Wikipedia:Reference desk/Archives/Science/2009 September 1

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September 1

Mice Trap

Hi evry one .... i have abig problem with mice and i tried evry thing , but it just wont work , this creature is so clever .

so is an act of dispair i tried to make an electric trap ...

strik one : i take a phase from the source and connect it to a plate then connect the other side of the plate with the earth,then i put some cheese , nothing happened except he got hte cheese.

strike two: i connect a fan to the power then connect the plate to the nutral coming back from the fan , also no use .

strike three : i talked to afreind about the secound try and he said i had should connect the plate to the phase comming from the source to feed the fan because the power coming back wont be enuogh to kill the mice , so i did that and still nothing.

please what should i do , these mice are killing me , and i'am running out of cheese.

why these trys failed....--Mjaafreh2008 (talk) 06:11, 1 September 2009 (UTC)

Mice are fast, but not really that clever. Try some of the many types of traps listed at Mousetrap? If they don't work, its unlikely your Heath Robinsonesque trap will. Rockpocket 06:28, 1 September 2009 (UTC)

Get a cat or two. They'll keep the mice under control. Baseball Bugs ^{What's up, Doc?} carrots 08:54, 1 September 2009 (UTC)

I understood that you connected the phase to the plate and then the other side (same connection!) to the earth. Then how would you expect to cause a small electric shock for instance? It is already a short circuit to the earth, and no voltage drop will go across the rat or mouse. The same problem is done in the second step, where the neutral has almost 0 volts with the earth at all.--Email4mobile (talk) 09:02, 1 September 2009 (UTC)

I had a mouse and tried many method of getting rid of it (I even tried the Manuel Noriega method of playing deafening music). What ultimately worked for me was glue traps, and, as a bonus, they also trap insects. Unfortunately, they are exceedingly cruel, leading the mouse to rip off half of it's face in an attempt to escape. When I saw this I finished him off by drowning, which surprisingly only takes about 12 seconds for a mouse. StuRat (talk) 13:35, 1 September 2009 (UTC)

Mice are not clever, but they are cautious. This is what our pest control professional told me. Start with a regular mousetrap, and bait it but don't set it (peanut butter works well). Leave the trap there until there are signs of the bait having been eaten. Then rebait and set the trap. DJ Clayworth (talk) 14:41, 1 September 2009 (UTC)

Why try to reinvent the wheel? Victor sells an effective battery powered electronic mouse trap [1]. It uses a computerized circuit to detect the presence of the mouse with a low voltage resistance measurement circuit, then when a mouse is present, it applies pulses of several thousand volts several hundred times a second for several seconds. It signals with a flashing LED when the mouse has been killed. The mouse is not left struggling on a glue trap for days, and it is not likely to get away like with snap traps. Nor does it get a leg caught in a snap trap. 4 AA batteries are enough to serially kill 50 mice. Edison (talk) 16:26, 1 September 2009 (UTC)

hi guys .... can any one explain to me whats wrong with my circuts , for example the first try ... why did'nt the mice died he stand over a plate connect to the phase from aside and with the earth from the other side...if any one of us grap awire connected to the power source while his legs touching the ground he will died , is that right or what...? its not the mice problem only ... i need to understand why it does'nt work..?--Mjaafreh2008 (talk) 18:26, 1 September 2009 (UTC)

Dude! If you are serious?? If you are i think you are lucky you haven't fried yourself yet. Do NOT play with mains power, far out. I'm surprised no one has said this yet. You must be crazy. Trust me, that is NOT the best way to kill the mouse, it's the best way to kill yourself! And if you do manage to kill the mouse, you'll quite possibly also start a fire and burn your house down. Firstly and hopefully your house is fitted with a Residual-current device which maybe has already saved your life. If you don't then it's probably just lucky you aren't already dead. STOP immediately and find a purpose built mouse extermination solution. Vespine (talk) 02:09, 2 September 2009 (UTC)

I agree, there's no need to risk your life to dispatch a mouse. As for the glue-board method, there's no need for them to last for days, just check your traps a couple times a day. The mice also seem to start squeaking when so trapped, so they act as an alarm to alert you that the trap has worked. StuRat (talk) 15:45, 2 September 2009 (UTC)

I agree that the experimentation you are doing is likely to result in someone dead. What if a child finds it and picks up the wires? Or a pet? Or a janitor/cop/grandma? From your talk of hooking a fan into the circuit, it just sounds like you are not skilled enough to be working with mains voltage electricity. Please stop before you or someone else gets injured or killed. Your vaguely described circuit had no provision to make sure the mouse was touching phase and ground at the same time. Even if he were standing on the grounded part, his whiskers might vibrate and alert him of the presence of high voltage on the other part, or he might touch it tentatively and just get a painful shock, causing him to jump off. The commercial electronic trap as I said uses a low voltage, probably undetectable, to make a continuous resistance measurement to detect resistance below one megohm, then applies thousands of volts in a series of pulses for the kill. It is built so there is no quick escape, and it is built so it is unlikely someone would poke a finger inside and get injured. Edison (talk) 15:48, 2 September 2009 (UTC)

Strange calculations of C

Can you have a look at this website [2]; although it is discussing the speed of light in a sort of religion or philosophy. I tried hard to understand the calculations but because I lack for sophisticated knowledge in astronomy, I think someone here might understand them better. What I need to understand; are those calculations completely true and accurate? or is there some trick in the calculations?--Email4mobile (talk) 07:56, 1 September 2009 (UTC)

here is a quick link to the calculations [3].--Email4mobile (talk) 07:59, 1 September 2009 (UTC)

It looks like a big fudge to me. Cutting through the verbiage and hyperbole, here's what seem to be going on at that site:

1. A rather strained interpretation of a verse from the Quran that says "this affair travels to Him a distance in one day, at a measure of one thousand years of what you count" is used to create the claim that the distance travelled by light in one day is equal to the distance travelled by the Moon in 12,000 orbits around the Earth. First fudge: there are not exactly 12 lunar orbits in one year - even if you use the synodic period of the Moon, 29.5 days, the Moon goes through approximately 12.4 cycles of phases in one year. But if you calculate the distance that the Moon travels in 1,000 years you get about 1.24 light days, which is 24% greater than claimed - so the time period has been rounded down to 12,000 lunar cycles to reduce this error.
2. A calculation of how far the Moon travels in 12,000 synodic periods gives about 1.2 light days, so this is still a 20% error. Second fudge: to further reduce the size of this error, the site switches from synodic periods to the shorter sidereal period, 27.3 days. In 12,000 sidereal periods, the Moon travels about 1.11 light days - so still an 11% error.
3. There is then a very complicated argument that claims that if the gravitational influence of the Sun is removed (why ??) then the Moon's orbital speed and the Earth's speed of rotation (and hence the definition of "one day") would change by just enough to compensate for this 11% error. This looks like one big third fudge to me.

Gandalf61 (talk) 09:28, 1 September 2009 (UTC)

I guess the website refers to 1 lunar year, because for Arab, the year deal with is around 354 days not 365, and that's why the arguement confirmed, "...of what you count" in the Quran. The 3rd point you mentioned was the one I couldn't get along with and thus asked for some more information.--Email4mobile (talk) 09:58, 1 September 2009 (UTC)

Even if the site uses a lunar year, i.e. 12 synodic periods, as the definition of one year, then point 2, the switch from synodic periods to sidereal periods, is still a big fudge. 12 sidereal periods, or 327.6 days, is a completely arbitrary time period, unrelated to any definition of a lunar year. A lunar year is much closer to 13 sidereal periods than to 12. Gandalf61 (talk) 10:42, 1 September 2009 (UTC)

Thank you very much, Gandalf61 for clarification. I don't lime like to mix science and religion by all means. It was just a curiosity to do so. I am still wondering how they could play with values till they got a so precise light speed value.--Email4mobile (talk) 13:47, 1 September 2009 (UTC)

This is obviously bullshit. They cheated - rounded things off, used fudge factors, were suitably vague about details. If you are allowed to do that, you can come out with any answer you like. Hey have you noticed that PI is exactly equal to the square root of 10 minus the phase of the moon in radians on the day that the IPU (mhhbb) first ordered a pineapple and ham pizza? Ha! Proof! May the world quake at the passage of her dung! SteveBaker (talk) 20:31, 1 September 2009 (UTC)

