Wikipedia:Reference desk/Archives/Science/2008 May 17

Science desk
< May 16	<< Apr | May | Jun >>	May 18 >

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.

May 17

fossil fuel

How is coal and oil formed and what is the difference —Preceding unsigned comment added by 203.102.161.75 (talk) 00:45, 17 May 2008 (UTC)

I suggest you look at the articles on fossil fuel, oil and coal. -mattbuck (Talk) 01:02, 17 May 2008 (UTC)

When researching oil as a fossil fuel, our article on petroleum may be more useful in this context. Nimur (talk) 23:57, 17 May 2008 (UTC)

DNA in multiregional model of human origins

As a layman, I don't understand how any species can emerge in more than one place or zone of places. My question is: under the multiregional model, how did genetic mutations become dominant traits in many different places, leading to speciation at different times and places? Are they suggesting that proto-members of the antecedent species had dormant DNA mutations, which in many different places and times became more successful because of similar environmental pressures? And that after the dormant DNA carried by the antecedent species spread out, it eventually flowered in different places in the same way? While still enabling successful breeding between members of the different groups constituting the new species, when eventually the geographically diverse people crossed paths again?

I guess the broader question is simply: Can any species emerge at different times and in widely-spaced locations? —Preceding unsigned comment added by 4.182.36.205 (talk) 04:59, 17 May 2008 (UTC)

Have you read Multiregional origin of modern humans? That details some of the different proposed mechanisms behind the multi-region hypotheses. Note that DNA isn't "dormant" per se, but genetic differences may not provide selective advantages until they "come into contact" the the right environment. If two geographically separated populations have the same source mutations and are exposed to a similar environment, they may Convergently evolve certain characteristics, but not all (since genetic mutation is an essentially random even). Its proposed that this happened to hominid species, who eventually met again to create the hybrid-origin of modern humans. Rockpocket 07:32, 17 May 2008 (UTC)

TY for this.

yes, I've read the source you refer to. Multiregional origin of modern humans

you wrote: "DNA isn't "dormant" per se, but genetic differences may not provide selective advantages until they "come into contact" the the right environment."

I meant to imply that point, in my badly written question. (i.e., I understand that.)

I'll read "convergent evolution."

It still seems like special pleading to me. —Preceding unsigned comment added by 4.182.36.205 (talk) 07:52, 17 May 2008 (UTC)

Convergent evolution, as I read it, refers to traits, not species -- of course, highly similar traits may evolve in widely divergent places and times ... but different species? I don't see how it makes sense. —Preceding unsigned comment added by 4.182.36.205 (talk) 07:56, 17 May 2008 (UTC)

and, WADR (with all due respect!), the article titled "hybrid-origin" is ridiculous and based on the fanciful notions of one S. Gooch, supported by no evidence ... —Preceding unsigned comment added by 4.182.36.205 (talk) 08:57, 17 May 2008 (UTC)

Convergent evolution does refer to traits, but speciation is the result of an accumulation of traits. Of course, the probability that multiple traits would convergently evolve gets smaller with each additional trait. However the hybrid origin doesn't require that exactly the same traits (and hence species) evolved independently, just that what traits did evolve were similar enough to permit interbreeding. The likelihood of that happening is open to debate, but it could explain some of the idiosyncrasies of the human genome, and other divergent species have been able to reproduce, so why not different hominids? Rockpocket 06:20, 20 May 2008 (UTC)

How Geology & Material Science are related?

Other than Crystallography, How Geology pictures in in Material science?59.95.69.97 (talk) 09:35, 17 May 2008 (UTC)

Also bulk properties of rocks or the crust can be addressed with material science concepts. On the applications side, many materials are derived from geological sources, such as quarrying, or mining followed by refining or smelting. Primary metallurgy studies this. Graeme Bartlett (talk) 12:09, 17 May 2008 (UTC)

Television screens

Is there some medical disadvantage in watching an old tv (with cathode ray tube) as compared with a new flat screen tv? MilkFloat 09:48, 17 May 2008 (UTC)

Our article lists a few (fairly minor) health concerns related to CRT displays. LCD doesn't cover health, unfortunately, and I can't speak for that. Algebraist 12:50, 17 May 2008 (UTC)

The short answer is "No". There are no significant electromagnetic emissions from LCD flat panels (apart from the heat and light they emit of course!) - but I'm not so sure about plasma displays. Certainly any effects are going to be exceedingly minor. There are measurable radiation outputs from CRT's - but if there were measurable health risks from CRT's, we'd know about it. Studies of people who use CRT's a lot in their jobs have shown health problems - but they mostly relate to posture and keyboard use - not from the displays themselves. Perhaps the most concern would be related to eyesight and eye-strain and that would depend on the relative sharpness and contrast ratio of the displays - on which grounds a good CRT might actually be better than a flatpanel display - but even that is pretty negligable. 66.137.234.217 (talk) 13:07, 17 May 2008 (UTC)

