Wikipedia:Reference desk/Archives/Science/2009 October 24

Welcome to the Wikipedia Science Reference Desk Archives
The page you are currently viewing is an archive page. While you can leave answers for any questions shown below, please ask new questions on one of the current reference desk pages.

October 24

Why don't cells make carbon fibers?

With all that carbon and all that elaborate machinery in the cell, why don't living things produce graphitic fibers or carbon nanotubes, which seem to have such nice properties? (and by the way should graphitic fibers redirect somewhere?) Thanks — Walrus heart (talk) 00:32, 24 October 2009 (UTC)

Well, for one thing, evidence is accumulating that suggests that carbon nanotubes are a pretty nasty health hazard. But evolution can't just magically "do the right thing". That's why animals don't roll around on wheels or communicate via radio waves or have digital circuitry in their brains. At each point, only teeny-tiny steps can happen and this very often results in amazingly evolved but hideously illogical solutions to life's little problems. For example, non-human mammalian babies reach an age where if they continued to consume their mother's milk, it would deprive their younger newborn siblings. Yet they have not evolved an efficient brain mechanism that makes them think "Hmm - I really don't want mommy's milk anymore because it would deprive my younger brother" - but instead we wound up with a mechanism that gives them indigestion and worse every time they try to drink milk of any kind! (lactose-intolerance) When you think about it - that's an incredibly dumb way to achieve the intended goal! Who wants baby animals getting sick for no particularly good reason? But that's all that the random shuffling of genes could do when the need arose. SteveBaker (talk) 01:14, 24 October 2009 (UTC)

The article Weaning implies that the initiative to end breastfeeding is taken by the mother. There are many cases where children continue happily to breastfeed up to age 8 and more. Lactose intolerance arises after weaning; about 25% of adults develop none and there is some evidence that there has been evolution in humans in the direction of lactose tolerance that allows the high modern consumption of milk and dairy products. Apropos sibling competion, mamma's standard-issue maternal endowments provide dual nourishment outlets for tandem feeding (triplets however present her a scheduling challenge). Cuddlyable3 (talk) 14:10, 24 October 2009 (UTC)

I did very carefully state "non-human mammals". Humans are very much the exception to the rule in this regard. SteveBaker (talk) 14:17, 24 October 2009 (UTC)

I apologise, I thought you were just being rude about other people's babies. Cuddlyable3 (talk) 14:41, 24 October 2009 (UTC)

Weaning is a little bit more complicated than that, Steve. There is compelling evidence that there is extensive rewiring on the locus coeruleus which facilitates the weaning process. The LC is thought to mediate the attachment of newborns to their mother's milk. This early sensory "imprinting" appears to be the mechanism through which young suckle. Around weaning, the LC undergoes neural changes which results in that attachment being weakened. Thus the young are less compelled to suckle and start to investigate other forms of sustenance.

Whats cool about this is the mechanism through which the LC works: it is linked to the young's ability to undergo positive and negative conditioning. During the suckling phase even negative stimuli (such as being bitten or trampled on by the mother) will result in positive reinforcement for suckling. This is important, because most mothers are not particularly good. But a bad mother is better than no mother, and it would be catastrophic if one developed an aversion to one's only form of sustenance. But after LC rewiring, the young will now respond to negative stimuli with aversion. So now when the mother bites or kicks the older young to chase them away while she suckles her new litter, the older young will now "get the message" and avoid her. We do not know exactly what precipitates this LC rewiring, but it appears to be developmentally regulated. Thus evolution appears to favored quite an elegant solution to the suckling problem. Even better, its all tied in with the ability for young to learn crucial avoidance behaviors (which are necessary for post-weaning survival, but catastrophic during suckling) at exactly the right time. Lactose intolerance has a slightly later onset in most mammals, and while it clearly functions to stop further suckling, it is unlikely to be the mechanism through which weaning occurs. Rockpocket 17:02, 24 October 2009 (UTC)

Second what SteveBaker stated. It's why living things can be amazingly complex; evolution often results in highly inelegent solutions to simple problems since not only must the current function be useful (or at worst, not harmful), every function along the way at each stage has to be useful (or at worst, not harmful). Furthermore, living things DO make some fantastically amazing polymers, consider silk. Spider silk, for example, has a tensile strength that, gram for gram, excedes that of steel, and is as strong as just about any synthetic fiber out there.--Jayron32 01:57, 24 October 2009 (UTC)

Mercury "burn"

In an episode of Diagnosis Murder, someone splashed a spot of mercury (liquid elemental mercury, that is) on their hand. They immediately wiped it away, but later a red "burn" mark appeared where the mercury had been in contact with the skin. I think it was termed a "mercury burn" in the script. I seem to remember coming into direct contact with mercury in the past and not suffering from any such problems. Is there any basis to this idea that mercury "burns" the skin, or is it just a fiction? —Preceding unsigned comment added by 86.138.41.250 (talk) 01:02, 24 October 2009 (UTC)

Sounds like bullshit to me. Mercury isn't an acute poison in this manner, it does not cause direct instant damage like that. Mercury poisoning is a chronic problem that comes from long-term exposure to mercury vapors or consumption of some mercury compounds. I don't see where elemental mercury would ever do something like that. Neither our article on Mercury (element) or on Mercury poisoning makes any mention of contact burns from handling mercury. The latter article notes that "Quicksilver (liquid metallic mercury) is poorly absorbed by ingestion and skin contact. It is hazardous due to its potential to release mercury vapour." --Jayron32 01:51, 24 October 2009 (UTC)

Agreed. Long-term exposure might but a quick splash wouldn't do more than a little rash at the worst. - Draeco (talk) 04:26, 24 October 2009 (UTC)

Unless it was hot, of course. Looie496 (talk) 05:49, 24 October 2009 (UTC)

Unless there was some allergy. We used to roll the stuff around on our school desks in the old days --BozMo talk 07:07, 24 October 2009 (UTC)

... agreed - I spent many happy hours playing with mercury in my younger days (and before I heard of mercury poisoning), having regular skin contact without any marks appearing. The health risk was probably about the same as that from my many amalgam fillings (i.e. minimal). Hot of very cold mercury might produce a "burn" mark. Contact allergy to mercury does exist, but is very rare. Dentists in the UK didn't test for it before using amalgam fillings. Dbfirs 09:39, 24 October 2009 (UTC)

IIRC (which I probably don't), something very similar was used in an episode of Jonathan Creek...A skull made of frozen mercury that would burn to the touch? Although, I guess that was probably because it was so cold. Vimescarrot (talk) 12:14, 24 October 2009 (UTC)

Sitting on Mercury. National Geographic published this picture of a Mercury miner. Cuddlyable3 (talk) 13:24, 24 October 2009 (UTC)

For the record, the mercury in said episode of Diagnosis Murder was at room temperature. If this "burn" thing is "bullshit" then it makes me wonder about the authenticity of the rest of the medical "facts" in that show. Perhaps it's all mumbo-jumbo...! 86.133.243.75 (talk) 13:27, 24 October 2009 (UTC).

