Wikipedia:Reference desk/Archives/Science/2008 August 30

= August 30 =

about electrons...
why are electrons easier to add or remove from an atom than a proton or neutron ? —Preceding unsigned comment added by 122.163.232.226 (talk) 04:20, 30 August 2008 (UTC)
 * Because they're held by the electromagnetic force, whereas protons and neutrons are subject to the much stronger nuclear force. Clarityfiend (talk) 04:53, 30 August 2008 (UTC)
 * Or, equivalently, because they are farther distant from the nucleus. --Ayacop (talk) 07:20, 30 August 2008 (UTC)
 * Protons will be taken up by bases. In fact a loose proton will react with almost any other substance, (even including helium to make helium hydride or hydrogen to make H3+) so a proton is not hard to add to something, it is extremely reactive! Neutrons are easily absorbed in many nuclei.  Electrons are very light, but yet not many negative ions are stable. Graeme Bartlett (talk) 09:00, 30 August 2008 (UTC)
 * Ayacop, that isn't equivalent. They're held by different forces. The protons are actually repulsed by the force that holds the electrons in.
 * Graeme, You're talking about adding and removing protons from a molecule. 122 is talking about adding it to an atom. If you added a proton atomically to a hydrogen atom, you'd get Helium-2, which would immediately decay into Deuterium, and emit a beta particle. This is nuclear fusion, and is very different, and far more difficult, than if you just added a proton molecularly and made H2+. — DanielLC 15:58, 30 August 2008 (UTC)

What's stopping electrons from actually hitting and joining the nucleus along with the protons and neutrons? ScienceApe (talk) 03:24, 31 August 2008 (UTC)


 * Among other reasons, the Pauli_Exclusion_Principle. However note that electrons on s orbitals have the peak of their wavefunction right in the center of the core. —Preceding unsigned comment added by 84.187.60.4 (talk) 12:08, 31 August 2008 (UTC)


 * In the case of the s orbitals, it's because the nucleus is so tiny. The waveform of the electron can collapse anywhere in the s orbital, and is more likely to do so closer to the nucleus, but it's still not likely enough to make the electrons hit such a small target. The Pauli Exclusion Principle just explains why not all of the electrons are in the smallest orbital, and doesn't have much to do with the question. — DanielLC 16:12, 31 August 2008 (UTC)

Purple skin
Why do some people's skin turn purple when they are cold? Does it have something to do with the blood? Minor Contributer (talk) 05:25, 30 August 2008 (UTC)
 * yes, the blood has its circulation rate reduced in the cold skin so that it does not lose heat. When the blood has oxygen consumed it changes colour from bright red to a dull red or purple. Combined with the bluish colour of empty skin it can look purple. Graeme Bartlett (talk) 09:04, 30 August 2008 (UTC)
 * Why do some people turn purple more quickly or more oftenly than others? Minor Contributer (talk) 15:24, 30 August 2008 (UTC)
 * Having poor circulation can do it. You may find Raynaud's syndrome interesting. --Tango (talk) 18:50, 30 August 2008 (UTC)
 * Another obvious thing, the lighter your skin the easier it would be to see any change in blood colour Nil Einne (talk) 01:16, 31 August 2008 (UTC)

Optical Activity
Hi all,

I was just wondering how do you determine if a moleucle is optically active and why (or explaining why it is/isn't optically active). I know if a molecule has symmetry (i.e. it is achiral) it is not optically active, also, if a molecule does not have a chiral carbon it too is not optically active. But how do i determine if something like 2-bromopentane or 2-bromo-3-methylpentane (both formed from an addition reaction) is optically active?

Thanks heaps —Preceding unsigned comment added by 122.108.169.75 (talk) 05:26, 30 August 2008 (UTC)
 * If a molecule is not symmetric, it is optically active. So 2-bromopropane is optically active (and you can look up its specific rotation in the usual literature sources, maybe even on wikipedia). Now whether a particular sample of the compound has optical activity, that takes a physical measurement in a polarimeter. If you have a 50/50 mixture of both enantiomers of a compound, the effect of each will cancel the other and the net rotation will be zero. DMacks (talk) 06:35, 30 August 2008 (UTC)