Furthermore, the OP is asking about TVs and not computer monitors. At normal TV viewing distances, the already minor hazards of CRTs become negligible. --Heron (talk) 13:23, 17 May 2008 (UTC)

Indeed - when you double the distance from something that's emitting any kind of radiation - you reduce the amount that reaches you by a factor of four - so if there is no problem with CRT's when you are sitting with your head a foot away from the screen then at ten feet, there is a hundred times less risk - so as negligable as the risk of sitting in front of a computer screen for 8 to 10 hours a day is, the risk for watching TV for less time at longer distances is VASTLY less. 66.137.234.217 (talk) 13:47, 17 May 2008 (UTC)

This simple equation (inverse square law) is only true with a point source, it's not true with a source that subtends a substantial angle (as does a CRT at practical viewing distances). Still, I'm just picking a nit here; soft X-rays aren't much of a risk from most modern CRTs and electromagnetic emssions from the deflection coils are likely no risk at all. On the other hand, throwing a brick through your CRT TV is probably a lot more hazardous than throwing a brick through a similary-sized LCD TV, so no matter how provoked you may be, don't throw a brick through your CRT TV.

Atlant (talk) 14:08, 19 May 2008 (UTC)

Atlant: I know about the restriction on the inverse square law but it is the gun at the back of the TV that radiates - it's pretty close to being a point source. The TV only appears to be an area source because the beam scans the faceplate - and even if there is some secondary radiation coming from the faceplate, it's a point source at any given instant - so inverse-square law definitely applies. 70.116.10.189 (talk) 22:40, 19 May 2008 (UTC)

No, it's not the cathode. It's the thing into which high-energy electrons smash that radiates X-rays and that's the screen-end of the tube (the shadow mask or aperture grill and the aluminized phosphor screen). That's the end of the 25 to 35 kilovolt acceleration chain; the electric fields around the cathodes are much lower and the electrons in the vicinity of the cathodes are, relatively speaking, just poking along. (In ancient times, the anode of the 3A3 high-voltage rectifier and 6BK4 shunt regulator vacuum tubes was another pretty-intense source of soft X-rays, but modern circuits have no such component.) But you don't have to take my word for it; go read our article about X-ray tubes.

Atlant (talk) 23:54, 19 May 2008 (UTC)

Photosensitive epilepsy would be less likely with an LCD display. But it is rather rare to begin with. Lisa4edit (talk) 05:37, 18 May 2008 (UTC)

Forest

How is forest a habitat?117.99.32.175 (talk) 10:29, 17 May 2008 (UTC)

I find it good to first make sure I understand the words being used. Our article, you guessed it, "Habitat" calls it "an ecological or environmental area that is inhabited by a particular species. It is the natural environment in which an organism lives, or the physical environment that surrounds (influences and is utilized by) a species population." What a bunch of crap. How are we supposed to understand all that jargon? What, pray, is a "species population"? In what universe does "surround" mean "influence and be utilized by"? Is "ecological area" a sensible juxtaposition of words at all? It gets worse, too, as the article continues, but I'll spare us that.

I suppose we'll have to resort to the ordinary meanings of the words involved and hope for the best. Forest can be a habitat for arboreal animals, who are specially adapted for life in the trees and can't get their food or evade their predators or nest away from them. Many epiphytic plants need trees to cling to. The trees themselves live in a forest habitat, I suppose. --Milkbreath (talk) 12:07, 17 May 2008 (UTC)

If the Habitat article is too complicated by jargon, consider reading Habitat Article at the Simple English Wikipedia. Nimur (talk) 23:59, 17 May 2008 (UTC)

Distribution restriction

In the first reference sited by the article on spinosad there is the following statement "DO NOT DISTRIBUTE THIS TECHNICAL BULLETIN IN THE STATE OF NEW YORK." What is the reason behind this statement? 71.100.14.205 (talk) 14:58, 17 May 2008 (UTC)

That is really odd! I'd say it's a matter of law in New York State. Perhaps the legal eagles at the humanities desk could help? I for one would really like to know. Fribbler (talk) 20:26, 17 May 2008 (UTC)

That is a bit strange - I checked, and spinosad is listed as a permitted pesticide by the NYS Dept of Environmental Conservation (here's the document (warning - large PDF). -Bmk (talk) 00:22, 18 May 2008 (UTC)

My guess is some of the claims, or 'information' contained in the bulletin are illegal under NY State law, or that something which should be in such a bulletin according to New York state law, is not Nil Einne (talk) 13:18, 18 May 2008 (UTC)

how to determine the rate of reaction of haloalkanes?

how to find the rate of substitution of different haloalkanes?? —Preceding unsigned comment added by Danielezzat9654437047 (talk • contribs) 18:02, 17 May 2008 (UTC)

Please read the notice at the top, we will not do your homework for you. Regards, CycloneNimrod^Talk? 19:18, 17 May 2008 (UTC)