What?! A work of television fiction with a lack of authenticity? NO! Nooooooo!!!! I won't believe it! SteveBaker (talk) 22:20, 24 October 2009 (UTC)

Fixing of insect fat body tissues

What is the procedure for fixing insect fat body tissue —Preceding unsigned comment added by 59.93.255.135 (talk) 05:25, 24 October 2009 (UTC)

Use Hoyer's mounting medium, maybe? I'm not sure. --Dr Dima (talk) 05:52, 24 October 2009 (UTC)

Cows

Why does a cow have four legs? --41.185.88.129 (talk) 07:01, 24 October 2009 (UTC)

Becuase it's a vertebrate, and most vertebrates are quadrupedal. Apparently they just evolved that way to get around on land. Mitch Ames (talk) 07:52, 24 October 2009 (UTC)

So, vertebrates were not predisposed to be four-legged before they tried to crawl onto the land? —Preceding unsigned comment added by 86.133.243.75 (talk) 11:21, 24 October 2009 (UTC)

It is connected with the layout of fins on the lobe-finned fish from which all tetrapodomorphs (and hence land vertebrates) have evolved. Creatures that did not evolve from this line have all sorts of other numbers of legs, there is nothing special about four, it is just an accident. SpinningSpark 12:29, 24 October 2009 (UTC)

Four makes sense, though - it's the minimum number of legs that allows you to walk without worrying about balance. Stability without any kind of active balancing requires at least 3 legs (that's simple mathematical fact) and in order to walk you need to lift at least one leg and want at least 3 still on the ground, that means you need at least 4 legs. Any superfluous legs are a potential cause of problems, so an animal with precisely 4 legs is a good choice. There are exceptions (in both directions), but that isn't surprising - there are usually many ways of solving a particular problem, so some animals have ended using other ways. Birds (often even flightless ones) will occasionally flap their wings to stop themselves falling over, humans will hold out their arms to balance themselves, etc. (interesting, both those other solutions still require 2 legs and 2 other limbs for a total of 4 limbs). There are creatures with many legs (eg. centipedes and millipedes), which I guess is the best way to carry a long thin body only a small distance above the ground. There are insects with 6 limbs, but only 4 legs, so that fits with my explanation. The only really weird land animals I can think of are spiders - I don't know why they have eight legs... --Tango (talk) 12:52, 24 October 2009 (UTC)

Tango, I think you are suffering from a bad case of OR there, with an added dose of anthropomorphism. I don't know of any evidence that evolution favours four legs for stability or has difficulty producing active controls for two legs - do you?. Many major branches of the tetrapod line have gone to locomotion on two legs: dinosaurs, birds, kangaroos and other marsupials. Nor does there seem to be any particularly heavy demand for processing power with many legs, millipedes do not have much of a brain. Early robotic research also came to the conclusion that one leg was the very easiest system to program, see Robotics#Walking robots. SpinningSpark 13:16, 24 October 2009 (UTC)

It's pure OR but I don't see how I can be accused of anthropomorphism when I gave humans as an example of an exception to the rule... While some aspects of life are "accidental" (eg. we use D-Glucose not L-Glucose for respiration - at least, I've never seen an explanation for that choice), most have evolved for a reason. I may be completely wrong about why so many animals have evolved to have 4 legs, but I doubt it is entirely random. --Tango (talk) 15:01, 24 October 2009 (UTC)

Yes, I should have said tetrapodomorphism, sorry. It is incorrect to say "so many animals have evolved to have 4 legs", they all belong to the same clade, that is, they have a common ancestor and have inherited a common characteristic. I cannot think off-hand of any species that has separately evolved into a 4-pod. Other groups are quite happy with other numbers, notably the wildly succesful Insecta are 6-pods. To address your question on spiders, the reason you cannot think of a good reason why spiders need to be 8-pods for their lifestyle is that they probably don't and it will never make any sense trying to analyse it that way. However, it immediately makes sense when you consider that spiders are related to crustaceans, they have merely inherited a common characteristic. SpinningSpark 17:32, 24 October 2009 (UTC)

As a minor counterexample, there is the Nymphalidae family of butterflies, sometimes called the "four-footed butterflies" because many species of the family have reduced front legs and use only the other four to stand and walk. These have, therefore, evolved from six-leggedness to functional four-leggedness. 87.81.230.195 (talk) 19:36, 27 October 2009 (UTC)

If you examine it in terms of clades you are obviously going to conclude that it is caused by the common ancestor having that characteristic because that is a feature of your examination. It makes more sense to examine it in terms of how widespread that characteristic is at the current time. If we restrict ourselves to land animals then we have small 6-legged, 8-legged and many-legged animals and large 4-legged (and a few tetrapods that walk on 2 legs). Why is it that only tetrapods have evolved to be larger than a few centimetres (at least, I can't think of any counter-examples)? It has been the case for an extremely long time so it isn't just that evolution hasn't had enough time to change it. Evolution is definitely capable of removing limbs (cf. aquatic mammals and snakes [which are a counter-example! I think this is an example of the exception proving the rule - snakes are unusual in their lack of legs]) and we see Polymelia quite often (although I don't know how often it has an inheritable cause) so it doesn't seem likely that there is some biological reason that prevents tetrapods from evolving more or fewer legs, that means that either by random chance it has never happened (which is unlikely, IMO) or there is some benefit to 4 legs over other numbers. --Tango (talk) 18:31, 24 October 2009 (UTC)

The point of looking at the clade is that inherited features within the clade are just neutral. If the same feature evolves in other clades then there is an indication that it is particularly advantageuous. I am fairly confident that 4-podism was not the driver to large size or that lack of it prevents groups evolving large size. Limitations in skeletal structure and vascular system are far more significant. Besides which, large examples of other clades can be found, eg coconut crab (10-pod). SpinningSpark 10:34, 25 October 2009 (UTC)

If a clade very rarely changes a particular feature (in this case only snakes, legless lizards and aquatic mammals have fewer legs and none have more legs, unless I've missed a species) that is also suggestive of an advantage. For example, tetrapods have a wide range of numbers of digits on their four legs, which suggests there is no great advantage to, say, 5 digits compared to 6 (some species have a number of digits that is advantageous to them, of course, but wouldn't be advantageous to tetrapods in general). The coconut crab is exceptional - in this kind of subject a single exception doesn't invalidate a rule, it just gives you a way to work out the reason for the rule by seeing what is different about the exception. --Tango (talk) 13:11, 25 October 2009 (UTC)

I'd have thought the more legs the merrier, since the more you have, the less your mobility is affected by damage to or loss of one or more of them. 86.133.243.75 (talk) 13:30, 24 October 2009 (UTC)