So does optical activity only depend on the symmetry of a molecule? what if I know that there was a satureated molecule (alkene) and it under went an addition reaction to give me 2-bromopentane ? does the addition reaction tell me anything? Also, does a chiral carbon tell me anything about the optical activity of a molecule? —Preceding unsigned comment added by 122.108.169.75 (talk) 07:04, 30 August 2008 (UTC)
 * Optical activity does not depend on symmetry, nor on chirality, i.e. asymmetric molecules need not have a specific activity. That's why you see compounds with (+) and (-) labeled in addition to D- and L- specification. See also cryptochirality. Optical activity has to be measured. --Ayacop (talk) 07:17, 30 August 2008 (UTC)
 * Knowing the optical activity of a sample of a compound that is not symmetric can tell you whether the sample is just a single isomer or is a mixture of stereochemistries. But you would need some (potentially difficult) experiment or (very easy) literature search to figure out which isomer you have based on its optical activity. If you know whether your sample is "optically pure" (i.e., a single enantiomer) vs a racemic mixture, you can know something about the reaction mechanism. DMacks (talk) 07:36, 30 August 2008 (UTC)

My homework question asks to explain why 2-chloropentane is not optically active after it is formed via the addition reaction of pent-1-ene and HCl. the molecule is not chiral, so i thought would be optically active, however, how do i explain it is not? —Preceding unsigned comment added by 122.108.169.75 (talk) 08:22, 30 August 2008 (UTC)
 * one molecule of 2-chloropentane is chiral, with the number 2 carbon being substituted with hydrogen chlorine methyl and propyl. However which side would the chlorine add on?  There is nothing to determine a left or right hand chirality, so both are made. Graeme Bartlett (talk) 09:11, 30 August 2008 (UTC)

Sorry i had meant to say that i know the molecule (2-chloropentane) is not achiral, so being that it is not symmetrical, shouldn't it be optically active? so why isn't it? —Preceding unsigned comment added by 122.108.169.75 (talk) 10:19, 30 August 2008 (UTC)


 * I think DMacks hinted at the answer above and Graeme Bartlett gave you an even stronger hint. Take a look at racemic mixture. Gandalf61 (talk) 15:46, 30 August 2008 (UTC)

Distortion in half-wave rectifier.
Why distortion happened in half-wave rectifier with high frequencysRonilove (talk) 09:29, 30 August 2008 (UTC)
 * Because it chops off half the wave, so the output does not resemble the input. This is if you are not using it as an amplitude modulation demodulator.  Distortion could arise in demodulcation, because after rectification you filter with a capacitor to remove RF, and this could reduce high frequencies. Graeme Bartlett (talk) 10:13, 30 August 2008 (UTC)
 * I wonder if there might be more distortion at high frequency because of the time of the rectifier to turn on, and the bias level for turn on, and if this might cause part of the waveform to be clipped off. What is "high frequency" depends on the response of the rectifier, its capacitance and other factors. Edison2 (talk) 19:30, 30 August 2008 (UTC)


 * Some context from the OP would help here. Fourier analysis of an unfiltered half-wave rectified sine wave shows that it has a very high harmonic distortion content anyway.  Sp in ni  ng  Spark  21:35, 30 August 2008 (UTC)

Cause of Quantum Decoherence?
According to the section Problems of the article Quantum computer, there are number of practical difficulties in building a quantum computer. One of the major difficulties is keeping the components of the computer in a coherent state. But I still get no sense of what interaction will cause the system to decohere... Is it environmental temperature? some form of noise? CMB? movement of planets, stars? cosmic inflation? Any ideas? Thanks - Justin545 (talk) 11:45, 30 August 2008 (UTC)
 * What I've usually heard is thermal noise. With say a silicon chip with a quantum dot on it, the chip emits a photon which hits the dot and ruins its superposition. Or the dot could emit a photon that hits the chip. Because of this, proper cooling is a preequisite of many quantum computer blueprints. EverGreg (talk) 15:43, 30 August 2008 (UTC)

Inventing VS Engineering
What exactly is the difference between an inventor and an engineer?

I was under the impression that an inventor is just an amateur engineer, is this true? —Preceding unsigned comment added by Kenjibeast (talk • contribs) 15:41, 30 August 2008 (UTC)
 * Inventor and engineer should help you differenciate the two. "Engineers are concerned with developing economical and safe solutions to practical problems, by applying mathematics and scientific knowledge while considering technical constraints." and "An inventor is a person who creates or discovers a new method, form, device or other useful means." are the key sentences. —Cyclonenim (talk · contribs · email) 15:46, 30 August 2008 (UTC)


 * An inventor is someone who had the idea for a device and usually also built it. This someone dosn't have to be an engineer by education. An engineer plan, develop, improve and build devices among other things, but can do their job fine without coming up with a new, original device. There's of course a grey area between an improved and a new device, but you can generally say that the person is an inventor of a new device if he/she can get a patent on it. Also see the term inventor in patent law EverGreg (talk) 15:53, 30 August 2008 (UTC)


 * There's no reason an inventor has to be an amateur. And there are plenty of inventors that are not engineers. Consider how many patents are filed each year by physicists, biologists, etc. --98.217.8.46 (talk) 17:22, 30 August 2008 (UTC)


