Wikipedia:Reference desk/Archives/Science/2007 December 9

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December 9[edit]

inverse sqaure law for vortices[edit]

for a traveling vortex of air, does the amount of pressure it produces decay occording to inverse square law [half the distance, four times the force] or along another line, if so what amount does the amount of pressure transported by the vortex decay by over distance?

Robin —Preceding unsigned comment added by 80.6.37.199 (talk) 00:03, 9 December 2007 (UTC)

I think this is a rather complicated effect. It's unlikely to be inverse-square because you get those amazing Vortex ring guns - or the more reasonable Air bazooka or Vortex ring toys that propel a stable vortex over large distances with seemingly little energy loss. SteveBaker (talk) 00:22, 9 December 2007 (UTC)

It is a complicated effect, however they do loose momentum over distance, and thus exert less pressure, doews anyone have any idea on the equasion or know the way it works?

Robin —Preceding unsigned comment added by 80.6.37.199 (talk) 00:42, 9 December 2007 (UTC)

Aspherical Lenses[edit]

Why are aspherical lenses so much harder to make? than spherical ones What's the optimum lens shape? --antilivedT | C | G 09:04, 9 December 2007 (UTC)

lenses with circular symmetry can be roughly made by spinning the glass against a grinder as in a lathe, but aspherical lenses are a more complicated shape! Graeme Bartlett (talk) 11:44, 9 December 2007 (UTC)
There isn't a single 'optimum lens shape' - it depends on the task you wish the lens to perform. For example, if you have one of those 'Laser line level' gadgets (especially one of the cheaper ones) that produces a fan-shaped beam of laser light - inside you'll find a cylindrical lens. On the other hand, most contact lenses are spherical - unless used for treating Astigmatism - in which case a non-spherical lens of some kind will be needed. SteveBaker (talk) 13:16, 9 December 2007 (UTC)
By optimum I mean one that has the least aberration. Spherical lenses has spherical aberration, so how about a parabolic lens, or some other shapes? Also, can't you just make the grinder aspherical is well and you can start grinding aspherical lenses in the exact same fashion? --antilivedT | C | G 04:55, 10 December 2007 (UTC)
The point is that there is no shape of lens that is optimal (aberration-free) for all conditions and applications. So, for instance, if you work out the best possible shape of a lens for imaging a person standing 20 metres away onto a camera film, then that lens will not be the best possible shape for imaging someone standing 25 metres away. The amount of spherical aberration isn't a fixed quantity for a given (spherical) lens, it also varies depending on exactly what you are using the lens for.
Lens grinding is a complex topic, but in the traditional method for grinding a spherical lens surface, the glass blank is ground against a tool with a hollow spherical interior surface. It doesn't just rotate around one point (like on a lathe), but moves sideways and rotates at the same time. A pair of exterior and interior spherical surfaces are the only surfaces which fit together like this, where they can touch each other at all points in any orientation and position.
The reason you can't just spin it like a lathe is that you end up scoring circular striations in the glass if it can't move and rotate at the same time. Though some modern aspheric grinding systems can place the cutting tool so accurately that they do work like a lathe, grinding away a tiny bit of the glass at a time. --Bob Mellish (talk) 05:42, 10 December 2007 (UTC)
Also, because lens materials have different refractive indices for different colours, you can't focus all colours at the same place - so some amount of fringing is inevitable ("chromatic aberration"). Various coatings and compound lenses (where two lenses with different refractive properties are bonded together) are employed on high quality lenses - but the cost of fixing it 100% correctly are huge. With aspheric lenses, fixing chromatic aberration is even more complicated - so very often people don't bother doing it. SteveBaker (talk) 13:52, 10 December 2007 (UTC)
I also wanted to mention the development of holographic lenses - which is exciting since it enables the production of more or less arbitary lens shapes using computer software with none of that tedious grinding of glass. SteveBaker (talk) 13:52, 10 December 2007 (UTC)

My Mobile Phone Underwent Strange Situation![edit]

