Wikipedia:Reference desk/Archives/Science/2011 February 12

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February 12

Boa constrictor scientific naming...

According to the Boa constrictor article - "Though all boids are constrictors, only this species is properly referred to as "Boa constrictor"; an almost unique instance of an animal having the same common and scientific binomial name". So, which came first - the common name, or the scientific name? --Kurt Shaped Box (talk) 04:15, 12 February 2011 (UTC)

I take that sentence to mean that it has always been known in English as a boa constrictor. Since that is its common name and its scientific name how can one say that either came first? (Of course its South American name jibóia was presumably used before that.)--Shantavira|^{feed me} 07:57, 12 February 2011 (UTC)

Linnaeus created this name in 1788, and he wrote in Latin, so if we equate "scientific binomial name" and "Latin name given by Linnaeus" then it would appear that the scientific name came first. The Penny Cyclopedia in 1836 referred to the name as "popular", so the transition from scientific to popular came between those two dates. The Latin word boa, incidentally, goes all the way back to Pliny. [Source: OED] --Heron (talk) 11:23, 12 February 2011 (UTC)

There are other species known in English by their binomial name. Aloe vera is another one. Dauto (talk) 15:57, 12 February 2011 (UTC)

In some cases, it would be redundant, as in Gorilla gorilla! --FOo (talk) 18:13, 13 February 2011 (UTC)

when the earth was shining

the first earth surface might be very hot between 2000~3000 degrees centigrade . this temperature cases the rocks be melt then this hot cases the matter such as the volcanic magma to shine . before the cooling surface of earth and when the sun was young it is clear that the earth was red color and shining . how much years was the during of this condition . (all remarks are collected from astronomic articles)--78.38.28.3 (talk) 04:54, 12 February 2011 (UTC)a. mohammad zade Iran 2011

We don't know very much about the Hadean period, but our article History of the Earth might be of interest. If the Earth initially shone, then it could have done this for only a comparatively short time, perhaps between 4,540,000,000 and 4,530,000,000 years ago (when the moon was formed and the Earth had a crust). Dbfirs 10:33, 12 February 2011 (UTC)

Use of the word "shine" in this context is potentially confusing, since it is frequently used in astronomy, as elsewhere, to denote the reflection of incident light, as in Earthshine. Colloquially, all the planets (and their satellites), including the Earth, "shine" by reflecting the light of the Sun (Earthshine being a double reflection), according to their individual albedos. For the phenomenon you are interested in, incandescence, or colloquially "glow", would be more appropriate. 87.81.230.195 (talk) 13:14, 12 February 2011 (UTC)

Which incident light does the Sun reflect to create sunshine, then? –Henning Makholm (talk) 15:22, 12 February 2011 (UTC)

Yes, that's all moonshine. Dbfirs 22:03, 12 February 2011 (UTC)

I said "potentially confusing", "frequently" and "more appropriate", not "wrong", "invariably" or "exclusively," and "solely appropriate". I was also pointing the OP towards articles on subjects relevant to his query, and providing hopefully polite guidance on vocabulary usages in a language in which he is evidently not fluent. How helpful do you suppose your comments were? *Stalks off in high dudgeon.* 87.81.230.195 (talk) 22:50, 12 February 2011 (UTC)

Yes, your reply was perfectly acceptable and you provided useful links, and we weren't really complaining about it, but we perfectly understood the usage of the OP whose first language is not English. Apologies for any offence caused

. Dbfirs 08:37, 15 February 2011 (UTC)

only one of my friends understood my meaning . I know that the earth is reflecting the sun shine all over the life of that and all matters such as dark matter has shining . the last planet that nasa discovers is melt and hot and is shining

• File:509304main kepler rocky planet full
• File:508881main KOI-72-STILL-6k-1 full full

so it can shine without reflecting the other star light--78.38.28.3 (talk) 04:51, 13 February 2011 (UTC) a. mohammadzade]] —Preceding unsigned comment added by 78.38.28.3 (talk) 04:46, 13 February 2011 (UTC)

Please see the articles on the Hadean period, Giant impact hypothesis, Cool Early Earth, and Late Heavy Bombardment. Because rocks from asteroids date back to 4.6 billion years, and rocks from Earth date back only to 3.8 billion years, it was thought the surface was largely molten for 800 million years, but now some people think that it was solid for most of this time, until a wave of asteroid bombardment. But according to Giant impact hypothesis, the existence of a magma ocean has never actually been proved at all! Unfortunately, the answer here is that it just isn't known yet. Wnt (talk) 07:55, 13 February 2011 (UTC)

Borosilicate glass

r light bulbs Borosilicate glass — Preceding unsigned comment added by Tomjohnson357 (talk • contribs) 05:15, 12 February 2011 (UTC)

If you're talking about ordinary incandescent lamps, then the outer envelope can be soft soda glass, which is cheaper to make and form. However, the inner glass stem that the metal supports are fixed to, are of borosilicate due to its low coefficient of expansion. Other lamps that run very hot, like vapour discharge lamps, are mostly borosilicate glass throughout.--Aspro (talk) 10:55, 12 February 2011 (UTC)