 * "engineer" is more of a job title. "Inventor" is just a description of something you've already done. APL (talk) 19:09, 30 August 2008 (UTC)


 * In modern times to call yourself an "engineeer" you need to be a graduate of an accredited college in an engineering curriculum. Experience and the passing of tests are required to be a "Professional engineer" entitled to do consulting for others. Some famous inventors of 19th century electrical gadgets, like Samuel Morse and Alexander Graham Bell knew virtually nothing about electricity and had assistants who understood electricity actually develop and build the invention, based on their idea. Others like Thomas Edison, Nikola Tesla and the Wright Brothers were knowledgable of the intricacies of most of their "inventions" and many qualified as engineers by the standards of the time. An invention is often an improvement on someone's earlier idea that did not quite work in a usable sense, perhaps incorporating other improved technologies. (edited)Edison2 (talk) 19:27, 30 August 2008 (UTC)

Spider in My Front Yard
Can somebody identify this spider I found in my front yard? Thanks.--Xp54321 (Hello! • Contribs ) 19:21, 30 August 2008 (UTC)
 * When asking this kind of question it's important to tell us what part of the world you live in. Australia, Scotland and Mexico have very different bugs.  --Sean 19:35, 30 August 2008 (UTC)
 * Oxnard.:)--Xp54321 (Hello! • Contribs ) 19:36, 30 August 2008 (UTC)
 * Actually it looks a lot like a Orb-weaver spider which is found in almost every corner of the globe. (If someone finds one on Mars I would be surprised but not shocked.) —Preceding unsigned comment added by 71.100.4.91 (talk) 21:00, 30 August 2008 (UTC)
 * I have re-arranged your photo so that it doesn't go into the next section. Right-floating is better for left-to-right languages like English. --antilivedT 00:18, 31 August 2008 (UTC)
 * Thanks!--Xp54321 (Hello! • Contribs ) 00:22, 31 August 2008 (UTC)

Helicopters unable to fly above a pit
Here is an interesting question found on this talk page of this article Udachnaya pipe

"should there be something noted about helicopters unable to fly above the pit? from what has been said or what ive heard, flight above the area causes damage to the helicopter, for example being completely destroyed. i am unsure of the research required, and would appreciate help in this area as with being unable to relocate the source of information, however use of the search query in google may provide sufficient sources of information. thanks."

16@r (talk) 19:48, 30 August 2008 (UTC)


 * It seems really impossible that flight over such a thing would result in some mysterious damage to the helicopter. THe person who posted that doesn't seem to have any references or proof of any kind.  I'd ignore it until/unless proof is produced.  If that person attempts to add that information into the article without some pretty decent references - just revert it.  Extraordinary claims require extraordinary evidence. SteveBaker (talk) 21:19, 30 August 2008 (UTC)


 * I'm thinking something along the lines of very strong, unpredictable updraughts coming up from the pit walls - or perhaps some odd wind vortex effect due to the particular shape of the excavation... --Kurt Shaped Box (talk) 22:05, 30 August 2008 (UTC)
 * Given the north latitude and the depth, it seems plausible that heat release from the relatively deeper earth might cause turbulent updrafting over the pit. Wind effects are also an interesting possibility. Franamax (talk) 22:25, 30 August 2008 (UTC)


 * I've heard many, many times before that you can't fly over it with a helicopter, precisely because air currents -- but downdrafts rather than updrafts. I don't know whether that's true or just a story, but in any case, the idea of something that big affecting air currents certainly doesn't sound implausible to me. That said, though, they certainly wouldn't damage the helicopter by themselves, but they would make flying over the pit dangerous and could cause a chopper to crash. An example of these stories can be found at here. -- Captain Disdain (talk) 01:24, 31 August 2008 (UTC)


 * A halfway decent pilot can cope with updrafts and downdrafts if he/she knows they are likely. Unexpected vertical air motion is kinda dangerous - but if you were flying over a big hole in the ground, you'd be expecting it.  Look at the number of helicopter flights they take into an out of the Grand Canyon for example.   Any steep slope or cliff-face can be the source of an updraft (if the wind is blowing up the slope) or a downdraft (if it's blowing the other way).  Helicopters can operate reasonably close to cliff faces and large buildings without problems.  So this can't be true.  I wouldn't find it hard to believe that helicopters have crashed there - perhaps even due to some careless piloting and that might be the source of some kind of an urban legend - but I used to make flight simulators for helicopters and we'd simulate all kinds of vertical air motion (both expected and unexpected) that pilots would train for.  This pit might make it harder to fly a helicopter over/into - but there is no way it's some kind of magical helicopter crusher that mangles anything that comes nearby - that's just ridiculous. SteveBaker (talk) 15:26, 31 August 2008 (UTC)


