Wikipedia:Reference desk/Archives/Science/2010 October 31

= October 31 =

Artificial reefs
After reading this article, I still remain with the question as to how long such reefs take to become populated with any life considerable enough to make it a bona fide reef -- I guess that definition as a factor of time can be debated, but assuming a range could be established, the article didn't seem to give one -- so when sinking ships or doing something like this, how long after construction does it take for a reef to appear?  DRosenbach  ( Talk 00:05, 31 October 2010 (UTC)
 * Technically a reef doesn't have to contain any life, so by definition as soon as the artificial reef is put in place it is a bona fide reef. But since you ask about the life, colonisation would begin almost immediately, but the time until you would define that to be a 'bona fide reef' would not be so clear cut as there's probably no hard and fast rules, at least not that I know about, for how much life is required. It would also be highly variable depending on where the reef was being established. For example, put in an artificial reef around the Great Barrier Reef and it will probably be teeming with sea-life within a week, put one up in under the Arctic sea ice and you're probably looking at considerably longer. Coral_reefs may also be helpful. --jjron (talk) 03:36, 31 October 2010 (UTC)


 * If you want to pursue this question further, you may wish to search some fishing message boards which will presumable contain some discussion of fishing conditions at newly created artificial reefs. -- 124.157.254.112 (talk) 04:58, 31 October 2010 (UTC)


 * This 2009 article is about the reef formed by sinking HMS Scylla (F71) near Plymouth in the UK. The National Marine Aquarium, Plymouth which organised the project, says "the scuttled former Royal Navy frigate is now home to about 260 sea species" in the course of 5 years. It also added £25 million to the local economy, because 42,000 divers had visited the site. Some more scientific information here and here. Alansplodge (talk) 17:18, 1 November 2010 (UTC)

Earthworms
How much of an earthworm is protein? I'd like my oscars and dinosaur bichir to grow at a more rapid rate than they are apparently doing now, and would a diet richer in live earthworms provide me with success? I figure that crickets have too much exoskeleton and wings to give me a good bang for my buck, seeing how earthworms are free from my backyard. Any comments?  DRosenbach  ( Talk 00:14, 31 October 2010 (UTC)
 * Well I really think you could type http://www.google.co.uk/search?sourceid=chrome&ie=UTF-8&q=earthworm+protein+content .. and take my word for it every article on the first page of results is a gem!
 * Anyway, nagging aside, we are here to serve: http://www.jstor.org/pss/2396435 - by dry weight well over 60% protein, <10% fat, variable carbohydrates
 * I know that earthworms are considered excellent food for other carnivorves eg salamanders, and are used as bait by fisherman - so I guess you are on to a money saving winner.
 * Excluding the usual warnings about fish overfeeding, and overloading filter systems with high nitrogen content waste.. which I'm sure you've already read or heard.77.86.42.103 (talk) 00:45, 31 October 2010 (UTC)


 * oh hang on . parasites and diseases - I don't recall ever having heard that earthworms are a parasite source, but diseases are a possibility .. I suppose. 77.86.42.103 (talk) 00:55, 31 October 2010 (UTC)
 * A good rule of thumb: diversify food! No single source of food can have it all. And, since oscars are serious predators, you should give them something more substantial than earthworms. They need fish flesh! East of Borschov 10:50, 31 October 2010 (UTC)
 * My 2 oscars are about 2 inches long, so I don't think there are many fish that I can feed them. I have two mollies in the tank with them for over a month and I suppose they are way too big because they are fine.  DRosenbach  ( Talk 12:57, 31 October 2010 (UTC)
 * If you keep the mollies (or guppies etc) in a separate tank they will rapidly breed - producing fry for the other fish to eat . I shouldn't really be promoting icthycide, but if you enjoy a bit of un-balanced 'gladiator style' fishkeeping it's the way to go.87.102.72.130 (talk) 13:02, 31 October 2010 (UTC)
 * Not necessarily live fish. The breeders that I know make their own frozen food from minced raw fish (the common supermarket white fish) with some cereal and vitamins. East of Borschov 22:53, 31 October 2010 (UTC)

