Wikipedia:Reference desk/Archives/Science/2007 May 22

Science desk
< May 21	<< Apr | May | Jun >>	May 23 >

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 22

Number of chromosomes in a Great White Shark?

I am trying to do a project in Biology concerning Great White Sharks. (You pick your own animal.) I have to include the number of chromosomes, and I have been told by my biology teacher that not all animals have the same chromosome number. I've searched on Wikipedia and Google to no prevail. I know every other aspect of sharks, but not this one. Does anyone know the number of them? Mr. Raptor 00:50, 22 May 2007 (UTC)

Chondrichthyes have among the largest genomes of any animals, and the most variable number of chromosomes between species (from 28 to 106). According to Stingo & Rocco (PMID 11841178): Carcharodon carcharias has "a diploid chromosomal number of 2n=82". The paper also notes that this includes "a single pair of sexual heterochromosomes." Rockpocket 01:30, 22 May 2007 (UTC)

I should note also the sexual heterochromosomes were recorded from a male shark. Rockpocket 01:33, 22 May 2007 (UTC)

Indeed. She did not say specifically, so this data will have to do, unless she says otherwise. Mr. Raptor 01:53, 22 May 2007 (UTC)

She did not say what, specifically? Rockpocket 06:46, 22 May 2007 (UTC)

Chromosome#Chromosomes in eukaryotes says that goldfish have 104; that is the closest species to shark listed. 128.12.131.62 01:31, 22 May 2007 (UTC)

"Closest" but not very close. --24.147.86.187 13:43, 23 May 2007 (UTC)

cost of aerogel

how much does aerogel cost ?

Well, a quick Google search gives a few different answers, depending on (a) the substance the aerogel is made of, (b) the manufacturer, (c) the form the aerogel comes in. Working off this order form, you can get 100g of silica aerogel in the form of "random pieces" (whatever that means) for $630, so about $6.30/g, or given the density of silica aerogel given in the article, about 6.3c/cm³. Given that another article suggests "a few dollars per square foot for quarter-inch thick material", which works out to roughly 600cm³ which would be about $30-40, that seems to be about right, to within an order of magnitude or so. Hope that helps ^^; Confusing Manifestation 02:50, 22 May 2007 (UTC)

tanks that helps a lot , by the way do you know how thick this material has to be to effectively isolate all the heat it has the potential to ?

Infinite! No amount of anything will perfectly stop all heat transfer. SteveBaker 16:59, 22 May 2007 (UTC)

Alternatively, *any* amount, as any amount is able to effectively isolate all the heat it has the potential to effectively isolate. Even none at all works, and with aerogel prices, that would be a terrific savings! --TotoBaggins 23:57, 22 May 2007 (UTC)

The picture in our Aerogel article appears to show a 5mm thickness insulating some matches against the heat from a blowtorch. -Arch dude 02:33, 23 May 2007 (UTC)

Laser pointer low on power

Why does my battery power 5mW, wavelength 650nm+-10 clas IIIA laser pointer have enough voltage to turn on the laser for less than a second and it won't show continuously? However, if I release the button and hit it again, I get another short intense burst followed by the same fading rate as before(a LR44 battery and a 1.5 volt 60mAh battery). It shines brighter at the center of the beam as well.

It's nothing special to do with it being a laser - a flashlight or any other battery operated gadget will do the exact same thing. When a battery has been left to 'recover' for a while, it'll get back a few seconds more charge...but eventually, even that goes away. Here are the ugly details of what's happening [1]. SteveBaker 03:25, 22 May 2007 (UTC)

Transistor breakage

I skimmed through the transistor article but could find no answer to my question. Suppose you have a typical NPN transitor. You have 15 MV running from the collector to the emmiter but zero volts from the base to the emmiter. The transitstor would be in cutoff mode but would there be an arc or would the transistor break due to the extreme voltage. Ozone 04:18, 22 May 2007 (UTC)

15 million volts? Most likely the transistor would have gone right past Zener breakdown (tunneling) and avalanche breakdown (impact ionization) and be well into toasty dielectric breakdown. 15 MV across a typical transistor implies absolutely huge electric fields which cannot be sustained by most (if any?) materials without breaking down, and the transistor would surely fry. I'm not sure what you mean by 'arc' since arcing is usually associated with dielectric breakdown in a gas. -- mattb 05:39, 22 May 2007 (UTC)