Forgive me friends for my non-native English and typos. I'd like when discussing such tricky calculations to understand them better in order to argue. I sent the website author some further questions and then argued with him but he replied "My physics is perfect. Maybe the problem is with your ignorance." --Email4mobile (talk) 23:00, 3 September 2009 (UTC)

Pin tumbler lock

What are pin tumbler locks usually made of? Would aluminium make sense? It's for a video script we're writing. Anthrcer (click to talk to me) 12:51, 1 September 2009 (UTC)

Aluminum is soft, so that would be surprising. I'd guess steel. --Sean 14:20, 1 September 2009 (UTC)

No, not aluminum. I think brass is the most common material for the pins and plug, and steel the most common material for the casing. This page also lists die-cast zinc as an alternate possible material for the pins and plug, and brass as an alternate possible material for the casing. Red Act (talk) 14:50, 1 September 2009 (UTC)

I key my own locks. I have a box of pins precut to the normal sizes. They are all brass. I have seen silvery ones (zinc, I assume), but the set I purchased came in all brass. -- kainaw™ 18:27, 1 September 2009 (UTC)

perchlorate

I'm a bit puzzled by the hypervalency of this ion ... is it anything like the hypervalency of sulfur? Am I right in thinking that this feat of oxidising a halogen prolly wouldn't work as well with fluorine (seeing as it's period 2?). I also wonder how the oxygens protect the chlorine so well ... to me the oxygens still have reactive lone pairs, don't have complete double bonds (7 electrons among 4 oxygens!). Why is the system so stable? I mean, overall I guess with all these electronegative atoms the system is strongly electrophilic ... but I guess the actual negative charge of the ion repels most electron donors away? Also ... how do you get resonance in a tetravalent system? Perchlorate isn't planar...John Riemann Soong (talk) 13:00, 1 September 2009 (UTC)

How polar is the Cl-O bond anyway (MO-wise)? If the bonding orbital is closer to period 2 than period 3, I guess that would keep the bonding electrons well protected? What would be the attack mechanism for reducing this system? John Riemann Soong (talk) 13:09, 1 September 2009 (UTC)

ClO4- is 'made' exactly the same way as SO4- , in fact the two are isoelectronic, both are tetrahedral, both can be considered to be made by electron donor bonds from Cl/S to O atoms (6 electrons). (the O can be considered a lewis acid)

First row elements including fluorine don't do this - the electron lone pairs are there - but they are too low in energy to effectively bond with other neutral atoms (under usual conditions), the lone pairs can bond with positive ions such as R2O + H+ >> R2OH+

There's a obvious exception for N which is R3N + O >> R3NO (see Amine oxide)

These donor bonds are also known as "dative bonds" or "donor coordinate bonds" - they usually occur when a lewis acid bonds with a lewis base, in this case the lone pairs on chlorine can be formally thought of as a lewis base, and the oxygen atom (6 electrons) as the formal lewis acid. In all cases of 'donor bonds' one atom donates both electrons for bonding, rather than each donating/sharing one such as found when two H atoms bond. —Preceding unsigned comment added by 83.100.250.79 (talk) 14:16, 1 September 2009 (UTC)

The perchlorate system is kinetically stable because all atoms have their 'octets' filled - it's a perfectly legitimate structure , plus the tetrahedral symmetry will help a bit. Of course perchloraate is not thermodynamically stable at all.

Compared to ClO3- , ClO4- lacks a free lone pair on Cl - this reduces the number of ways it has to react, and contributes to it's higher kinetic stability.

You don't need planarity for resonance (this is a complex topic: sometimes you do - depends on the bond type)- the resonance (per bond) in this case includes :

Cl+-O-  <>  Cl:→O (donor bond) 

The Cl-O bond is expected to be polarised with negative charge on O

As for attach mechanism - as mentioned above Cl04- lacks attack mechanisms that other ClOn- species have, it also is non polar overal (due to symmetry) which doesn't help.

I don't know the mechanism of perchlorate reduction, though at a guess it would involve electron transfer to give an unstable Clorate(6) radical *when in acid solution - it would be the major difference in stabilities between the two chlorates that cause this reaction to have a high activation energy

ClO4- +2H+ +e- >>> H2O + Cl03· (radical)

(Note that the reduction of ClO3- to ClO2· results in a radical that can be stabilised by resonance onto a Cl lone pair

          O                O
          |                |
        O-Cl-O           O-Cl:          (the Cl-O bonds are donor bonds , not single bonds
          ·                ·                I can't find the correct symbol though...)
    
     ClO3 radical        ClO2 radical (can stabilise radical via resonance onto lone pair)

Cl03- , Cl02- , Cl0- all react much faster in general than ClO4- due to this effect (I think) —Preceding unsigned comment added by 83.100.250.79 (talk) 13:49, 1 September 2009 (UTC)

There's a possibilty that you might have noticed of stabilising the radical onto the oxygen

          O                O
          |                |
        O-Cl·            O-Cl:
          |                | 
          O                O·

The reason that this doesn't matter much is that second row elements are much better at stabilising radicals that first row (and so on for 3rd and 4th row..) - I general - moving down the periodic table the stability of non-standard molecular structures increases - due to various reasons..

So the stabilising effect of a Cl is much more than that of an O where radicals are concerned.

(Don't quote me on the reduction mechanism in an exam! I'm not sure what it is - that was a guess)83.100.250.79 (talk) 13:54, 1 September 2009 (UTC)

Wonderful as always. Thanks! Hmm, I am very curious now about possible attack mechanisms though. The thing I find interesting is that perchlorate is thermodynamically electron deficient (thermodynamically a strong oxidiser), but it carries a net negative charge, which would probably repel most electron donors away. Yet however, the pKa of perchloric acid is like -10, so I imagine H+ doesn't want to stay on perchlorate for very long! And of course, in explosives, you don't need the presence of magic acid (or sulfuric acid), do you? Or is that what some of the sulfur components in explosive mixtures are for? Is there an "attack mechanism" under high heat? Would it be as simple as high heat ripping the Cl-O bond apart? Given that both Cl and O are strongly electronegative, I don't think the bond energy would be very high, would it? What about polarisability? Would it be possible that at certain moments in time, one of the oxygens would become a bit bluer than the rest, leaving a small window for electron donation to occur or something to pull the oxygen off?

Say, something is burning and you have a hydrocarbon radical being formed or something (C-C bonds broken apart by high heat?) So I'm thinking something happens along the lines of thermal cracking... forming something like say, R2HC* to perform some sort of radical attack on the oxygen? John Riemann Soong (talk) 22:06, 1 September 2009 (UTC)

Also, what about the nature of the donor bonds? In perchlorate, the bonds are shown as a combination of solid and dotted lines ... but aren't there 8 bonding electrons (mostly being kept by the oxygens)? Do they have "double" donor bonds? I note compounds like chlorine pentafluoride or iodine heptafluoride ... in those cases the chlorine or iodine doesn't have a "full" shell! (Would iodine octofluoride be potentially possible? And why doesn't chlorine heptafluoride occur? Is it too small?) John Riemann Soong (talk) 22:18, 1 September 2009 (UTC)

ok first paragraph - mostly yes - sulphur is in gunpowder to burn - once the reaction gets started - above about 500C the compounds just fall apart into atoms - ending up with whatever is stable (usually oxides)

Yes combustion is mostly radicals (plus some ionised gas once it gets really hot) see Radical_(chemistry)#Combustion and Combustion#Reaction_mechanism the reaction

CH4 + O2 >>>> CH3· + HOO·

is an important one - the carbon radicals become more important when there is not enough O2 - and can polymerise to form soot.

Second paragraph (donor bonds often look like double bonds to a lot of people - but they aren't )

I tend to think of it as Cl- and 4 oxygen atoms - the Cl- has a full octet and four lone pairs, the oxygen atoms have only six electrons - and so need to get 2 to complete the octet.

As for the interhalogen compounds - there are difficult to explain - chlorine heptafluoride doesn't exist because chlorine is too small - 7 fluorines wouldn't fit (or be far too crowded to fit round a Cl atom)

Iodine octofluoride is unlikely at any temperature - maybe IF₈⁺ might exist - you'd need a very powerful fluorinating agent - and I imagine it would just oxidise any counter ion.