My nokia model 3310 (oldest known version) has now got a strange problem! It's accepting any other sim except my sim! If I'm putting my sim it was saying "simcard rejected"! At the same time it was accepting every other sim i tried in it! Same is the case with my sim card! It was working in all other models like sony errickson, motorola, LG, etc, but it was not accepted in any of the nokia model.."simcard rejected"! What might be the reason? & solution? Temuzion (talk) 09:57, 9 December 2007 (UTC)

Must say I'm stumped! I've been using this model of mobile phone since 2001 and nothing like this has happened to me (although I've got one N3310 acting very weird too). Did you do anything just before you experienced these problems (drop the phone, change the battery, tamper with it in any way, receive any strange/unusual SMS/picture message) or did it just happen suddenly? I like the way you spelled sony errickson ;) Cheers, Ouro (blah blah) 10:36, 9 December 2007 (UTC)
I'd expect it to be a problem with your sim card. I expect it's possible to have a sim card fault that your phone cares about but others don't. I don't know where you are, but in Australia the phone company owns the sim card, so I suggest you go to your mobile service provider and tell them that their sim card is faulty. They should replace it. --Psud (talk) 12:07, 9 December 2007 (UTC)
This happened to me once, and was because I had been swapping simcards with people a lot, as well as removing and reinserting the sim a lot because the phone was glitchy. Basically, the sim got scratched in a way that was specific to my phone, because it was the connectors on my phone that did the scratching. I was never able to get that sim and that phone to work together again, so I think you may have to choose between them and replace the other. Needless to say, I now remove the sim very carefully if it's necessary. Skittle (talk) 13:02, 9 December 2007 (UTC)
If I'm reading you correctly, it sounds like you could have abbreviated your situation as, "No phone accepts my SIM card," in which case the answer seems obvious: there's a problem with your SIM card. Your phone is probably fine, you just need to get your SIM card fixed. -- HiEv 00:01, 10 December 2007 (UTC)
But my sim is active in all other models except nokia range! May be nokia software is rejecting the sim! & one more thing! I just dropped my phone twice in the last month & another thing is thatI took the message offer by which I got 500 free messages to be sent to any mobile & by that offer I m using the sms service extensively! & previously once the problem rose when dealing with sms only! But none of the components can be avoided b'coz both are working but in but in diffrent models..so ultimately what should I do? Temuzion (talk) 03:05, 10 December 2007 (UTC)
If it were me nowadays, I would take my sim to get a backup made at one of the places that does that (Carphone Warehouse? I imagine most phone places do it), then see if the backup works in the phone. This is because I suspect something has physically happened to your sim that is specific to the way it connects with the nokia. Getting a physically different sim would solve this problem. If it still doesn't work, something much odder must have happened. In that case I'd get a new phone, unless you don't mind swapping your number and re-inputting your phonebook and losing the pictures saved to your sim in which case you could get a new sim. Skittle (talk) 04:36, 10 December 2007 (UTC)

Journal Article Database[edit]

Can anyone suggest a good free-to-use journal database (is there such a thing? I'm sort of spoiled by my memories of computer labs at school...they always seem to have access to those kind of things)? I'm writing a short research paper, and I'd like to flesh out my works cited with a few off-line resources. Love,70.181.41.1 (talk) 16:33, 9 December 2007 (UTC)

Pubmed is free and pretty good. Does not cover some plant journals. An excellent non-free, but often available at libraries, database is Web of Science. David D. (Talk) 16:40, 9 December 2007 (UTC)
Thanks. I'll make a note of that (the PubMed is going to be fun to dig around in, I appreciate it). I should be more specific. I'm writing a quick five page overview of lunar formation, focusing on the strengths and weaknesses of the giant impact hypothesis. The reference links from the article were helpful, but I feel like some off-line citations will give it an air of respectability. 70.181.41.1 (talk) 16:59, 9 December 2007 (UTC)
You may also want to take a look at scholar.google.com. I've had luck with articles linked from their database, when going to the journal's website doesn't work. I'm not sure if perhaps Google is a subscriber (much like your library was at school) or what, exactly, but it seems to work often. (EhJJ) 18:43, 9 December 2007 (UTC)
What's your field? Physics preprints are almost always nowadays posted at the ArXiv. SamuelRiv (talk) 19:01, 9 December 2007 (UTC)
See also the NASA ADS for anything sky-related. --Tardis (talk) 16:26, 10 December 2007 (UTC)

