Wikipedia:Reference desk/Archives/Science/2008 March 14

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.

March 14

physic

richard fenyman —Preceding unsigned comment added by 71.10.27.69 (talk) 00:14, 14 March 2008 (UTC)

We just happen to have an article on Richard Feynman, if that's who you're looking for.--VectorPotential^Talk 00:17, 14 March 2008 (UTC)

Quantum Physics

Why does each atom have different allowed energy levels for its electrons? I understand that each electron is actually a "particle-wave" and its orbits are only the ones allowed so the electron-wave won't cancel itself. So, why are there different orbits allowed for each compound and atom? Every electron is the same so how are there different orbits in each atom that don't make them cancel? For example, in one atom the orbits may be consecutively 2eV then 5eV, but in another, there may be an allowed orbit of 3.5eV where in the other atom this would not have been allowed. Why is this? Many thanks, Zrs 12 (talk) 00:18, 14 March 2008 (UTC)

The article on atomic orbitals will give you the full answer on this. However, my simplified answer to this is that the charge in the nucleus is different for each type of atom. Consequently, the electric potential energy is different in different atoms. The full quantum mechanical solution for atoms more complex than helium is not trivial and cannot be expressed as a neat exact mathematical formula. Numerical solution techniques must be used. For hydrogen, an exact expression is possible as derived in the Bohr model but this relies on simplifications that cannot be applied to more complex atoms. SpinningSpark 00:47, 14 March 2008 (UTC)

Mathematical side note: Roughly speaking, the calculation of energy levels involves solving the time-independent Schrödinger equation

$E\Psi =\left[-{\frac {\hbar ^{2}}{2m}}\nabla ^{2}+V(\mathbf {r} )\right]\Psi$

where $E$ is the energy to be solved, $V(\mathbf {r} )$ is the potential energy of the electron. The $V(\mathbf {r} )$ depends on different kind of atom since $V(\mathbf {r} )$ is the integral of electrostatic force $\mathbf {F}$ experienced by the electron and $\mathbf {F}$ is affected by the proton number in the nucleus of different kind of atoms. Therefore, the difference in $V(\mathbf {r} )$ makes different set of energy levels solved. - Justin545 (talk) 02:05, 14 March 2008 (UTC)

And atoms with multiple electrons, have the electrons influencing each other. The Pauli exclusion principle applies as electrons are fermions. It forces electrons to adopt different positions in the atom. Graeme Bartlett (talk) 00:53, 14 March 2008 (UTC)

For a very simplified explanation, see the energy levels article SpinningSpark 01:15, 14 March 2008 (UTC)

What species of seal or walrus is this?

http://www.youtube.com/watch?v=DDg7kWgs5e0 --Sonjaaa (talk) 02:17, 14 March 2008 (UTC)

It is definitely an eared seal, meaning either some species of sea lion or - less likely - some species of fur seal. That leaves a choice of about 17 species. Many of the archetypal performing pinnipeds in circuses etc. are California sea lions. --mglg_(talk) 16:03, 14 March 2008 (UTC)

RL circuit

I am working on a physics laboratory project dealing with an RL circuit (the resistor and inductor are in series with a battery). I am trying to find out the work done by the battery over a period of time from when the circuit is connected, and the energy dissipated from the resistor in that same time period. I realize that to find the work done I need to integrate the expression for the power (P=IV) over time, but I'm not sure how I ought to go about doing that. —Preceding unsigned comment added by 134.129.115.87 (talk) 02:19, 14 March 2008 (UTC)

The work done by the battery is equal to the energy dissipated in the resistor. Is that any help? —Preceding unsigned comment added by SpingMander (talk • contribs) 02:36, 14 March 2008 (UTC)

I don't think that's right. The battery also did work creating the magnetic field around the inductor; the magnetic field represents potential energy which can be returned to the electrical circuit at a later point in time. Integrated over infinity, your answer is correct, but it's not correct over some discrete, finite portion of time.

Atlant (talk) 12:23, 14 March 2008 (UTC)

Total work down is given by the expression: (1/T)(integral 0 to T of (P dt)) where P is the power dissipated in the resistor over time t and T is the time period per osscilation of the RL circuit.