Optical computing
Why haven't optical computers become widespread? The technology exists and it has many advantages over electronic computing. --75.33.217.61 (talk) 00:26, 31 October 2010 (UTC)
 * Have you read Optical computing? The simple answer is that the technology is still in its infancy, and many problems have to be solved first. Clarityfiend (talk) 01:26, 31 October 2010 (UTC)

why do plants use so much more potassium than animals?
Almost all the biochem taught to me so far has been systemically biased towards animals ... I think my professor is biased in favour of his own race. Anyway...I can't seem to find an explanation. Do they use the potassium to bring water into the cells? Do animals not do this because their cells lack cell walls? I'm really curious why potassium is a micronutrient in animals but a macronutrient in plants. Furthermore, plants usually lack sodium. Do terrestrial rocks have more potassium than sodium? John Riemann Soong (talk) 00:40, 31 October 2010 (UTC)


 * First sentence in Potassium in biology: "Potassium is an essential mineral macronutrient and is the main intracellular ion for all types of cells." Furthermore it goes on to say: "Potassium is the major cation (positive ion) inside animal cells, while sodium is the major cation outside animal cells." This would seem to suggest an error in your premise that it's more important in plants than animals. Is this what you're looking for? (And BTW, animals are not a 'race') . --jjron (talk) 03:47, 31 October 2010 (UTC)


 * Potassium is a micronutrient, not a macronutrient for animals. Extracellular sodium concentration as I recall >>> cytosolic potassium concentration. Is it potassium a micronutrient because animal cells are usually close to isotonic? John Riemann Soong (talk) 04:51, 31 October 2010 (UTC)


 * Macronutrient and Micronutrient both agree with the Potassium in biology article that potassium is a macronutrient. Micronutrient says the distinction is at 100 micrograms/day (though that sounds very low and seems to disagree with the list it gives later - maybe it's meant to be 100 milligrams/day; I've added a citation needed tag), while dietary mineral says the dietary requirement for potassium is 4700 mg a day. And yes, as suggested above, sodium concentrations are greater outside the cells. See also Sodium. --jjron (talk) 05:48, 31 October 2010 (UTC)
 * I meant that sodium usage >> potassium usage, sorry. In the potassium article, it is cited as a micronutrient. John Riemann Soong (talk) 07:02, 31 October 2010 (UTC)


 * Eh, to be honest I would have instinctively said it was a micronutrient, but overall the articles seem to be calling it a macronutrient - however there seems to be no clearly defined cut-off and it could just be playing semantics. To put it another way, you'd undoubtedly say that carbohydrates, protein and fats were macronutrients for animals, but this is not the case for plants as they largely manufacture their own, which means something like potassium could be considered relatively more important. So to compare fairly you really need to look at elements and talk about the macronutrients being carbon, oxygen, hydrogen, nitrogen, etc. On this list potassium now ranks considerably lower for plants and the key elements for plants and animals are more similar. Regardless of this, yes it's true that plants require/use far more potassium than sodium, and less sodium than animals. The main point is that animals use a lot of sodium and potassium in the sodium-potassium pump. In plants, a similar 'pump' system utilises hydrogen and potassium rather than sodium. Additionally, some plants require considerably more sodium than others, halophytes for example, and some plants can substitute sodium for potassium in some functions if necessary. --jjron (talk) 14:31, 31 October 2010 (UTC)


 * I think this is simply a matter of growth -- plants grow much more rapidly than animals. Animal cells require very large amounts of potassium, but we don't lose it very rapidly because almost all of it is intracellular, so we don't need a large intake.  If we were growing rapidly, we would need a lot more, to supply the new cells we are creating. Looie496 (talk) 18:00, 31 October 2010 (UTC)
 * Mammals also use sodium for excretion: the sodium is actively pumped across a membrane towards the outside world, and the water follows it by osmosis. In urination, this helps to prevent the organic contaminants from crystallizing out of the urine; in sweating, it allows water to be "pumped" onto the surface of the skin to evaporate and cool it. Potassium cannot be used for this purpose, because it has other physiological roles as described above. Physchim62 (talk) 18:08, 31 October 2010 (UTC)

Contradiction in the solution to a Physics question?
Read Question 10 (c) here: http://www.tqa.tas.gov.au/4DCGI/_WWW_doc/003139/RND01/PH866_paper99.pdf

The solution can be found here: http://www.tqa.tas.gov.au/4DCGI/_WWW_doc/003068/RND01/PH866_report99.pdf