Yet very few animals have more than 4 legs. If more was better, one would expect most animals to have more legs. --Tango (talk) 15:01, 24 October 2009 (UTC)

Eight million species of insects (all of whom are 6 legged) would like to differ with your viewpoint! From a stability point of view, 2 and 3 legs clearly suck - 2 legged animals have to actively balance, which costs them energy on a continual basis. 4 legs solve the stability problem - but only at the cost of relatively slow locomotion. If (as you said) they gain this stability by moving one leg forward at at time (which they rarely do), then their top speed would be awful! Most quadrupeds don't actually take advantage of the stability you're talking about - except when standing still. 3 legs is plenty to do what pretty much all quadrupeds need to do. Insects (6 legs) DO get the benefit you claim - they stand stably on 3 legs while they move the other 3 forwards to get a new stable position. This certainly simplifies the control issues - exerting sinusoidal patterns onto the lift and move-forwards muscles with the right phase is really all you need to make a 6 legged animal run pretty fast. SteveBaker (talk) 22:15, 24 October 2009 (UTC)

Sorry, I should have said very few large animals. I realised I had forgotten the small animals and clarified that above. You are wrong that tetrapods rarely move one leg at a time - see Horse gait#Walk. When they have no need to go fast, they use a nice stable gait and only use less stable ones when there is a need for speed. I think that same is true for other animals. Now, I may be completely wrong about stability being the main benefit for 4 legs - the point I am trying to make is that there almost certainly is a benefit. While evolution does result in random choices that just get stuck, it doesn't happen very often with features as widespread as tetrapodism (for things like which isomer of glucose we use there is a benefit of using the same isomer as every other lifeform uses [ie. you can eat], even if there isn't a benefit to one isomer over another, that isn't the case for number of legs). --Tango (talk) 13:11, 25 October 2009 (UTC)

Start removing them and you'll see why. Vranak (talk) 13:19, 24 October 2009 (UTC)

I've seen a three-legged cow before. She seemed to be managing okay-ish. I'm guessing that she would probably struggle with only two. --Kurt Shaped Box (talk) 13:45, 24 October 2009 (UTC)

Okay, I'll bite. Why does a cow have 4 legs? Cuddlyable3 (talk) 13:26, 24 October 2009 (UTC)

to get to the other side. John Riemann Soong (talk) 13:35, 24 October 2009 (UTC)

except when said cow is already on the other side in which case the question is udderly moot. Cuddlyable3 (talk) 13:40, 24 October 2009 (UTC)

I want a reference that the even have four in the first place! SpinningSpark 13:42, 24 October 2009 (UTC)

I don't want to lower the tone of the desk, but I feel that you need to know that I just found this in an internet search for this question. SpinningSpark 13:54, 24 October 2009 (UTC)

I nominate that video for the Golden Globes Award. Cuddlyable3 (talk) 14:22, 24 October 2009 (UTC)

Ok, just one more, then I promise to leave this thread alone. What to you call a cow with no legs? Give up?

Ground beef! SpinningSpark 14:38, 24 October 2009 (UTC)

Because its Hox genes encode four legs. Rockpocket 15:36, 24 October 2009 (UTC)

Because we live in 3 dimensions. In two dimensions they have three legs and in four cows have five legs  ;-) Dmcq (talk) 17:32, 24 October 2009 (UTC)

Well, that would be consistent with my proposed explanation. --Tango (talk) 18:31, 24 October 2009 (UTC)

_{Well, we seemed to have milked that for all its worth! Richard Avery (talk) 22:28, 24 October 2009 (UTC)}

Four - four - any advance on four? yes I have a bid of five - no I don't need another bid of five - any advance on five ? yes I have a bid of six - going at six - going - GONE. Cuddlyable3 (talk) 22:50, 24 October 2009 (UTC)

I thought everyone knew that cows have 6 legs (the two in the rear plus the forelegs in front). StuRat (talk) 04:27, 25 October 2009 (UTC)

high dc voltage and current measurement

how are high dc voltage and current measured ??? —Preceding unsigned comment added by Waghmare (talk • contribs) 10:11, 24 October 2009 (UTC)

It depends what you mean by "high". A simple resistive divider probe (such as this) used with an ordinary Multimeter can measure voltages up to about 50kV DC - Hall effect ammeters (such as this) can measure up to about 2000A. Tevildo (talk) 13:27, 24 October 2009 (UTC)

Measurement of high voltage and high current. Cuddlyable3 (talk) 13:37, 24 October 2009 (UTC)

:You could use VTs (voltage transformers) and CTs current transformers. See also article on Transformers —Preceding unsigned comment added by 79.75.37.202 (talk) 22:47, 24 October 2009 (UTC)

phosphate (and diphosphates) as leaving groups

I see that a lot of Wikipedia likes to use the high school explanation of a "high energy bond" for why phosphorylation drives certain processes. Recently I learnt about role of ATP in fatty acid synthesis and now the use of ATP makes so much sense! But are there other uses of ATP besides using phosphates as leaving groups? Mechanistically, how does ATP drive a reaction forward? I get the idea that a triphosphate group is sort of like the phosphoric version of acid anhydride, and thus cleaving ATP into a phosphate ester and ADP is favourable (diphosphate is a good leaving group). And phosphorylating an already phosphorylated substrate is still thermodynamically favourable...? John Riemann Soong (talk) 13:46, 24 October 2009 (UTC)

Under physiological conditions, there is a large free energy difference between ATP plus water substrates and the ADP plus phosphate products. Enzymes like the sodium potassium ATPases, which "simply" hydrolyze the ATP without forming a covalent phosphoryl intermediate take advantage of this by mechanistic coupling of the desired reaction with the ATP hydrolysis reaction. (The enzyme sets things up such that you don't get hydrolysis without the reaction.) While the desired reaction is ΔG positive, the overall hydrolysis+desired reaction is ΔG negative, making it spontaneous. You can phosphoylate a phosphate (that's what nucleoside-diphosphate kinase does, but the ΔG for the reaction is near zero, so you have to drive the reaction forward with other effects (e.g. using Le Chatelier's principle). -- 128.104.112.179 (talk) 17:54, 26 October 2009 (UTC)

Swine flu vaccine

On average, about how long does it take after being vaccinated for an individual to be fully protected from swine flu? --75.28.168.53 (talk) 14:08, 24 October 2009 (UTC)

The U.S. CDC estimates that it takes about 2 weeks. Note that kids under 10 years need 2 doses (spaced about 4 weeks apart) and your use of the word "fully" might reflect a misconception about its efficacy (perhaps not your misconception, but other readers might be confused) - the flu vaccine reduces incidence, severity, and mortality, but does not completely prevent infection in all recipients. -- Scray (talk) 14:49, 24 October 2009 (UTC)

And remember, it protects you from swine flu but not other strains of flu (including other strains of H1N1). So, just because some people get the flu after having the jab doesn't necessarily mean the vaccination hasn't worked. --Tango (talk) 15:21, 24 October 2009 (UTC)

Why would a portable radio's signal go out for one station?