Ozone in the interstellar medium

Molecular oxygen has been found in the interstellar medium. Why has ozone not? There is UV radiation in the ISM from nearby stars, surely this would produce ozone? Is it just that it has not yet been discovered. NB: I know that ozone is short lived but if there is O2 and UV radiation continously present, you will get ozone....right? --Shniken1 (talk) 03:38, 14 March 2008 (UTC)

You also need sufficient density of O₂ to generate the ozone, which is unlikely. — Lomn 04:53, 14 March 2008 (UTC)

Especially, you need a sufficient fraction of the total gas being O₂, because else the atomic oxygen produced by photodissociation will also react with other atoms or molecules, and only a small fraction with O₂ to form O₃. Icek (talk) 22:25, 14 March 2008 (UTC)

Additionally, the formation of ozone needs a 3-particle collision to conserve the momentum (maybe it could - but with a much lower probability - be conserved by the emission of a photon), whereas the destruction of ozone doesn't; this is another reason for the virtual non-existence of ozone in the low-density interstellar medium. Icek (talk) 06:59, 15 March 2008 (UTC)

what decides the direction of rotation of diesel engines?

section created AlmostReadytoFly (talk) 10:20, 14 March 2008 (UTC)

what decides the direction of rotaion of doesel engines--Shoby 2001 (talk) 07:09, 14 March 2008 (UTC)shoby 14 mar 08

4 stroke or 2 stroke? (just kidding) Seriously it depends on the timing of the injections of fuel into the cyclinders - which in turn must match the arrangement of the crankshaft. Also the starter motor must start in this direction.

If you are still bemused I recommend you read a piece on "how diesel engines work" search for this.. Once you know how they work it the answer should be obvious.

Need more explanation?87.102.83.204 (talk) 11:51, 14 March 2008 (UTC)

Aren't some supercharged/turbocharged diesel designs valve-less? Assuming you inject the fuel at Top Dead Center, they'll run in either direction. And aren't some marine diesel engines designed to run in both the "forward" and "reverse" directions?

Atlant (talk) 12:29, 14 March 2008 (UTC)

Your experteese not mine - all I can say is that assuming more than one cylinder I'd still have to control the timing of the fuel injections of the cylinders differently for each rotation direction. eg I'm assuming a 4 cylnder crank has the offsets at 0,90,180,270 degrees?

I'm willing to learn more though, or be wrong.87.102.83.204 (talk) 12:45, 14 March 2008 (UTC)

(correct myself) If the fuel injections are controlled by tappets on the crank then yes - I think you're right that they would work both ways..

As for 'valveless' there need to be something preventing the diesel gas escaping back out (ie a one way valve) - I don't have full understanding of the exact terminology87.102.83.204 (talk) 12:47, 14 March 2008 (UTC)

No, that's the interesting thing with a diesel: The only thing that goes in is air and the only thing that goes out is exhaust. (Unlike an Otto engine that forces in an expensive fuel-air mixture.) Because of this, you can arrange the engine so that it simply has valveless exhaust and inlet ports in the cylinder very near the bottom of the stroke. The super/turbocharger (the "one-way device" you mentioned) then rams a whole lotta air in, forcing out the previous cycle's exhaust gas. The piston then rises, closing off the ports and compression begins. It's not the most volumetrically-efficient way to arrange the engine (because 1) lots of extra volume of air is pushed through the engine and 2) because some of the stroke is "wasted" until the piston passes the ports) but it sure is simple and reliable.

Also, injection pumps used to be driven by a gear-train from the crank so that's effectively the same as if they were driven by tappets on the crank.

Atlant (talk) 16:26, 14 March 2008 (UTC)

Yes. I was thinking in terms of a four stroke 'otto like' diesel - two stroke will work in reverse as you say. So it looks like that the starter motor will determine the rotation in the case of a 2 stroke.