[1] this book says the same. The halogen lamps are made from high silica glass or aluminosilicate “hard” glass.--Stone (talk) 10:59, 12 February 2011 (UTC)

capacitance

4 μF capacitor is charged by 250V.Find energy stored in it.It is connected in parallel combination to an uncharged 3μF capacitor. Find energy stored. —Preceding unsigned comment added by 180.215.44.196 (talk) 07:41, 12 February 2011 (UTC)

Welcome to Wikipedia. Your question appears to be a homework question. I apologize if this is a misinterpretation, but it is our policy here not to do people's homework for them, but to merely aid them in doing it themselves. Letting someone else do your homework does not help you learn nearly as much as doing it yourself. Please attempt to solve the problem or answer the question yourself first. If you need help with a specific part of your homework, feel free to tell us where you are stuck and ask for help. If you need help grasping the concept of a problem, by all means let us know.

Have you read our Farad article? It gives various equations that relate the charge, voltage, and stored energy to the capacitance. For the first question, use the appropriate equation directly. For the second question, the first capacitor will discharge (dropping its voltage) into the second until the voltage on both is the same; no charge is lost. Find the initial charge on the first capacitor, then find the corresponding volatage on the combined system. CS Miller (talk) 10:20, 12 February 2011 (UTC)

That sounds an awful lot like my homework from last semester. I already had the pleasure of doing it, I don't want to rob you of the same experience. Dismas|^(talk) 10:22, 12 February 2011 (UTC)

Radio activity

A radio active material at a given instant emit 3000 particles per minute,10 minute later it emits 1500 particles per minute. find decay constant and half life. —Preceding unsigned comment added by 180.215.44.196 (talk) 07:45, 12 February 2011 (UTC)

We avoid homework questions as if they were radioactive. Clarityfiend (talk) 07:55, 12 February 2011 (UTC)

I suggest you read decay constant and half life.--Shantavira|^{feed me} 08:36, 12 February 2011 (UTC)

Legal chemistry

Is there and article regarding legal chemistry?--Email4mobile (talk) 10:35, 12 February 2011 (UTC)

Probably forensic chemistry. Someguy1221 (talk) 10:39, 12 February 2011 (UTC)

Or forensic toxicology, as the original use of the term 'legal chemistry' was all about poisoning. Mikenorton (talk) 10:43, 12 February 2011 (UTC)

Thanks a lot. I will add Arabic interwiki to that articles.--Email4mobile (talk) 11:01, 12 February 2011 (UTC)

Changing the length of a second/minute/hour/day/year

The recent leap day related questions left me with a question that I don't think has been addressed. Has the idea of changing the official length of a day (or whatever would be necessary) so that we don't need as many/any leap days been seriously proposed? Or are we at an optimum now? Dismas|^(talk) 10:44, 12 February 2011 (UTC)

Well, if you change the length of the day, so that the year has exactly 365 days, then noon will be shifted from 12:00 to almost 18:00 within a year - I don't think that's acceptable. Icek (talk) 11:11, 12 February 2011 (UTC)

Exactly. The fundamental problem is that the length of the tropical year – essentially, the length of one full cycle of seasons – just isn't an integer multiple of the solar day. If we insist that our timekeeping system stays in sync with the sun (12:00 noon is always in the middle of the daytime) and with the seasons (the winter solstice will always fall in late December), then these little corrections are unavoidable. TenOfAllTrades(talk) 15:35, 12 February 2011 (UTC)

Proposal to abolish leap seconds have been put forward by people who don't understand the issue properly. I think the current arrangement will survive for sometime yet and is as good as is reasonable and workable. --Aspro (talk) 11:12, 12 February 2011 (UTC)

The second is an arbitrary human unit, unlike the solar day and tropical year, though in practice it's too entrenched to be redefined. As I software engineer I don't like the current system of random leap seconds announced six months in advance. It's akin to announcing leap years on March 1 of the previous year. I'd much prefer a predefined schedule. The schedule doesn't have to be "no leap seconds ever", it just has to be predictable. -- BenRG (talk) 18:18, 12 February 2011 (UTC)

That's why things are as good as can be expected . The change in the Earth's rotation can't be modelled very well yet based on our current understanding. Even changing the way a second is defined will not get around this fact. To expect more at present is just wishful thinking. --Aspro (talk) 18:47, 12 February 2011 (UTC)

The problem is that the need for leap seconds is inherently unpredictable; the rotation rate of the Earth just isn't regular enough for us to predict the need for a leap second well in advance. Between 1988 and 1998, there were eight leap seconds added; between 1998 and 2008, there was just one: File:Leapsecond.ut1-utc.svg. In principle, if one were willing to tolerate an increased drift in UTC versus mean solar time, one could implement a longer period between leap second adjustments or allow a larger deviation to accumulate before corrections were applied — but it still wouldn't prevent those adjustments from being at 'random' intervals. TenOfAllTrades(talk) 19:00, 12 February 2011 (UTC)

A perfectly workable solution would be simply to abolish leap seconds and let UTC float with respect to the Earth's rotation. The offsets between local mean solar time and civil time that we already accept as unproblematic everyday consequences of zonal time are far larger than the UTC drift that could accumulate until several centuries pass. And once those centuries do pass, countries can simply choose to switch to another UTC offset for their civil timekeeping. Many countries implement such switches twice per year already, and modify their rules for these changes relatively frequently. Simply skipping one of the diurnal switches in order to move one timezone east or west will be a fairly routine exercise, compared to the practical benefits a predictable "universal time" scale would bring.