I had a portable radio/CD player a few years back, where the station I listened to all the time gradually faded to where it wouldn't play anymore. Not sure if it was analog or digital (I got it around 2000, if that helps), but I was just wondering what would cause it to do that? It seemed to play other stations well. And, it wasn't the radio station, becuase I tried on other portable radios that I have.

I suppose the receiver part for that one could wear down, but I'm really unsure. SO, I thought I'd ask if anyone had any ideas.209.244.187.155 (talk) 14:20, 24 October 2009 (UTC)

Reception of FM stations can be affected by standing waves due to interference from reflected signal paths; the signal wavelength is about 3 meters. Even though a receiver is not moved signal reflections can change over time e.g. when a nearby building is put up. Maybe that happened with the OP's favourite program. Cuddlyable3 (talk) 14:36, 24 October 2009 (UTC)

sky

If water is blue because of sky reflection then how come water is still bright blue when they show photos of earth from outer space? it's not reflecting space, that's for sure. —Preceding unsigned comment added by 82.44.55.2 (talk) 14:30, 24 October 2009 (UTC)

Where did you get the idea that it is caused by reflection? According to our article Ocean#color it is not. SpinningSpark 14:49, 24 October 2009 (UTC)

I imagine the OP got the idea because it is an extremely common misconception. Water is usually only noticeably blue in large quantities, which means a lot of people don't realise that it genuinely is blue. --Tango (talk) 15:03, 24 October 2009 (UTC)

See also: Color of water. --NorwegianBlue ^talk 15:40, 24 October 2009 (UTC)

As for the sky, check out Rayleigh scattering#Reason for the blue color of the sky. ~ Amory (u • t • c) 17:06, 24 October 2009 (UTC)

In addition, to the extent that sky reflection contributes to the water's color, this is just as true for water photographed from space as it is when photographed from the same angle from a point in the air. The fact that you're looking down from above sky doesn't change anything about the light that reaches and reflects off the water. --Anonymous, 18:44 UTC, October 24, 2009.

(OR) Many postcards show lakes having an intense emerald blue colour that one never actually sees. This looks good in tourist advertisements. An explanation I have received is that colour films are sensitive to invisible ultraviolet light reflected from the water, and render it as a saturated blue. Only a select few have actually seen earth from outer space. Cuddlyable3 (talk) 22:33, 24 October 2009 (UTC)

Photographs may have been taken using filters to enhance the colour. These days colour enhancements are likely to have been done digitally. Mitch Ames (talk) 00:15, 25 October 2009 (UTC)

"Momma, don't take my Kodachrome away". StuRat (talk) 04:16, 25 October 2009 (UTC)

Too late! --Anonymous, 21:10 UTC, October 25, 2009.

You can see the blue color of water in an indoor pool. StuRat (talk) 04:16, 25 October 2009 (UTC)

... but the faint turquoise-blue of the water is often swamped by blue paint on the sides, by reflection from the sky or ceiling, and by chemicals in the water. Interestingly, heavy water is not blue. Dbfirs 08:57, 25 October 2009 (UTC)

If by any chance it's green, then I have several tons of heavy water in my back-yard pool right now! Who should I call? SteveBaker (talk) 20:18, 25 October 2009 (UTC)

Is the OP sure that the sky isn't blue because it's relecting the water? B00P (talk) 04:52, 26 October 2009 (UTC)

Humans after extended time in outer space

What happens to humans after they land on earth after an extended stay in outer space, say 5 months? --Reticuli88 (talk) 14:40, 24 October 2009 (UTC)

A ticker-tape parade and pictures in the newspapers. Cuddlyable3 (talk) 14:45, 24 October 2009 (UTC)

I meant physically and mentally.--Reticuli88 (talk) 14:46, 24 October 2009 (UTC)

Of course you did, some people around here just like to muck about making unhelpful jokes. —Preceding unsigned comment added by 82.44.55.2 (talk) 18:09, 24 October 2009 (UTC)

See Human spaceflight#Medical issues. Rockpocket 15:20, 24 October 2009 (UTC)

Natural gas from the outer planets?

Would it be possible to commercially extract methane,hydrogen gas,hydrocarbons or helium from the outer planets or moons and parachute it back to Earth?80.0.99.159 (talk) 14:58, 24 October 2009 (UTC)

It would certainly be possible to do it (Uranus, Neptune or the moons of the outer planets would be the best choices for methane and other hydrocarbons - they exist in very small proportions in Jupiter and Saturn and the increased gravity makes extraction most costly even if you can isolate it). I'm not sure it would be economically viable to return it to Earth, though - big solar panels in space beaming energy to Earth by microwaves is at least as easy an engineering problem and probably far cheaper. It might make sense to extract it for use in the outer solar system, though - there is less solar energy out there and you don't have to move the extracted gasses as far, so the costs would be more favourable to the extraction. I'm not sure there is really any point extracting helium, though. If you can get Helium-3, it might be useful in a fusion reactor, but we don't have any viable fusion reactors yet. --Tango (talk) 15:12, 24 October 2009 (UTC)

To use These hydrocarbons as a source of energy in the outer solar system you would also need a source of oxydiser. There is no molecular oxygen out there. Dauto (talk) 16:37, 24 October 2009 (UTC)

That is a good point. We could, in theory bring the oxygen from Earth (or produce it in the inner solar system using solar power to electrolyse water). We would then only have to transport half our rocket fuel from the inner solar system. I'm not sure that would be more efficient than other means of generating energy in the outer solar system, though (tidal power is one option). --Tango (talk) 18:36, 24 October 2009 (UTC)

There's quite enough energy on earth without importing energy. Some solar panels in the Sahara or Utah or Gobi or some wave power and good electrical transmission could solve lots of problems for instance, and would be cheaper cleaner and safer than getting methane from other planets or beaming microwaves from space. Dmcq (talk) 17:29, 24 October 2009 (UTC)

I agree about the methane - we have enough problems with global warming as it is - without importing another source of carbon from outside! We're never going to run out of fossil fuels because we're going to have to stop burning them long before they run out.

However, having large amounts of Helium-3 (which is thought to exist in useful quantities on the Moon) available would make building super-clean, efficient fusion reactors much more feasible. Such reactors would need very little helium-3 to produce vast amounts of energy - so the "shipping cost" isn't such a concern as it would be for almost any other product mined in space. It isn't so hard to imagine some scheme whereby robotic mines on the moon could produce the stuff and ship it back here by hurling containers of it off the lunar surface using a linear induction motor launch track or something. The mines themselves could be powered using the same helium-3 fusion technology - and could be more or less self-sustaining.