I think the problem with IF8 is this reaction

2 IF₈ >>> 2 IF₇ +F₂

Not only does it increase entropy, but is likely to exothermic as well - especially since the I-F bonds are going to be weaker than a standard I-F bond (too many electrons).83.100.250.79 (talk) 01:43, 2 September 2009 (UTC)

Okay I'm kind of confused because it seems to me that Cl and O are interacting in a kind of polar covalent bond (I mean otherwise water could easily solvate away the O) ... it's actually a single donor bond? In terms of MO theory, what's the exact nature of this bond and how do the bonding orbitals form? (The bonding orbitals are located closer to the oxygens, right?) John Riemann Soong (talk) 02:18, 2 September 2009 (UTC)

Does oxidisation get kinetically easier as valency increases from tetravalency? Perchlorate's bonding electrons are well shielded from attack ... but IF8's wouldn't? John Riemann Soong (talk) 02:19, 2 September 2009 (UTC)

Also, wouldn't it be IF8- ... an anion...? (I'm thinking IF7 + e- from an alkali metal + F radical). Or does the fact that the iodine is "crowded" provide an exception to the octet rule? (Too bad astatine isn't stable enough to see if we could get AtF8 ... damn you radioactivity.) John Riemann Soong (talk) 05:06, 2 September 2009 (UTC)

Donor bonds are covalent, and usually polar too. In ClO4- the electrons for bond can be considered to come from the Cl (if you are constructing the molecule from Cl- and 4 O atoms), they are shared between the Cl and O - so this means that the electrons have moved a bit from the Cl towards the O.

Does the reaction:

H3N: + BF3 >>>> H3N:→BF3

make sense - that's a donor bond too - the product can also be written as:

H3N+-B-F3

That's a covalent bond with a strong electric dipole in it.

The situation in ClO4- is similar - except Cl has a more complex set of molecular orbitals - in general any molecular orbitals that are close to both the two atoms will contribute to bonding.

The ease of oxidation depends on many things- mostly the big factor is the stability of the the intermediate, it depends a lot on what type of atom is being oxidised. I can't be more specific.

IF7 + an electron plus a fluoride radical would definately make IF7 plus F- , getting the F- to react is with IF7 is debateable - the F- is a lewis base - to react it needs a lewis acid to bond to. For IF7 to act as a lewis acid it would need to add two more electrons to it's already expanded 'octet'

I agree that IF₈^- is not a bad idea - it has certain properties going for it. It's difficult to show with hand waving arguments, or using calculations that it will not exist. What about

 IF8- >>>  IF6- + F2

Would the IF6- be more or less stable than IF8- ?

See also Xenon hexafluoride specifically the part about XeF²⁻
₈ which does exist - it's only one proton and 2 electrons more than IF8-, but otherwise identical.83.100.250.79 (talk) 10:56, 2 September 2009 (UTC)

IF8- and XeF8

Mmmh- according to this IF8- exists http://www.chemthes.com/rxn.php?side=RHS&entity=2542 however there's no reference at all, I'm not sure.

More reliably it seems that XeF8 might exist (which is isoelectronic with IF8- making it's existance very probable) -see http://www.google.co.uk/search?hl=en&q=Xenon+octafluoride+synthesised++sealed+tube&btnG=Search&meta= unfortunately I can't read the whole article.

It's interesting to note the decreasing bond strengths of XeF4 XeF6 as the number of F increases (see here "xenon+octafluoride"&source=bl&ots=NvqEqQNe_P&sig=TpJNiK2cW_6_fSFdgDqfM7LQ0Pk&hl=en&ei=plyeSqPuEsvt-QbUgO3aCw&sa=X&oi=book_result&ct=result&resnum=8#v=onepage&q=%22xenon%20octafluoride%22&f=false google book result)

This paper claims in a footnote [4] page 13 footnote 5, that XeF8 will be unstable to loss of fluorine atoms - but without reference to the calculations unfortunately.

If XeF8 exists then IF8- might exist - however I can't find cut and dried evidence for either. I'll ask a question to see if anyone else knows.83.100.250.79 (talk)

Hmmm, I just discovered xenon tetroxide.... it's tetrahedral with its bonding electrons well-shielded but now I wonder why the perchlorate ion isn't as explosive ... is it that the anionic nature of perchlorate stabilises the thermodynamically electron-deficient perchlorate somewhat? Or does the large size of xenon make it easier for oxygen to break off spontaneously? (And shouldn't they be resonating in the zrticle's Lewis structure?) John Riemann Soong (talk) 17:22, 2 September 2009 (UTC)

That's a good question - the isoelectronic species is (periodate) iodate(VII) IO₄^- which is stable as a salt and in solution just like perchlorate - the obvious answer is as you say - that the ionic bonding in the salt, or solvation energy of a the salt in solution gives enough addition stabilisation. I can't think of another reason for extra stability - though the difference in stabilities is vast.

It's worth noting that there are other compounds that are stable as salts or in solution, but attempts to make the free (covalent) acid (or the acid anhydride/acidic oxide) are generally met with failure.

An example of this is permanganate - potassium permangante though a strong oxidant is stable enough to be sold in chemists - it does decompose above ~230C (according to the article - I think this is right) but not explosively on it's own (usually) - however the free acid HMnO₄ is unknown as a pure compound, and the corresponding oxide Mn₂O₇ is dangerous above room temperature see Manganese heptaoxide and has a reputation to explode.

So thats a difference of over 180C in the decomposition temperature going from covalent substance to the ionic salt. I think the stabilisation from making an ionic salt must make a huge difference.83.100.250.79 (talk) 18:51, 2 September 2009 (UTC)

Almost forgot there's also Dichlorine heptoxide to compare with perchlorate as well - similar comparisons apply - perchlorate salts can be bought/transported etc fairly safely. The oxide is far to unstable to do the same.83.100.250.79 (talk) 19:13, 2 September 2009 (UTC)

And the XeO4 structure is problematic - I'd tend to write it as a donor bond, Xe:→O for each oxygen, as this sticks to the octet rule, Xe⁺-O^- is also valid (and also sticks to the octet rule) (the two forms are basically equivalent)

As it's drawn in the article it uses double bonds ie Xe=O , this is ok, but implies an extent of bonding that might not be there - ie the bond strength isn't that of a double bond : bond strength ~210kJ/mol , compare with ~220kJ/mol in XeF4 (expected to be single bonds) , though there will be some percentage of double bonding I personally don't think the extent is big enough to justify writing it as double bonds. (the comparison with XeF4 may not be the best one, but I couldn't find other data off hand).83.100.250.79 (talk) 19:39, 2 September 2009 (UTC)

MO wise, what's happening in a donor bond in perchlorate or XeO4? I guess I'm puzzled what happens here (my sense of chemistry totally breaks down). There's no pi system so that's why it's not a double bond, right? Would the donor bond be .... an sp3 orbital? Would the oxygens have 3 lone pairs each? Would most ab initio calculators be accurate if I used them in this situation? Ahhhh...John Riemann Soong (talk) 22:49, 2 September 2009 (UTC)

Re: ionisation -- I wasn't thinking solvation stabilisation so much more as the fact that both XeO4 and perchlorate are quite electron-poor systems and that perchlorate is an anion. John Riemann Soong (talk) 22:51, 2 September 2009 (UTC)

The orbitals are tricky, for O each O is in a triangularily symmetrical enviroment - therefor orbitals with triangular symmetry are sensible eg sp2 or sp3. In general the highest unoccuppied molecular orbitals will be the ones that do the bonding (provided they point in the right direcion - not a problem here) - so for O this will be the p orbitals - it could well be that most of the bonding comes from a pure p orbital, however since the lone pairs will want to repulse away from the Xe, probably sp3 is the best bet.

For Xe (as a donor) the highest occupied orbitals will be doing the bonding - I'm not sure - this could be some combination of the highest s, p or d orbtials. (see HOMO/LUMO for more details, as well as the Xenon page for its electron configuration)

Calcuations might help - I'm not sure how good they can be nowadays - with the increase in computing power to optimise molecular orbitals, years ago such calculations would have been impossible/useless.

As an aside if the possibility of pi bonding is considered it could be constructed like this:

- consider the hybridisation on O as p + 3 sp2 orbitals - if the p orbital forms the main bond to the Xenon (remember that Xe is spherically symmetrical so there are no problems with symmetry problems) - then the sp2 orbitals (at right angles to the p bond) can point between the other Oxygens - eg when looking down a Xe-O bond there would be a six pointed star - Oxygen/sp2orbtial/oxygen/sp2orbital/etc )

The other oxygens can do the same. Xe is a big atom so there should be some molecular orbital overlap (side on as in all pi systems) between the Xe orbitals and the sp2 orbitals.