At 15 million volts, the transistor would not be the only thing breaking. This is well above the dielectric breakdown threshold of air (assuming a standard sized 3-pin transistor package); so unless the transistor is very unusual, this voltage will be wreaking havoc, arcing between pins, leads, circuit traces, and anything else. Nimur 05:58, 22 May 2007 (UTC)

Thank you, Mattb and Nimur. Mattb, I meant arc as in if the transistor didn't fry would there be a arc from the collector to emmiter leads. I'll take a screenshot of the circuit I'm thinking of and post it here soon. Ozone 06:25, 22 May 2007 (UTC)

Ahh, here it is, Circuit. Please ignore colors. My idea was to create a circuit which every few seconds arced (arc in the past tense I assume) between those two leads at the bottom. Would this lower the voltage strain on the transistor or would it be better If I use a relay instead? Or would the relay itself arc or break? Ozone 06:39, 22 May 2007 (UTC)

I have no idea what you're trying to do here, but there's not really an open circuit between the collector and emmitter, meaning there will still be current through it, albeit small. Although, with 15 megavolts, or 15 million volts, that current would still be huge and will easily fry any NPN transistor. Did you maybe mean millivolts? --Wirbelwindヴィルヴェルヴィント (talk) 07:42, 22 May 2007 (UTC)

Here's my idea, the current runs through the inductor for a couple of seconds then it forced to stop by a transistor, this fast change creates a huge emf, it either arcs between those two points or fries the inductor. I do mean MV, that particular number might just be BS from my simulator though.

No BS, that transistor would be toast. It sounds like you need an Autotransformer to step up the voltage, just like Electronic Ignition systems do. In fact, you could use a surplus Ignition coil to experiment with before you try to wind that 7 henry monster. (_Donovan|Geocachernemesis|^Interact) 09:51, 22 May 2007 (UTC)

Power electronics devices such as thyristors can switch several thousand volts. Ratings such as 150kvDC are obtained by stacking the devices. See [2] for a description of some uses. A history of the devices is given at [3]. See also Power electronics. There is a free online course in the topic at [4]. Edison 17:32, 22 May 2007 (UTC)

Judging by the users name, he is probably trying to make an ozone generator. Heres how I would go about it; get hold of a car ignition coil, drive the low voltage winding with the transistor (make sure to use protection diode connected to +12v). Then attach your spark gap across the secondary. Be careful, you could get about 30kV at the output!

This link [5] looks like the circuit you need.

Thank you all. I was not looking to make ozone although that is a surprising coincidence. I'll try to do it with the induction coil. Ozone 03:16, 23 May 2007 (UTC)

Pronunciation of scientific Latin

Is there a standard pronunciation system for binomial nomenclature? Of course different Latin pronunciations are used by different people generally, but I'm wondering if scientific institutions have ever established any standards on the matter. I was thinking of this question specifically in connection with WP:SPOKEN biology articles. Thanks.--Pharos 04:39, 22 May 2007 (UTC)

My understanding is that the purpose of Latin in science traces back to historical written communication between scientists of different native languages. In biology, the use of Latin is to eliminate common-names, for many reasons listed at this article. I doubt there is a strong tradition of formalizing the pronunciations; I imagine a lot of scientists use common names for day-to-day communication unless they specifically need the unique benefits of the Latin version. Of course, in some species (such as microbes?), the common name may be more obscure than the Latin... Nimur 06:02, 22 May 2007 (UTC)

I'll pass along a few hints my Latin teacher gave me (as best as I can remember them). 1-pronounce every letter, 2-C's are always hard, 3-V's are pronounced as W's, 4-J's are pronounced as Y's. There are a few caveats: by every letter, I mean every Latin letter; a lot of AE's are supposed to be Æ's, so you'd pronounce one sound not two. More importantly, not everyone is "up" on conversational Latin anyway and it will sound weird to many folks anyway. Lastly, a lot of taxonomic names are "Latinized" versions of people's names, places, and other stuff. If you're trying to pronounce S. garylarsoni, you're better off to forget Latin and read up on The Far Side. Hope this helps. Matt Deres 16:50, 22 May 2007 (UTC)

Check out this PDF: [6], an excellent, concise (4 pages) guide to all the mysteries of Latin pronunciation. --Tugbug 18:40, 22 May 2007 (UTC)

% of light hitting a solar panel

i know that the best solar panel transfer around 30% of the light that hit them and that the rest is turned to heat , i would like to know if it remains the same % of transfer if the light is concentrated with a lens or if its only with normal sunlight?