For the reasons you mention I always find it suprising that 2 strokes are more efficeint than 4 strokes. Maybe the 1/2 utilisation of each stroke for power in a 4 stroke tips the balance.87.102.83.204 (talk) 16:42, 14 March 2008 (UTC)

Ahh, here's one article I was thinking about: Opposed piston engine. I should have thought of it sooner seeing as how I took one of the pictures that graces that article ;-),

Atlant (talk) 23:30, 14 March 2008 (UTC)

And here's something similar (with animation!) Napier Deltic 87.102.21.171 (talk) 09:57, 15 March 2008 (UTC)

Knee problems

Does anyone know why traditional Zulu's don't have knee problems? I have searched everywhere and come up with nothing related. Leepylal (talk) 07:53, 14 March 2008 (UTC)

Where did you hear they don't? - Nunh-huh 08:32, 14 March 2008 (UTC)

I'm doing a course in exercise science. They make the statement and ask what factors contribute to this phenomenon. —Preceding unsigned comment added by Leepylal (talk • contribs) 10:40, 14 March 2008 (UTC)

Haven't a clue but people more in line for knee problems (but don't seem to have any to report) are the Maasai for jumping. Julia Rossi (talk) 11:21, 14 March 2008 (UTC)

try this http://pharyngula.org/index/weblog/print/1572/ specifically comment number 27 ? Does that help87.102.83.204 (talk) 11:47, 14 March 2008 (UTC)

The idea 'use it or lose it' comes to mind. Perhaps these traditional Zulus use their legs a great deal traversing the veldt, so they are kept in good repair. Vranak (talk) 21:02, 14 March 2008 (UTC)

Omnipresent God

Does evidence of the superposition of particles that would collapse if observed, such as elelctron difraction, not rule out an omnipresent god, as if he was all seeing and all knowing, he would see mhich gate the electron went through, and therefore destroy the wave diffraction pattern. —Preceding unsigned comment added by 81.140.151.57 (talk) 09:34, 14 March 2008 (UTC)

It's not quite true that electrons intrinsically know that they are being observed. It's simply the case that all of our detection methods alter the electron's behavior, though I'm sure someone can better describe how this works.

That's right, but please note that it is not the clumsiness of measurement that causes the alteration. It is a necessary consequence of the measurement no matter what method is used. Detection requires energy from the detected particle to be transferred to the detection device. Transfer of energy necessarily means the particle must change state or even be destroyed. If you are trying to detect a train, the change in energy is inconsequential. If you are trying to detect fundamental particles you must find them to be in one of the allowed quantum states, which are usually radically different from one another. SpinningSpark 11:53, 14 March 2008 (UTC)

When you get into God and logic, there really is no falsifiability as any number of explanations could be used to allow for exceptions. Perhaps God is incorporeal. Perhaps an all knowing God doesn't need to observe the electrons since He already knows its paths. — Ƶ§œš¹ _{[aɪm ˈfɻɛ̃ⁿdˡi]} 09:51, 14 March 2008 (UTC)

The point the original poster was probably trying to make is that according to mainstream interpretations of various phenomena that are studied in quantum mechanics there is no fact about certain things except that they are in a superposition. And if Bell's Theorem is correct, there cannot be underlying variables which we are either ignorant of or unable to detect due to imprecise equipment.

So the OP wants to know if the universe operates like this, with certain characteristics of matter being in a superposition without clear facts about the certain characteristics other than the fact they are in a superposition, then does that invalidate the notion that a deity knows everything. This conception of the universe is at odds with what most Western religions have thought for quite some time; and it also really perplexes some philosophers of physics. Traditional theology probably defined omniscience as knowing everything that is knowable, so you can't merely object that God can't know false things. This definition seems to evade the worry the OP mentioned since, if there is no fact of the matter in respect to certain characteristics of certain pieces of matter then there is not anything that God would fail to know.--droptone (talk) 16:38, 14 March 2008 (UTC)

