Wikipedia:Reference desk/Archives/Science/2008 October 15

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October 15

physics

why the electric field inside the conductor is zero?if the electric field inside the conductor is zero then how can current flow through it. —Preceding unsigned comment added by 221.120.210.41 (talk) 06:20, 15 October 2008 (UTC)

• You're totally right. The eletric field inside a conductor in only zero in static situations, i.e., if no current flows. baszoetekouw (talk) 08:54, 15 October 2008 (UTC)
  • But if it is a perfect conductor there is no electric field. Current can still flow, but because there is no resistance there is no electric field. Also the current has to flow on the surface of a perfect conductor, the magnetic field lines cannot penetrate. See superconductor Even in an imperfect conductor such as sea water the magnetic field is slow to penetrate and therefore alternating currents cannot go deep into the sea. see skin depth. Graeme Bartlett (talk) 11:29, 15 October 2008 (UTC)

Practical conductors, even a copper bus bar of 100 sq cm cross section, have resistance and have a measurable and significant voltage drop when carrying rated current. A superconductor may be different story. Edison (talk) 14:55, 15 October 2008 (UTC)

The OP was talking about the electric field inside a conductor (not between its ends) being zero. see this [1]--GreenSpigot (talk) 15:10, 15 October 2008 (UTC)

The "sides" of a conductor and the "ends " of the conductor are all "inside" the conductor. It is relevant to the discussion that in a practical conductor, there is a measurable voltage gradient throughout the substance when it carries substantial current. The OP referred to current flowing, and your example shows a conductor in equilibrium, with no current flowing. Edison (talk) 15:53, 15 October 2008 (UTC)

Baszoetekouw is right. Superconductors have zero resistance at DC, but non-zero reactance and even losses (see here under "Resistance") at AC. This means that a changing E field (e.g. a step function) can penetrate the material and thus set up a current. When the E field stops changing, the current keeps flowing (assuming you have a closed circuit) until you stop it by applying a reverse field. --Heron (talk) 19:00, 15 October 2008 (UTC)

Hückel's rule

I am having difficulties grasping the concept of how the aromaticity of a compound is determined in cases such as that of furan. The oxygen atom has two lone pairs, yet just one of the pairs is counted while counting the no. of pi electrons in the cyclic ring (whereby the no. of pi electrons comes to 6, satisfying the condition for huckel's rule). I read about the relation between the hybridization concept and huckel's rule on the furan article page but could not understand it very well. What exactly is the state of hybridization on the oxygen atom in furan? Additionally, how are we supposed to count the pi electrons in a ring in such cases where multiple lone pairs occur? Leif edling (talk) 06:41, 15 October 2008 (UTC)

It's been awhile but here's how I remember how this one works: 3 of the oxygen's electron pairs are sp2 hybridized, and the third pair is unhybridized in the pi orbital. The pi electrons contribute to to aromaiticity. the sp2 hybridized electrons all lie in the plane of the ring: 2 pairs make up the sigma bonds with the neighboring carbon atoms, and the third electron pair is a lone pair. When counting electrons for huckels rule, use whatever electrons are available to you that are committed to pi orbitals, or in this case CAN BE committed to pi orbitals in the correct plane. The lewis structure shows you 2 pairs of pi bonding electrons already in the ring and 2 more available pairs on the oxygen. One pair contributes to the aromaticity, the other becomes a sp2 hybridized lone pair pointing straight out from the molecule (normally it would be sp3 because there are 4 pairs of electrons associated iwth the oxygen right? BUT we lose one p orbital to that pair of electrons stolen for aromaticity, so instead of sp3 we're downgraded to sp2! It looks just like the hybridzation on the carbon atoms in benzene). The combination of aromaticity and significant resonance causes the lone pair to be very tightly bound: loss of the lone pair would destroy the aromaticity, so the molecule is much 'happier' (more stable/lower energy) with the electrons bound. --Shaggorama (talk) 07:23, 15 October 2008 (UTC)

