Wikipedia:Reference desk/Archives/Science/2011 October 23

= October 23 =

Women & Yeast Infections
What causes some women to get a yeast infection after going swimming? Why swimming? If a pool has chlorine in it, shouldn't it help prevent it? Why does it happen and why are some women more prone to it? — Preceding unsigned comment added by 76.173.30.220 (talk) 00:40, 23 October 2011 (UTC)
 * First of all, public swimming pools are notorious exchanges of microorganisms -- because so many people swim in them, it's a near-certainty that at least some of them will introduce harmful microorganisms into the pool, which can then spread very efficiently through the water. And while chlorine can reduce the sheer amount of pathogens in the water (ironically, by oxidizing them), it cannot ensure the elimination of all the nasty bugs in the water (especially since there are just so many of them, and since some kinds are immune to chlorine altogether).  As for why some women are more prone to it, some of the reasons may be: differences in the woman's vaginal environment (more acidic = less susceptible), different patterns of sexual activity (promiscuous sex makes the woman more prone to infection, while frequent monogamous sex may decrease her susceptibility), immune system health (obviously, immunocompromised = MUCH more susceptible), stress levels (more stressed = more prone to infection in general), and just the woman's general health level (healthy = more resistant to infection, not feeling well = more susceptible).  This list is by no means all-inclusive. 67.169.177.176 (talk) 01:16, 23 October 2011 (UTC)
 * You claimed "promiscuous sex makes the woman more prone to infection, while frequent monogamous sex may decrease her susceptibility." There is a need for a reliable source to substantiate this puritanic claim, unless it just means that more partners means a higher likelihood of one having a STD. Edison (talk) 05:00, 23 October 2011 (UTC)
 * Quote from the article Vaginitis: "Further, either a change in pH balance or introduction of foreign bacteria in the vagina can lead to infection known as vaginitis." The first condition is always the case during unprotected sex, while the second is much more likely with multiple sex partners, because more sex partners = more varieties of bacteria, funguses, etc., that are likely to be introduced into the vagina.  Therefore, promiscuous unprotected sex is definitely a risk factor in developing vaginal infections, even if venereal disease is not a factor.  As for frequent monogamous sex, although it does change the pH balance as well, there is circumstantial evidence that receiving semen from one man only on a regular basis could improve the woman's immune response through repeated exposure to pretty much the same antigens all the time; however, at this time it is not certain that this significantly improves resistance to yeast infections, which is why I put "may" before this claim, to show that it's not proven.  I also should have qualified these claims as applying only to unprotected sex; obviously, protected sex (using a condom) is not a risk factor for developing vaginal infections, even if promiscuous. 67.169.177.176 (talk) 20:55, 23 October 2011 (UTC)
 * With your later admissions, it seems clear your original statement is dubious. In fact it seems likely someone who is not 'promiscious' but in a stable committed relationship is less likely to use a condom. If that is they case, they could easily be at higher risk (compared to both the 'promiscious' woman who uses condoms all or nearly the time and the woman who rarely has sex with any partner) despite only having one partner but frequent sex, even with an unproven reduction in susceptibility. In other words, if you're talking averages, the info you have provided does not establish your original claims or at least suggests they could be misleading as originally presented, a 'promiscious' woman may be less likely to get an infection then a woman having frequent monogamous sex (with the close to celibate woman possibly being the lowest risk). Unless you can provide some actual sources showing otherwise of course.... Nil Einne (talk) 00:07, 25 October 2011 (UTC)
 * Your statement ignores the effects of the antigens found in the semen on the woman's immune system, which, in the stable committed relationship scenario, would tend to counteract the effects of the pH changes inevitable during unprotected sex. This is the same mechanism that greatly reduces the risk of miscarriage when the woman has frequent, monogamous, unprotected sex during pregnancy -- I just don't remember the scientific name for this effect, or else I'd have given you the article link by now. 67.169.177.176 (talk) 04:52, 25 October 2011 (UTC)
 * Follow-up: the phenomenon I was thinking of (and vainly trying to find a link to for 2 days) is paternal tolerance. This has been proven to prevent miscarriages, and is hypothesized (but not proven at this time) to also reduce the risk of vaginitis (all forms, including yeast infections).  67.169.177.176 (talk) 05:09, 25 October 2011 (UTC)
 * Not I did not. Read my comment carefully. I said 'even with an unproven reduction in susceptibility'. In other words, even if there is this purported reduction in susceptibility, it doesn't automatically follow that the woman in the stable committed relationship who does not use condoms will have a lower risk of yeast infections or vaginitis then either the woman who rarely has sex or the woman who has sex with multiple partners but nearly always uses a condom since the pH changes and introduction of a great number of yeast, bacteria etc (despite the lower variety present on the sexual organ of her partner then would be present on multiple different partners) could easily be greater given the lack of protection. (Note I did not say then and I'm not saying now it's definitely the case, I'm just saying from the evidence presented thus far, both scenarios seems equally plausible as well as a whole host of other scenarios so your original statement was misleading.) You cannot look at factors in isolation if you are talking about overall risk. And unfortunately without the source you are thinking of we cannot even know if the source said the woman may have a lower susceptibility (as you said at first) or a lower risk (as you suggest now).
 * I'm aware of the miscarriage theory but that's a somewhat different thing, the hypothesis is that womans immune system develops tolerance due to frequent exposure to paternal antigens and this reduces the risk her body will respond in an unwanted way to said antigens coming from the fetus. This it would seem to suggest it would lead to a greater susceptibility to yeast infections from her partner because she develops some tolerance to the antigens including those present on external bodies like yeast her partner introduces. However the immune system is a complex thing so I'm not denying it is possible she would develop some sort of improved immune response to the antigens on external bodies her partner introduces simply that isn't so much paternal tolerance. Of course it's also possible a woman who has frequent unprotected sex with multiple partners may in fact have the lowest susceptibility (but perhaps not the lowest risk) because the frequent introduction of a variety of different antigens best primes the immune system to respond.
 * Nil Einne (talk) 09:53, 26 October 2011 (UTC)
 * In other words, nobody can tell for sure which woman will have the lowest risk/susceptibility -- the slut, the nun, or the ardent lover. I will not dispute this statement, especially since there are so many other confounding factors as well. 67.169.177.176 (talk) 05:55, 27 October 2011 (UTC)