 * I think (well, I've always assumed, anyway) that this has involved situations where the chopper is coming in low and actually going into the pit rather than flying a safe distance above it. That's got no basis on anything other than my imagination, though. -- Captain Disdain (talk) 17:59, 31 August 2008 (UTC)
 * But even that should be no worse than landing on a helipad on top of a tall building. The up/down-drafts around those things are phenomenal but pilots seem to have no problem sticking landings on them all the time. SteveBaker (talk) 20:15, 31 August 2008 (UTC)


 * Sure. I mean, maybe it was some cowboy pilot with a couple of shots of vodka under his belt. Or just someone who wasn't really qualified to pilot a helicopter; after all this is Russia; I wouldn't be surprised to hear of someone who would never pass the FAA's standards screwing up. And I'm not putting the country down, mind you, but, uh, a lot of the time, they do things a little differently over there. Or maybe it never happened at all. Who knows?


 * (Speaking of the way things are done in Russia, an unrelated, but amusing anecdote: friend of mine was shooting a science fiction action movie over there, and the production included cooperation from the local army guys, who were hired as extras. Someone in the prop department screwed up and they didn't have the scifi guns for them, so they simply arranged to dress up the troops' rifles on the spot. That was a couple of hundred AK-74s that were actually army property, altered so they could no longer be fired or even properly aimed, because of the scifi crap they welded on them. I mean, I'm sure they compensated them, but from what I understand, the decision was pretty much made on the spot, because they were in a hurry, but I kind of doubt the guy making the sale actually had authorization to sell working weapons to a movie crew. Nobody thought it was a big deal, but my friend, not being Russian, was kinda nervous about the whole thing. Later on they also blew up a whole bunch of aircraft... allegedly because it was cheaper, faster, and easier than using special effects. So, you know, the idea of some unqualified guy just jumping in a helicopter and crashing it when the air currents prove surprisingly treacherous strikes me as entirely credible. But really, who knows?) -- Captain Disdain (talk) 00:48, 1 September 2008 (UTC)


 * I personally blame the Russian army's experimental EMP howitzer firing range just down the road. The one they haven't told us about yet. --Kurt Shaped Box (talk) 19:27, 31 August 2008 (UTC)

bipolar transistor
Is there a replacement or substitute for the old (1980) bipolar 100mhz, 115 volt, 10 amp, 100 watt complementary PNP/NPN transistors? —Preceding unsigned comment added by 71.100.4.91 (talk) 20:57, 30 August 2008 (UTC)


 * Do you have a part designation for that?  Sp in ni ng  Spark  21:24, 30 August 2008 (UTC)


 * You might find something here that will do the job.  Sp in ni ng  Spark  22:01, 30 August 2008 (UTC)


 * Are you sure about the 100MHz? Finding a 100W Pdiss, 115v Vcb, 10A Ic transistor with that fT is going to be difficult if not impossible.--79.76.176.172 (talk) 02:15, 31 August 2008 (UTC)

Butterfly Bee Insect (Unknown)
Hello,

I have a question about an insect that keeps returning to my Butterfly Bush here in lower Delaware. The insect aforementioned is about the size of a large Bumble-Bee, it consists of a black and yellow striped pattern, wings like a bee, mouth and feelers of a butterfly, and an odd fan shaped tail. It does not seem aggresive as I have been studying it for the last two months, and mingles with the other insects feeding on the bush without incident. I could not find it or any reference in any insect books I have, or google. Any help you could give me would be outstanding.
 * Thank You,
 * Brent —Preceding unsigned comment added by 209.244.188.217 (talk) 23:13, 30 August 2008 (UTC)
 * A bee hawk moth perhaps? Nanonic (talk) 23:23, 30 August 2008 (UTC)
 * Or a related bug in the family Sphingidae. Deor (talk) 00:12, 31 August 2008 (UTC)


 * Hemaris thysbe is found in Delaware and would fit the bill.--Eriastrum (talk) 00:28, 31 August 2008 (UTC)


 * Side note: I planted a few Butterfly Bush plants near the end of my sojourn in my ex-Ontario home with the hard-fought backyard naturalized (wilderness) area. This was for the express purpose of atrracting more butterflies, which they did. Much to my chagrin, given my "naturist" mindset, when I looked up the plant species, they both turned out to be of Asian origin and listed as potential invasive species in temperate North America. Luckily, I didn't water them enough at the start, so I didn't have to actually kill a living thing, my problem was solved after two months. But beware! - Butterfly Bush is not a native-adapted plant. Franamax (talk) 00:37, 31 August 2008 (UTC)