Thanks to everyone who responded. The reports been turned in (with a little rounding out care of ADS and ArXiv). We'll see how it's received. 70.181.41.1 (talk) 23:47, 11 December 2007 (UTC)

THE ABC's OF ONLINE INDEXES
For those who are reading this who might not be completely familiar with what is being discussed, I thought I'd offer a brief explanation, and then proceed to provide a couple of suggestions. I offer my opinions from having had direct experience as a professional Reference Librarian who worked for years at a busy Serials Desk. First off, when a person wants to dig in to deep research about something, he or she will very quickly (depending on the topic) discover that not everything may be found on the open Internet. Instead, the more complex research being done in the world is done through perusing indexes (note: I use the word "indexes", not "indices" for better comprehension here) that are compiled by for-profit companies, and then sold to libraries and research facilities on a subscription basis. Each company organizes its indexes differently than the others, but there are a few similiarities from one index to the next. Most of these indexes have been around for many years, and only recently (in the 1990s and this decade) have most of them acquired an online version of their indexes, which at one time were only available in print format.
Just for clarification, I want to add that indexes do not necessarily provide links to the articles for you to read online. This feature is called "Full Text", and some databases offer it extensively, others a little bit, and still others not at all. So if you're using an online index thinking you will find full text journal articles on your subject, look twice because it may be that the index you're using doesn't do this. However, if you have the time, you can usually order copies of articles to be faxed or emailed to you through your local library - providing you supply them with the full citation (which of course you'd get from the online index).
In recent years the information profession has been attempting to find a way to create software that could search all of these online journal index databases at one time. This kind of global search is known by several terms: federated search, broadcast search, or metasearch. The industry isn't quite at the place where a person may search for a subject that would dig through all the databases available, but that's more or less the direction that things are heading. Until then, one must go through these online indexes one at a time (or more than one, if there is a family of databases connected together, such as FirstSearch).
However, not everything is online yet. Due to the costs of digitizing data and then storing it, most online databases provide access to older archival material only back a couple of decades or less. Depending on what your subject might be that you're researching, this might be a problem. But it is easily solved by going in person to a library that has the print back copies of the index on its shelves, and then doing research the old fashioned way by actually opening books.
Knowing which database to use for what subject material you'll be looking for is part science and part art. Each index lists which journals it covers, and also includes an explanation for what subjects it focuses on. That's the science part. The art part comes in by knowing which databases to search. Here's an example: say that you are searching for information about how agricultural runoff effects fishes in California rivers. There would be some indexes that would cover biological science, some that would cover medical science, some environmental science, and others agricultural science. In each of these databases there may be information indexed that would provide information about this subject. However, some of these databases would do a better job of it than others. Knowing which database would be the BEST choice to look in (and thus saving time and effort on the part of the user) is what provides job security to people like me - Serials Librarians.
Even IF there comes a day when a universal search engine is available that can search all knowledge everywhere, the overwhelming task of finding something specific still is very daunting, because until artificial intelligence is invented, these search engines must use computer algorithms which use the search terms entered to try to find relevant material. However, computers are very stupid and do not really know what is relevant or not much of the time. Nothing can replace the human mind (yet) when it comes to being able to know where to go for the most relevant material for a given search. So it is that if someone makes his or her living doing searches for people on all kinds of subjects all day long, then this person would soon develop a knack for knowing where the best places are for different subjects. Okay, I'll lay off of the plug for Librarians, except to say: it costs nothing but time to go into a library and ask a reference librarian for help in finding something - what a deal!
Now, addressing your question about which databases may be accessed for free online - this is tricky. Of course most of these journal indexes are "for-profit" companies, and they make their money by selling subscriptions to libraries and research facilities, and they do not take kindly to people using their services who are not connected to an institution that has paid for their service. However, the good news is that you don't have to be a scientist or a college student to have access to these subscription databases! Most public libraries have subscribed to multiple journal databases, and most of these libraries have now been able to implement systems in which their patrons may connect to these databases remotely from their own home. If not, then at the very least they will provide access to the databases from the computers in their library building. I'd say the first step for you would be to start by calling your local library and asking whether they offer remote access to their online journal databases.