As far as Huckel's rule or hybridization goes, lone pairs can either be part of the hybridization scheme or part of the pi-system as needed. Because lone-pairs are not "tied up" in a specific location, they are free to "shift" to produce whatever geometry will result in the lowest-energy configuration. All other things being equal, oxygen which is singly bonded (as in, say, dimethyl ether,) will assume an sp3 hybridization. However, in furan, the presence of unhybridized p-orbitals on neighboring atoms makes it so that one of the lone-pairs in the oxygen will assume an unhybridized p-orbital state as well, because of a property called conjugation. Basically, p-orbitals in conjugation (aka "delocalized pi-system) are at a lower energy state than the sp2-sp3-sp2 system which would exist if the singly-bonded oxygen maintained its expected geometry. Again, this is all because the lone pairs on oxygen are "mobile" in a way that bonding pairs are not. Analysis of furan shows that it is, indeed a planar molecule, unlike the similar Cyclopentadiene, where the two hydrogens on the lone sp3 carbon lie out of the plane. --Jayron32.talk.contribs 17:52, 15 October 2008 (UTC)

Gem quality Epidote mineral

I was wondering, and looked everywhere for that matter, the odds of finding gem quality Epidote in the united states. The only things i have read have stated Brazil as the main occurence for gem quality. Also i have read that people buy gem oddities such as this, i was wondering what a gem quality would catch per carat. Thanks to anyone that can find, or knows more information then me. —Preceding unsigned comment added by 216.52.133.15 (talk) 07:43, 15 October 2008 (UTC)

'Landing on Jupiter'

We were discussing in physics class yesterday about the gravitational field strength of Jupiter. We then got onto the subject of what happens if you genuinely do try to land on it. I'd love to know what would happen assuming that whatever we 'land' with is able to cope with the high pressures and the pummelling it'd get on the way down. Can anyone talk me through it? Would there eventually be a rocky, or icy, core because of the high pressure or would it still remain gasous all the way through?

Thanks, —Cyclonenim (talk · contribs · email) 07:51, 15 October 2008 (UTC)

• See Jupiter#Internal_structure baszoetekouw (talk) 08:52, 15 October 2008 (UTC)

• Also note that it's been done, insofar as it's possible. --Sean 13:15, 15 October 2008 (UTC)

It sounds like at some level of the Jovian atmosphere conditions are such as to allow a balloon, or a more solid probe with a degree of bouyancy, to float for an extended period. The article about the Galileo probe indicates that it lasted down to a pressure of 23 atmospheres and 150 Celsius, but a probe with a gas bag or a bouyancy chamber filled with gas or liquid could be deployed to float at a higher and cooler level of the atmosphere, or it could ascend or descend like a submarine. Like the bathyscaphe, it could have a large flotation chamber which was not shielded from ambient pressure and a small instrument chamber which was braced to keep lower pressure inside. It could also have a cooling system to keep the instrument chamber cooler than the ambient temperature, with a refrigerant system which transferred heat from the instrument chamber to the flotation fluid or gas in its larger chamber, which would in turn reject heat to the atmosphere. The pressure of 23 atmospheres was not all that high. The manned Bathyscaphe in 1960 descended deeper than 10,000 meters in the Challenger Deep on Earth, where the pressure should have been about 1000 atmospheres, 43 times the pressure the probe was able to endure. Modern submarines are tested to a depth equivalent to 50 atmospheres, and are estimated to be able to survive over 70 atmospheres. Edison (talk) 15:18, 15 October 2008 (UTC)

Have you seen Floating city (science fiction)? It's a similar idea, although not going so deep. --Tango (talk) 16:01, 15 October 2008 (UTC)

You really sould be thinking about a balloon, not a submarine. 50 atmospheres of gas still only has about 5% of the density of water, and hence only ~5% the bouyant force. I don't think anything that looked like submarine could sustain high pressures with so little mass. You could probably design a balloon-like system to be bouyant at a few atmospheres of pressure though. Dragons flight (talk) 16:39, 15 October 2008 (UTC)

You might be interested in the article Comet Shoemaker-Levy 9. --Shaggorama (talk) 16:09, 15 October 2008 (UTC)