The idea of beaming energy down from orbit (probably in the form of microwave laser beams) has it's attractions. Without all of that atmosphere in the way, being able to park yourself in an orbit where you'd get almost 24 hours of sunlight per day and with zero-g meaning you wouldn't need huge supporting structures - you could greatly increase the efficiency of solar panels. Whether that increase would be worth the enormous cost of getting them up there and getting the power back down again - is anyone's guess at this point. SteveBaker (talk) 22:02, 24 October 2009 (UTC)

Lunar He-3 mining is almost certainly a good idea if we can invent the fusion reactors. Getting things from the lunar surface to Earth's surface is far easier than getting it here from the outer solar system. --Tango (talk) 22:23, 24 October 2009 (UTC)

I'm not too sure about that. Overall considering the extraction costs it may be cheaper to get it from an outer planet. Travel is fairly cheap, the main problem is the money tied up in the time. Dmcq (talk) 22:53, 24 October 2009 (UTC)

The question with He3 fusion is whether you can develop the fusion technology without a sufficient supply of He3 to power it...and then, whether it's worth setting up all of the infrastructure to mine the stuff on the moon with the fusion plant design being merely a theoretical concept. It's a chicken and egg problem. The problem with mining He3 in the outer planets is that it's such a trek out there to fix problems, shuttle spare parts, etc. You can get to the moon in three days - if your lunar soil excavator needs a new reverse spurling frangulator, you'll not lose much production fixing it. But it's gonna take the best part of a year to get to Mars - and then only when the Earth/Mars system are in the right places in their respective orbits. SteveBaker (talk) 04:37, 25 October 2009 (UTC)

Using solar and RTG powered ion-electric engines might make it economic,but importing methane would not be green.However, forgetting about helium3, isn't normal helium running out and becoming very expensive?80.0.108.29 (talk) 13:21, 25 October 2009 (UTC)

Yes, regular helium is becoming scarce - but not sufficiently to warrant the spectacular costs associated with bringing it back from the Moon (or anywhere else for that matter!). The effort to produce (say) just 20 kilograms of helium-3 from the moon would be insane - but those few kilo's are enough to produce all the energy needed by an entire 1000 megawatt power station for an entire year...that's enough to replace about 10 million tons of coal. I could believe that the cost to mine and transport 10 million tons of coal could be more than the cost to mine and transport 20kg of Helium-3 from the moon. However, 20 kg of regular helium is unlikely to be enough to fill the kids party balloons for one major US city for a day and would probably take a comparable amount of effort to produce! Hardly likely! However, if we were mining He3 on the moon - then immense amounts of regular helium would be produced as a by-product - and it's possible that it might be worth shipping that back to earth if the stuff get really scarce down here. SteveBaker (talk) 20:14, 25 October 2009 (UTC)

Could you power an ion drive with an RTG? RTGs are good for low levels of power production for long periods of time (which makes them perfect for probes to the outer planets). Ion drives need quite a lot of power (existing ones use several kilowatts, I think) and existing RTGs generate at most a few hundred watts, and that reduces over the lifetime of the RTG. --Tango (talk) 23:19, 25 October 2009 (UTC)

Dunno - but once you get the processing pipeline 'primed', you don't really care how long it takes to get the stuff back to earth - there's really only a need to nudge it in the right direction and let the orbital dynamics do the rest. IMHO, you wouldn't want elaborate rocket motors on the shipping containers - hence my suggestion that you launch them with solar-powered linear induction motors or something. The containers could probably contain just a few kg of He3 apiece and be launched at spectacularly high g-forces from the lunar surface. Alternatively, a lunar version of the space-elevator could do the job very cheaply. SteveBaker (talk) 02:03, 26 October 2009 (UTC)

I was assuming we were talking about the outer planets - there is really no point using an RTG in the inner solar system, solar power is much better. I agree that getting things from the lunar surface to Earth orbit should probably be done using some kind of mass driver. --Tango (talk) 02:08, 26 October 2009 (UTC)

Pterodactyls

Were pterodactyls large and strong enough that a person could ride on its back, similarly to the way dragons are used in some fantasy settings? --70.129.184.103 (talk) 16:34, 24 October 2009 (UTC)

No. Large birds even today have difficulty taking off. Their weight is really at a premium. See bird flight.--Shantavira|^{feed me} 16:47, 24 October 2009 (UTC)

Is there any large, flying animal (modern or extinct, it doesn't matter) that's capable of carrying a human? --70.129.184.103 (talk) 17:35, 24 October 2009 (UTC)

Unlikely. It takes a lot of energy to feed muscles and the animal wouldn't benefit from having big enough muscles to lift itself and a human when it has only ever needed to lift itself (and maybe its young or some food for its young). You might be able to intentionally breed such a bird, but it wouldn't be easy. (Land animals can carry humans since the extra muscle required to walk with extra weight is so much less than the extra muscle required to fly with it.) --Tango (talk) 18:40, 24 October 2009 (UTC)

What about Haast's Eagle? If the bird was indeed strong enough to carry off small/young humans, then it may also have been the case that it could've been used as a (probably extremely unwilling and likely short-range) mount. That said, the Ostrich is a much more substantial bird and is (AFAIK) the only living bird that can be ridden by adults - yet (from what I remember reading somewhere a long time ago) it struggles to even stand up with a human weighing more than 120lbs on its back. --Kurt Shaped Box (talk) 20:06, 24 October 2009 (UTC)

According to our article, the heaviest Haast Eagles weighed in at 33lbs. So, no - it couldn't have been ridden by a human. Even an anorexic 5 year old would weigh more than the largest Haast Eagle...and let's not forget the increase in drag by adding a very un-aerodynamic human to that smooth, slippery Eagle body. SteveBaker (talk) 05:18, 25 October 2009 (UTC)

No - the problem is that of volume versus area. In terms of flying, one critical number is the "wing loading" - the body-weight to wing-area ratio. When you double the size of an animal, you pretty much increase it's weight by a factor of 8 and it's surface area by a factor of 4. When you double the size of a bird, the wing loading doubles - making it twice as difficult to get off the ground. That's why insects seem to fly so effortlessly - they are tiny, and halving the size of an animal halves the wing loading. For an animal to have enough spare lift capacity to get a human into the air, it would have to be huge - and as you make the bird larger and larger in order to have that spare capacity - the drastic increase in wing loading rapidly overtakes their ability to fly. Birds also have to have a whole lot of adaptations for flight - including very lightweight bones - that makes them fragile creatures. Quite unable to bear lots of extra weight.

Pterodactyls had to operate with the same laws of physics - there is no reason to assume that they did any better - and their lack of fancy flight feathers strongly suggests that they did a lot worse. Modern aircraft have higher wing loading than birds and pterodactyls - which they have to compensate for by having high forward speed and powerful engines, but for animals that get their forward thrust from flapping - the mechanical difficulties of moving those gigantic wings up and down makes that solution impossible too.

When you look at the Gossamer Albatross (the first human-powered plane) - you can see just how much wing area you'd need in such a bird. That plane was incredibly fragile - and there is no way you could make those big flimsy wings actually flap!

