Wikipedia:Reference desk/Archives/Science/2007 December 28

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December 28

Heating value of Nitrogen

I know that heating values are usually used for fuels, but in real-world applications, many fuels are not pure but actually have some nitrogen absorbed in them. In particular, I'm trying to calculate out the BTU/standard cubic feet (megajoule/normal cubic meter would be fine, too) for various compositions of natural gas depending on how much nitrogen is added or removed post-production. So my questions:

1. What is the heating value of Nitrogen?
2. Am I correct that if I know the BTU/scf of the pre-processed gas, then all I need to know is the % of Nitrogen added or removed and the Heating value of Nitrogen, and I can do a simple ratio calculation to get the BTU/scf of the post-processed gas?

Thanks!--M@rēino 00:49, 28 December 2007 (UTC)

In the conditions found in a residential or commercial heating furnace, Nitrogen will not react and therefore has no heating value. It is possible to obtain nitrous oxide which will serve as an oxidizer (not as a fuel), but this is an extremely unlikely component of your natural gas. As far as your second question, you can probably assume the Nitrogen will not react in any way in your setup, so you can simply neglect it in the calculations (assuming that an inert gas does not affect a reaction is a common first-pass approach, and as long as the percent quantity is low, is very accurate). Simply calculate how much actual fuel (natural gas) you have present. Nimur (talk) 01:49, 28 December 2007 (UTC)

If nitrogen is burnt to form various nitrogen oxides, it actually consumes energy. If you burn the natural gas with air, the air will provide far more nitrogen (78% of the air) than the natural gas. Nitrogen oxides are only created at high temperatures and you can probably neglect them. You can look up thermochemical data in the NIST Chemistry Webbook; e. g. formation of 1 mole NO₂ consumes 33.1 kJ. Icek (talk) 20:38, 28 December 2007 (UTC)

• Thanks to both of you! --M@rēino 17:58, 29 December 2007 (UTC)

What is the least damaged ecosystem/biome?

That is, what biome has the largest percentage of its original area remaining? (I assume the Antarctic ice cap would be the answer, but I mean besides that...)

Apologies if this is obvious/easily found information ... Vultur (talk) 00:58, 28 December 2007 (UTC)

Maybe the ocean, although fishing and whaling threaten many species. I think your question leaves a lot of vagueness - do you mean to equate "undamaged" directly to "least loss of area"? That approach has limited applicability, as human encroachment does not necessarily mean total destruction and replacement of a biome. Nimur (talk) 01:43, 28 December 2007 (UTC)

I'd guess Antarctica. That "lost world" place (see Foja Mountains) is worth mentioning too. —Pengo 03:03, 28 December 2007 (UTC)

There are completely isolated biomes. Lake Vostok is one example. Biomes growing up around Black smoker is also fairly isolated from our effects. Taemyr (talk) 14:13, 28 December 2007 (UTC)

• The Planet Earth TV series showed an ecosystem in a completely isolated cave system which will probably survive any nuclear holocaust we humans dream up. --Sean 14:28, 28 December 2007 (UTC)

If said ecosystem is "completely isolated", how did the cave's discoverer find it, and the film crew get to it and and film it? No, THAT ecosystem has now had a wide array of microbes introduced to it, courtesy of the discoverers and the film crew. Perhaps, if it is somehow sealed away from further visits, and the last visitors left some kind of automated remote-sending equipment, we can see what effects there are without any more visits. -SandyJax (talk) 17:46, 28 December 2007 (UTC)

• I just meant that there's no regular communication with the outside world (via underground streams, etc.). You could say that the moon is completely isolated from humans, even though they did tramp around on it a bit, and might do so again some day. --Sean 00:34, 29 December 2007 (UTC)

You're all right - I worded the question badly. Let's see: Of the major land habitat types (tropical rainforest, temperate deciduous forest, grassland, desert, tundra, etc.), which has the smallest percentage of its original land area converted by farming, ranching, logging, oil extraction, or urbanization?

I know the Antarctic is #1, but what's #2? Tundra? Desert? —Preceding unsigned comment added by 74.162.57.116 (talk) 18:54, 28 December 2007 (UTC)

Through desertification and persistent drought, land area of much of the world is turning into desert (so that biome is growing). You might want to read up on biomes to learn how they are classified; depending on which system you choose to apply, the "ranking" will change. Nimur (talk) 16:26, 31 December 2007 (UTC)

Blood Clot and Cancer

Could coagulation contribute to uterine cancer or vice-versa? —Preceding unsigned comment added by 70.216.253.139 (talk) 04:00, 28 December 2007 (UTC)

Coagulation of the liver to be exact. —Preceding unsigned comment added by 70.216.253.139 (talk) 04:05, 28 December 2007 (UTC)