The article on haloalkane has some information. If you don't have a text of some kind to look in, try google or Wikibooks. --Bmk (talk) 19:41, 17 May 2008 (UTC)

it's not my homework!!!! i have three samples and all i know about them is that they are haloalkenes!!!! how can i differentiate between them???? I've thought of adding sodium hydroxide but the problem is they all turn to alcohols i need a way to differentiate between them!!! —Preceding unsigned comment added by Danielezzat9654437047 (talk • contribs) 20:45, 17 May 2008 (UTC)

I assume you've read and understood the information at haloalkanes. So, ... What measurement equipment is available to you? Can you measure any of: density, heat capacity, molecular weight, boiling point, hygroscopicity, index of refraction, dissolution limits of other halo-compounds, melting point? Can you identify any gas that may be evolved or any solid that may be precipitated during your NaOH experiment (which may suggest which halide has been substituted)? Can you measure diffusion (or effusion) rates of the gases? Can you perform a chromatography differentiation between your samples? Can you run your samples through a mass spec to limit the range of potential candidates? Are your samples flammable (suggests incomplete halide substitution)? How much oxygen is consumed/mass lost during burning (suggests unsubstituted fraction)? What gases are evolved during burning (may indicate which halide(s) is/are substituted)? What gases are evolved under oxygen-poor conditions and aggressive heating (may suggest unsubstituted fragments in the original molecules)? Have you attempted a fractionation (to determine whether your samples are single components or mixtures? Et c. ... -- Fuzzyeric (talk) 00:09, 18 May 2008 (UTC)

I need an experiment based on the isomeric halogenoalkanes with the formula C4H9BR reacting with water!!!! Plus i don't have any of this stuff, i can't perform a chromatography and i don't have any of the measurement equipment you just mentioned.When haloalkanes react with NaOH they release halides which are easy to identify if passed through silver nitrate. But i need to identify whether the haloalkane is primary, secondary or tertiary. And i think the only way to do so is by measuring the rate of the substitution of the halide. (There is no need to attempt a fractionation my samples are single components).Danielezzat9654437047 (talk) 03:29, 18 May 2008 (UTC)

Hydration in chemistry

What's it called when a bunch of water molecules gather round an entity as in MgCl₂*6H₂. We did something about it in A level chemistry but I guess those brain-cells haven't seen lot of use since. ----Seans Potato Business 18:56, 17 May 2008 (UTC)

Water of hydration? --Bowlhover (talk) 19:16, 17 May 2008 (UTC)

When it's a salt like that, it's referred to as a Hydrate. Wisdom89 _{(T / ^C)} 00:29, 18 May 2008 (UTC)

A well defined demographic barycenter

Hello,

sorry if this belongs in Humanities or Mathematics. One sometimes reads about "the demographic barycenter". It is used to denote where most people live in a certain region or state. But is this well defined? Has someone ever tried it?

My definition would be : make a vector (from the center of the earth ) pointing at an individual, add them all up and divide by the number of people in the region you want to consider.(note that we would be also taking into account some differences in height). Or would this not be suitable?

Has someone ever used a definition like this? Are there articles/websites/books actually providings maps to illustrate where the "demographic barycenter" is?

Many thanks,

Evilbu (talk) 19:13, 17 May 2008 (UTC)

Well, it's hard to say if it is a useful measure unless you know what you want to use it for. The definition you describe will indeed give you the demographic barycenter (or population barycenter, as I think it's more often called). Note that for regions that are large enough to have dimensions comparable to the earth's radius, that definition of barycenter will usually land you far underground; for instance if you do it for the entire planet, you will get a location deep inside the earth - that doesn't seem particularly informative to me. Of course, you can sidestep that issue by projecting the barycenter point radially onto the surface. --Bmk (talk) 19:23, 17 May 2008 (UTC)

Or, you can use the spherical coordinate system; simply express each person's position using latitude, longitude, and perhaps elevation above mean sea level. I don't understand the need for vectors; would scalars not work just as well? --Bowlhover (talk) 16:44, 18 May 2008 (UTC)

The problem with using latitude and longitude is that functions of latitude and longitude can have branch points. For instance, suppose a population is located such that the center of population is located approximately on $180^{o}$ longitude; i.e. half the population is east of the 180 longitude line, and half is west of it. Then the calculated center of population will be on the other side of the earth, near 0 longitude. This is only a problem if the region you are interested in includes the 180 longitude line. Luckily (actually by design) that line is in the ocean, so few people live near there. So the short answer is for most regions, averaging using latitude and longitude coordinates should work fine. --Bmk (talk) 18:22, 18 May 2008 (UTC)

The average-and-normalize method is indeed the most well-defined one. The depth at which the unnormalized barycenter ends up simply tells you how meaningful the normalized barycenter is: if the unnormalized barycenter was near the center of the Earth, even small population shifts could make the normalized center swing wildly around the surface. And, of course, if the unnormalized center just happened to end up at the exact center of the Earth, then the distribution of the population would be balanced and there wouldn't be any meaningful barycenter on the surface. —Ilmari Karonen (talk) 20:13, 18 May 2008 (UTC)