm not sure.

A Razor (scooter) or a foldable scooter of similar design might suit your needs. --Shaggorama (talk) 04:37, 22 May 2008 (UTC)

Heelys are the shoes with the wheels in the heels. My kid used them when the craze was at it's peak - they are very effective but (obviously) take practice. Some styles of Heely's come in adult sizes and they look just like regular sneakers at a glance. They also make 'work boots' in adult sizes with the same trick heels. You can snap out the wheels and stick them into your pocket for a completely "stealth" look and more 'normal' walking - you can just about do it with your fingers - but they have a tool that makes that easy to do. 71.155.164.147 (talk) 14:05, 22 May 2008 (UTC)

How about a fully collapsible folding bicycle? These things aren't much bigger than a briefcase when fully folded. Heelys, rollerskates and skateboards aren't very useful, because you'll still have to walk uphill; they'll only save you time & energy on flat surfaces or downhill. — The Hand That Feeds You:^Bite 23:36, 22 May 2008 (UTC)

I met a guy on a train in Europe who had a custom made bike that fit in a roll-aboard carry on (the old style that had room for more that a magazine and a toothbrush). I quit asking when he told me of the Euro 7,000.00 price tag. Commercially available and affordable is this one [6] claiming to be the world's smallest. Other options might be: comes in a carry bag [7] or in a suitcase [8] and on the expensive side [9]. All still bigger than a briefcase, though. --71.236.23.111 (talk) 06:17, 23 May 2008 (UTC)

diketopiperazines

How do diketopiperazines occur in humans?68.148.164.166 (talk) 17:48, 21 May 2008 (UTC)

The article explicitly tells you how they are made and gives examples/links for some specific examples. DMacks (talk) 18:40, 21 May 2008 (UTC)

Um, no actually it does not explain how diketopiperazines occur in humans.68.148.164.166 (talk) 21:56, 21 May 2008 (UTC)

Perhaps you'd better explain further what you mean by "how". Do you mean what creates them ("how"=="method") or what compounds have them ("how"=="form") or why they are there and what they do ("how"=="how come")? DMacks (talk) 22:07, 21 May 2008 (UTC)

My original question was if diketopiperazines even occurred in humans at all. What creates diketopiperazines, what compounds have diketopiperazines, why are diketopiperazines in humans, and what do diketopiperazines do?68.148.164.166 (talk) 23:06, 21 May 2008 (UTC)

They are found in mammals per the article; chasing the cited refs says specifically rat/mouse. They are present in biological systems as the result of a certain type of enzyme per the article; googling for the enzyme and "human" gives many hits that say that enzyme is present in humans. I don't know if anyone has specifically looked for this particular structure itself in humans. So they almost certainly are present in humans (both the enzyme and substrate are present). DMacks (talk) 23:37, 21 May 2008 (UTC)

Aspartate

Where and how does aspartate occur in humans, if even?68.148.164.166 (talk) 23:24, 21 May 2008 (UTC)

"Aspartic acid is also a metabolite in the urea cycle and participates in gluconeogenesis." DMacks (talk) 23:43, 21 May 2008 (UTC)

Aspartate would be everywhere, as it is a common amino acid present in virtually all proteins. Woodlore (talk) 01:25, 22 May 2008 (UTC)

Computer Mice

Do computer mice use plastics that might contain Bisphenol A? Are plastics that are made from Bisphenol A used in Bisphenol A? Do speakers use plastics that are made from Bisphenol A used in Bisphenol A? Do wires use plastics that are made from Bisphenol A used in Bisphenol A?68.148.164.166 (talk) 23:31, 21 May 2008 (UTC)

The Bisphenol A article may be of some use. It exists in most plastics as an antioxidant and/or polymerization inhibidor, and is necessary for polycarbonates and epoxies. I believe computer mice and speakers use polycarbonate, although wire cladding is likely PVC (which also contains bisphenol A). The chemical is an aid in plastic manufacture, it's there for the convenience of manufacturers. ~Amatulić (talk) 23:42, 21 May 2008 (UTC)

Why? Are you licking any/all of these things? —Preceding unsigned comment added by 96.242.34.177 (talk) 00:33, 22 May 2008 (UTC)

Some (sick) peolpe like to lick PVC! —Preceding unsigned comment added by 79.76.238.57 (talk) 03:23, 24 May 2008 (UTC)

Licking PVC will most likely make you sick Think outside the box 19:24, 25 May 2008 (UTC)

I guess some people like to like mouse balls. --jpgordon^∇∆∇∆ 14:26, 24 May 2008 (UTC)

It could reach your food via your hands, I guess. Imagine Reason (talk) 00:45, 28 May 2008 (UTC)