The density question was not clearly adressed in the article about the Jupiter probe of a few years ago. I recall a sci-fi story of many years ago about a metal craft which dropped down into the Jovian atmosphere until it reached the layer where its density allowed it to float. With today's technology a Blimp might be a better model than a bathyscaphe, to stay at a higher and cooler level. Lightning and tumultuous wind would be a challenge. I wonder how far down into the atmosphere sunlight penetrates before it is as dark as a cloudy moonless night on earth? Edison (talk) 22:39, 15 October 2008 (UTC)

It seems like it depends on wavelength - see the image caption on the right hand side here. --Tango (talk) 22:54, 15 October 2008 (UTC)

Another interesting feature is that Jupiter's clouds are generally thought to give way to an essentially transparent Hydrogen/Helium sky within 100km or so. So if you could float a blimp there, you could get to look up at the swirling clouds from below. Dragons flight (talk) 01:11, 16 October 2008 (UTC)

Arthur C. Clarke wrote about balloon travel in the atmosphere of Jupiter in his 1971 novella A Meeting with Medusa. Gandalf61 (talk) 13:30, 16 October 2008 (UTC)

orchestra

why doesn't the sound emitted by the instruments of an orchestra interefere with each other? on an unrelated note, are the different instruments arranged in any particular order? thanks —Preceding unsigned comment added by 65.92.231.82 (talk) 08:25, 15 October 2008 (UTC)

Q.2. Yes, they are usually, although various conductors have experimented with alternative layouts. I'm surprised we don't seem to have an article with the traditional orchestral seating plan. Anyone? -- JackofOz (talk) 08:42, 15 October 2008 (UTC)

This is the usual arrangement. -- JackofOz (talk) 21:15, 15 October 2008 (UTC)

Q.1: They do; however, most composers work to make that interference pleasant to the listener. The concept of acoustic beats may also be of interest (tuning the orchestra is used to prevent this) — Lomn 13:05, 15 October 2008 (UTC)

A band or orchestra has the sounds of the instruments combine in various harmonies and dissonances, intended or unintended. "Tuning" is an exercise musical ensembles go through to get all instruments on the same pitch, and if two or more are not tuned exactly the same, audible "beats" are heard, which sound like the volume increasing and decreasing by a number of times per second equal to the difference in the frequencies of the two instruments. String tensions are adjusted, brass instrument tuning slides are pushed in or out, and woodwind mouthpieces are pushed in or out to alter the physical characteristics of the instrument. Edison (talk) 15:24, 15 October 2008 (UTC)

As the preceding responders have stated, the sounds certainly interfere with each other. Each instrument produces a complex wave, and the waves from each instrument add up, producing a very complex signal indeed. The amazing thing is that your brain more often than not is capable of doing the reverse transformation, i.e. to decompose the composite wave into its constituent parts, to allow you to distinguish the oboe from the violin. The article psychoacoustics might be of interest to you, as well as some of the articles it links to, notably missing fundamental and auditory masking. --NorwegianBlue ^talk 21:21, 15 October 2008 (UTC)

Just to make sure i understood what was said, when the orchestra is 'practicing', they're also making sure that there isn't much unwanted interference? Also, for my second question, yes I do know that the orchestra has a certain, fixed arrangement. I was wondering if there was any reason (acoustically) for, say, the violin's being closer to the centre than the cello's etc. THanks

This is one of those ikky things where pure, simple Physics-101 doesn't really cover the bases.

When we think of interference, we're thinking of nice, simple sine-waves that go on for a long time so that when two waves are mixed together, they add together to produce extra large peaks and troughs when the peaks and troughs of the two waves happen to line up - and they cancel out when the peaks of one wave line up with the troughs of another. This is all very wonderful - and you can have it happen before your very eyes in a wave tank or with electronic systems that produce perfectly pure sine wave audio.

But in reality, the waveform produced by a musical instrument is a mixture of an insane number of sine waves - all of different frequencies. Each part of the instrument is vibrating and resonating - each part producing different harmonics and noise. Each sine wave is starting and ending - or changing amplitude or frequency as the note progresses. Look at almost any instrument on an oscilloscope or a spectrum analyser and it's a total mess to look at. The waves within a single instrument are continually interfering with each other for short periods - those effects shift around millisecond by millisecond as the note builds up and dies away.