This can go both ways. There's a general rule in medicine that "tumors love blood," so anything that broadly impedes blood flow is unlikely to assist in tumor growth. Beyond tumor growth, however, cancer can benefit from blood clotting; studies have shown that metastasis of cancer cells may be aided by blood clotting, which may help malignant cells settle down inside a blood vessel (cancer cells have even been observed to generate chemicals that promote coagulation). Just keep in mind that is entirely related to the spread of cancer, not the appearnce of it. Further, blood clotting may be a side affect of certain cancer treatments, or a side-effect of drugs used to treat anemia, which can itself be a side-effect of certain cancer treatments. Someguy1221 (talk) 04:13, 28 December 2007 (UTC)

Eclipses

Looking up the page at Image:Total Solar Eclipse Paths- 1001-2000.gif, it strikes me that the southern half of the Earth has far larger gaps (where no eclipses have taken place in the last 1000 years) than the northern half. Any physical reason for this, or is it just chance? GeeJo ^(t)⁄_(c) • 04:28, 28 December 2007 (UTC)

Don't know for sure, but I suspect it's the same reason that the Southern Hemisphere gets shorter summers than the north. Something to do with the shape of our orbit. --Psud (talk) 12:09, 28 December 2007 (UTC)

First of all, you're more likely to get a solar eclipse in your hemisphere's summer. (From your POV on the earth, a solar eclipse has to happen when the sun is up, and it's up longer in the summer. From the sun's POV, the visible half of the earth is dominated by the hemisphere that's in summer.) Now, you know that by remarkable coincidence, the apparent size of the sun and moon are almost exactly the same; the moon can just barely cover the sun. Since the earth's orbit around the sun and the moon's orbit around the earth are both slightly elliptical, the apparent sizes of the moon and sun can vary a bit, as they get closer and farther from us. In some cases, the moon appears slightly smaller than the sun, and the result is an annular eclipse instead of a total eclipse—there's a ring (annulus) of sun sticking out from behind the moon. During the southern hemisphere summer—when the southern hemisphere is most likely to see an eclipse—the earth is closest to the sun, and so the sun is at its largest apparent size. This means that you're more likely to get an annular eclipse instead of a total eclipse, compared to the northern hemisphere summer, when the sun is at its smallest apparent size. -- Coneslayer (talk) 13:34, 28 December 2007 (UTC)

Another contributory factor is simply the way that particular map was drawn. Because it's using a Mercator projection, the north and south poles are disproportionately large compared to equatorial regions. Most of the 'gaps' in the eclipse coverage map are in the south polar region and while they seem like massive holes in the map, they are in reality quite small compared to the ones nearer the equator. So, while there are undoubtedly more gaps in the southern hemisphere, they are much smaller than they appear and the discrepancy is nothing like as big as the map implies at first glance. SteveBaker (talk) 14:46, 28 December 2007 (UTC)

percentage of caffeine in coffee

percentage of caffeine in coffee - Greatest -59.94.153.78 (talk) 07:00, 28 December 2007 (UTC)

That is really going to vary a great deal from coffee to coffee. Coffee#Caffeine content gives the common masses of caffein in various coffee-like beverages (which is the major factor), but doesn't give the density of the coffee. So, it's easy enough you want a mass/volume ratio. In espresso, for example, you have 30 mg of caffein in 30 ml of espresso, so you have 0.1% caffein (mass/volume). The mass/mass ratio shouldn't be too far off, as most water-based solutions have a density pretty close to 1mg/ml. Someguy1221 (talk) 07:19, 28 December 2007 (UTC)

This is really unscientific but I used to wait tables and the packets we used said that regular coffee had 3% caffeine while "decaf" coffee was 98% caffeine free. To me, bad at math, this seems to say that there is only a 1% difference between decaf and regular. And that rationalising always made me feel not so bad when giving people the wrong kind of coffee when I was "in the weeds". There - since I am not Catholic (or religious at all) my confessional is the Wikipedia:Reference desk/Science! Saudade7 20:33, 28 December 2007 (UTC)

The '98% caffeine free' statement is intended to mean that 98% of the caffeine has been removed, leaving just 2% of the original concentration. Note that the 3% number is much too high under any sensible measuring scheme—as Someguy notes, the correct figure is less than 0.1%. (This page suggests that an eight-ounce (225 mL) cup of coffee contains between 80 and 175 mg of caffeine; that's a concentration between 0.03 and 0.08%. Espresso, with its smaller volume, has a caffeine concentration closer to 0.2%. For comparison, a cup of decaf coffee contains 2 to 4 mg of caffeine—roughly 98% less than the regular. TenOfAllTrades(talk) 21:23, 28 December 2007 (UTC)

That's cool. Like I said it was unscientific! That's just what the individual brewing packets said! But I totally see your reasoning and it sounds right to me! Thanks for being Math-ier than I! Saudade7 23:11, 28 December 2007 (UTC)

It's possible the packet was simply being vague, or there was some notation regarding an unusual measuring scheme. 3% caffein could refer to the mass ratio of caffein in the packet itself, sans water and milk. Someguy1221 (talk) 07:47, 29 December 2007 (UTC)

You're really gonna persist in that spelling, huh? Come on, it's an amine, it ends in -ine. --Trovatore (talk) 07:57, 29 December 2007 (UTC)

It's a well established fact that my spelling sucks. Someguy1221 (talk) 08:03, 29 December 2007 (UTC)

Don't take reduction as the intended meaning for all things, though. I've seen items labeled "98% fat-free" meaning "2% of the total weight is fat", rather than "only 2% of the usual fat content". It all depends on what sounds better to marketing, and what they can legally get away with. -- 128.104.112.236 (talk) 18:38, 29 December 2007 (UTC)

Snail mail

Where I am (Canberra, Australia) we've just passed summer solstice, and the weather's warming. Temperatures are sitting in the mid 20s (Celcius), occasionally touching 30. We've also had loads of rain. As a result, snail populations are booming.