The solution for part i) has the red light going below the blue light, but the solution to part ii) shows it going above the blue light. Can someone explain this apparent contradiction?-220.253.253.75 (talk) 00:48, 31 October 2010 (UTC)
 * Simple. Your brain automatically assumes the light came in a straight line. If you project the red and blue rays straight back, it looks like the red blue one came from a higher point. Clarityfiend (talk) 01:24, 31 October 2010 (UTC)
 * Oh OK. However, that still poses a contradiction between that question (Question 10 (c) (i), 1999 paper), and Question 4 (f) (i) from the 1997 paper found here:http://www.tqa.tas.gov.au/4DCGI/_WWW_doc/003429/RND01/PH866_paper97.pdf whose solution can be found here: http://www.tqa.tas.gov.au/4DCGI/_WWW_doc/003378/RND01/PH866_report97.pdf Could someone resolve this contradiction as well?-220.253.253.75 (talk) 03:10, 31 October 2010 (UTC)
 * The solutions are AFTER all of the examiner's comments by the way (i.e. scroll down).-220.253.253.75 (talk) 03:18, 31 October 2010 (UTC)
 * I thought that first answer looked a bit odd, but I was too lazy to pursue it. It's wrong, as shown in the colorful picture in Prism (optics). Clarityfiend (talk) 07:27, 31 October 2010 (UTC)

Solar cells, dark current and current direction
Hi, a little knowledge is a dangerous thing and I have tangled myself trying to understand current in PV cells.

Light strikes the depleted (middle) region of the p-n junction, creating electron/hole pairs which are separated by the electric field established by bringing doped semiconductors into contact with each other, leading to accumulation of electrons (and negative charge) at the p-side and holes/positive charge at the n-side. Got it. Now my questions are:

1) Is the potential difference between the n- and p-sides that drives a working current through an external circuit *the same as* the potential difference that drives the dark current back through the cell? Or are those two currents driven by different phenomena?

2) More fundamentally, I can see that (negatively charged) electrons accumulated at the p-side would be attracted to flow through an external wire to the (positively charged) n-side. But doesn't this mean we have a current flowing from a negative terminal to a positive terminal? A positive current flows from a positive terminal to negative, right?

I hope I've explained my confusion and appreciate any help untangling myself. —Preceding unsigned comment added by 146.23.212.21 (talk) 12:20, 31 October 2010 (UTC)
 * I think you've got that backwards. The electrons produced by the photoelectric effect accumulate on the n-doped side of the p-n junction, and the holes accumulate on the p-doped side.  See the arrows in this diagram.  That diagram is used at Theory of solar cell, which unfortunately doesn't use words to provide support for the direction of the arrows in the diagram.  Red Act (talk) 16:36, 31 October 2010 (UTC)

Thanks Red Act, yes you're absolutely right, sorry for that silly mistake. I've actually done quite a lot of reading today and think I understand my original confusion: it is not the built-in electric field that drives the external current; rather it is better to think of a solar cell as a current producer that happens to create a potential difference across the cell when it's connected to an external load. Indeed the n-side does wind up as the negative "terminal", and the p-side the "positive", which neatly describes the direction of current and electron flow through the circuit (embarassingly, I forgot they are opposite!) —Preceding unsigned comment added by 146.23.212.21 (talk) 13:43, 1 November 2010 (UTC)

Creature in Thailand
There is a bunch of similar images, but I became specifically interested in these ones from Thailand. What's the hell is that? A mock-up or some underdeveloped baby? —Preceding unsigned comment added by 85.222.86.130 (talk) 21:31, 31 October 2010 (UTC)
 * Here's a previous thread on the topic, with a link to a Snopes discussion. Deor (talk) 21:58, 31 October 2010 (UTC)

Curious phenomenon
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Birefringence
Can someone tell me what exactly the x,y, z axes in a uniaxial crystal are? The article birefringence fails to identify them in any images, making me highly confused. Is the z-axis the optical axis? John Riemann Soong (talk) 22:29, 31 October 2010 (UTC)
 * It's somewhat arbitrary, but usually the standard 3 dimensional cartesian coordinate system has the x axis going in-out, the y-axis going left-right, and the z-axis going up-down. -- Jayron  32  01:33, 1 November 2010 (UTC)
 * Thanks, but that doesn't help me so much because I still don't know where the optic axis and the anisotropy or any of the relevant properties are defined! Which axis is on the optic axis on? Does the table match the image? John Riemann Soong (talk) 02:01, 1 November 2010 (UTC)


 * In the calcite images -- which axis are the letters being seen through? John Riemann Soong (talk) 02:02, 1 November 2010 (UTC)