So all the 4 O sp2 orbitals will be pointing (in sets of 3) to the points of a tetrahedron on the surface of Xe, this tetrahedron, and the tetrahedron of the molecule itself will be inverts of each other (ie consider the octahedron made up of 2 tetrahedra)

If this happens the that will be a spherical 'pi' system around the xenon, between the O atoms, (and made up of sp2 orbitals not the usual p (!)) - this is getting close to what starts to happen in clusters of metal atoms. However because Xe and O are electron rich both the bonding and antibonding orbitals will tend to be filled - this the 'pi' system, if it existed wouldn't contribute much to bonding, if at all... Yet another reason to expect sp3 on oxygen.83.100.250.79 (talk) 14:09, 3 September 2009 (UTC)

Surface tension image - comment

Surface tension this image

To me it looks like a huge paperclip on a blue cushion, with some drops of water added for effect - can anyone explain how it looks? 83.100.250.79 (talk) 13:21, 1 September 2009 (UTC)

It looks...like it looks--not sure what you're asking really. There is a blue glass surface covered with water and a paper clip floating on the water. DMacks (talk) 14:06, 1 September 2009 (UTC)

I think it's actually the rim of a drinking glass. It's a close-up photo. The uneven edges of the water and reflections (click the image for a closer look) mean it doesn't look like a cushion to me. AlmostReadytoFly (talk) 14:10, 1 September 2009 (UTC)

It looks more like a paperclip on the surface of a can of paint to me. Googlemeister (talk) 14:17, 1 September 2009 (UTC)

Maybe the skin on top of some blue paint? with a paper clip on, plus some water drops around the loops of the clip?83.100.250.79 (talk) 14:33, 1 September 2009 (UTC)

Ah yes, it actually is top lip of a blue drinking glass. DMacks (talk) 14:20, 1 September 2009 (UTC)

The glass sides are not blue, but the rim is, and the 'water' surface is exactly the same colour as the blue rim!!83.100.250.79 (talk) 14:29, 1 September 2009 (UTC)

Also look at the bottom (picture) two loops of the paperclip - there appears to be water droplets on the loops, on top of the "blue water" surface :)

83.100.250.79 (talk) 14:31, 1 September 2009 (UTC)

No, what you identify as droplets are the reflections I mentioned earlier. It's more obvious if you look at the full size image. What the reflection is of, I'm not certain. Something interesting on User:noodle snacks' ceiling, perhaps.

At the full size, you can also see part of the paperclip underwater, without there being any join from water to "cushion". AlmostReadytoFly (talk) 14:45, 1 September 2009 (UTC)

this other variant makes it more clear what is going on. The "cushion" effect is just how the light is hitting the glass on the other side of the water. -98.217.14.211 (talk) 14:31, 1 September 2009 (UTC)

I'm very suspicious - where is the ref desk fraud squad when you need them?83.100.250.79 (talk) 14:36, 1 September 2009 (UTC)

Right. The whole point is that surface tension does make something of a cushion effect, or a fabric layer rather--the water bulges up out of the glass like a muffin and the paper clip depresses the surface a little still on it without actually falling completely into the water. You could always email the photographer and ask him the details. DMacks (talk) 14:39, 1 September 2009 (UTC)

I assume that the inside of the cup is blue. And the reflections are of the scaffolding used to hold up a set of professional photography lights. ... But what's causing the hard edge between the reflecting portions of the fluid and the blurry (See-through?) portions of the fluid. Are those bubbles?

This is an amazing photograph. Anyone can balance a paperclip on water and get that "Cushion" effect, but to photograph it so dramatically takes a lot of skill and effort. APL (talk) 15:05, 1 September 2009 (UTC)

That picture looks pretty realistic to me. I don't think it's a fraud. Dauto (talk) 16:24, 1 September 2009 (UTC)

You can do this right now. The trick is to slowly place the paperclip on the water so that it doesn't break the surface tension. Generally, the easiest way to do this is to bend a second paperclip into an "L" shape (just pull the middle portion out at a 90 degree angle to the outer portion). Use this "L" to very slowly lower the other paperclip into the water. The paperclip will float gently off and look exactly like the above picture. --Jayron32 16:55, 1 September 2009 (UTC)

But it doesn't look like that image - it looks like this (see), the image doesn't look right for a blue glass either - it looks like the liquid is blue - see the striations in pigmentation on the surface (to the left of the paper clip, starting about the middle of the paper clip going roughly horizontally left) - looks like paint to me?83.100.250.79 (talk) 17:08, 1 September 2009 (UTC)

As I said before, if you look at the full size image, you can also see part of the paperclip underwater, without there being any join from water to "cushion" (or paint). If it were paint, that wouldn't work. AlmostReadytoFly (talk) 20:26, 1 September 2009 (UTC)

Why is there a total absence of refraction or reflection effects except at the loops of the paperclip? 83.100.250.79 (talk) 20:42, 1 September 2009 (UTC)

Most of the surface is actually a reflection from the main light source per below. Noodle snacks (talk) 03:09, 2 September 2009 (UTC)

If you are just concerned about the coloration of the water, any number of lighting effects could lead to that; it could just be a result of the type of room lighting, camera flash, or an artifact of the way the camera recorded the image which could result in such coloration. I see nothing in the color of the water that makes me to believe it isn't water. --Jayron32 20:47, 1 September 2009 (UTC)

The rough lighting behind the shot

Speaking here as the photographer. Per the caption in Surface tension the water is in a blue glass. I have updated the file caption to reflect this as well. The explanation for the appearance can come purely from the lighting used. The water facing in the right direction (including flat) is reflecting the light source, a white umbrella. Other parts of the surface are letting the light through from below the surface. That light is blue due to a tinted glass. The reflections that User:AlmostReadytoFly refers to are the silver support arms of the umbrella. I've thrown some bits and pieces on to the kitchen table and took a shot of it all, giving you some idea what is going on, though things haven't been put together very carefully or in exactly the same way. And yes, my poor old radio trigger is on it's last legs, held together with glue and duct tape. I feel that this method is initially a little less clear, but it does let you see the shape of the surface very well. I will add a link to the lighting demonstration in the file caption to help clarify this. --Noodle snacks (talk) 03:08, 2 September 2009 (UTC)

Oh, as far as Surface tension goes, it wouldn't matter if the fluid was paint. Surface tension is a property of many liquids. Noodle snacks (talk) 03:20, 2 September 2009 (UTC)

Thanks for coming and explaining - (I still can't persuade my eyes to see it right) - I suppose the 'flat' or 'silk' effect on the surface of the water comes from the surface of the special reflecting umbrella then.83.100.250.79 (talk) 11:39, 2 September 2009 (UTC)

I think it should be replaced with a more realistic image, OK?--Mikespedia is on Wikipedia! 11:17, 15 June 2010 (UTC)

Chemistry

Can Hafnium be mistaken for Thorium 232 during gamma spectrum testing —Preceding unsigned comment added by Radioactiveman6552 (talk • contribs) 14:25, 1 September 2009 (UTC)

Well chemically they could be confused, and likely to be found together in nature. Both form similar compounds. I have attached the spectrum as found on Wikipedia, can some one make a readable version? For Hafnium read Induced gamma emission: Hafnium controversy and Hafnium bomb. Hafnium could emit a 2.5 MeV photon. Graeme Bartlett (talk) 09:35, 5 September 2009 (UTC)

Optical beam combiner?