Coincident to that, letters in my mailbox started to appear eaten, with up to about 15% of each envelope, and occasionally a bit of the enclosed letters gone. With about 5 letters arriving a day, that's quite an appetite.

Investigation turned up a colony of snails - about 10 in the letter box, and another dozen or so clustered around the base of the post on which the box stands. The snails seemed healthy, and the chickens enjoyed eating them, but I can't see how a snail could live on such un-nutritious food as bills and bank statements. Is there really enough in paper (predominately blue and white paper) to support a snail family? Or were they simply snacking on the paper while making a meal out of the nearby plants? Note that the letter box stands on a post a little less than a metre tall and is about half a metre from any greenery.

--Psud (talk) 11:20, 28 December 2007 (UTC)

The snails I'm familiar with seem to live on a cellulosic diet, so I'd imagine that the cellulose in paper isn't all too far off of the cellulose they'd normally consume from living plant matter. In the long run, they might lack some essential nutrients, but in the short run, there goes your mail. Maybe you can train them to just eat the bills and adverts?

Atlant (talk) 12:46, 28 December 2007 (UTC)

You seem to have been running a popular little snail snack bar. What you need to do now is recycle the envelopes and junk mail to grow more snails to feed your chickens. Richard Avery (talk) 14:11, 28 December 2007 (UTC)

It may have been more than paper they were after. Your letters probably contained glue (to seal the envelopes) and even the inks involved may have been made by vegetable oils. Sounds like a healthy diet. Maybe you should get a tiny tub of seltzer water and offer it as a snail spa? Matt Deres (talk) 19:01, 29 December 2007 (UTC)

Oil converted to jelly

I was deep-frying papad made from mung bean and some black pepper in safola(brand's name) oil (containing edible Blended edible Vegetable oil which is a mixture of rice bran oil and kardi oil in ratio 3:7),...after frying i saw that the oil had turned black and got converted into jelly.......Can u tell WHY it happened --59.94.132.222 (talk) 17:07, 28 December 2007 (UTC)

Can't speak to specifics in your case, but you should be aware of our article on Drying oils. A large number of oils will turn solid after exposure to air, due to oxidation. This is the basis of oil painting - the paint goes on "wet" and then "dries" solid. -- 128.104.112.236 (talk) 20:59, 28 December 2007 (UTC)

Was this jelly state when the oil was cool? In my experience, this happens when something from the food is dissolved into the oil. The melting point of the mixture is raised and when the mixture cools below that point it becomes a jelly. The something could be sugars, starch, or protein. I don't know which, but I suspect protein, because it happens most often when I fry meat or green beans (legumes). The oil can be used again, so long as it did not scorch. Black means the oil got too hot and scorched. Each kind of oil has a specific temperature at which it begins to scorch. For high-temperature deep frying I use peanut oil; its scorching temperature is higher than most. By the way, I don't fry papad anymore; I toast them in a toaster oven. --Una Smith (talk) 04:02, 29 December 2007 (UTC)

Microwaving works, too. It takes about a minute on high for one papad. MrRedact (talk) 17:17, 29 December 2007 (UTC)

Kardi, by the way, is nothing other than safflower. Rmhermen (talk) 19:46, 29 December 2007 (UTC)

Paper tearing question

At work I've been tearing up old bills that are approximately 8.5x5.5 (half the size of a normal piece of paper) into 4ths. So I tear them half down the length then tear each resulting portion into half. I've found that it's much easier to tear the paper on the sides that were factory cut rather than the side that resulted from my previous tearing. Why is this? I have some suspicions but I don't see why any one of them would necessarily be the case.--droptone (talk) 17:28, 28 December 2007 (UTC)

Paper has a grain, and will tear straight along the grain, but erratically perpendicular to the grain. -- Coneslayer (talk) 18:53, 28 December 2007 (UTC)

I suspected this sort of answer. But I wondered about why this would be different than the changes in the grain of the paper after it is cut to the various sizes (A4, etc, etc). Would it be because the device that cuts the paper into those sizes would have a much smaller distribution of energy than the imprecise motion of my hands?--droptone (talk) 19:37, 28 December 2007 (UTC)

Let's be clear. Are you saying:

1. it's harder to tear the paper in the side-to-side direction than in the up-and-down direction?
2. it's harder to tear the paper (in any direction) when starting at a freshly-torn edge, than when starting from a factory-cut edge?