Probably the best chance of such a creature existing would be if it survived only by gliding over steep, windy mountains where the slope lift would allow them to stay aloft despite crazily high wing loading. For a long time, scientists believed that the larger species of Pteranodons could only fly in this way. SteveBaker (talk) 21:45, 24 October 2009 (UTC)

Size comparison of the two known Quetzalcoatlus species and a human

Wait, why the comparison with the Gossamer Albatross? It's a very unusual craft. The GA was constructed like that,because it had such incredibly little horsepower. Aren't you making an assumption about the muscle strength of pterosaurs? Even professional paleontologists have not come to agreements about the flight mechanics or even the total body weight of pterosaurs because they're so different than any living animal.

If a pterosaurs wanted to launch from a cliff, and then ride thermals, it doesn't seem entirely beyond belief that it could carry a small human. Nine meters is an entirely reasonable wingspan for a hanglider, and Pterosaurs have been found with wingspans greater than 10meters. (And it's suspected that a complete Hatzegopteryx skeleton would be even larger if one is ever found!) APL (talk) 04:47, 26 October 2009 (UTC)

If you're interested in a theoretical creature that could carry an adult human in the air, I suspect that it would need to be something like a blimp, perhaps with many hydrogen "balloons", each made of highly elastic sacs, like a frog's throat. It might also have wings to control it's flight. I would suspect that the evolutionary path to such a flying animal would be from some fish or other marine animal, as many of them already use the concept of an expandable ballast "tank". StuRat (talk) 04:07, 25 October 2009 (UTC)

Yes - that would work - but it's hard to see how a proto-gasbag creature would evolve hydrogen production. SteveBaker (talk) 04:27, 25 October 2009 (UTC)

My knowledge of biochemistry is minimal, but plenty of process created hydrogen ions (eg. glycolysis). I expect some mechanism exists to get the extra electron. I agree that the most likely route is from a water dwelling animal with a gas bladder changing the gas used from air/oxygen to hydrogen. --Tango (talk) 13:29, 25 October 2009 (UTC)

Two caterpillars were shaken when a passing butterfly vibrated the leaf they were crawling on. One caterpillar grumbled "I'll never go up in one of those flying machines. Cuddlyable3 (talk) 16:47, 26 October 2009 (UTC)

Artificial Chocolate Flavour

Hello. Is there an ester that tastes like chocolate? Thanks in advance. --Mayfare (talk) 16:35, 24 October 2009 (UTC)

As I understand it, chocolate is a rather complex flavor, consisting of many different molecules. So, no, there is no single ester that tastes much like chocolate. The only chocolate substitute I'm aware of is carob, and it's not all that close. StuRat (talk) 03:52, 25 October 2009 (UTC)

Any chance colonoscopy can be replaced?

I just read about the preparation for the procedure. No wonder there're so many public service ads for it! Is there any chance that in the future this procedure will be replaced by something more agreeable? Imagine Reason (talk) 17:35, 24 October 2009 (UTC)

See Virtual colonoscopy I guess someone could swallow a camera and the footage viewed after it had passed through the person being examined, but as the article states, optical colonoscopy means you can do procedures while you have a look. Endocopic procedures would be preferable to open surgery, so even if virtual colonoscopy was used, if a procedure was to take place the unagreeable preparation would have to occur.MedicRoo (talk) 17:41, 24 October 2009 (UTC)

It's not that bad... you take a laxative, and don't eat for a day. Other than the laxative, it's not much different than the preparation you do before getting your blood drawn. As far as medical procedures go, it's pretty straightforward, and you are usually somewhat sedated during it. --98.217.71.237 (talk) 18:50, 24 October 2009 (UTC)

I'll go further and say the experience was more interesting than bothersome. I watched "my" picture and chatted with the nurses while it went on. I had no idea that the colon seen from the inside is not round but triangular. For anyone longing to know, my colon was declared to be in excellent condition. Cuddlyable3 (talk) 22:14, 24 October 2009 (UTC)

The laxative is the part that sounds really disagreeable. Plus I've read that it's more like a day and a half of little food ingestion. Imagine Reason (talk) 01:06, 25 October 2009 (UTC)

The laxative is to make sure one does what one does anyway. It's not a good idea to go straight from the colonoscopy to an important meeting because the colon will have been inflated with air that has to be expelled the way it went in. Cuddlyable3 (talk) 19:24, 25 October 2009 (UTC)

One of the main benefits of physical endoscopic procedures as opposed to "virtual" radiological imaging is the potential to take biopsies or perform therapeutic manoeuvres. Colonoscopy is unlikely to be replaced any time soon. Axl ¤ [Talk] 22:09, 25 October 2009 (UTC)

Can you identify this bug?

Unidentified bug in my home

I have seen this little guy and others like him in my home in Kentucky. Any idea what he is? -- KathrynLybarger (talk) 17:52, 24 October 2009 (UTC)

I think this is him!

The lighting is a bit bad, but it may be a species of Soldier beetle; there are many many kinds of soldier beetle, but this would be my first guess. --Jayron32 20:00, 24 October 2009 (UTC)

Looks like a true bug to me, maybe in Coreidae family. Hard to say, though. Can you please take a picture with the camera flash on (assuming the critter is still there)? --Dr Dima (talk) 00:43, 25 October 2009 (UTC)

Isn't that the rare Blind Venetian Beetle ? :-) StuRat (talk) 04:40, 25 October 2009 (UTC)

Good call, Dr Dima! He did look quite a bit like the guy in the Leptoglossus article. Thanks!! -- KathrynLybarger (talk) 21:07, 25 October 2009 (UTC)

Air composition

what is the percentage of o2 in the air? —Preceding unsigned comment added by 24.60.6.106 (talk) 18:37, 24 October 2009 (UTC)

We have articles about air and oxygen, both of which seem good places for you to learn to look up this type of information. DMacks (talk) 18:41, 24 October 2009 (UTC)

Roughly 21%. SteveBaker (talk) 21:25, 24 October 2009 (UTC)

This figure is also dropping at roughly just over twice the rate that CO₂ concentrations are rising (roughly 100 ppm, or 0.01% in the past 100 years for CO₂), mainly because each molecule of carbon dioxide is composed of two oxygen atoms. ~AH1^(TCU) 00:14, 26 October 2009 (UTC)

Worrysome as the rise of CO2 levels is - the reduction in oxygen content as a result of that is truly negligable. The CO2 problem is measured in hundreds of parts per million (we're currently at about 390ppm) - the effect of that on oxygen levels is therefore also hundreds of parts per million - which is just nothing compared to 21%. SteveBaker (talk) 01:57, 26 October 2009 (UTC)

yeah, but it is a good rejoinder to those whose religious beliefs tell them that the additional CO2 is all released by volcanos. Gzuckier (talk) 03:29, 26 October 2009 (UTC)

"This figure is also dropping at roughly just over twice the rate that CO₂ concentrations are rising ... mainly because each molecule of carbon dioxide is composed of two oxygen atoms."