I am working on a DIY project that involves mixing the output from an array of red, green, and blue LEDs to create a single color. Is there some sort of way to easily mix the light output from the different LEDs besides putting some sort of diffuse, semi-opaque sheet of material in front of them? Could I use lenses or mirrors to achieve the same effect? Thanks so much. Ilikefood (talk) 15:48, 1 September 2009 (UTC)

EDIT: Sorry, forgot to mention, the LEDs will be red, green, and blue Luxeon Rebel Stars from http://luxeonstar.com/ , arranged in a hexagonal grid. Ilikefood (talk) 15:52, 1 September 2009 (UTC)

You can buy a beam splitter, and run it "in reverse"; but you're going to lose significant energy to reflections, so it will suffer from brightness problems. Also, these tend to be expensive, and large compared to an LED. I think a diffuse surface will be better suited to a small hobby project. Thin tissue paper will work. Nimur (talk) 16:00, 1 September 2009 (UTC)

Are there any materials that are a bit more durable than tissue paper, but will diffuse the light about the same without losing too much brightness? (Like some sort of readily-available plastics?) Ilikefood (talk) 16:10, 1 September 2009 (UTC)

Sure - you can search for frosted glass, or frosted acrylic / frosted plexiglass, which are available from suppliers. (If you search your phone book for "acrylic", you will probably find some light-industrial suppliers in your area); or you can mail-order frosted acrylic or frosted plexiglass via the internet. Various thickness are available; depending on your needs, these may be suitable. Nimur (talk) 18:30, 1 September 2009 (UTC)

It seems like in theory a prism could be used to line up the images of the 3 LEDs. There might be practical problems with that. I don't really know. Rckrone (talk) 16:43, 1 September 2009 (UTC)

How about a lens with a really bad spherical aberration? Would that work as well as the tissue paper method? Ilikefood (talk) 16:47, 1 September 2009 (UTC)

Why should we bleed when blood pressure is lesser than atmospheric pressure?

The atmospheric pressure is 790mmHg and our blood pressure measures 120/80mmHg. Blood being a fluid is supposed to flow from high pressure to low pressure. Then we are not supposed to bleed when cut, on the contrary air must rush in to cause embolism. I am quiet convinced that my understanding went wrong somewhere or does it mean that the blood pressure measured is 120/80 mmHg above atmospheric pressure(since the atm pressure acts on the blood pressure cuff too), if yes why should'nt we say 910/870 mmHg? please help me out.Neduncheralathan (talk) 16:31, 1 September 2009 (UTC)

Blood pressure is the pressure exerted on the walls of the blood vessels. Think of a balloon if it makes it easier. Just sitting there, uninflated, there is some air inside. That air is exerting the same pressure on the inside of the balloon as air on the outside of the balloon. If I say that I put 10 pounds of air in a balloon, I mean that there is 10 more pounds of pressure inside than outside. Blood pressure is the same. There is 120mmHg more pressure inside than outside. As for why don't add atmospheric pressure to it... It doesn't matter. I brought the following up earlier and some others felt the need to say it was completely wrong. We don't need to know our exact blood pressure. We need to know our blood pressure relative to everyone else. When we say "below 140/90 is controlled blood pressure", we mean that those who get a reading of below 140/90 on a blood pressure measuring machine have a blood pressure that does not pose a high risk for heart attack or stroke. If the machine used letters instead of numbers, we would measure a lot of people and, perhaps, discover that anyone with a blood pressure letter below J was not at high risk of heart attack or stroke. The numbers don't have nearly the importance as the comparison of values among the population. -- kainaw™ 16:46, 1 September 2009 (UTC)

(edit conflict with above reply) Blood pressure is always the measurement above the current atmospheric pressure. Average atmosheric pressure is defined by international standards to be 760 mmHg (see Standard conditions for temperature and pressure) not 790. But this is a defined standard; not an actual measurement of current atmospheric pressure, which could vary up or down considerably from that average. However, since the blood pressure cuff is currently under that same atmospheric pressure, it is measuring relative to that, not to some hypothetical vacuum state. --Jayron32 16:50, 1 September 2009 (UTC)

Such readings, BTW, are called gauge pressure, as opposed to absolute pressure. See Pressure measurement#Absolute, gauge and differential pressures - zero reference. Your car's tire pressure is also an example of gauge pressure—a flat tire at "0 psi" contains air at the ambient atmospheric pressure, not a vacuum. -- Coneslayer (talk) 17:01, 1 September 2009 (UTC)

(more ec and weird interleaving) The idea of relative (gauge) pressure is right-on here. As to what that's used instead of absolute, it's certainly more convenient to set up a mercury manometer for a pressure range 0–250 mmHg (only has to be 25 cm tall) than 750–1000 mmHg (need a meter height and 4x as much Hg) and probably easier to set up/fill an open-ended manometer than a closed-ended one. I wonder if it really does matter more what the relative pressure is than the absolute because people are kinda squishy: have to overcome the crush of the atmospheric pressure to keep vessels open? DMacks (talk) 17:06, 1 September 2009 (UTC)

Some of the blood pressure is probably overcoming the "crush" of the atmosphere, but not all of it, which makes the attempt to define the absolute blood pressure by addition to be an incorrect method. Most of the pressure in our vessels comes from the integrity of those vessels themselves; if we put our bodies in an absolute vacuum, and measured the blood pressure by the standard method, we would not find a pressure of 800+mmHg. I would anticipate the pressure to be higher than 120/80, but not that much. --Jayron32 20:44, 1 September 2009 (UTC)

What's the closest human equivalent to what a bird feels when going through a heavy moult?

Any suggestions? When my parrots and parakeets have their big moults, it's clear from their body language and demenour that they're feeling like crap. Apart from the obvious all-over itching and possible pain from the pinfeathers coming in, it also seems to suck the energy and playfulness right out of them. I'm just trying to imagine what it might feel like. --84.69.205.30 (talk) 16:44, 1 September 2009 (UTC)

Have you had the flu? Because when I had it, I felt a general lousiness. Vranak (talk) 01:15, 2 September 2009 (UTC)

I have no idea since no one here can definitively state exactly how the bird feels. Maybe a rash would be similar. Just multiply it across your entire body. Googlemeister (talk) 13:43, 2 September 2009 (UTC)

How about if you have an all-over sunburn and your skin starts to peel ? That has the same combo of pain and itchyness, I would think. StuRat (talk) 15:39, 2 September 2009 (UTC)

Combine that with the sensation of teething, perhaps? It's also the case (AFAIK) that whilst still growing in, a bird's feathers have sensation in them - and will bleed heavily if damaged. I can't think of anything that humans experience that could compare to that. The long, thick wing and tail feathers must be hell. --Kurt Shaped Box (talk) 05:30, 3 September 2009 (UTC)

Astronomy: Stray Stars?

Are there thought to be any stars that exist, at least for a time, independent of any galaxies? If so, wouldn't they then inexorably fall into orbit around one galaxy or another, and wouldn't they then to cease to be stray stars?68.166.245.91 (talk) 16:52, 1 September 2009 (UTC)

• Yes, they exist, but no, they will not necessarily be captured by a galaxy within their lifetimes. Stars don't live forever, and galaxies are far apart. -- Coneslayer (talk) 17:04, 1 September 2009 (UTC)

• There are certainly stray stars outside of galaxies - at least those that have been ejected from their original host galaxy via gravitational interaction. Google for intergalactic stars for some hits. Hubble has found some, see [5], and ESO caught one on the run [6]. And while they would, of course, always be gravitationally attracted to some galaxies (well, actually to all in the visible universe), there is no automatism that would ensure that a star would become gravitationally bound to another galaxy - that would require a n-body interaction so that the star could shed some of its kinetic energy. --Stephan Schulz (talk) 17:06, 1 September 2009 (UTC)

If such a star fell into a galaxy it would travel so fast through it, that it would probably come out the other side in a few dozen million years in a hyperbolic orbit. Graeme Bartlett (talk) 22:21, 1 September 2009 (UTC)

It might even fly faster, gravitational slingshot. Sagittarian Milky Way (talk) 22:50, 2 September 2009 (UTC)

Electromagnet Vs Headphones

When an active (recieveing message/phonecall etc) mobile phone is near to a speaker or a set of headphones, it creates an intermittant buzzing sound, and disrupts the sound being produced by the speaker. I've come to understand that this comes from the amplifier in the the speaker picking up the much higher mobile phone frequencies and translating them into frequencies the speaker can produce.

My question is whether an electromagnet can create the same phenominon or disrupt the speakers/headphones in a similarly effective way. How strong would that electromagnet have to be, and what sort of design would it require?

When I last put this question to someone, the answer was that it was theoretically possible, however, it would require excitation of the electromagnet to about a few kHz. This answer didn't make much sense to me, as my studies of electromagnets did not extend to them having their own frequencies, unless this answer refers to a pulsing field? The same answer also said that mobile phones are entirely incapable of interfering with speakers, so I am less than inclined to think of it as overly authoriative.