If #1, it's definitely the grain. If #2, I'm quite surprised. There are some materials (notably plastic films and plasticized paper) that are very difficult to start tearing from a clean edge, but are quite easy to tear once they've begun tearing. Often, to tear these at all requires starting a cut with a knife or scissors. But since a torn edge contains all sorts of jaggies and other irregularities, I'd expect tearing from a torn edge to be easier, not harder. —Steve Summit (talk) 22:22, 28 December 2007 (UTC)

1. 2 is what I was trying to describe. I don't think it matters that these papers have residue from a type of carbon paper but it is worth mentioning.--droptone (talk) 22:56, 28 December 2007 (UTC)

This really does seem unlikely - but I suppose we might consider a mechanism like this:

• Suppose the paper is not evenly tough - it has weak regions and strong regions.
• Imagine that it's easy to start a tear in a weak region but harder in a strong region.
• When you tore the paper the first time, the tear progressed by tracing an irregular path through the weak regions.
• Hence, on either side of the tear, we now have strong regions.
• On the other hand, the straight cut edge goes randomly through strong and weak regions of the paper.
• Ergo, when you attempt a second tear from the already-torn edge, you immediately encounter strong regions of the paper and it's hard to get a tear started - but when you start from the pre-cut edge, you easily find a weak region to get the tear started in.

I don't like this explanation much - but I suppose it's possible.

SteveBaker (talk) 13:44, 29 December 2007 (UTC)

animals living on humans

I was talking with my 5yo the other day about how there are lots of bacteria on and in our bodies which are either helpful or at least not harmful (generally). Someone else said that there were also animals, and mentioned dust mites as one example. I see from dust mite that sunlight kills them, so it seems that dust mites don't live on people for very long, just on our stuff. Are there other animals that live on or in us, but are not parasites? That is, I know we can have various kinds of worms, lice, fleas, etc., and I'm wondering about microscopic or symbiotic situations. Let me know if I'm not making sense, and I'll try to elaborate. Ingrid (talk) 18:45, 28 December 2007 (UTC)

Your gut flora (mostly but not entirely bacteria) provide an invaluable job in digestion - that's surely the strongest symbiotic relationship we have with any (modern) organism. But if you're looking for something really cool, consider the endosymbiotic theory, which hypothesises that mitochondria (vital structures present in almost all cells) were once independent creatures that have becoming inextricably symbiotically linked to us. -- Finlay McWalter | Talk 18:56, 28 December 2007 (UTC)

Here's demodex,[1] our little friend in the follicles. Richard Avery (talk) 20:04, 28 December 2007 (UTC)

More critters Saudade7 20:26, 28 December 2007 (UTC)

Not to stray off topic but rather to reveal a better understanding of the development of such relationships consider what individual humans are to larger "organisms" such as corporations. Multimillionaire (talk) 20:47, 28 December 2007 (UTC)

Except that Capitalist corporations are the "parasites" living off the life-time of human beings. Maybe if we were part of the fantasy / image in the frontispiece of Hobbes' Leviathan! In other news, Helminthic therapy is interesting! And here is a YouTube Video Ingrid might like!! Saudade7 23:17, 28 December 2007 (UTC)

Concentrations Question

I have a chemisty concentrations question:

What volume of 0.5M sodium hydroxide solution will completely neutralise 35cm³ of 0.25 sulphuric acid?

2 NaOH + H₂SO₄ -> Na₂SO₄ + 2 H₂O.

My answer has turned out at 35cm³ but the answer says 70cm³. Could someone please tell me which one is correct? 88.108.150.120 (talk) 20:09, 28 December 2007 (UTC)—Preceding unsigned comment added by 88.108.150.120 (talk) 19:56, 28 December 2007 (UTC)

As sulfuric acid has two titratable protons per mole, the (effective) concentration of hydroxide ion in 0.5 M NaOH is the same as the (effective) concentration of hydrogen/hydronium in 0.25 M H₂SO₄. Equal volumes of the two will thus neutralize each other, resulting in no net gain or loss of hydrogen ions (and thus no shift in pH). You should be aware, however, that there is a difference between molarity and normality - Normality takes into account the difference in titratable hydrogen numbers. One volume of 1 N base will neutralize one volume of 1 N acid. So if the sulfuric acid is 0.25 normal versus 0.25 molar, you would need two volumes of 0.25 N H₂SO₄ to neutralize one volume of 0.5 N NaOH. - But that doesn't explain the 70 mL answer, either. -- 128.104.112.236 (talk) 20:50, 28 December 2007 (UTC)

One mole of H2SO4 is required to neutralise 2 moles of NaOH: 1M = 35cm3, thus 2M = 35*2 = 70cm3. 86.146.170.122 (talk) 20:45, 29 December 2007 (UTC)

But arent they different concentrations as the sulphuric acid is only 0.25M where Sodium Hydroxide is 0.5M concentration —Preceding unsigned comment added by 88.108.150.120 (talk) 22:19, 30 December 2007 (UTC)

Psychological basis for racial prejudice

Apart from the difficulty of others being able to visually detect dark soil on dark skin, can one basis for racial prejudice be traced and attributed to negative outcome or consequence for the offspring or descendants of racial mix, such as the inheritance of Crohn's and other genetic anomalies or defects? Multimillionaire (talk) 19:57, 28 December 2007 (UTC)