Alternatively we could start defining civil time everywhere in terms of hourly offsets from TAI and abolish the entire concept of a separate UTC scale. This would be just as workable in the long term, but the half minute of TAI-UTC difference that has already accumulated would make the switchover impossibly chaotic as a practical matter, and for no real benefit compared to a floating UTC. –Henning Makholm (talk) 02:27, 13 February 2011 (UTC)

Floating universal time (GMT) was abandoned because it became impractical and got replaced by UTC. Satellite navigation may remove some of the current dependence on this, but one still has the problem of how to constantly synchronise all the independent time pieces at odd times throughout the year which may actually require more disruption than the present system. Transaction in commerce are completed and logged in milliseconds. Television transmission networks which are slightly out of sync falter. However, the suggestion to use ITA with offsets, would not only work, but is what is done now, but to let ITA loose and let it drift away as civil time and away from the astronomical time datum, would make future interpretation of data (especial astronomical data) more difficult. All the pros and cons added together leave us with what we have now, as the best compromise, and so we wont start going backwards on the basis of spurious arguments. --Aspro (talk) 12:38, 13 February 2011 (UTC)

Um, GMT does not float; on the contrary it is explicitly tied to the Eath's rotation by definition. In a sense, what is done now is to use ITA with offsets, except that those change unpredictably, leaving gaps and overlaps in the otherwise continuous time scale. Sure, astronomers need to keep accurate track of the Earth's rotation for their own particular reasons -- they shall be welcome to keep historical records about that so they can interpret their observations in the future -- but that is not a good reason to shackle all of the rest of humanity to an unpredictable, non-continuous, non-arithmetic civil timekeeping scale. –Henning Makholm (talk) 16:34, 13 February 2011 (UTC)

You say: Um, GMT does not float; on the contrary it is explicitly tied to the Eath's rotation. Please read Non sequitur (logic). See image . I don't mind explaining things but please don't come back with absolute tosh!--Aspro (talk) 18:16, 13 February 2011 (UTC)

A "floating" time standard is one that isn't tied to the Earth's rotation. -- BenRG (talk) 22:45, 13 February 2011 (UTC)

The question is, if you add $\lfloor {\tfrac {1}{2}}a(t-t_{o}+86400s)^{2}\rfloor -\lfloor {\tfrac {1}{2}}a(t-t_{o})^{2}\rfloor$ seconds to the end of every day, where t_o is circa 1820 and a ≈ 0.0014/day/century, what's the expected maximum drift from solar time over, say, the next thousand years? I don't know the answer, but if it's, say, ten minutes, then I think it would be a huge improvement over what we've got now. I'm pretty sure the number of people who don't need to correct for the current 1-second drift but would need to correct for a ten-minute drift is small compared to the number of people inconvenienced by the current system. For heaven's sake, we tolerate a 0.3% variation in calendar year length and a whopping 10% variation in calendar month length. Astronomers have to correct for those too. -- BenRG (talk) 22:45, 13 February 2011 (UTC)

The short term fluctuations are random and 1820 was the nadir of a parabola, which means the drift does not follow a liner increase - so that doesn't work. Recalculating every day or other regular period and adjusting, goes against every other method of standardization so that's not on either. --Aspro (talk) 19:55, 14 February 2011 (UTC)

I know it's a parabola; that's why there's a ² in the formula. I agree that it's unlikely to be adopted, though weirder things have happened. -- BenRG (talk) 21:27, 14 February 2011 (UTC)

Not to mention that we tolerate a ±30 minute deviation from solar time just so that our time zones can change on one-hour boundaries rather than one-minute or one-second. Why is one-second accuracy in UTC so important? -- BenRG (talk) 23:07, 13 February 2011 (UTC)

It is not about looking out the window a noticing anything different. The current time systems have evolved as technology has advanced to benefit from evermore sophisticated time standards. Leap seconds are one of those steps to bridge the gaps until the next advance is ripe for adoption. Abandoning them because some people don't care about the problems it will cause in multiple other places is not the way to go. For computer systems the Network Time Protocol adjusts for leap seconds automatically. Systems for which this approach is unsuitable, can stick with ATI as standard time reference.--Aspro (talk) 19:55, 14 February 2011 (UTC)

I'm aware of the existence of NTP and TAI. Other than that, I can't make any sense of your reply. Isn't "the next advance" what this crazy subthread is about? -- BenRG (talk) 21:27, 14 February 2011 (UTC)