If such is the case that you do not live in an area that is covered by such a library, then the next step would be to try to do what you're doing now: finding out which online databases are accessible for free. There actually are (as has been mentioned already here) a couple of databases that are fairly decent for finding information. Here is an annotated list of some (the annotations are my own):
  • GoogleScholar. GoogleScholar is fantastic, and I use it all the time. However, it has some very serious limitations. The good points are that it covers a LOT of material, most of which is in professional peer-reviewed academic journals. Its limitations are that searching in it can be very difficult sometimes if you're doing anything complicated. It also (contrary to what it claims) does NOT provide full text access to everything it claims to. In addition, one is forced to use the Google search algorithm, which although is a fairly good one, does not allow a lot of flexibility when it comes to fine tuning a search, as many of the other online databasees do. However, GoogleScholar is still a very useful tool. It is not, though, a solution for everyone.
  • Most countries in the world have a National Library whose mission is to take a leadership role of the libraries in that country, offering many services and providing for an oversight on quality control, so to speak. The U.S., however, has no National Library. Although many believe the Library of Congress is the National Library, it is not - it is specifically the Library of the United States Congress. However, there are three major libraries in the U.S., that taken together, might represent certain aspects of what a typical National Library does in other countries. These three libraries are the Library of Congress in Washington, D.C., the United States National Library of Medicine in Bethesda, Maryland (which is the largest medical library in the world), and the United States National Agricultural Library (which is the largest library of agriculture in the world), also located in Washington, D.C. Each of these libraries has its own vast collection of journals, as one might imagine. Both the National Library of Medicine and the National Agricultural Library offer a freely-accessible index to their journals. In other words, both of these libraries have a staff of people who index the journals they receive, creating a database that they then share freely to the public. The Library of Congress, however, does not do this. They do maintain a very popular database called thomas.loc.gov whose focus is on legislation and some U.S. history, but instead of providing a free index service to the journals they subscribe to, as does the other two libraries, they sell the rights to do this to an independent company called OCLC. I'll discuss all of these databases below:
  • The online journal index database for the National Library of Medicine is called MedLine, whose database is called PubMed. It may be found here: http://www.ncbi.nlm.nih.gov/sites/entrez If you go there, you may set up a free account and perform searches for journal articles that are in their huge collection. However, their search engine software is quite klunky to use. Two indexing companies also offer MedLine through their own services, OCLC's FirstSearch, and Cambridge Scientific Abstracts. I would recommend either of these other two indexes over that of PubMed, due to their more superior searching capabilities. However, in order to access them, you need to go through a library that has a subscription to either of these companies. Fortunately, most libraries (at least in the U.S.) have subscriptions to OCLC.
  • The online journal index database for the National Agricultural Library is called Agricola. It too maintains its own search engine, but which is better than that of PubMed. Again, OCLC has also provided a mirror database for Agricola, using its search engine, which is probably easier to use than that of the original. Here is the location of Agricola: http://agricola.nal.usda.gov/
  • Thomas is the name given by the Library of Congress to its database for legislation and history. It may be found here: http://thomas.loc.gov/
  • OCLC is NOT a free service. However, it has by far the most extensive subscription journal indexing around. Most libraries subscribe to OCLC, which is not a single database, but a family or collection of over 50 different databases, including some that are absolutely superb for what they do. One of these databases, called WorldCat, is fantastic, as it combines of thousands of libraries' online catalogs in one place - it is literally like going to an online catalog that includes almost every library in the U.S., many in Europe, and Asia. WorldCat does not index journal articles, but books (or monographs, if you will). OCLC also has many other journal databases, making it perhaps the single most powerful journal search index in the world.
In closing, I'd like to reiterate that you will not find one single place where everything is located - not even Wikipedia does that (yet). Knowing where to look is very important if you're trying to find out about something. And again, the best person to ask where to look is your friendly local Reference Librarian. Or us here... Saukkomies 04:29, 11 December 2007 (UTC)
Wow, I'm quite impressed. Thank you for the helpful lesson. Ƶ§œš¹ [aɪm ˈfɻɛ̃ⁿdˡi] 06:16, 13 December 2007 (UTC)