I believe the limitation of most solar cells (and plants which engage in photosynthesis, as well) is that they only act on a few narrow light frequencies (colors), while the rest of the light frequencies are turned into heat or reflected. This would be true regardless of how much light was present. However, if you used a prism to only shine the proper frequencies on the solar cells, you could raise the efficiency considerably. Although, unless you do something useful with the "reject frequencies", like heat water with them, there isn't any advantage to separating them out. StuRat 07:27, 22 May 2007 (UTC)

is there a limit to how much of the right frequencie of light a solar panel could tranfer?

Yes the current limitation with silicon-based solar panels is about 20 to 30%. StuRat is quite right in what he says. I believe there is current groundbreaking research into other materials that trap the other frequencies of light (infrared, ultraviolet, etc.) to turn over 50% of solar light into energy. I believe for it to replace current energy sources, the conversion rate has to be over 55%. I can't remember the exact figures or new materials though; saw this on the third episide of 2057... Sandman30s 11:28, 22 May 2007 (UTC)

You can already convert infrared and ultraviolet wavelengths, you just have to choose appropriate material systems. The highest efficiency PV cells to date are called tandem cells (~33% efficiency if memory serves) and use a stack of materials with different band gap energies (generally wider to narrower bandgaps from top to bottom) to effectively convert more of the incident spectrum into electrical energy. The best single-junction PV cells on cheap materials (silicon, usually) actually can acheive internal quantum efficiency close to the maximum theoretical value (limited by the band gap energy of the material and the solar spectrum at a particular airmass). The trick with PV isn't so much trying to hit an impossible efficiency number as trying to make efficient cells extremely cheap. That is, single-junction single-crystalline silicon PV cells just aren't going to get a whole lot more efficient save for a major breakthrough in our understanding of solid state physics. Rather, a lot of focus is on making the existing technology we have cheap enough to be cost effective for more applications. Tandem cells are very expensive to manufacture in bulk due to all the epitaxy typically involved. Even high (external quantum) efficiency single-crystalline silicon cells are pretty expensive, so you see a lot of modern PV research going into polysilicon/amorphous cells and nanocrystaline films. -- mattb 20:01, 22 May 2007 (UTC)

Incidentally, the peak efficiency of a well-made PV cell occurs at a wavelength near its band gap energy and can easily be in the high 90% range. If only the sun was monochromatic! :) -- mattb 20:06, 22 May 2007 (UTC)

A given solar panel has a limited power and temperature rating. If you used a giant magnifying glass or reflector to concentrate excessive sunlight on a normal one, it could destroy it. Full sunlight hits with about 1000 watts per square meter, so about 700 watts or more is either reflected off the panel or turned to heat. Obviously if you concentrated 2, 3, or 4 times full sunlight on an ordinary photovoltaic panel, eventually the heating could become enough to melt solder connections, set wires on fire, or destroy the semiconductor. Special-purpose solar panels could be made for such high temperature applications. Perhaps a dichroic mirror could be used to reflect onto the panel only those wavelengths it is most responsive to, while excluding perhaps the longer wavelengths which produce most heating. A small panel I once owned (sold by Radio Shack) included angled mirrors on each side to increase the sunlight hitting it and increase the output. Edison 15:17, 22 May 2007 (UTC)

If possible, you want to keep the panel at right-angles to the rays of light too - the power drops as the cosine of the angle. SteveBaker 16:54, 22 May 2007 (UTC)

Defibrillate

What would someone experience if they were defibrillated whilst conscious? Just out of interest >.> 86.146.169.68 13:31, 22 May 2007 (UTC)

A man I knew who had a permanently implanted defibrillator as a result of a series of heart attacks said that when it fired, it felt like he had been "kicked in the chest by a mule". 71.57.125.95 13:49, 22 May 2007 (UTC)

Probably a similar feeling to electrocution, the above post seems to be almost putting it lightly -- Phoeba Wright^OBJECTION! 14:07, 22 May 2007 (UTC)

electromagnetic waves

please any one kindly help me with proper theoretical evidence

when light(an electromagnetic wave) falls on an electrical conductor why no emf/induced current is produced.since flux of light with magnetic component changes hencce emf must be induced.(YES/NO?)