Thank you for any light or theory you can shed on this query80.46.18.68 (talk) 17:54, 1 September 2009 (UTC)

The suggestion is to drive the electromagnet with an alternating source current at an audible frequency (in the kilohertz range). This will probably send out some interference and couple to the speaker or amplifier; but kilohertz wave propagation isn't very easy to achieve. Alternatively, you can try to hit an RF frequency and hope to mix back down to audio range via the audio amplifier; this is what causes the buzzing interference you hear when a cellular phone is sitting near a HiFi amplifier. Nimur (talk) 18:52, 1 September 2009 (UTC)

An electromagnet is generally driven with a DC electrical signal - the magnet turns on - it stays on. The might produce a momentary click when turned your earphones (if they are magnetic ones) - but otherwise not. However, if you drive the coil with an AC signal - it will emit electromagnetic waves at that frequency (and probably harmonics of that frequency too)...and if it's a frequency that's disruptive to the headphones - you may get a horrible noise. It's not always the amplifier that picks up this stuff - sometimes it's the cable, which may happen to be a nice exact multiple or submultiple of the wavelength of the radio frequency stuff. Often it's more a matter of luck whether something is screwed up or not. Also, with a cellphone, there are a lot of other frequencies involved - the cellphone may output a bluetooth or WiFi signal when it gets a call. The internal electronics of the phone will be waking up - so you'll see the clock frequency of the microprocessor - and of the sound generator that drives the ringer. There are all sorts of other frequencies involved that could be causing the interference. I have a set of wireless headphones at home that go nuts if there is a microwave oven turned on...either in my own home OR in my next-door neighbors house! SteveBaker (talk) 20:11, 1 September 2009 (UTC)

What psychological aliment or psychosis does this person have?

A guy enters a new relationship with a female. They agree to be exclusive with one another. However, he starts to snoop through her things: cell phone, social pages, photographs, etc. He finds some evidence of the female's past relationships. So he begins to get very jealous and angry and demands that she erase the evidence. She states that what he did was an invasion of privacy and she doesn't have to address him in this state of mind. Then he calls her foul names and starts drinking alcohol. What sort of psychological aliments/psychosis does the male have? --Reticuli88 (talk) 19:59, 1 September 2009 (UTC)

FYI - this plot comes from a fictional short story I wrote for English. I wanted to sound 'professional' by trying to diagnose with something even if it is fake. Thought someone could throw me some bones. --Reticuli88 (talk) 20:10, 1 September 2009 (UTC)

Something along the lines of a person who asks us to diagnose a medical condition when the top of the page clearly states that we will not do so. -- kainaw™ 20:02, 1 September 2009 (UTC)

Even hypothetical ones? Is this medical? --Reticuli88 (talk) 20:05, 1 September 2009 (UTC)

Diagnosing a hypothetical person is diagnosing. We won't do it. -- kainaw™ 20:08, 1 September 2009 (UTC)

Firstly, it's entirely impossible to diagnose mental problems from third-party reports over the internet, even if one were a psychiatrist (and even if psychiatrists had a much better track-record of diagnosing things than they really do). And secondly don't mistake being a jerk for a mental illness. -- Finlay McWalter • Talk 20:04, 1 September 2009 (UTC)

This question falls solidly into the category of request for medical/psychiatric diagnosis (even if it is "hypothetical"). It is an explicit request for diagnosis.

Nimur (talk) 20:10, 1 September 2009 (UTC)

If it is in a work of fiction, you could just make up some rare condition and say your character has it. It is pretend, so accuracy is not required. Googlemeister (talk) 20:22, 1 September 2009 (UTC)

Unfortunately, jealousy is not that rare a condition. In this case, it's whatever the medical term is for "control freak". Baseball Bugs ^{What's up, Doc?} carrots 20:45, 1 September 2009 (UTC)

It's called being paranoid, an alcoholic, and a shitty boyfriend. No special diagnosis. ~ Amory (user • talk • contribs) 20:51, 1 September 2009 (UTC)

The internet is full of sites where you enter symptoms and it tells you possible diseases. Google for one and enter "paranoia, aggression" and pick one of the conditions it comes up with that you like the look of. Or just go with my colleague's suggestion above and say he's a jerk. --Tango (talk) 21:34, 1 September 2009 (UTC)

How about trying things the other way around. Read up on psychiatric conditions. If you just start with psychiatry and/or mental disorder you can probably read and follow links until you find a condition that seems "interesting" to write about. Do more research about that condition. Then construct your character and story to fit a condition that you are now knowledgeable about. No internet diagnosis needed, just some background research. The really good authors do insane amounts of research on their subjects, even for works of fiction. --- Medical geneticist (talk) 21:40, 1 September 2009 (UTC)

Your character's kinda flat. I agree with the idea of reversing your search -- pick a few conditions, give them a twist and write your character from there. John Riemann Soong (talk) 00:45, 2 September 2009 (UTC)

And if you need more inspiration, pick a few symptoms, look them up to find a suitable disease/condition - then read about the other, more obscure symptoms and pick some of the more weird ones. If that's not enough to make your character what you want, you could look up the treatments for this condition - some of them are bound to be drugs with even nastier side-effects - so you can have your character stuck between the symptoms of the disease and the symptoms of the side-effects of the cure...the character might even be able to swing back and forth between these symptoms when (for example) the plot causes him to lose his medication or attempt to overdose on the stuff. If you want, you could find two diseases with a similar set of symptoms and have the poor schmuck get mis-diagnosed so he gets the symptoms of one disease and the side-effects of the ineffectual cure for some other disease. (It's a hard life being a character in this kind of novel!) SteveBaker (talk) 01:05, 2 September 2009 (UTC)

A rather similar type of behaviour is described in the novel Before She Met Me by Julian Barnes. AndrewWTaylor (talk) 13:48, 2 September 2009 (UTC)

cause of death

What gets put on the death certificate as the cause of death when the person was executed by lethal injection in the US? It obviously would not be ruled accidental, and they probably could not call it homicide or suicide. Googlemeister (talk) 20:17, 1 September 2009 (UTC)

Timothy McVeigh's says "lethal injection". -- Finlay McWalter • Talk 20:26, 1 September 2009 (UTC)

Manny Babbitt's said "homicide", which is a perfectly valid and non-judgemental use of the word homicide - as that article says, not all homicides are crimes. -- Finlay McWalter • Talk 20:29, 1 September 2009 (UTC)

Homicide is not a synonym for murder. Homicide includes legal killings such as justifiable self-defense and, yes, executions. --Anonymous, 20:40 UTC, September 1, 2009.

And possible accidental overdoses of anesthetics applied to pop singing stars. Baseball Bugs ^{What's up, Doc?} carrots 20:42, 1 September 2009 (UTC)

I'm not a lawyer, but here's a classification of different kinds of homicide, as far as I can remember:

• Justifiable/excusable homicide: legal killing such as self-defense, capital punishment, killing an enemy during war, or unavoidable accident (such as, e.g., if someone falls on the tracks and a train hits him/her).
• Involuntary manslaughter: killing someone due to negligence (like if you run a red light and hit a pedestrian in the crosswalk); this can be either a misdemeanor or a felony, depending on the circumstances.
• Voluntary manslaughter: deliberate killing upon "great provocation" that nevertheless falls short of a true self-defense situation; usually a felony.
• Third-degree murder: killing someone due to "wanton recklessness" (for instance, if you race your car on city streets while passing-out drunk and kill someone); can easily get you a 10- to 20-year sentence.
• Second-degree murder: deliberate, unprovoked killing without premeditation; 20 years to life is a reasonable sentence to expect. Most murders fall into this category.
• Felony-murder: murder in the course of committing another felony, such as rape, bank robbery, kidnapping, etc...; this is a good way to get "life-plus" (or in some cases even two consecutive life sentences without parole); in some cases, such as kidnapping-rape-murder of a child under 12, a death sentence is likely.
• First-degree murder: the most serious kind of homicide, defined as a deliberate, premeditated killing (i.e. planned in advance); standard sentence is either life without parole, or the death penalty.