Sure. Why not? Humans are great at rationalizing. As a reference desk, you need to specify if you are looking for studies on this topic or just references in journals and books. This is not a soapbox or discussion forum for tossing around opinions. -- kainaw™ 20:19, 28 December 2007 (UTC)

Understanding the basis for racial prejudice is an anti-soapbox effort, not the other way around. Multimillionaire (talk) 20:35, 28 December 2007 (UTC)

There is differences in opinion about the genetic basis of race see What We Know and What We Don’t Know: Human Genetic Variation and the Social Construction of Race by Joseph L. Graves and other excellent essays for an overview. However, any systematic negative outcome observed from inter-racial breeding these daysis almost certainly a social consequence and not a genetic one. If you believe race is a good correlate for genetic differences then one would generally expect the opposite, due to heterosis, though of course there will always be examples of outbreeding depression also (and obvious example, to use genetic parlance, would be how lighter skin from a black/white cross could be disadvantageous in Equatorial Africa, compared to a black/black cross). If you don't buy a strong race/genetics association, then using race as a basis for anything biological is a rather artificial construct. Rockpocket 20:54, 28 December 2007 (UTC)

I'm reminded of this previous question, which may provide insight, although it's distinctly unpleasant reading. —Steve Summit (talk) 22:24, 28 December 2007 (UTC)

Although the referecned question is more detailed regarding association of skin color and color in the enviroment my question is not hypothetical or personal. I'm seeking a resolve as to why racial prejudice seems to be in many cases much more ingrained. For instance, (excuse please, offensive language may follow) when Blacks hear their skin color described as the same color as shit they often counter with "...we look like chocolate not shit..." Invariably the reaction is not positive but on the order of "...that's the problem... shit looks like chocolate to you so you have no natural objection to shit..." Multimillionaire (talk) 04:34, 29 December 2007 (UTC)

I don't think you are going to find any reliable information on biological justification for that sort of reasining outside proponents of scientific racism. Its a bit like asking if there is good reason for calling red-heads "carrot top" (hint: there isn't). If you wish to learn about why people are racist, Racism#Ideology is a good place to start. Rockpocket 05:57, 29 December 2007 (UTC)

Chewing Wintergreen Lifesavers with your Mouth Open in the Dark! SOLVED!

Resolved

When you perform this trick with friends at summer camp or with your significant other - and the sparks fly! is this a demonstration of Triboluminescence or Piezoelectricity or both or neither? Thanks smart people! Saudade7 20:23, 28 December 2007 (UTC)

Well, it's discussed in Triboluminescence, and that's what I've always read. I doubt that it's piezoelectricity, because you don't feel an electric shock. You can feel a shock from piezoelectric cigarette lighters, for example. -- Coneslayer (talk) 20:27, 28 December 2007 (UTC)

(ec) They demonstrate triboluminescence, but the visibility is greatly enhanced because the lifesavers also fluoresce in response to the light generated from tribo. Dragons flight (talk) 20:29, 28 December 2007 (UTC)

Thanks Coneslayer and Dragons flight! I looked on the Triboluminescence page and didn't see anything so I was going to add it. Apparently my brain is not working correctly today. I think I am coming down with something. Thanks again! And Happy New Year! Saudade7 21:02, 28 December 2007 (UTC)

See also Cecil's answer: http://www.straightdope.com/classics/a1_129.html. —Steve Summit (talk) 22:00, 28 December 2007 (UTC)

Why is toe-walking so bad?

I've noticed that on all articles I can find on toe-walking, it says it should be treated. If it's not related to any other conditions and is most comfortable for the person, why should it be treated? SprayTape (talk) 20:41, 28 December 2007 (UTC)

Toe walking is often a symptom of another problem. The problem should be treated, not necessarily the toe walking. I checked and found no procedures for treating toe walking by itself in the CPT codes. I couldn't even find an ICD for toe walking (just a general "difficulty walking" code). So, unless I'm missing something, it is not normal for U.S. doctors to identify and treat toe walking - otherwise they'd have an ICD to identify it and a CPT to treat it. -- kainaw™ 00:35, 29 December 2007 (UTC)

Toe-walking by itself can contribute to tripping and falling. This may or may not be a problem for the person in question. I have been told many classical dancers are natural toe-walkers. --Una Smith (talk) 03:47, 29 December 2007 (UTC)

I know someone that seems to walk on his toes a bit, and it's apparently because he has a short achilles tendon (which it also mentions in the article toe walking). I don't know if he had treatment when he was younger, but he's now in his 20s and he seems to function just fine. Rfwoolf (talk) 18:52, 29 December 2007 (UTC)

I used to toe-walk until I was in my teens when I gradually grew out of it. In my case it was a symptom of Aspergers Syndrome - it's also prevelant in Autism cases too. In such cases, 'treating' the problem is pointless since you're only attacking a relatively benign symptom. SteveBaker (talk) 17:19, 30 December 2007 (UTC)