Theology isn't really my subject, but I don't see why omniscience would imply knowing properties of a system that, according to quantum mechanics, the system doesn't even have. What quantum mechanics says is that a system can at a given time only have some properties out of the collection of all properties a similar system might have; having certain properties precludes having certain others. (Very roughly speaking it can have half of all properties at a time.) If you know which properties it has, you can in principle measure the values of those properties without affecting the system. If you try to measure a property that it doesn't have, you're effectively forcing it into a state where it has that property at the expense of some other property it used to have, and it's in that sense that you're altering it. There's nothing that would preclude a deity knowing which properties a system has at any given time, and the values of those properties. -- BenRG (talk) 11:58, 14 March 2008 (UTC)

Indeed. Asking which slit the electron went through is like asking what colour is Thursday. If a question has no answer then even God cannot answer it. He could change the universe so that the question did have an answer, but He chooses not to. Gandalf61 (talk) 12:40, 14 March 2008 (UTC)

I see that as a particularly poor example of trying to diminish any respectability to the question, the fact is in two slits experiment if you measure which slit the electron went through, even if you do this by measuring it's "wake" as such, it did go through a slit. But if you dont measure, then the superposition occurs, and patterns of interference with itself are clear. Thursday does not have a colour, even if you try and measure it. If god genuinely saw everything (wether this is a to rigorous definition of omnipresence is another question) then he would know which slit the electron went through and no interference pattern would be made. —Preceding unsigned comment added by 81.140.151.57 (talk) 09:44, 15 March 2008 (UTC)

Well, anyway, this is the old "Does God play dice with the universe?" question, really. Which has as much to do with your ideas about God as it does what flavor of quantum mechanics you ascribe to. Omniscience is pretty poorly defined, in the sense that its actual physical properties are usually not defined, not to the degree necessary to make sense of quantum mechanical implications . Perhaps He would know the possible future outcomes for all quantum configurations? Is God an "observer" in the physical sense or in some other, supernatural sense? Who knows? It isn't exactly a rigorous set of terms. --98.217.18.109 (talk) 12:45, 14 March 2008 (UTC)

It really depends on which god you're talking about. There's this Nordic one I just heard about that might be able to do exactly what you're looking for... Imagine Reason (talk) 00:45, 16 March 2008 (UTC)

Or perhaps God, as it were, could be seen in in the electron...Vance.mcpherson (talk) 20:29, 17 March 2008 (UTC)

Again, which one? Imagine Reason (talk) 03:50, 18 March 2008 (UTC)

Hu? How can you have more than one? The God who created the universe obviously. How's it possible to have more than one? Ariel. (talk) 21:12, 19 March 2008 (UTC)

I myself thought there were two, actually. 66.65.143.85 (talk) 21:02, 20 March 2008 (UTC)

Because God doesn't watch the electron do anything - God causes the electron to do what it does. Every single motion of every single particle in the universe is individually 'animated' by God. Ariel. (talk) 16:18, 18 March 2008 (UTC)

No wonder so many children are starving. Imagine Reason (talk) 17:02, 19 March 2008 (UTC)

Did you post in the wrong section? Ariel. (talk) 21:12, 19 March 2008 (UTC)

Indian pharmaceutical companies

Which pharmaceutical company is first in India Org ratings & how much turn over per year —Preceding unsigned comment added by 121.247.109.61 (talk) 10:49, 14 March 2008 (UTC)

Hope you don't mind, I turned down the volume. No caps lock please. Julia Rossi (talk) 11:23, 14 March 2008 (UTC)

maybe this http://iis-db.stanford.edu/pubs/11893/Smith.pdf will have the answer (p24+?)87.102.83.204 (talk) 11:43, 14 March 2008 (UTC)

or Category:Pharmaceutical_companies_of_India should help if you do the neccessary research.87.102.83.204 (talk) 11:44, 14 March 2008 (UTC)

PS what is "India Org"?87.102.83.204 (talk) 12:16, 14 March 2008 (UTC)

Chemical Process Pipe Labeling

I need the following question answered: When having a mixture of two or more chemicals in a vessel and the mixture is being tranfered to another vessel thru process piping what type of labeling is needed or required? Im not looking for the ansi color code standard, but the exact nomenclature to label the process pipe.

The mixture does consist of the following:

BioDiesel, Methanol, Glycerin, and Sulfuric Acid

So how do I make the correct label in correlation to the most hazardous,the most concentration, specific mixture?