Did this current system come about over-night? No, it didn't! From the time when the ancient Egyptians just counted the days and ignored the astronomical year, the advancement of the measurement of time has advanced stepwise. The introduction of leap seconds is just one of those many steps. At each step, some individuals have objected, as it has forced them to adopt new ways of doing things. I don't think there is any point in trying to explain this in simpler terms as you appear to be not trying to understand. A trait that others, who object to leap seconds, seem to share IMHO.And which you appear to be now confirming.--Aspro (talk) 21:54, 14 February 2011 (UTC)

±30 minutes is only the minimum necessary deviation. In practice, there are places that use civil timescales as much as 90 minutes from local mean solar, such as Galicia (Spain) (150 minutes in the summer), Iceland or Western Sudan. –Henning Makholm (talk) 00:34, 14 February 2011 (UTC)

Physics Entries in General

I am new to wikipedia. I am writing a book on evolution and my areas of expertise are biology, animal behavior and evolution. I am however writing an introductory chapter on atoms in order to define where life does not appear to exist ( in order to later identify the first molecules which could be considered to have "life"). As I have been reviewing the elementary particles such as quarks and gluons, etc I have found a very disturbing trend to not clearly define right up front which aspects of particles and theories have been clearly "proven" and observed and which concepts are theoretical in nature. I feel that even as not being a physics specialist, that this concept regarding scientific theories is extremely basic. I added some comments to the gluon page in this regard and they were removed. I have no wish to "soapbox" about this concept. But I have noticed for physics based articles, often the pages will immediately start going off on very mathematic based formulas without even stating whether there is any proof of the particle or concept. In biology I can talk about tree bark and vascular systems and photosynthesis and we can go in and very soundly prove and display and observe the science right down to the molecules which house the chemical reactions from sunlight to sugars. But the particle folks go into arcane mathematics which is almost religious in nature without ever clearly stating whether for example particle spin (1/2 or full integer) is theorized or has actually been observed. It is like smoke and mirrors with math being the mystical gate. In other words "regular" people cant consider the accuracy of these theories as they immediately made opaque by the insertion of mysterious terms and equations. I think this concept is very basic and extremely unscientific. How does one taker on these articles in order to demand very basic descriptions of what exactly is proven and what exactly is theoretical up front? Thanks so much for your time. — Preceding unsigned comment added by Maplelanefarm (talk • contribs) 14:47, 12 February 2011 (UTC)

Again..thanks for your time...I think this is a huge issue with most of the particle pages. — Preceding unsigned comment added by Maplelanefarm (talk • contribs) 14:49, 12 February 2011 (UTC)

Indeed, you do need to know mysterious terms and equations to make sense of the subatomic world. There isn't much of a way around that. There are experiments, say, that show that spin certainly exists. But even interpreting what is going on in the experiment requires you to buy into a lot of complicated maths. Most of the standard model is "proven" (as far as that goes) pretty definitively. The stuff that is wholly "theoretical" lies just on the edges — e.g., quantum gravity, string theory, etc. The presence of math does not indicate that something is not experimentally verified. As just an indication, the classic experiment proving the existence of spin is the Stern–Gerlach experiment. It doesn't rely on heavy maths, but it still requires quite a deep theoretical understanding to make sense of what is going on. I'm inclined to think this is an artifact of the physics, not so much Wikipedia, though I do agree that the high levels of technical jargon in such articles make them pretty impenetrable. --Mr.98 (talk) 15:15, 12 February 2011 (UTC)

Why do you say you can "prove...and observe the science right down to the molecules..."? The tools you use to do that such as electron microscopes and mass spectrometers all rely on physics concepts like particle spin and electron orbitals. To properly complete your "proof" you will need these concepts too. Franamax (talk) 15:56, 12 February 2011 (UTC)

There is no royal road to physics. If you really want to understand the subatomic world you will need to learn some math along with it. Said that, I think you do have a point that many of those concepts can be partially explained in plain language but that s not a particularly easy thing to do. Plain language is just inadequate for physics concepts and that is why we use math to begin with. Also, there isn't a bright line separating "known facts" from "theoretical constructs" since the theories are an indispensable component of how physics is understood. Dauto (talk) 16:22, 12 February 2011 (UTC)

consider the first sentence of gluon : "Gluons are elementary particles ..." vs tachyon "A tachyon is a hypothetical subatomic particle ..." and magnetic monopole "A magnetic monopole is a hypothetical ...". Probably, if the article does not mention "hypothetical" in the first few sentences, you may consider it "proven". I suspeci it is the same in biology, where the articles about real animals will not mention they are real, while articles about mythical animals mention they are mythical. 83.134.173.228 (talk) 16:52, 12 February 2011 (UTC)

You should not be surprised that your comments were removed since you added them to the articles themselves. Every article has a talk page associated with it that should be used for comments like yours. Dauto (talk) 17:12, 12 February 2011 (UTC)

Here is a quote from the lead of quark's article:

"Quarks were introduced as parts of an ordering scheme for hadrons, and there was little evidence for their physical existence until deep inelastic scattering experiments at SLAC in 1968.[6][7] All six flavors of quark have since been observed in accelerator experiments; the top quark, first observed at Fermilab in 1995, was the last to be discovered.[5]"

Makes me wonder what are you complaining about?