How much does solar energy cost to produce ?[edit]

How much does solar energy cost to produce? —Preceding unsigned comment added by 68.218.19.20 (talk) 17:20, 9 December 2007 (UTC)

(Added title to question) SteveBaker (talk) 17:34, 9 December 2007 (UTC)
For the time being solar energy is free. --Ouro (blah blah) 18:31, 9 December 2007 (UTC)
I assume they mean the capital costs, etc., of a power system which converted solar energy to electricity. --24.147.86.187 (talk) 21:47, 9 December 2007 (UTC)
It's not just capital costs either - eventually they fail and have to be replaced. In a large scale installation, the panels need to be cleaned to remove dust and dirt that would otherwise reduce their efficiency. However, it's not going to be anything like as expensive as maintaining any other kind of power station. The biggest problem is the capital cost. SteveBaker (talk) 23:21, 9 December 2007 (UTC)
Indeed. Not only is solar energy not free in practice, it's so not free that in general, it can't really compete with fossil fuels yet. That is, the equipment required to capture and use that "free" energy is so expensive per unit of energy captured that it's usually cheaper to use "conventional" sources instead. (The situation is changing, but slowly.) —Steve Summit (talk) 23:38, 9 December 2007 (UTC)
Well, it all depends on how much solar energy you're producing. Small solar panels, like ones for solar powered calculators, sell for under $10. But solar power plants, like these, can cost quite a bit more. This solar power tower, for example, cost about $40 million (USD) to build. So, your question is like, "How much does a motor powered vehicle cost?" It depends on whether you're talking about a scooter or a helicopter and if you're including other things, like long term maintenance. -- HiEv 00:42, 10 December 2007 (UTC)
But even there, in a calculator, is an example of the problem in microcosm. You can buy a AA battery for about 50c - so for a $10 solar cell, you can get 20 batteries. The little 4-function calculators that come with solar cells use so little current that they'll easily run for a year of normal use on one battery - so if you'd spent your $10 on batteries instead, it would take maybe 20 years for you to finally discover that you'd have been better off using a solar cell. The trouble is that nobody keeps a calculator for 20 years because they break, get lost or are outmoded. What's worse is that if you'd invested your $10 in the stock market (with an average return of about 5% per annum), then your interest would be 50c per year - which means that you could keep yourself in batteries for the rest of your life for the price of those solar cells. Sure, you can quibble with the numbers (I don't think calculator solar cells cost anything like $10) - but you get the point. It's not always obvious that "free" electricity is something you can afford to buy. SteveBaker (talk) 03:50, 10 December 2007 (UTC)
But (I have to have this but) there's the additional costs that come into play. Say you went with the batteries, and not the solar cell, it would be good to count in the costs of disposal of the batteries and/or their recycling. I know it's usually something negligible or something that the usual calculator user never ever ponders on or cares about, but still, somebody has got to take care of the refuse. There's also the costs (to the environment) of the production of those umpteen batteries vs the costs (to the environment) of solar cell manufacture. If we're discussing everything, then it should be everything. --Ouro (blah blah) 06:59, 10 December 2007 (UTC)
I think the $10 figure for a calculator solar cell is too pricey, by an order of magnitude or so. Cheap solar-powered calculators are a giveaway item at conferences and such. Here's a calculator for $2.75, retail, in single quantity. Here are calculators for as little as $1.09 in quantity. The solar cell itself must be under a dollar. -- Coneslayer (talk) 12:43, 10 December 2007 (UTC)