Or do I need to label with all materials within the mixture?

Thank you George Leighlitner CTIBioFuels (email address removed to prevent abuse) Richard Avery (talk) 14:57, 14 March 2008 (UTC) —Preceding unsigned comment added by 66.212.144.50 (talk) 14:30, 14 March 2008 (UTC)

Right I honestly think you are taking the piss here. I apologise for my frank reply.

http://www.ctibiofuels.com/ which I assume is your company appears to be a legitimate business. We are volunteers and are not paid. I would not expect any professional company to be relying upon the advice of unpaid volunteers to give what is essentially health and safety advice - a matter which most likely is covered and governed by applicable laws in your region.

Typically such safety and safe working practice issues are taken very seriously and failure to comply with them can result in criminal prosecution. As such I would imagine that it is your responsibility to hire or employ someone who is professionally qualified and certified to give you advice on this matter.

Please do not rely on volunteers for this information.87.102.83.204 (talk) 15:33, 14 March 2008 (UTC)

You should include the words "Run away!" somewhere in your labelling scheme. --Heron (talk) 20:03, 14 March 2008 (UTC)

Maybe one pipe has Sodium Hydroxide in it instead of Sulfuric acid ? Polypipe Wrangler (talk) 05:56, 15 March 2008 (UTC)

Vance.mcpherson (talk) 20:40, 17 March 2008 (UTC)

Echo above, and add that this will differ heavily depending on your province / state / country. And the intended use of the product, and who's going to be using it. Legal implications about this sort of thing are dire. Especially with that mix of joy-in-a-jar. The good news is that these regulations are easy to find and it's usually inexpensive or free to get yourself trained. See Dangerous Goods. I've never seen any reg's about pipes per se, but there may be such regs in some jurisdictions. I wouldn't be using an oxidizable metal to make that pipe, though.

Evolutionary biology and sexual selection in humans

Hi. I remember reading that studies are now showing that a significant percentage of women trick men into raising children who are not theirs, but I can't find any such article now. Is there a reliable source I can go to for the figure? Thanks. —Preceding unsigned comment added by Imagine Reason (talk • contribs) 15:58, 14 March 2008 (UTC)

Unfortunately, our article on cuckold has no worthwhile references. -- kainaw™ 16:04, 14 March 2008 (UTC)

Mmh women who trick their husbands.. I doubt it would ever be possible to get a reliable figure from people who carry out such a deceit.? Also did you specifically want women who 'trick their men' or to include all step-father type upbringings eg accident, drunk, uncontrollable fling, don't know exactly who the father is,,, etc.?87.102.83.204 (talk) 16:55, 14 March 2008 (UTC)

What the OP is asking about is called "extrapair paternity", and we have a little bit on it at Incidence_of_Monogamy#Incidence_of_genetic_monogamy. --Allen (talk) 17:23, 14 March 2008 (UTC)

To clarify, "extrapair paternity" doesn't apply to stepfathers, but only to cases where the mother's partner does not know he is not the genetic father. That's because rates of extrapair paternity are used to study how males behave differently (e.g. put less effort into raising their partner's offspring) when they cannot be sure that they are the genetic father. --Allen (talk) 17:33, 14 March 2008 (UTC)

The studies looking at this looked at DNA (or maybe just blood types, can't remember) of parents and children.. and found very high rates of cuckolding (maybe 1 in 4).. however the methodology was very flawed.. as the "fathers" were a self selecting group who wanted to know if their child was legit.. so unsurprisingly their suspicions were often founded much more often than you'd expect from the general population. The stats were publicized by an agency which specialized in this sort of test. I don't know if any "legitimate" studies have been done in this area. I also don't have any references. —Pengo 04:36, 15 March 2008 (UTC)

I think that was the study that I read about. A shame, then, that it is not so reliable. Imagine Reason (talk) 15:18, 15 March 2008 (UTC)

It boggles my mind the bad methodology that one often sees in such sorts of surveys. You'd think someone involved would have seen the very obvious problem with that approach. --98.217.18.109 (talk) 20:31, 15 March 2008 (UTC)