Did you even read the lead?

Dauto (talk) 19:14, 12 February 2011 (UTC)

It's worth remembering that all serious extensions of the Standard Model (SM) are equally compatible with the experimental facts. There are no known facts that completely rule out supersymmetry or technicolor and yet there is no experimental evidence that we should accept them over the plain old SM. How would you say one is "proven" and the other not? Certainly neither is falsified. 129.234.53.49 (talk) 18:57, 12 February 2011 (UTC)

All of the standard model particles except for the Higgs boson have been experimentally observed. None of the additional particles required by either supersymmetry or technicolor have been experimentally observed. That's the difference. Dauto (talk) 19:09, 12 February 2011 (UTC)

Can you explain what you mean by "experimentally observed"? As far as I'm aware, an extension of the SM with appropriately broken SUSY would have the same low-energy scattering signatures as the SM itself. The fact that at some point the two models would lead to different pheneomena doesn't mean that there is any more experimental evidence to support one over the other. 129.234.53.49 (talk) 20:26, 12 February 2011 (UTC)

I'm not saying SUSY is wrong. But there is no indisputable experimental evidence that it is right either. It is still a hypothesis. For instance, there is no experimental evidence that the selectron actually exists .On the other hand, there is plenty of experimental evidence that the particles of the standard model actually exist. That is a huge difference. Of course that doesn't mean that SUSY is wrong. Dauto (talk) 20:40, 12 February 2011 (UTC)

Well I'm not trying to defend or refute SUSY, that was just an example. My point is that I don't think there is such a thing as "experimental evidence that [some particles] exist". You don't observe proof that something exists, you see experimental results which agree (or disagree) with your theory. Any reasonable extension of the SM will agree with the data gathered so far just as well as the SM does. They are both hypotheses, which are currently not falsified; all current support for the SM is also support for the MSSM and many other theories too. 129.234.53.49 (talk) 23:02, 12 February 2011 (UTC)

Yes, I understand what you're saying but I think the point you're making is a bit disingenuous. The SM is consistent with the data gathered so far because we have gazillions of experiments confirming it. SUSY is consistent with the experiment because we have gazillions of experiments that neither confirm nor refute it. It is not the same thing. There is a difference between being confirmed by experiment and being consistent with experiment. Dauto (talk) 06:02, 13 February 2011 (UTC)

It looks like we may have to agree to differ here. I simply disagree with your last statement; I just can't see how "confirmation" by experiment would work. 129.234.53.49 (talk) 22:35, 13 February 2011 (UTC)

The OP is writing a book, and complaining that Wikipedia's content makes it difficult to evaluate scientific facts. If the original poster is writing a serious book, they should consider hiring a subject-matter expert to review any scientific writing. You may be able to find a local scientist or professor who can contract to review your work for scientific accuracy. You can also contract to various "review shops" on the internet. Physics is difficult; and presenting it correctly and accurately is a challenge. But if any content you wrote were ever challenged, it would be far better to say "...but the content was reviewed for scientific accuracy by Professor So-and-So at Podunk University," rather than "...the content was based on my own interpretation of Wikipedia." A more serious book should be reviewed by somebody with more serious credentials. Physics is an ultra-precise subject, and subtle variations that you may use when wording your presentation of a topic can mean the difference between "completely validated by scientific experiment" and "completely wrong." This applies even to the classical and well-established (non-mind-bending) areas of physics, like Newtonian descriptions of force and energy. "Regular people" can evaluate anything they want for accuracy - but if "regular people" are disinclined from being extremely precise in their thought-process, they will never be able to evaluate physics - no matter how simple and straightforward any presentation may be. Nimur (talk) 20:47, 12 February 2011 (UTC)

I'm not sure whether I'm on the right track here, but the OP seems particularly concerned about counterintuitive notions like spin. The article there explains that spin does not obey the usual rules we expect spinning objects to, so it isn't the same sort of thing that a spinning beach ball has, but perhaps our articles don't make these things terribly explicit. As I understand it, spin is best seen as a mathematical idea that works in quantum mechanics in a way similar to how orbital momentum works for beach balls. The resemblance could stop there, if it should happen to violate intuition, or it could go further, though I myself do not know any more about it. To say that an electron has spin 1/2 is impossible to picture, but it simply means that when you do the math, you find the electron takes a 720 degree rotation (or something mathematically equivalent) in order to reach a state isomorphic to its original one. To say further that it has been demonstrated experimentally only means that we have so much evidence that we have got the mathematics right that we can teach it at undergraduate level without fear of embarrassment. Someone might come up with some deeper mathematics, or a better verbal description, later, but for now we're doing alright. Can a physicist please tell me if I've got this right, and can the OP perhaps tell me if it helps any? It's been emotional (talk) 16:56, 13 February 2011 (UTC)