98.234.126.251 (talk) 06:45, 5 September 2009 (UTC)

See also felony murder rule. —Tamfang (talk) 07:01, 7 December 2009 (UTC)

In England and Wales (when we had the death penalty) it was "execution of a sentence of death" according to Coroner#Verdict. --Tango (talk) 21:39, 1 September 2009 (UTC)

That was the Coroner's verdict: the death certificate went into more details - according to this page Ruth Ellis's gave the cause of death as "Injuries to the central nervous system consequent upon judicial hanging." AndrewWTaylor (talk) 08:18, 2 September 2009 (UTC)

European Space Agency's Bernard Foing

I've created an article for Bernard Foing who was the Principle Project Scientist for the SMART-1 mission to the Moon. I'm trying to figure out what exactly his current job title at ESA is. I originally wrote "Bernard Foing is Chief Scientist and Senior Research Coordinator at the European Space Agency (ESA)" but now I'm wondering if this is one job title or two? Either way, is he at this/these position(s) for ESA as a whole or for the Research and Scientific Support Department? Can anyone help on this? The sources I've used for the article aren't 100% clear. Thanks! A Quest For Knowledge (talk) 20:26, 1 September 2009 (UTC) 20:24, 1 September 2009 (UTC)

Here's his page from Ecole Polytechnique Federale de Lausanne. What more information do you need? Nimur (talk) 20:46, 1 September 2009 (UTC)

And this source[7] describes him as "Chief Scientist and Senior Research Coordinator". (I only created the article a few days ago and it's already #10 on Google's search results. If any false information gets spread because of the article, it's my fault. I'm just trying to be accurate.) A Quest For Knowledge (talk) 22:31, 1 September 2009 (UTC)

Read WP:BLP. This is a very serious issue, and it's a topic that has a lot of Wikipedia policy to help guide your article. Tread carefully and be sure to cite reputable information sources. Nimur (talk) 23:36, 1 September 2009 (UTC)

I've changed the article to say "scientist" until I can get some clarification. A Quest For Knowledge (talk) 04:09, 2 September 2009 (UTC)

Does the gravitational constant implicitly include 4 pi?

We all know that the force due to gravity is:

$F_{g}=G{\frac {m_{1}m_{2}}{r^{2}}}$

where the m's are mass, r is distance, and G is the gravitational constant.

It is clear that the force due to gravity attenuates as the distance between the objects squared, and I was wondering if since this is the case, it is similar to something like brightness where as the spherical shell around an object of constant luminosity enlarges, the brightness diminishes as the surface area of that spherical shell ($4\pi r^{2}$). I won't be surprised if someone who knows something about gravity says that this is a totally bogus comparison, but assuming it isn't, then I would think that:

$F_{g}=a{\frac {m_{1}m_{2}}{4\pi r^{2}}}$

where a is a constant, and therefore

$G={\frac {a}{4\pi }}$

So does the gravitational constant implicitly include a 4 pi? Awickert (talk) 20:53, 1 September 2009 (UTC)

Sure, but unlike the electrical analog, (Coulomb's constant has a meaningful physical interpretation $k={\frac {1}{4\pi \epsilon _{0}}}$, where ε₀ is a relevant physical constant), your rearrangement of constants does not seem to have any practical purpose. In your construction, you introduced a new parameter, a, and as far as I know, this has no useful meaning (except that it is G * 4π). Nimur (talk) 21:00, 1 September 2009 (UTC)

Right, it's totally trivial, but I was going for something more like whether gravity's decay with distance is fundamentally based on spherical shells. Awickert (talk) 21:07, 1 September 2009 (UTC)

Which constants do and do not include factors of pi is entirely convention. We choose definitions which lead to the key formulae being as simple as possible, although there is some disagreement over which formulae should be considered key. The current conventions are pretty entrenched, though, so I doubt they will change. --Tango (talk) 21:16, 1 September 2009 (UTC)

(EC) Sorry, I didn't elaborate enough. The above electromagnetic counterpart, Coulomb's force law, can be derived via a 3D Stoke's integral transformation (what you're calling "spherical shells" are, in general, surface integral solutions). But in the case of gravitation, at least as far as I am aware, there is no counterpart that can be converted to a surface-integral. So, it makes no physical sense to represent the constants in a form which elucidate that relationship. Nimur (talk) 21:19, 1 September 2009 (UTC)

I'm not sure exactly where they come from (I did see and understand the derivation once, but I've forgotten it!), but there are pi's in the Friedmann equations, alongside a G. It has been proposed that G should be redefined to be 8piG to make that formula simpler. --Tango (talk) 21:28, 1 September 2009 (UTC)

The factor of 4π is always there in more modern statements of the law of gravity like Poisson's law (∇²Φ = 4πGρ), the GR field equations (G_μν = 8πGT_μν), and the Einstein-Hilbert action ($\textstyle \int {\tfrac {R}{16\pi G}}\,dV$), and, as Awickert says, you can make a good case that it ought to have been in the 1/r² law too. I think there's no question that 4πG is more physically relevant than G. By historical accident we ended up with the "wrong" gravitational constant, and the "wrong" Planck's constant too (h instead of the more important ħ). I don't understand the distinction you (Nimur) are trying to make between the electrical and gravitational force laws. The mathematics is essentially the same. -- BenRG (talk) 21:32, 1 September 2009 (UTC)

The mathematics are virtually the same, but the physics are totally different. Is there a gravitational dielectric constant? We can represent gravity in whatever equational form we like, but unless those equations correspond to useful physics, it's not worth redefining our constants to make those equations prettier. Nimur (talk) 21:36, 1 September 2009 (UTC)

Come on Nimur, I think BenRG gave you three examples where the factor 4piG does correspond to physically useful quantities (Poisson's Law, Einstein's Field Equations, and Einstein-Hilbet action) and you yourself gave us a fourth one (Stokes theorem). Dauto (talk) 23:08, 1 September 2009 (UTC)

Thanks, Ben, that's what I was looking for. Sorry, Nimur, but whether or not it is "worth it" is outside of what I was trying to ask (as I tried to get across above); thanks for the comments but I am wandering in "what-makes-math-pretty land". My question was more "is the 1/r² relationship due to the 'dilution' of that force over a larger and larger area as one moves further away, with the result that for a point mass, that force should be diluted across a spherical shell". But I am pretty sure that once I look at those equations that Ben mentioned my question will be answered; thanks. Awickert (talk) 02:27, 2 September 2009 (UTC)

Sorry - years of stodgy engineering have worn my appreciation for "pretty equations" away. I just want four decimal places and no more than a cubic polynomical fit. For everything. If it's worse than that, I'll take a data table.  : ) Nimur (talk) 03:09, 2 September 2009 (UTC)

It's all right. I work with field data. Slap it on log-log, fit a power law, and hope for the best. (Well, I usually try to do better, but sometimes there is nothing else I can do!) :) Awickert (talk) 06:03, 2 September 2009 (UTC)

(outdent) After doing some research, the answer seems to be yes; Newton wanted to make his statements as simple as possible, and the fact that the Poisson expression includes the 4 pi seems really to be to compensate for the fact that G incorporates a 4 pi in the denominator. Awickert (talk) 17:09, 3 September 2009 (UTC)

Density

The Sulfuric acid page states that its density is 1.84 g/cm³. Is it possible to calculate the density of a .1M solution, or would I need to measure it physically? Nadando (talk) 23:19, 1 September 2009 (UTC)

The densities of solutions are usually very complicated, because volumes of solvent and solute do not add linearly. If you find an equation for calculating the density at 0.1M, it is probably an empirical formula; your safest bet is to measure it yourself or consult a chemistry reference table. Nimur (talk) 23:39, 1 September 2009 (UTC)

If this is homework at a stage below college, it's possible that they're assuming linear addition and simply wanting you to understand dimensional analysis. If that's the case, consider whether you know how to convert grams of sulfuric acid to moles of sulfuric acid, and whether this helps you get to the density of a dilution of sulfuric acid in water. If indeed this is homework, we won't do it for you, but let us know where you're stuck and we'll try to help you go in the right direction. --Scray (talk) 23:45, 1 September 2009 (UTC)

No, nothing like that, I was just curious. Nadando (talk) 23:48, 1 September 2009 (UTC)

I see that Wolfram seems to know, although the calculations probably being simplified, like Nimur said. Nadando (talk) 00:02, 2 September 2009 (UTC)

Wolfram is wrong, as ever.