Maybe the less conscientious scientists do it more often. It certainly gets them into the news. Imagine Reason (talk) 00:47, 16 March 2008 (UTC)

The conventional rate of "non-paternal events" in a (human) genetic lineage is 10%, though more recent studies suggest a lower rate of about 5%. Even at the lower rate, it is more likely than not that any given chain of 14 generations has been interrupted by an (otherwise unsuspected) non-paternal event. - Nunh-huh 04:57, 15 March 2008 (UTC)

I trust your numbers, but "non-paternal event" seems to include adoption, stepfather relationships, and other situations where the mother's partner knows he is not the genetic father [1]. So they wouldn't play the same evolutionary role as unknowing extrapair paternity. --Allen (talk) 13:36, 15 March 2008 (UTC)

No, these are unexpected non-paternal events within a "known" genealogy. These are non-paternal events unsuspected by genealogists, and in most instances it's reasonable to presume also unsuspected by the (falsely) identified father. - Nunh-huh 01:56, 17 March 2008 (UTC)

3000 Farad "ultracapacitor". Is it a joke?

I've recently seen this [2] on a webpage. Could it be true? A 3000 Farad capacitor? Shouldn't it be larger than the Earth? And needing a Dyson sphere to power it up?

Oops, sorry, I see we have an article about it.. so they must exist for real. So how can it be, that it didn't solved our energy problems? As I know, the storage of power is one of our biggest problems, that's one of the causes why we are trying to develop superconductors. To store solar energy for the night. To store energy in cars and other vehicles. To create man-portable particle beam weapons and railguns. To stop wasting fuel by heating cities at nighttime and cooling them at daytime. --V. Szabolcs (talk) 19:07, 14 March 2008 (UTC)

It exists, but farads do not equal joules. You can only store a small amount of energy in one of those things, and you can only inject or extract a small amount of power. They have an amazing capacitance per unit volume, but not a very high energy per unit volume or per unit mass. If you look at our energy density article, you will see that a typical ultracapacitor has an ED of about 0.02 MJ/kg, compared to a lead-acid battery with about 0.1 MJ/kg. --Heron (talk) 19:48, 14 March 2008 (UTC)

It only operates at 2.7 volts. 3000 farads at that voltage can only store 2250 mAh of charge. About the same as a regular AA cell. SpinningSpark 20:33, 14 March 2008 (UTC)

Yes, but the nice part is that the charge/discharge cycle is substantially more efficient with an ultracapacitor and you can run it hundreds of thousands of times, unlike with rechargeable batteries that do something like 1,000 cycles. For this reason, ultracapacitors are starting to figure into the power trains of hybrid vehicles.

Atlant (talk) 23:40, 14 March 2008 (UTC)

Point taken, Atlant, but ultracaps haven't yet been economically scaled up for big applications like load-levelling of wind and solar power. (Ultracaps are used in the internal works of some wind turbines, but not for load-levelling, as far as I know.) My virtual money is on flow batteries for really big installations. --Heron (talk) 14:03, 15 March 2008 (UTC)

Actually, I saw a talk recently about a demonstration project using ultracaps for fluctuation management on a ~70 MW wind farm. Dragons flight (talk) 16:37, 15 March 2008 (UTC)

Windy metro entrances

Why is there a strong airflow at the ground-level entrances to subway stations? 91.156.148.161 (talk) 19:51, 14 March 2008 (UTC)

Two reasons come to mind. First, to ensure a constant supply of fresh, clean air at platform level, some stations may use forced ventilation to draw in clean surface air. Second, moving subway trains act as large piston heads within their tunnels, forcing air out of the tunnel ahead of them and drawing in air behind. Both of these factors can mean that a subway station will be at a higher or lower air pressure, resulting in a constant (sometimes strong) airflow. TenOfAllTrades(talk) 19:56, 14 March 2008 (UTC)