I certainly agree that Wikipedia has an ongoing problem with math and science articles written by experts, for experts, as opposed to the general population. The sticking point in simplifying such articles is always that the simplified model is not quite as correct. For example, all Newtonian physics is technically "wrong", since it doesn't account for time dilation, etc., and other effects of relativity. So, you can't say that a train going 10 km/hr will travel 10 km in an hour, if any physics PhDs are around, as it all depends on the locations of the observers, etc. StuRat (talk) 04:26, 14 February 2011 (UTC)

Salaries

what is the salary of an engineer working for government after passing IES (Indian Engineering Services exam)? —Preceding unsigned comment added by 1.23.10.106 (talk) 15:13, 12 February 2011 (UTC)

Determining the salary of an entry-level engineer in a government position (I presume it's India that your enquiry is based in) is difficult, as salaries can change from year to year. Additionally, public-sector roles are reorganised regularly, so the job itself may change. Perhaps this article on the Indian Engineering Service, or this link to the Union Public Service Commission of India, might be of use to you. Malusmoriendumest (talk) 08:11, 13 February 2011 (UTC)

Relationship between island size and its freshwater supply

Consider a hypothetical island somewhere in the subtropics. If you want to put a group of say 20 people on the island year-round and want them to be self-sufficient in terms of freshwater needs, how big does the island need to be to have the needed freshwater supply? I know the question is not precisely answerable, but I'd like to see some estimates that has some rational basis. Thanks. --173.49.82.30 (talk) 19:06, 12 February 2011 (UTC)

"Subtropical" describes a zone of temperature not rain fall, so there is no answer to you question. You need to first establish the total water requirements for a 'typical' person multiply by total number of people etc. Err.. just curious: Why do you want to know this – have you discovered the World comes to an end next year and are looking for a bolt hole? --Aspro (talk) 19:26, 12 February 2011 (UTC)

Maybe the OP meant tropical? Near the equator rainfall is regular for most of the year. Here twenty people would not need a large island to meet their water needs assuming they had reasonable equipment for capturing rain water. I can't estimate any kind of specific size, but I think it would suffice to say that in the tropics, water is unlikely to be an issue before food or building materials. --Daniel 19:38, 12 February 2011 (UTC)

Just to add to my earlier thoughts, very small tropical islands usually don't have any naturally occurring fresh water. Rain would be the only source so collection and storage would be very important. --Daniel 19:49, 12 February 2011 (UTC)

The questioner can use the Hawaiian Islands as an example. Size is not as important as elevation. Simply look at Oahu. One side of the island has regular rainfall. The other does not. It is due to the mountains. The wind predominantly blows from the same side. The mountains force the moist wind upward where it cools. Rain forms. For more on this, see orographic precipitation. -- kainaw™ 20:17, 12 February 2011 (UTC)

Another example is Yakushima in Japan. It's relatively small, but also has a very high peak, so clouds bump into the island all the time and it rains almost every day. TomorrowTime (talk) 01:30, 13 February 2011 (UTC)

The most straightforward way to evaluate this is to calculate the size of the drainage basin (which only depends on topography), and average precipitation rate for the drainage basin (which can be easily measured). Excluding ground permeability (that is - water that goes down into the ground, instead of flowing on the surface), this will approximately give you the net surface water outflow rate. This subject can be very complicated - specialists in environmental engineering and water quality management spend years trying to accurately model the water cycle for specific regions.

The next thing to worry about is the nontrivial effect of water-pollution on a very small island. Will humans effectively manage their sewage to guarantee that it doesn't contaminate their drinking water? If so, you need to calculate how much water that would require. Will sewage from livestock, domesticated animals, or wildlife contribute to water-pollution?

Finally, will the humans have any technology (even primitive filtration systems) to clean up polluted or microbe-infested water? If so, they can survive on much smaller quantities of water; otherwise, you will need "fast-moving" streams or rivers - this necessarily requires more water throughput. Nimur (talk) 20:54, 12 February 2011 (UTC)

Freshwater aquifers are important to consider for island water supplies. Smaller and flatter islands will probably have more saltwater intrusion. ~AH1^(TCU) 01:07, 13 February 2011 (UTC)

Odd One out of n Resistors

Hello. There are 10 resistors, nine of which are 1 Ω each. How do I identify the odd resistor and its resistance in the least number of ohmmeter readings? I am allowed zero-resistance wires but no batteries. Thanks in advance. --Mayfare (talk) 21:20, 12 February 2011 (UTC)

Note that the ohmmeter has a battery in it, but I expect that you are allowed to use it. Then, just divide and conquer. If you divide them into two groups of 5 resistors and connect them in series, each should read 5 ohms unless one of the piles has a resistor that is not 1 ohm. Once you identify the pile with the odd resistor, do the same. Divide into two piles and connect all the resistors in the pile in series. Continue until you have only 2 resistors. -- kainaw™ 21:25, 12 February 2011 (UTC)