The road I used to live on was scattered with many small jagged rocks. When I would go out without shoes, walking on my toes minimized foot-to-road contact and lessened the chance that I would hurt myself. From time to time, I catch myself toe walking. However, looking at the wear patterns on the bottom of my shoes, I don't do it all the time. HYENASTE 04:31, 2 January 2008 (UTC)

Electronic Circuit for Generating a Gaussian Signal

Is there a basic circuit that will create a Gaussian (e^-x^2) signal? I would like to send a constant voltage for a fixed amount of time through the circuit, and receive (as closely as possible) a Gaussian as output (either voltage or current). Thanks! 99.135.13.90 (talk) 22:42, 28 December 2007 (UTC) -Craig Calcaterra

Per a document that Tektronix wrote ages ago, a large number of cascaded RC circuits will approximate a Gaussian response. I don't remember whether or not the RCs needed to be isolated by amplifiers. They designed their oscilloscope vertical amplifiers to display this characteristic.

Atlant (talk) 23:20, 28 December 2007 (UTC)

An infinite number of low pass filter sections will give a perfect Gaussian pulse at the output when fed with a pulse of the correct width (if you can wait for ever). A practical implementation would require the use of buffer amplifiers (to avoid distorting the previous stage's response) and, of course, a finite number of stages. Also, I rather think that the Gaussian response of Tek scopes was more a matter of 'accident' rather than design. After all there are a rather large number of low pass stages all in series inside a scope. This will naturally give give the overall time and frequency response a Gaussian shape.--TreeSmiler (talk) 14:46, 31 December 2007 (UTC)

No, Tek made the statement (in this book) that they designed to achieve a near-Gaussian response. It makes it far easier to evaluate "what you're seeing" compared to the original form of the signal you're viewing, particularly with respect to the signal's rise time and fall time.

Atlant (talk) 18:34, 31 December 2007 (UTC)

Hmmm. Current thinking about the desirability of a Gaussian response appears to be different! [2] Personally I think I prefer Gaussian, as you know what youre getting.--TreeSmiler (talk) 20:26, 31 December 2007 (UTC)

Thanks -- that was a great article! (I do have to say, though, that it did sound to me a little bit like Agilent trying to take some lemons (insufficient sampling rates) and make lemonade. ;). I'll have to pay more attention to what Tek's current policies are vis-a-vis bandwidth versus sampling rates. )

Atlant (talk) 19:42, 3 January 2008 (UTC)

Interesting. I'm imagining the Central Limit Theorem from probability comes into play ("large number of cascaded RCs") to get the Gaussian. But I admit to being a little disappointed. I'm quite ignorant of electronic design, but I was hoping to get a simple "bump" function like the Gaussian there might be a simple circuit. The Gaussian itself would be preferable, but might there be another circuits that generate other symmetric bumps?

99.135.13.90 (talk) 00:07, 29 December 2007 (UTC) -Craig

I agree the central limit theorem comes into play here. If you take a large number of any old band pass filters and put them in series, the amplitude-frequency response will be nearly Gaussian. Also The time domain pulse response will be Gaussian in shape. I believe that the Gaussian filter is the only one that gives the same shape in its frequency domain as it does in the time domain. Interesting? or not true?--TreeSmiler (talk) 00:29, 29 December 2007 (UTC)

Interesting I think: the Fourier transform of the Gaussian function yields a Gaussian function, --TreeSmiler (talk) 00:45, 29 December 2007 (UTC)

Yes it does. It is actually impossible to build such a circuit to do this precisely as it breaks some physical laws. The Gaussian shape goes from negative to positive infinity, so it has to be chopped off. It also means that you have to have a delayed response to your input, whether you like it or not. Graeme Bartlett (talk) 20:44, 29 December 2007 (UTC)

What do you mean by the statement the Gaussian shape goes from negative to positive infinity,?--TreeSmiler (talk) 01:59, 30 December 2007 (UTC)

Oh! in time. I see--TreeSmiler (talk) 14:36, 31 December 2007 (UTC)

Try A CMOS Analog Circuit for Gaussian Functions by J. Madrenas, M. Verleysen, P. Thissen, and J. L. Voz (PDF). This uses 5 CMOS transistors and is intended to be built as an IC, but perhaps you could adapt it for discrete MOSFETs. This version is an approximation of a half-Gaussian pulse that goes to zero in a finite time, thus avoiding the problem rightly pointed out by Graeme Bartlett. Also, I think that you have to drive it using a linear ramp, not a constant voltage. --Heron (talk) 14:29, 31 December 2007 (UTC)

Nice. I'm not worried about the infinite domain, since the exponential vanishes so quickly it's zero for *all* practical purposes after a short time. The larger concern is whether I can "invert(?)" one of these circuits and juxtapose it with an original to get a full bump. 99.135.13.90 (talk) 21:45, 1 January 2008 (UTC) Craig

The paper you mentioned answered this question positively. Thank you all very much for your help. I'll acknowledge your contribution in any publication. -Craig

The Burning of the atmosphere.