I don't actually know anything about this (other than many years commuting) but from experience the wind at the entrances seems to me to be particularly steady and therefore unlikely to be anything to do with the trains pushing the air along. This is not the case on the platforms where clearly the train is having a marked effect. Could it be that underground heat is forcing the air to rise? The air is certainly very much warmer underground. One can imagine that the air escapes through one entrance and is replaced by colder air coming in through another lower entrance (possibly even at a different station). The constriction at the entrances would magnify the air velocity so even a quite small effect of this sort could produce a strong wind at the entrance. SpinningSpark 20:46, 14 March 2008 (UTC)

Forced ventilation. There are utility buildings that house huge fans. People ruin "fresh air" pretty fast, between body heat, O2-CO2 respiration, water vapor addition, smoking, body odor, engine exhaust, you name it, we go through air FAST. And, large groups of people go through it faster. So, any restricted area (read as "small cave filled with people") will have massive air turnover systems designed in. And, rather than have three openings (people access, air in, and air out), it is cheaper to have the people opening also serve as one of the air ducts. Next, since the whole purpose is to maintain a constant supply of fresh air, and the people opening is right next to where the bus is idling on the street, belching blue smoke, it is standard to use the people opening as the air exhaust. Go to any subway. You'll note that the wind is coming FROM the subway TO the opening on the sidewalk. Now, how did I get logged out, and where did the SignBot go? -SandyJax (talk) 21:58, 14 March 2008 (UTC)

Thanks for the explanations! Yeah, the air indeed flows from inside the metro station to the opening. 91.156.148.161 (talk) 07:10, 15 March 2008 (UTC)

There is another possibility, although from the description of the air always blowing out, I doubt if this is what's happening here. In a large cave system, with only tiny openings to the outside, wind will blow in or out to equalize the pressure whenever the surface barometric pressure changes. At times this can cause strong winds at cave entrances or narrow points inside the cave. I suspect that this effect is slight in the subway, in comparison with the forced air system, due to the large number of entrances to any subway. StuRat (talk) 18:26, 16 March 2008 (UTC)

Gustatory table of elements

Do molecules, and more specifically elements, have flavours? Could you arange complex molecules in a taste map similar to the colour thing, where they fade into one another? I mean to say, do chemically similar elements taste similar? I think I'm phrasing it badly but that's the best I can do really. 81.96.160.6 (talk) 20:57, 14 March 2008 (UTC)

Yes, but I believe it would be insufferably difficult to do. However, salts taste salty, acids taste sour, bases taste bitter, most sugars taste sweet, etc..etc.. Wisdom89 _{(T / ^C)} 21:03, 14 March 2008 (UTC)

There were informative answers here on a previous question regarding the taste of salts which concluded that all salts do not taste salty. There is some grouping by chemical species but the overall picture is clearly much more complex.

If you were looking for a taste equivalent of colour space it really does not exist. For one thing, human colour vision has only three receptors making the colour space possible to plot graphically. Taste has many more (given that smell forms a large part of taste). Secondly, the receptors are still being discovered and documented and a definitive "map" in terms of the fundamental sensations is not yet possible. And it would need to be multi-dimensional even if all the information were now available. SpinningSpark 22:01, 14 March 2008 (UTC)

Having said all that, many "flavours" that come to mind when you say the word are actually derived from the same family of molecules -- the terpene family. Among the tasty assortment of delicacies associated with terpenes or terpenoids are citrus (limonene), black pepper (sabinene), ginger (zingiberene and other sesquiterpenoids), mint (pulegone, although menthol is not an example), cloves (eugenol), strawberry (linalool and nerolidol), among many, many others. Our olfactory senses seem to have evolved to be particularly discriminating of of terpenes and terpenoids. Recall that the tongue can only perceive sweet, salty, sour or bitter, and the nose does the rest. Take a peek at [3] for a little fun.Vance.mcpherson (talk) 21:21, 17 March 2008 (UTC)

Speed of fire ant destruction

How fast can a bunch of fire ants devour something? I guess it depends on how many there are, so let's say a door is opened and a big room full come out into another room; could a person be threatened? This is a a parody story of a soap opera I'm helping a person write, and I'm using an old idea I had years ago as a kid about an Evil Overlord threatening to take over the world using fire ants to blackmail it. Obviously, I don't need to know exactly, but just ballpark figure of what they could do.