I think there may be a more efficient way to do it. At the moment this is just a hunch, because I haven't worked out all the details or verified that it actually works, but suppose you do this: First, number all the resistors, so you can distinguish them. Connect the first two resistors in parallel, and then connect that network in series with the third resistor, and then connect that whole thing in parallel to the fourth resistor, and so on, and then measure the equivalent resistance of the resulting network. You can work out what the equivalent resistance should be if all the resistors were 1 ohm; the reading you get will at least tell you whether the odd resistor is higher or lower than 1 ohm. Additionally, you can figure out things like, "If resistor 1 is the odd resistor, then its resistance must be _____; if resistor 2 is the odd resistor, then its resistance must be _____; etc." Now connect the resistors together in a different configuration (I'm being vague here, because I don't know for sure what this configuration should be), and measure the resistance again, and recompute "If resistor 1 is the odd resistor, then its resistance must be _____, etc." Perhaps it is possible to design the two configurations of resistors in such a way that, for any pair of ohmmeter readings, there is only one consistent possibility. If so, then you've solved the problem with just two ohmmeter readings. The details of this idea are left as an exercise to the reader. :-) —Bkell (talk) 21:40, 12 February 2011 (UTC)

A guess: Perhaps the second configuration should be like the first, except that you connect resistors 9 and 10 in parallel, and then connect that network in series with resistor 8, and then connect that network in parallel with resistor 7, and so on. Will that do the trick? It seems plausible to me. —Bkell (talk) 21:49, 12 February 2011 (UTC)

This is a pretty common mind-game or final-exam-question for circuit-design and related EE courses:) Divide-and-conquor is the minimum "student thought about it" answer, based on powers-of-two. There are a million variations/applications of this approach ("how many yes/no questions do you need to ask to determine which playing card someone has picked", etc.). The parallel/series-combinations approach is the really clever solution, solvable in a single measurement for certain numbers of resistors. The question is all over google (both actual science/hobbyist sites and "just tell me the damn answer to my homework question" boardd), but the parallel/series way is only on some of their answers:) DMacks (talk) 21:57, 12 February 2011 (UTC)

Can you really solve it in a single measurement if you don't know the resistance of the odd resistor? I thought about that for a while, and I convinced myself that at least some measurements of the equivalent resistance of the series-parallel network could not lead to a unique conclusion. In particular, if one of the two "innermost" resistors is the odd one out, you have no way of telling which one. —Bkell (talk) 22:02, 12 February 2011 (UTC)

Yes, I'm pretty sure 2 is the best for worst-case scenario or if you need to figure out if the odd one is high vs low. But it can be solved in 1 depending on where the odd one winds up. For example,$-{\begin{bmatrix}-R1-R2-\\-R3-\end{bmatrix}}-$ solves in 2 if the odd one is R1 or R2 as you say, but solves in 1 and even know what the odd one's resistance actually is if R3 is the odd one. Is 4 completely knowable (which R#, what value) in$-{\begin{bmatrix}-{\begin{bmatrix}-R1-R2-\\-R3-\end{bmatrix}}-\\-R4-\end{bmatrix}}-$ by a single measurement unless the odd one is R1 or R2? Is this the start of a pattern where there is always only that one pair that cannot be distinguished by single measurement? DMacks (talk) 22:30, 12 February 2011 (UTC)

Hmm, let's consider that three-resistor network. I agree that for some values of R3, a single measurement is enough to identify it. But if R3 is 1.2 ohms, for example, you will get an equivalent resistance of 0.75 ohm, which is the same equivalent resistance you'd get if R1 was the odd resistor, having a resistance of 2 ohms. So I still think, even if you get lucky and R3 is the odd resistor, you'll sometimes need at least two measurements. —Bkell (talk) 22:54, 12 February 2011 (UTC)

Oooh good point! This is all based on vague recollections of working it out years ago, maybe I was assuming it was known if the problem stated if the odd one was high or low. DMacks (talk) 23:08, 12 February 2011 (UTC)

Well, in the counterexample I just showed, both possible cases have a "high" resistor: the first case has R3 too high at 1.2 ohms, and the second case has R1 too high at 2 ohms. So even knowing whether the odd resistor is too high or too low isn't enough information. —Bkell (talk) 23:18, 12 February 2011 (UTC)

And in your four-resistor network, R3 is in parallel with R4, so if one of those was the odd resistor you couldn't tell them apart. My guess is that you meant to put R4 in series with the three-resistor network. —Bkell (talk) 22:59, 12 February 2011 (UTC)

-R4- is outside the R1/R2/R3 bracket (parallel to the whole 3-resistor circuit. Lemme try again with longer leads...$-{\begin{bmatrix}-{\begin{bmatrix}-R1-R2-\\--R3--\end{bmatrix}}-\\---R4---\end{bmatrix}}-$(wikipedia's TeX doesn't seem to support boxes). DMacks (talk) 23:08, 12 February 2011 (UTC)