Kindly clear my confusion.Sameerdubey.sbp

When 'light' (an electromagnetic wave) eg radio falls on an electrical conductor an EMF is produced (how radios work..) - you've given the reason yourself in the question. Answer seems YES.83.100.183.229 14:53, 22 May 2007 (UTC)

We do have an article on electromagnetic waves. Splintercellguy 14:56, 22 May 2007 (UTC)

For the real (complicated) answer, see plasmon. But simply put, current is induced, but is of such a high frequency that it does not penetrate the surface to any appreciable extent. All that happens is that the light gets reflected.

See also photoelectric effect. DMacks 03:46, 23 May 2007 (UTC)

empirical formula

How can I work out the empirical formula of arsenic sulphide which has a percentage mass of 70% arsenic and 30% sulphur, I have tried several ways and have gotten different answers!

Thank you! xxx

Consider water: it's 89% oxygen and 11% hydrogen by mass. To convert to the relative amounts of atoms, we'll divide mass by atomic weight: (8/9)/16 and (1/9)/1. This gives 1/18 atoms of oxygen and 1/9 atoms of hydrogen. Of course, the empirical formula doesn't have fractional atoms, so we'll multiply all atoms by a constant (in this case, 18) so that all the coefficients are relatively prime. That yields 1 O and 2 H -- the familiar H₂O. Does that help? — Lomn 18:10, 22 May 2007 (UTC)

Light Antenna

Why can't an antenna by used to transmit and receive light? --Tugbug 18:44, 22 May 2007 (UTC)

Take a look at our article on antenna (radio) and especially the section on Resonant frequency. Then think of some common devices or structures that do emit or receive light waves, and think about which parts of them are resonating at the appropriate frequency. Gandalf61 19:07, 22 May 2007 (UTC)

Well... By one definition of "light", antennae can be used to transmit and receive it. It's just a matter of frequency, as Gandalf pointed out. -- mattb 19:48, 22 May 2007 (UTC)

Or, another definition of "antenna" might include optical transmitters and receivers. Nimur 06:08, 23 May 2007 (UTC)

I guess the answer is that an antenna (in the usual sense) that transmits & receives light would have to be extremely small. Thanks. --Tugbug 18:54, 23 May 2007 (UTC)

Snail question

How do you easily take a snail out of its shell? PitchBlack 19:25, 22 May 2007 (UTC)

dead or alive?

Why? Tomgreeny 20:41, 22 May 2007 (UTC)

Dead. But if you'd like to tell me how to while alive, that'd be interesting too. PitchBlack 21:33, 22 May 2007 (UTC)

Our heliciculture article says, Boil about eight minutes, then drain and plunge the snails into cold water. Drain. With a needle or small fork, pick the snails out of their shells. --TotoBaggins 00:09, 23 May 2007 (UTC)

breaking the shell off piece by piece is quicker.

QED photons

According to quantum electrodynamics a photon has a small chance of travelling faster or slower than the speed of light at short distances. How does this work? Why does it cancel out at long distances (probaility or fast-slow cycle)? Thanks, Max. *Max* 21:26, 22 May 2007 (UTC)

On the second question: It does not cancel out. If the speed of the photon over the short distance is not detectable (measured), the speed does not exist (is undefined). If the speed is detectable, and was slower, the delay remains existent. —Preceding unsigned comment added by 84.187.55.83 (talk • contribs) 14:34, 22 May 2007

Great question! Unfortunately, there's not one simple answer. If you break the question down into several smaller pieces, they can be answered one at a time. I hope this helps.

• Why the different behavior on small scales and over long distances?

Theories like QED are made by first writing down a theory of "free particles." A free particle is a single particle in space, unconstrained and not interacting with anything. It just travels forever without changing. Now, you start to "couple" things. The photons and electrons in QED are allowed to interact with each other. But if the coupling constants are not large, you can think of the electrons and photons as being free particles most of the time, with occasional incidents where something different happens. That's basically how perturbation theory works. When there's an interaction taking place (maybe in the middle of some messy Feynman diagram), you can get things that are very, very far from being free particles, like photons going faster or slower than the speed of light. But the theory is weakly coupled, so it always goes back to looking like the free particle theory away from interactions. And in the uncoupled theory, photons always have no mass and travel at the speed of light. "Off-shell" particles get less and less probable.