— AH1

Each molecule of carbon dioxide is composed of two oxygen atoms and a carbon atom. Each molecule of oxygen is composed of two oxygen atoms. Axl ¤ [Talk] 13:19, 26 October 2009 (UTC)

Right statements, wrong association. As Axl points out, O2 and CO2 contain equal amounts of oxygen per molecule (and, by ideal gas approximation, quite nearly per volume). O2 is sinking at twice CO2 increase, because at the moment about 50% of the anthropogenic CO2 is absorbed by natural sinks - in particular, it's dissolved in the ocean, forming carbonic acid and increasing ocean PH. --Stephan Schulz (talk) 14:14, 26 October 2009 (UTC)

Er, decreasing pH surely? Axl ¤ [Talk] 18:37, 26 October 2009 (UTC)

I'm not a directionalist. Forward/backward, left/right - it's all the same to me. --Stephan Schulz (talk) 12:42, 27 October 2009 (UTC)

yeah, that's what i meant the whole time. (^_^)Gzuckier (talk) 19:19, 26 October 2009 (UTC)

What is this called?

I'm preparing for a trip to the literal other side of the world, and I'm trying to force my sleep schedule towards the time zone I'll be working in. Is there a term for this practice? SDY (talk) 19:41, 24 October 2009 (UTC)

Its been referred to in academic studies as Preflight sleep adjustment Rockpocket 20:12, 24 October 2009 (UTC)

Anabolism in Heterotrophs

In heterotrophs, anabolic processes synthesize organic compounds. Would the following be a true statement?

Heterotrophs are able to synthesize all their needed organic compounds solely through anabolic reactions.

Heterotrophs, of course, must get the carbon for these organic compounds and the glucose to undergo cellular respiration from other organisms, but isn't the synthesis solely anabolic? —Dromioofephesus (talk) 20:25, 24 October 2009 (UTC)

Not quite. Many organisms are unable to synthesize certain compounds, hence the classes of essential nutrients (including essential amino acids, among others), for example. DRosenbach ^{(Talk | Contribs)} 04:50, 25 October 2009 (UTC)

That's a good point. I didn't think about that. —Dromioofephesus (talk) 05:00, 25 October 2009 (UTC)

Eggs (the kind in a woman's body)

When was it first discovered that a man's "seed" was not enough -- that a woman's egg, too, was needed? —Preceding unsigned comment added by 75.40.136.211 (talk) 20:43, 24 October 2009 (UTC)

Leonardo da Vinci 1452-1519 drew dissections of a woman and embryos which may be the earliest documented investigation of woman's reproductive organs. Cuddlyable3 (talk) 22:08, 24 October 2009 (UTC)

What I mean was, when was it first discovered that a woman's reproductive organs were not simply a ground of sorts into which a man's "seed" was planted? —Preceding unsigned comment added by 76.210.248.231 (talk) 22:21, 24 October 2009 (UTC)

It wasn't really a Eureka! moment, in as much as it was the accumulation of lots of different scientists work. William Harvey proposed that all creatures arise from eggs ("ex ovo omnia") rather than just sperm, back in the 1600s. In 1826 Karl Ernst von Baer discovered the ovum by examining the ovarian follicle of the dog. In 1876, Oscar Hertwig reported that fertilization requires the penetration of a sperm cell into an egg cell, but it wasn't until October 6, 1928 that the first human ovum was recovered and described (by Allen at al). Rockpocket 00:42, 25 October 2009 (UTC)

Wow -- and I thought Hertwig was famous for his work on teeth. DRosenbach ^{(Talk | Contribs)} 01:22, 25 October 2009 (UTC)

But, of course, it should have been readily apparent even to the ancients that the mother contributed genetically to the resultant child, since the child will often at least partially resemble the mother. Of course, this is most obvious when the parents are quite different from one another. A white father and a black mother producing a brown child would be pretty good evidence. Those that insisted that the mother was only an "empty vessel", in the face of such obvious evidence to the contrary, must have been willfully ignoring the facts to push their POV. StuRat (talk) 03:48, 25 October 2009 (UTC)

The first generation doesn't actually demonstrate that the woman contributes any heritable factors to the child. It could easily have been argued that the father contributes all heritable factors, and the mother-like characteristics are caused merely by developing in her "environment" and consuming the nutrition provided therein. Although, the perpetuation of said characteristics to subsequent generations would quite surely kill that theory. Someguy1221 (talk) 03:56, 25 October 2009 (UTC)

What if it were a sex-linked recessive allele? —Dromioofephesus (talk) 05:14, 25 October 2009 (UTC)

It would kill the theory just as easily, except that the data would be more confusing to our pre-genetics observers. The key is multi-generational heritability, which an "allele" necessarily is, as opposed to an epigenetic situation. Although then there's still the possibility of pan-generational epigenetic inheritance, which would just have been damn confusing to anyone predating modern understanding of DNA regulation. Someguy1221 (talk) 05:23, 25 October 2009 (UTC)

It was rarely held that the mother contributed nothing (though since genes were not discovered for another few thousand years, I'm not sure how readily apparent it could be that the mother contributed genetically). Aristotle thought that the man's sperm mixed with the woman's blood to form the child. Menstrual blood was thought to be the fertile soil that the seed was planted in. Thus the Greeks realised brothers are blood relatives through the mother, a relationship that would be meaningless if the mother contributed nothing to their generation. Rockpocket 05:28, 25 October 2009 (UTC)

I thought somebody was going to get picky about my use of the word "genetically". I was using the word to mean "the observed similarity in features between parents and offspring", which existed long before the formal study of genetics. I believe the Bible even mentions it ("like shall beget like", or something along those lines). It you have a better term to use, please let me know. StuRat (talk) 21:35, 25 October 2009 (UTC)

The term for the historical concept you are referring to is heredity. Rockpocket 22:13, 25 October 2009 (UTC)

Also, remember that incorrect theories are suprisingly resiliant. People tend to massage the old system in suprisingly convoluted ways than to simply ditch it entirely to adopt a new, more elegent, theory. Consider the whole "geocentric-heliocentric" conflict in astronomy. The geocentric system had to devise some pretty rediculous explanations, like epicycles and things like that, just to avoid having to remove the Earth from the center of creation. So, any explanation of how a mother's traits could be passed to a child even though only the "sperm" was supposed to develop into the child needs no further explanation; it doesn't have to make sense with our modern understanding of genetics, it only needed to make sense to people back then and apparently, given such theory's resiliance, it did. --Jayron32 06:16, 25 October 2009 (UTC)

I think I will get Jayron a spell-checker for Xmas... :-) StuRat (talk) 21:39, 25 October 2009 (UTC)

hedron

what is 'the great unified theory' —Preceding unsigned comment added by Johny anu15 (talk • contribs) 21:10, 24 October 2009 (UTC)

May be you're looking for grand unified theory? Grand unified theories are models that atempt to explain all three interactions of the standard model of particle physics (electromagnetic, weak and strong interactions) as different aspects of a single interaction —Preceding unsigned comment added by Dauto (talk • contribs)

Also, a hadron (note spelling) is a particle made of quarks held together by the strong nuclear force. Jkasd 22:44, 24 October 2009 (UTC)

help

This question appears to be a request for medical advice. It is against our guidelines to provide medical advice. You might like to clarify your question. Thank you. Responses containing prescriptive information or medical advice should be removed and an explanatory note posted on the discussion page. If you feel a response has been removed in error, please discuss it before restoring it.