I'm running a role-playing game that I'm trying to keep as "hard science fiction" as possible, despite my ridiculous concepts. A certain element of the back story involves a large asteroid being stopped by a force field via friction - which sets the atmosphere on fire, and I have the following questions about such an activity:

1. Is it even possible to set the atmosphere on fire?

2. If it is possible, can friction provide enough heat for ignition?

3. What would happen after such an ignition? Would the surface of the Earth be affected much? (i.e. Would the crust melt slightly or anything?)

4. Is it possible for the planet to somehow regain its atmosphere over time (and how long would it take), or would it take an outside source (e.g. technological terraforming)?

I know this is a strange request, but I'd figure I'd ask anyway. Chris16447 (talk) 23:05, 28 December 2007 (UTC)

For the atmosphere to chemically burn, oxygen must combine with something flammable. There isn't enough flammable material in normal air because most of air is nitrogen, and conditions such as needed for a dust explosion are quite localized. Water needs very high temperatures to separate into hydrogen and oxygen, and that separation consumes energy so will cool the area. A Mars-sized object would create enough heat to make those reactions take place, thus destroying the atmosphere, but the heat would be the driving force of the combustion rather than the combustion causing enough heat to be self-propagating. I'm basing this on the giant impact hypothesis for creation of the Moon, which might have created a rock vapor atmosphere for a few hundred years. -- SEWilco (talk) 00:42, 29 December 2007 (UTC)

I realized I have to correct myself. Depending upon the type of change in the atmosphere which you need, one possibility is to remove the oxygen and increase the carbon dioxide by using a carbonaceous chondrite asteroid. Smashing your asteroid into coal dust and hydrocarbon vapors could certainly produce a lot of burning. -- SEWilco (talk) 01:00, 29 December 2007 (UTC)

Before the Trinity nuclear test, it was debated whether the atomic blast would be sufficient to ignite the atmosphere (burning the nitrogen and oxygen into various nitrogen oxides). This might be an interesting starting point for your reseach.

Atlant (talk) 23:24, 28 December 2007 (UTC)

No, chemical reactions to create nitrogen oxides are endothermic so will cool the area. An exothermic reaction is needed for a continuing fire. -- SEWilco (talk) 00:42, 29 December 2007 (UTC)

Could a larger (i.e. really big, gigaton range perhaps?) nuclear weapon ignite the atmosphere, as a matter of interest? --Kurt Shaped Box (talk) 23:31, 28 December 2007 (UTC)

A large nuclear weapon can not ignite the atmosphere because what was briefly discussed was igniting fusion of the atmosphere. The Manhattan Project scientists dismissed that possibility in short order, I think because the nuclear properties of the atmosphere would counteract the conditions needed for a fusion wavefront. I'd have to fetch one of the books on the Project to give the details. Another indication that high-energy nuclear activity does not easily ignite the atmosphere is that cosmic rays do not ignite it every day. -- SEWilco (talk) 00:42, 29 December 2007 (UTC)

Did you ever specify that the atmosphere on the world you're role-playing in is made of the same stuff as ours? You could say it has a gas, or possibly a dust, in it that can make an exothermic reaction, but has to get to an incredibly high temperature to do so.— Daniel 03:41, 29 December 2007 (UTC)

This is plain old Earth, with the amount of pollution one would expect in the year 1412 (the current time is 1905 - alternative history and all). That is, almost no pollution. The event happened in 1412. I've heard of atmospheric ignition from the Trinity Test article, but I don't know if it is possible. Especially just from an asteroid being heated up with friction (in addition to the normal air compression). If it is deducted that an asteroid simply can't do it, I want to know what natural event can. Chris16447 (talk) 04:42, 29 December 2007 (UTC)

As noted, chemical ignition of the atmosphere is impossible, and nuclear "ignition" cannot be started even by a thermonuclear weapon -- we know that for certain because we're still alive today. So there's no way that an asteroid impact could do anything like that. Massive destruction, yes, but not that way.

So what you need is a way to change the chemical composition of the atmosphere. For example, if you could add enough methane to make up about 6% of the air, that would be a flammable mixture and then an ordinary spark would be enough to blow up the world. Similarly for other gases flammable in air, like hydrogen; of course, the percentage needed would vary.

Considering that you're already positing a science-fictional force field that could prevent an actual impact, I wondered if an encounter with Titan could be made to do the trick -- but it turns out that its atmosphere is mostly nitrogen with only about 1.5% methane. Still, there might be something imaginable along those lines with a fictional foreign body that would supply a greater amount of methane or hydrogen.

--Anonymous, 05:52 UTC, December 29, 2007.

Methane requires hydrogen, so requires a cometary object from the cold Oort cloud. The requested asteroid will do OK. Chondrite#Composition says the CI type of meteorite (or asteroid) is the most similar to the solar composition except for Hydrogen and Helium. Formation and evolution of the Solar System#Pre-solar nebula has a table of isotopes, just toss the H and He and figure out how much carbon you need to burn, and then you can calculate how large a rock you need after including the rest of the stuff. Looks like the iron will need about all the oxygen in the rock when heated, so the carbon in the rock will be consuming Earth's oxygen. If that is a suitable effect for what you need; a blast of coal dust which ignites over a large area (maybe worldwide if rock is blasted into space and reenters). -- SEWilco (talk) 06:21, 29 December 2007 (UTC)

Am I who I was last year?