When you add more instruments, it just gets more and more complicated. Certainly the instruments are interfering with each other periodically. But consider why two seemingly identically tuned violins playing the same note at the same time sound completely different from one violin played at twice the volume. The two violins will never completely cancel each other out and never completely add together because they are subtly different - their dimensions are not the same, the wood they are made from has different thickness and flexibility because they have aged differently and have different amounts of humidity inside and varnish on the surface. The wood grain density is a little different. The bows have different amounts of rosin on them - and the horse-hair the bows are made from came from horses of different genetic makeup giving them different frictional characteristics. The two musicians are applying different amounts of force to bow and string for different durations throughout the note. The instruments are not identically or perfectly in tune and the position of the musician's fingers on the fingerboard are not identical. The distance of each violin from the walls and ceiling of the room are different - and since the musicians are not perfectly still (compared to a wavelength of sound at least), that distance is changing throughout the note. The audience has variable refractive indices depending on how they are dressed - this bends the sound around in yet more complex ways.

The result is that even with two seemingly identical instruments, those interference patterns are totally chaotic - changing so fast throughout the note that it would be utterly impossible for the two instruments to perfectly add or perfectly cancel each other out for more than a tiny fraction of the time. The result (to our ears) is that we hear two separate instruments...which is an amazing feat of calculation in our brains!

Two perfectly tuned electronic music synthesizers - playing the same, simple waveform through really high quality amplifiers and speakers in a large anechoic room WOULD interfere...but we are never in that situation with a real source of live music.

SteveBaker (talk) 13:14, 16 October 2008 (UTC)

OK that's it now. I'm going to start collecting these q's and responses to publish in my Book of SteveBaker. It will sell a million copies and I will get all the money, thank you very much GFDL. :) Franamax (talk) 06:30, 17 October 2008 (UTC)

My understanding on some of these issues-- the ways violins are held tends to make them tilt to the right, which means by putting them on stage right the instruments mainly point their fronts toward the audience. Violins being so important to an orchestra this is more important than for say, violas. Also given the importance of the strings, it makes sense to place them toward the front of the stage. The woodwinds make sense being more in the center, but in front of the louder brass and percussion.

Also on the ability of our brains to hear separate instruments within a mass of sounds-- this isn't so much the case with masses instruments of the same type. For example, while a solo violin has a somewhat biting and piercing sound, a mass of violins, as you get in an orchestra, sounds less like 20-30 individual instruments and more like a single sound, but one notably different from a single violin. Massed violins (or clarinets for that matter) tend to sound smoother, less piercing, richer, etc. Pfly (talk) 05:36, 17 October 2008 (UTC)

Doesn't that depend somewhat on the skill level of the players? The difference between massed violins and clarinets on the one hand, and bassoons on the other hand, is that the clarinets and violins are easier to light, but the bassoons burn longer. (Classic joke from brass players) Edison (talk) 02:45, 18 October 2008 (UTC)

Molecular Formula of Gasoline

What is the molecular formula of gasoline(petrol)? —Preceding unsigned comment added by PunarvasuOMEGA (talk • contribs) 08:26, 15 October 2008 (UTC)

Gasoline is not a pure substance, so it doesn't have a single molecular formula; it is a mixture of different hydrocarbons. Our article says "typical gasoline consists of hydrocarbons with between 5 and 12 carbon atoms per molecule ... benzene (up to 5% by volume), toluene (up to 35% by volume), naphthalene (up to 1% by volume), trimethylbenzene (up to 7% by volume), MTBE (up to 18% by volume) and about ten others". Gandalf61 (talk) 09:34, 15 October 2008 (UTC)

Would a car run well on a single carefully selected hydrocarbon, for instance if someone found a way to add hydrogen molecules to carbon dioxide with the addition of energy from solar or nuclear to create a synfuel? In other words would some single compound like octane serve as auto fuel, or is a blend necessary to achieve proper combustion? Edison (talk) 15:45, 15 October 2008 (UTC)

Propane is used as a fuel, and so is natural gas which is mostly methane so I don't think a blend is necessary. I believe gasoline is a blend because the energy it would take to separate it doesn't actually give you a benefit.-- Mad031683 (talk) 16:23, 15 October 2008 (UTC)