Cuddlyable3 (talk) 21:52, 24 October 2009 (UTC)

stocking ground

in the context of quarrying, what is a stocking ground? —Preceding unsigned comment added by 86.128.188.238 (talk) 21:47, 24 October 2009 (UTC)

From this link [1], it seems to be an area where the quarried material is stored before being distributed. Richard Avery (talk) 21:54, 24 October 2009 (UTC)

Confused about transformers.

In the transformer article, it says that the flux in the core cuts the secondary winding to give secondary voltage. This voltage can then deliver a current to a secondary load. But if a load is connected to the secondary, the secondary current flow will introduce an equal and opposite flux in the core. So how is the flux in the core related to the power transfer when the transformer is working?--79.75.37.202 (talk) 23:10, 24 October 2009 (UTC)

No, the direction of the secondary current is such that it corresponds to the same flux in the same direction (which you can determine by the right hand screw rule. If the secondary current instead, was being sucked out of the transformer by a generator, exactly the same flux would result and a current would be induced at the primary equal to the original current as in the first case. SpinningSpark 02:32, 25 October 2009 (UTC)

But, of course, to fully answer this Q, we would need to know if the Transformer is in the form of a race car, jet plane, or rocket ship. :-) StuRat (talk) 04:45, 25 October 2009 (UTC)

The OP is right to be confused. His question cannot be dismissed as easily as SpinningSpark suggests. The flux due to the secondary current does in fact (neglecting leakage) cancel out the flux due to the primary. In an "ideal" transformer, you would be left with zero net flux and no power transfer. Luckily for electrical engineers, "ideal" transformers do not exist. In a real transformer, there is a non-zero flux called the magnetising flux in the core, but even this is not an indicator of the amount of power flowing between the windings. The magnetising flux is proportional to the primary EMF (if we're talking about sinusoidal AC), and does not depend on the load. The magnetising flux is a necessary condition for power transfer, but it is not what transmits the power. The power actually snakes its way around the windings outside of the core, guided by the magnetising flux in the core.

If you studied transformers at school then at this point you will be saying "No! No! No! That's not what they taught me!" and you would be right. What they teach at school is a theoretical model that gives the right answers in exams but does not describe physical reality. Please see the archived talk pages for the Transformer article for a lengthy and enlightening discussion on this, but be warned, it may mess with your head. --Heron (talk) 11:17, 25 October 2009 (UTC)

Suppose a transformer is built up as follows. A) The primary winding is just a coiled wire, a solenoid. B) Add a core of iron that is an easier magnetic path than air. Flux in the core links together the magnetic fields of each turn of the coil and that increases the self inductance of the primary. C) Apply a fixed alternating voltage to the primary and the current is limited by the primary inductance (and usually to a lesser extent by its resistance). D) Add the secondary winding. The alternating magnetic field induces an alternating voltage across the secondary terminals but until they are connected to a load there is no secondary current nor change in the primary current. E) When current flows from the secondary into a load the nett magnetic flux in the core gets reduced just as the OP says (though Spark denied). Less flux means the inductance seen at the primary gets reduced so more primary current flows. Thus power taken from the transformer secondary causes the primary to draw extra power. Transformers for power conversion can be made almost 100% efficient when power losses in wire resistance, eddy currents and stray magnetic field are minimised. Ideally there would be no field outside the core. To Heron, I don't see that "power snakes around the windings outside the core". Cuddlyable3 (talk) 18:48, 25 October 2009 (UTC)

Hi Cuddly, I knew that somebody would pick me up on that. I have since realised that I was describing the special case of bifilar windings. In the archetypal transformer with two separate windings at opposite sides of a toroidal or rectangular core, the power actually flows across the air space bounded by the core. In any case, it does not flow through the core material. --Heron (talk) 19:13, 26 October 2009 (UTC)

A transformer does two power conversions: electric-to-magnetic, then magnetic-to-electric. In likeness to the way an electric field causes an electric current to follow the path of least resistance, a magnetic field causes magnetic flux to follow the path of least magnetic reluctance which is through the core. Suppose the reluctance of a core material is arbitrarily low. In the limit one gets a very good transformer where virtually all the magnetic circuit is through the core and the stray field through air is insignificant. Using Bifilar wires for primary and secondary obtains tight coupling of their magnetic fields by their close proximity through air but that coupling is through their self-inductance which is too small to be effective at typical low power frequencies.Cuddlyable3 (talk) 21:08, 27 October 2009 (UTC)

But what is your answer to the OP's question? I say that there is no relationship between the flux in the core and the power being transferred (here's a reference: particularly the 'Transformer Principles' section). You haven't explicitly stated your view, but you seem to be implying that the flux in the core is the power. If that is what you think, then I am going to enjoy trying to convince you otherwise. Although perhaps we should retire to the Argument Desk to save space here. --Heron (talk) 19:04, 28 October 2009 (UTC)

HIV and RNA

The HIV virus contains two strands of RNA in its capsid. This video is pretty good and shows that it uses reverse transcriptase to form the RNA into DNA. My question is, why does the capsid contain two strands of RNA? Surely only one strand is needed? 81.154.211.84 (talk) 23:58, 24 October 2009 (UTC)

Great question!! I can start with a couple of possibilities: (i) RNA can be hydrolyzed easily, resulting in strand breaks - since the RT enzyme is really good at template-switching, a broken strand can be complemented by its companion, allowing completion of replication; (ii) recombination is a major mechanism that accelerates HIV evolution, and the two strands can be from different HIV genomes if the cell producing the virions is dually infected with two different genetic variants. This latter point is almost certainly the genesis of circulating recombinant forms of HIV that make up a significant fraction of HIV strains worldwide. -- Scray (talk) 01:05, 25 October 2009 (UTC)

Along the same vein, DNA doesn't have to be double-stranded either. While it's great to speculate, many things will remain just that -- speculation. DRosenbach ^{(Talk | Contribs)} 04:46, 25 October 2009 (UTC)

Ummm...that seems to be a non sequitur. The RNA in the HIV virion is not double-stranded - the two strands are both positive-sense. So, your statement is correct, DNA need not be double-stranded, but I don't see how that is relevant here. -- Scray (talk) 13:38, 26 October 2009 (UTC)