I was wondering: are áll my cells replaced by new ones after a period of time? If so, are these new ones produced with completely new molecules or with waste of the old ones? Thanks 217.136.141.199 (talk) 23:42, 28 December 2007 (UTC)

In short, the answer is no. Lots of cell types of the human body are replaced completely, after varying periods of time. For example, circulating red blood cells, the lining of your stomach, the cells that detect smells in your nose, and your skin cells are replaced pretty rapidly. However some cells, such as some nerves and muscles cells, are not replaced. Instead we make lots of them (or have modifications to ensure they survive a long time) so when they die off, we still have enough left to carry on their job. When we eventually begin to run short, the brain and muscles stop working so well. This is what happens when we get old. Rockpocket 00:02, 29 December 2007 (UTC)

I heard you are a completely new person after 7 years. Therefore i have a few years to go before completing my metamorphosis.--TreeSmiler (talk) 00:04, 29 December 2007 (UTC)

Thats one of those things that are "common knowledge" but not entirely true, I'm afraid. We know for sure that some cells certainly do not regenerate, though the vast majority do. See here for some more info. Of course, one can certainly become a new person in other ways, if not molecularly. That doesn't take seven years. Rockpocket 00:15, 29 December 2007 (UTC)

Correct. That only takes about 1 year.--TreeSmiler (talk) 00:22, 29 December 2007 (UTC)

Hmm, it took me about 2 1/2 years ;-) hydnjo talk 00:31, 29 December 2007 (UTC)

To do what? —Preceding unsigned comment added by TreeSmiler (talk • contribs) 00:41, 29 December 2007

To become a new person ;-) hydnjo talk 00:50, 29 December 2007 (UTC)

There have been some remarkable publications from Jonas Frisén's group that look at this type of question. (Here's the fulltext of one.) During the Cold War, there was a period of extensive aboveground testing of nuclear weapons in the late fifties and early sixties; most testing moved underground in the mid-1960s. This testing put a large amount of radioactive carbon-14 into the air – much more than is naturally present and that has been historically used for radiocarbon dating – which was incorporated into living things at that time. Since cells retain the same DNA for their entire lifetime, the amount of carbon-14 in a cell's DNA will be related to the amount of carbon-14 in the biosphere at the time that the cell was created. This provides a crude clock to determine the age of cells in the body. Frisén's group demonstrated (among other things) that non-neuronal cells in the brain's neocortex turn over with time, but that neurons did not. TenOfAllTrades(talk) 00:30, 29 December 2007 (UTC)

The carbon-14 in the DNA of 50 cells a second reverts to nitrogen,[3] so some small amount of the stuff doesn't stay in the DNA. -- SEWilco (talk) 01:14, 29 December 2007 (UTC)

If neurons are permanently post-mitotic, might it be because some neuron networks hold our memories which might fade with neurogenesis? hydnjo talk 00:48, 29 December 2007 (UTC)

There are neural stem cells wandering through the brain which do create new brain cells. Whether old memories fade depends upon the mechanisms by which memories are stored, and it is possible that remembering/replaying memories will create duplicate copies of old memories. We need to learn more about memories. -- SEWilco (talk) 01:14, 29 December 2007 (UTC)

Seen some google results saying that 98% of atoms in your body are replaced in 1 year. I also remember Carl Sagan saying something like that. Shniken1 (talk) 06:05, 29 December 2007 (UTC)

If the section title, Am I who I was last year?, is the question, then I'm not sure it matters whether cells are replaced or not, or whether they reuse the same matter. If the question here is more along the lines of "what is selfness?", then perhaps browsing pages like Person, Self (psychology), Self (philosophy), and, as an overall jumping off point, List of basic self topics, might be of use. The gist of it is, as I understand, that whether or not the "you" of this year is made up of new cells and matter, and whether this has implications about who "you" are, is about an meaningful a question as whether a whirlpool in a bathtub is the "same" whirlpool even after a minute has passed, when clearly the water spiraling into the vortex is wholly new. Isn't it the pattern that counts? Pfly (talk) 06:15, 29 December 2007 (UTC)

See: Ship of Theseus Saudade7 13:20, 29 December 2007 (UTC)

Operating Department Practitioners

Hi there, I am considering taking a DipHE course at university in ODP but can't seem to find a lot of information on the job. I know that there are 3 areas, but I mean, are ODPs allowed to administer oral and IV drugs etc, can I do other jobs like.. that aren't regular ODP jobs such as working in A+E Departments, and I read something about working in helicopter emergencies etc. What would that entail? Finally, would I use a stethoscope and are there any courses in scotland that allow me to then study to become a nurse in an accelerated programme?

Any help would be greatly appreciated! Christopher —Preceding unsigned comment added by 79.67.160.202 (talk) 23:47, 28 December 2007 (UTC)