• What about relativity? How can anything go faster than light?

For this one, fortunately, you don't need to think about field theory. It's the same way for all kinds of waves. If you splash around in a swimming pool, you can make large waves that travel at a particular, constant speed. You can also make a bunch of small waves that slosh around and run into each other. Once in a while, some of the little waves will come together in a way that looks like a single, faster wave. If you spent enough time splashing the water in the pool, you might eventually see what looks like a super-fast wave going all the way across the pool. But it's not really a single wave, it's just a bunch of little waves that happened to line up in the right place at the right time. Well, at some scales, QED is really sloshy. You can picture virtual particles popping in and out of existence in the vacuum, like the little waves in the pool. Sometimes, you can get particles moving faster than light. But it doesn't mean that you can actually send signals faster than light over those short distances. You can think of it as the photon field sloshing around in a way that looks faster than light, without an actual particle or information going that fast. --Reuben 22:13, 22 May 2007 (UTC)

This was a fantastic answer. And I didn't even ask the questio! Thanks, Reuben! Nimur 06:14, 23 May 2007 (UTC)

Thanks, that was very helpful *Max* 21:10, 23 May 2007 (UTC)

Ekman Layer

Hello. I need some help estimating the thickness of an Eckman layer, in terms of atmospheric physics. The correct answer is 654m, but I cannot see how this has been obtained. Hopefully someone can help me out.

The two components of horizontal velocity are given as:

$u=u_{g}[1-exp(-\gamma z)cos(\gamma z)]$

$v=u_{g}exp(\gamma z)sin(\gamma z)$

$\gamma$ has been calculated to be $1.53x10^{-3}$.

→Ollie (talk • contribs) 22:12, 22 May 2007 (UTC)

I have never done this particular calculation before, nor worked with this theory. But if your approach is to set u = v, you might be interested to note that there is more than one value of $(\gamma z)$ that will satisfy that equation. Can you explain your procedure a bit more? Nimur 06:17, 23 May 2007 (UTC)

I was approaching it by saying that the top of the Ekman layer is where $u=u_{g}$, but working that through doesn't give the correct answer - I assume therefore that this is not the correct criteria. I also know that $u=v$ near the surface, so that isn't it either. Beyond that, I'm stuck.

One thing I do note is that $\gamma$ has units of $m^{-1}$ and that the correct answer, 654m is equal to $1/\gamma$. I am beginning to wonder if that is the manner of estimation that my lecturer is after. →Ollie (talk • contribs) 11:19, 23 May 2007 (UTC)

Dimensional analysis is an important tool in physics (of all levels). I think you have already answered your issue. Nimur 14:39, 24 May 2007 (UTC)

Airport x rays

How do those airport x ray machines give such detailed color images?

They use false color imaging; also see Explosive detection#X-ray machines. -- MarcoTolo 00:12, 23 May 2007 (UTC)

You might also enjoy our article on backscatter X-ray. --TotoBaggins 00:13, 23 May 2007 (UTC)

Shot in the heart

How long can a person remain conscious after taking a bullet directly to the heart? I think I read somewhere that it can be several minutes, though how much of that is 'useful' consciousness (i.e. aware enough to scrawl the name of the guy who just killed you on the wall using your own blood), I'm unsure... --Kurt Shaped Box 23:59, 22 May 2007 (UTC)

There are too many factors to consider to give a general answer. You can look at a few articles such as stopping power and terminal ballistics which describe that the bullet it self doesn't actually do that much damage and people have survived a bullet to the heart. Usually shock and blood loss are the major effects from bullet wounds, not trauma caused by the bullet it self. It is the brain, spine and heart which will speed up the process greatly but it's very complicated and there are a lot of variables. I imagine most of the time a bullet to the heart would cause some sort of massive heart attack or complete failure, in which case I don't think you are in any state to be scrawling anything, but that would be most of the time, not all the time.. Vespine 00:57, 23 May 2007 (UTC)

I know that in Martial arts, you can knock someone out in about 10-20 seconds by applying a choke hold that blocks the carotid artery. This stops blood flow to the brain. I imagine that a bullet to the heart would have to be as fast or faster if you assume that the heart is effectively destroyed and stops pumping blood.--Czmtzc 12:32, 23 May 2007 (UTC)