As a side note, I recall as a kid, seeing on...maybe That's Incredible on TV, how a colony of fire ants can devour an animal, all but skeleton, in minutes. But, that's pretty vague, too, it was long ago I saw it. Thanks.63.3.19.1 (talk) 21:18, 14 March 2008 (UTC)

This isn't a science reference but I guess the author researched it – there's an informative scene of fire ant destruction in the Congo in Barbara Kingsolver's novel The Poisonwood Bible though our article is brief and doesn't mention it. Fire ant mentions them in relation to small animals. Julia Rossi (talk) 23:10, 14 March 2008 (UTC)

Can you look up this physical property for me? I've exhaused my available references

I need to know the surface tension of a saturated solution of magnesium nitrate. Can you help? If so, I also like to know where you found it... Thanks! ike9898 (talk) 21:57, 14 March 2008 (UTC)

It's possible that someone has measured this - if you can find chemical Abstracts you could look it up there. It's equally possible I think that this has never been measured before.87.102.21.171 (talk) 09:52, 15 March 2008 (UTC)

Thunderstorms

What's the capacitance of a thunderstorm? --Carnildo (talk) 21:58, 14 March 2008 (UTC)

Not at all sure that anyone has ever analysed thunderstorms in terms of capacitance. However, to have a go at this question, the thunderstorm article states that an average thunderstorm contains 3.6 x 10¹³ joules of energy. The lightning article tells me that 10⁹ volts are required to break down air in a 1000 ft lightning strike. Using the equation (see capacitance article);

$E={\frac {1}{2}}CV^{2}\,\!$

yields,

$C={\frac {2\times 3.6\times 10^{13}}{10^{18}}}$

Which is a piddling 72 μF. Even a big storm with 100 times the energy only comes out to a surprisingly small 7200 μF. It is turning out this way because the square of a very large voltage is an even more enormous number. Whether anyone has any "real" figures for thunderstorm capacitance I don't know. SpinningSpark 23:51, 14 March 2008 (UTC)

Um, let me correct that, I was misreading the energy figure from the thunderstorm article which is the energy of the storm, not the electrical energy. The lightning article gives 5x10⁸ joule for the energy of an average lighting strike. So translating that to capacitance yields;

$C={\frac {2\times 5\times 10^{8}}{10^{18}}}=1\ nF$

Even smaller. SpinningSpark 00:08, 15 March 2008 (UTC)

Another possible approach is to calculate the capacitance directly from the geometry of the storm. Probably, the closest model is a spherical storm above a ground plane. But since some pretty wild assumptions are being made, we might as well use the slightly simpler parallel plate model. For a 500m radius storm, 300m above the ground,

$C=\epsilon _{0}{\frac {\pi \times 500^{2}}{300}}=\epsilon _{0}\pi {\frac {2500}{3}}$

putting in $\epsilon _{0}=8.8541878176\times 10^{-12}$ results in $C=23\ nF$

I make the result for the spherical/ground plane model;

$C=\epsilon _{0}\pi {\frac {4800}{11}}$

just over half the parallel plate answer. SpinningSpark 11:26, 15 March 2008 (UTC)

Being creatively lazy, I just Googled for the answer. The top hit for "thunder cloud capacitance" is an interesting PDF [4] on the subject. It arrives at a figure of 60 nF, so you guys are in the right rounders field.

Digression: That PDF also has an calculation of the power in all the world's thunderstorms, which turns out to be just enough for the electrical needs of one large city. I'll try to remember that fact for the next time someone asks how to harness the power of a thunderstorm. --Heron (talk) 11:55, 15 March 2008 (UTC)

Actually, the 60 nF calculated in that reference is not the same capacitance. They have calculated 'self-capacitance'. That is, the notional capacitance between the storm and an inifinitely large sphere centred on it. On the other hand, my calcualtion above is the capacitance between the storm and ground. The next paragraph in that paper points out that the important capacitance as far as the discharge of lightning is concerned is the capacitance between the storm and the ionised channel once this has been struck. This capacitance they calculate at 9 nF for the same size storm. SpinningSpark 12:55, 15 March 2008 (UTC)