0.1M Sulphuric acid is ~0.2% sulphuric acid by moles, or 0.2 x 98/18 by weight which is ~1.1% by weight - water has a density of 1. So 0.1M sulphuric acid will be 99% water - and hence will have a density very close to 1g/cc (within 1%). Any simple calculations you could make might get a slight better figure - but not meaningfully - you'd have to measure the density to get a density figure better than 1% error (even that is not easy without proper measuring equipment).

The answer is 1g/cc within 1% error.83.100.250.79 (talk) 01:06, 2 September 2009 (UTC)

See here for a table - 1.0049g/cc [8]83.100.250.79 (talk) 01:20, 2 September 2009 (UTC)

You might want to go straight to the original source for that number, which was the CRC Handbook of Chemistry & Physics. This is a handy reference, and your school or library probably has a copy or ten floating around. Nimur (talk) 05:57, 3 September 2009 (UTC)

Buying a telescope.

I'm not really sure if this is the sort of thing that would be appropriate here, but if this doesn't fall within the scope of the rules for posting on the science reference desk, then feel free to delete my post.

I'm getting interested in astronomy and especially with astro-photography, but I have very underpowered equipment because I have limited monetary resources. I have a cheap $80 Wal-Mart telescope I was given for Christmas when I was 12 (you know...the kind that makes you buy it using impressive pictures of galaxies and nebulae on the box, but you actually get better magnification from a pair of binoculars). I also have a pair of binoculars my father gave me. I can see the moon and a few nebulae pretty well with the binoculars, but the telescope is so difficult to align perfectly, that I almost never use it unless I want to take photographs through it, but the results are never anything to write home about.

So I've been researching about buying good entry-level quality telescopes. It turns out that good telescopes for this purpose are extremely expensive and out of my price range. E-bay seems to have better prices on used telescopes, but I want to make sure I'm getting a good beginners telescope which good enough optics to keep me from getting bored with it and the unfamiliar technical specifications for these products confuse a newbie like me.

So my question is: what sort of specifications should I look for in a telescope to meet my needs? Price is a major factor, but I will probably end up getting a used one from E-bay (if I can find one) and that will reduce the price a lot. Also, if there are any websites or primers on the internet for beginners like me, I would greatly appreciate links to such sites. Thanks for your time. 63.245.144.68 (talk) 23:51, 1 September 2009 (UTC)

Your first choice is the type of telescope, e.g.:

• Refracting telescope
• Schmidt-Cassegrain telescope
• Newtonian telescope

These are the most common commercial distinctions. They each have various characteristics. You might read Sky and Telescope Magazine, which is a sort of good place to get your bearings and learn some terminology. Many of their back-issues are available online, and they have a beginners' guide and a How To Choose A Telescope tutorial. Nimur (talk) 00:04, 2 September 2009 (UTC)

IMHO astro photography and "budget" are mutually exclusive. Sorry, it is an expensive hobby full stop. I've been to several astro camps and there are very few people who have cheap gear who do successful astro photography. They generally fall into a couple of categories, either you stick to wide field and maybe moon photos, or you are extremely dedicated and patient and willing to learn A LOT about the tricks like squeezing resolution out of specially modified web cams; home building accurate tracking stands and stacking and processing images. Don't underestimate how patient and dedicated you have to be, it takes years of learning to get any good at it. I've probably come across 3 such individuals while countless have tried and given up because they weren't satisfied with the speed of their progress. It is really not something you can pick up and be good at. On the other hand, 95% of the photos you see from amateurs are taken with gear adding up to thousands of dollars. I think $3k or $4k is a starting point. Even then that's not a guarantee, there's still lots to learn, you see just as many rubbish pictures from people with expensive equipment too. I own a nice 12" Dobson scope, cost me $1.5k, not including a few accessories, I went for viewing aperture because I knew my budget wouldn't stretch to astro photography and I wanted a telescope I could enjoy actually looking through. Dobson = cheap for big aparture, but no good for photos (wihtout complicated and/or expensive accessories). A great forum is iceinspace.com it's australian based but has members all over the world. It has very good amateur photographers there which you can learn a lot from. Vespine (talk) 01:51, 2 September 2009 (UTC)

I endorse the idea of getting an issue of Sky and Telescope or Astronomy Magazine. You will see tons of ads for real astronomic scopes. More than that, you will probably find a listing for an astronomy club somewhere near you. Talking direcrly to the members will allow you to ask detailed questions beyond what the ref desk can provide. Additionally, some members may be getting ready to upgrade themselves and looking to sell their old scopes at a good price to you. B00P (talk) 01:45, 2 September 2009 (UTC)

But yes! Don't let me put you off! Most astro clubs have a "guest" night, you'd be silly not to look up a club near you and give them a visit. Just be aware that a high percentage of astro club members are total hopeless gear junkies so even though they are certain their advice is the best advice you can get, you should still indapendantly weigh up your own options. Vespine (talk) 01:59, 2 September 2009 (UTC)

Yeah. I'd second the $3-4,000 benchmark as a good "introductory" level. It only goes up from there... Nimur (talk) 03:05, 2 September 2009 (UTC)

Bear in mind that some Astronomical Clubs and Societies also own (or have regular access to) observatories, telescopes and other gear, which as a member (and with supervision/after training) you would be able to use. If like me you've had a somewhat nomadic home life and/or live in a light-polluted location and/or have limited personal funds, precluding a decent home-based astronomical setup, this can be a way to obtain the use of otherwise unobtainably expensive and/or large equipment, as well as advice and help from more experienced members. 87.81.230.195 (talk) 12:55, 3 September 2009 (UTC)

When you're choosing a new telescope, make sure you don't fall for the magnification (power) trap! Magnification is not important; a decent aperture is! For example, if you see an advertisement for "500 POWER!" on a 3-inch scope, ignore it. The field of view will be so small that even if you do happen to find anything, it'll be dim and move out of the field in a few seconds, and it'll be too shaky to add any photographic equipment. I own a $150 CAN reflecting (Newtonian) telescope (it was at reduced price; regular price $250) and a regular digital camera (price varies on this one, SLRs are usually better but not the type I used because I can easily hold this one). By using different eyepieces (you have to have those to change the magnification), a reasonally stable mount (this includes the tripod AND the ground condition; pavement is better than soil), and simply holding the camera up to the eyepiece, the most I've managed to take are some decent pictures of the moon and some blurry photos of Venus and Saturn, too blurry to find any detail but possible to identify the shape (cresent of Venus, thin rings of Saturn). Having some photographic filters also helps, in case the object is too bright, some details need to be shown, or even to compensate for light pollution (a lunar filter, an extra eyepiece, and a barlow lens cost me $50). Please be aware that some fainter objects require longer exposure times, and some of the pictures I've taken required several seconds of exposure, which means that you need a steady automatic or manual tracking system (an Equatorial mount helps), and a way to hold the camera steady (which may require a special camera adapter). The amateur photos of deep-sky objects (which I haven't managed to produce with my setup) that you see in magazines and on boxes of department store telescopes (that's the other thing–AVOID department store telescopes!) are usually taken on exposures of several hours (you'd need a very stable tracking system and equipment costing thousands of dollars, and excellent sky transparency conditions, or good seeing for planets), or alternatively hundreds of several-minute exposure photos stacked together using image processing software (which is sometimes free). There's also the option of CCD cameras, USB connections, and computer software to guide the telescope and control the camera's shutter (so you don't have to be outside while the astrophotos are being taken), ranging from a few hundred to several thousand dollars). Some telescopes also have automated GoTo mount controls and even GPS, so you can find a specific target with the push of a button (again ranging from a few hundred to several thousand dollars). If you have a low budget, I suggest sticking to simple lunar and some planetary images. Of course, there's also the option of piggyback mounting (my telescope has this option), where the camera can be attached on top of the telescope (thus pointing in the same direction), and can be used to take pictures of the constellations, the moon, the Milky Way (limited to locations with low light pollution) and conjunctions, and the trails of bright satellites like the International Space Station (requiring an exposure time of perhaps 15 seconds to a minute. There's also the option of recording star trails (okay, that's the wrong link, but the image shows star trails) while on piggyback, which requires exposures of several minutes to several hours (and again, low light pollution). For close-up photos using this technique, you need a good telephoto lens. And yes, star parties and observatories are good places to learn more about astrophotography and amateur astronomy in general, and there are some good books and online sources as well. Good luck! ~AH1^(TCU) 16:58, 3 September 2009 (UTC)