I suspect one could almost certainly find pure substances that a normal car would happily run on. It's less obvious whether a pure substance would outperform a mixture when it comes to variables like efficiency of combustion and pollution control. Some of the things in gasoline are explicitly added to improve performance, and I suspect that even synfuels are likely to be blended for best performance. Dragons flight (talk) 16:28, 15 October 2008 (UTC)

(ec) With minimal modifications, most cars today can be easily adapted to run on pure propane and natural gas (primarily methane). Brazilian flex-fuel vehicles can run on pure ethanol, while flex-fuel vehicles in other jurisdictions are often able to handle up to 85% ethanol (E85 gasoline). I would expect that an unmodified car today could run quite comfortably on pure iso-octane (2,2,4-trimethylpentane), as such a fuel would by definition have an octane rating of 100. TenOfAllTrades(talk) 16:31, 15 October 2008 (UTC)

Orthogonality in OFDM

Why does the fact that the subcarriers used in a OFDM system are equally spaced mean that they are orthogonal? Can anyone provide a clear and simple demonstration of the subcarriers' orthogonality in OFDM? 85.243.50.175 (talk) 15:32, 15 October 2008 (UTC)

Have you read Orthogonality#Radio_communications? --Shaggorama (talk) 16:12, 15 October 2008 (UTC)

Yes, sure. I'm just looking for a couple of explanations from other people in order to formulate a more comprehensive view on the issue. —Preceding unsigned comment added by 85.243.50.175 (talk) 16:33, 15 October 2008 (UTC)

If you build a perfect coherent detector for any one of those carriers it will not respond to any of the other carriers. To demonstrate this integrate over one symbol time the product of one carrier, with another one. When you multiply sine functions you will get a sum frequency and a difference frequency. If this difference frequency is the same as the symbol rate, the integral will be zero. Graeme Bartlett (talk) 20:41, 15 October 2008 (UTC)

Mining collapse question?

What is it called when the surface ground (way above where mining took place) suddenly sinks a few metres because of shifting of the earth or collapsing in mines underground? Or if a deep hole/shaft suddenly opens up like a pothole on the surface due to collapsing hollowness and mining underneath? Are there terms for these changes to the surface's geology?--Sonjaaa (talk) 17:55, 15 October 2008 (UTC)

"Subsidence". Axl ¤ [Talk] 18:15, 15 October 2008 (UTC)

"Subsidence" refers to the process, but does not necessarily imply that it reaches the surface; it could be entirely underground. If a hole forms in the surface, it is called a sinkhole. But both terms could apply to wholly natural effects as well as the consequences of mining. You need to use additional words if you want to limit it to that cause, I think. --Anonymous, 21:40 UTC, October 15, 2008.

"Sinkhole" explicitly refers to subsidence caused by the action of water. It is not a consequence of mining, but may be due to artificial alteration of water courses. Axl ¤ [Talk] 22:14, 15 October 2008 (UTC)

Well, the "sinkhole" article says "Sinkholes also form from human activity, such as the ... collapse of abandoned mines..."; perhaps there are conflicting definitions in use. --Anon, 04:05 UTC, October 16, 2008.

From Sinkholes: "Many ground collapses are labelled sinkholes when they actually belong to a more general category: subsidence." Axl ¤ [Talk] 09:09, 16 October 2008 (UTC)

Acorn

Are all varieties of Acorns edible?--76.28.73.16 (talk) 20:18, 15 October 2008 (UTC)

According to our article Acorn all are, with the exception of toxicity to horses. Some have higher tannin content which takes some processes to deal with whether you're a wildlife creature or a human. Is all there under "Nutrition". Cheers, Julia Rossi (talk) 22:24, 15 October 2008 (UTC)

Other sources say acorns are poisonous, unless specially treated:[2] , [3] , [4] . Edison (talk) 22:33, 15 October 2008 (UTC)

Koreans eat them as a jelly called Dotorimuk. Acorns are poisonous only to the extent that they contain a lot of tannins from some species of oaks, as such if you eat them straight you may get all sorts of nasty stomach problems or nutritional deficiencies. Sjschen (talk) 16:20, 20 October 2008 (UTC)