Like I said - you can certainly quibble about the numbers...I merely present that little exercise as a way to show the problems with solar power in general. For calculators, solar actually is cost-effective or we wouldn't see them on store shelves. I suspect that in reality, the solar cell is actually cheaper than the empty battery compartment - and it has the advantage that it's thin so you get a smaller calculator as a result. As Ouro points out, these assessments (both on the level of calculators and on the level of bazillion watt power stations) really ought to factor in the cost of other power sources on the environment. Solar can't compete with coal on price - but if you were to factor in the likely costs in terms of global warming of the CO2 and other junk put out by coal, then solar power would probably turn out to be more cost-effective despite the cost of capital. SteveBaker (talk) 13:40, 10 December 2007 (UTC)
A 3000 mAh alkaline battery at 1.5 V that costs 50 cents would be delivering power at a rate of $100 / kWh. A nice solar system has a life cycle cost closer to $1 / kWh, while traditional energy sources are more like $0.1-0.25 / kWh. Batteries are by far the most expensive energy source in common use. In virtually every circumstance where it is practical to replace batteries with an alternative energy source (be it electricity from the grid, or distributed power like solar) it is more cost effective to do so. Or put another way, the ~0.1 W required to run a calculator can be provided by a ~15 cent solar panel, not a $10 one. Dragons flight (talk) 15:12, 10 December 2007 (UTC)
Recent large orders for Stirling engines (40,000 or so ordered for Southern California) and solar trough plants (see List of solar thermal power stations) show that the expense is not that far off coventional sources. Rmhermen (talk) 18:53, 10 December 2007 (UTC)
For the record, I never said solar cells for calculators cost $10. I said they cost under $10, and gave an example of a 6 volt one for $9. The same site had a 3.5v one for $2.49, and if you buy 'em in bulk you can often get 'em even cheaper (as Coneslayer pointed out earlier). So, if a 1.5v AA battery costs $0.50, as argued earlier, then that 3.5v solar panel takes the place of $1.50 worth of batteries, and doesn't need replacing. In other words, it's paid for itself by the time you'd have put in that second set of batteries, and it doesn't create the additional pollution that batteries do either. -- HiEv 22:51, 11 December 2007 (UTC)
<nitpick>Technically, Ouro got it right the first time. Solar energy is produced completely for free: the panels just capture and convert the energy. --Storkk (talk) 15:18, 12 December 2007 (UTC)

Does Cefdinir dehydrate you, or am I getting seriously windburnt?[edit]

This question has been removed. Per the reference desk guidelines, the reference desk is not an appropriate place to request medical, legal or other professional advice, including any kind of medical diagnosis, prognosis, or treatment recommendations.

Since you've found our article on cefdinir already, there's not much more we can add. The volunteers at the Reference Desk aren't qualified to diagnose your condition—and even if they were, it's difficult to do a proper examination over the internet. You should speak to a trained and licensed professional – your physician or pharmacist – about your concerns. TenOfAllTrades(talk) 19:43, 9 December 2007 (UTC)

Oh, sorry. I'll ask the doctor at my followup. MalwareSmarts (talk) 19:55, 9 December 2007 (UTC)

NASA's COBE (Cosmic Background Explorer) Satellite[edit]

COBE produced an Infra-Red picture, that was unveiled by Eli Dwek at a meeting.

Where and when was this meeting?

Was it, for example, at the Conference held in College Park, MD - in April 1995? Anyone know? - I'm Peter Lamont. —Preceding unsigned comment added by Astrocat0-1 (talkcontribs) 18:52, 9 December 2007 (UTC)