Yes, I understand where R4 is. It is still in parallel with R3. The network as a whole has three branches in parallel: one branch consists of R1+R2, the second consists of R3, and the third consists of R4. The network you have drawn is electrically equivalent to $-{\begin{bmatrix}-R1-R2-\\--R3--\\--R4--\end{bmatrix}}-$. The resistors R3 and R4 are perfectly symmetric in this network. I think what you want to draw is $-{\begin{bmatrix}-R1-R2-\\--R3--\end{bmatrix}}-R4-$, where R4 is in series with the three-resistor network. —Bkell (talk) 23:14, 12 February 2011 (UTC)

Ah yeah. There was some pattern for extending each next resistor. And there was also definitely some more specific parameter on the odd-one, since even now you could swap in any of them and get the same result by different choices of the odd-one's value. I'm gonna quit trying to work out the details, been too long since I've forgotten the cleverness:( DMacks (talk) 04:04, 13 February 2011 (UTC)

No two resistors in the network are interchangeable, except for two pairs at the bottom. If all the voltages bridged by the resistors are different, the effect of changing one should be different... I think.

I'm thinking that the circuit I diagram at right should work, provided that you know the unknown resistance, and you're not so unlucky as to put it in one of the spots I failed to make unique (is there a better way to do it? I always end up with some odd resistors out. But I think that if you scramble the resistors and measure twice it should work. But the circuit I draw does bring back fearful memories of the dreaded Wheatstone Bridge from bygone days... and for this to work I'd have to figure out V2, V3, V4, V5... Sigh, let's see, Kirchhoff's circuit laws... Wnt (talk) 06:58, 13 February 2011 (UTC)

Good God, but I'm coming up with stuff like "v5 = (-v1/R1 - v1/R2 +v2/R4 +v2/R1 +v2/R3 +v3/R5+v3/R2+v3/R6-v4/R3)/(1/R4 + 1/R5)". And I still have v4, v3, v2, and v1 to solve for. Wnt (talk) 07:40, 13 February 2011 (UTC)

The kind of network I was suggesting is shown to the right. It is built by connecting resistors D and E in parallel, and then connecting C to that network in series, and then connecting B to that network in parallel, and then connecting A to that network in series; it is built up one resistor at a time by alternating series connections and parallel connections. I believe that no two resistors here are interchangeable except the two innermost ones (D and E). This kind of network has the advantage of being series-parallel, unlike the "grid" of resistors Wnt is building; so you can analyze it using just simple series and parallel reductions, rather than having to perform Y-Δ transforms. I conjecture (though I haven't proved this) that if you build a second network like this, but connect the resistors in the reverse order (swap A and E, swap B and D, C stays where it is) then the measurements of the equivalent resistances of the two networks will identify the odd resistor and its value. —Bkell (talk) 08:02, 13 February 2011 (UTC)

Wouldn't you need 3 readings in the worst case? Say you take 1 reading for the first five resistors. But those resistors are identical. You take another 2 readings to find the odd resistor in the other set of five resistors. I may be confused. --Mayfare (talk) 23:20, 13 February 2011 (UTC)

(To Mayfare:) Oh, yeah, sorry for the confusion. The picture there is only for five resistors; that's only because I had already drawn that picture for a paper I wrote several years ago, so it was easy to just upload what I had rather than drawing a new one. So my explanation was for a puzzle where you just have five resistors. But the idea extends to any number of resistors. If you have ten resistors in all, then just continue that construction: add the next resistor in parallel with these five, and then the sixth resistor in series with that, and so on, adding each resistor one at a time and alternating series and parallel connections. Then find the equivalent resistance of that whole 10-resistor network. Now build the network again, but use the resistors in the reverse order (so the "innermost" resistors from the first network become the "outermost" resistors in the new network, and vice versa), and measure the equivalent resistance of that network. My conjecture is that these two measurements will uniquely identify the odd resistor and its value (ignoring the issues about measurement errors and resistor tolerances that Gr8xoz raises below—I'm assuming that since we are allowed to use zero-resistance wires we also have a perfect ohmmeter and perfect one-ohm resistors). —Bkell (talk) 00:22, 14 February 2011 (UTC)

This method is very sensitive to measurement errors and errors in the resistance of the nine resistors that are suposed to be 1 ohm. 10 resistors connected in this meaner where one of the innermost resistors have a resistance of 2 ohm and the rest are 1 ohm have an total resistance of 0.618055 ohm while if it is the third innermost resistor the resistance becomes 0.618320 ohm. With normal ohm meters and resistors this is easily masked by the errors in measurement and the nine "1 ohm" resistors. I have not analysed the "grid" suggested by Wnt but I would expect it to have similar problems. In order to answer the question we need to know the measurement error, the allowed errors in the resistances and the minimum allowed difference between the odd resistance and 1 ohm. Gr8xoz (talk) 10:57, 13 February 2011 (UTC)

Now you are trying to turn a useless theoretical exercise into something practical. Why would anybody want to do that? Dauto (talk) 15:26, 13 February 2011 (UTC)