ice age
is it true we're entering an ice age — Preceding unsigned comment added by 69.5.89.125 (talk) 02:13, 23 October 2011 (UTC)


 * Read further up the page, a related question is posted under 19 October. Plasmic Physics (talk) 02:20, 23 October 2011 (UTC)


 * See global cooling, global warming and Milankovich cycles. ~ AH1 (discuss!) 15:02, 24 October 2011 (UTC)

Neutrino interactions
Hi, I watched a program recently which, as I understood it, seemed to say that neutrinos interact with matter only when they hit an atomic nucleus, and that the great expanses of empty space around atomic nuclei were the reason why such interactions were so rare, and why, for example, neutrinos can easily fly straight through the Earth. However, according to my calculations based on the separation of atoms and the sizes of atomic nuclei, it seems to me that, barring unfortunate exact alignment with a huge perfectly regular crystal, a particle of negligible size could expect to hit a nucleus after only a few tens of centimetres, or at most a few metres, of some reasonably dense material like rock. So, what gives? Have my calculations gone wrong, or can neutrinos also fly straight through atomic nuclei? 86.179.116.118 (talk) 02:53, 23 October 2011 (UTC)


 * That program was wrong in two accounts. First, neutrinos can interact with nuclei but they also can interact with electrons which are found outside of the nuclei. Second, the reason they can go through the earth is because they interact very weakly. Dauto (talk) 06:20, 23 October 2011 (UTC)


 * Expanding on Dauto's excellent but brief explanation: the nucleus of an atom is not a solid object. It consists of neutrons and protons, which in turn are made up of quarks. The figures usually given for the "size" of an atomic nucleus - around 1 to 10 femtometres - are a measure of the range of the strong force, which keeps the neutrons and protons (or, more precisely, their constituent quarks) bound together in the nucleus. But neutrinos do not feel the strong force (or, indeed, the electromagnetic force which keeps electrons in orbit around the nucleus). They only interact via the weak force, which has a range of about 1/100 of a femtometre. So to a neutrino even an atomic nucleus is mostly empty space - the neutrino has to come very close indeed to a quark (or an orbiting electron) before it will interact with it via the weak force. Gandalf61 (talk) 10:06, 23 October 2011 (UTC)
 * Thanks guys. 86.179.116.3 (talk) 13:06, 23 October 2011 (UTC)

Why not ? (RDX)
Why they don't use RDX in gun-cartridges ? Jon Ascton    (talk)  06:47, 23 October 2011 (UTC)


 * As a propellant, you mean? It's because RDX "burns" extremely fast so that the pressure would shatter the weapon, instead of propelling the bullet through the barrel. For a gun propellant you want something that "burns" slower than that.Sjö (talk) 09:04, 23 October 2011 (UTC)


 * (EC) For one thing, RDX would be too brisant. Gunpowder works well as a propellant because it's a low explosive that deflagrates at subsonic speeds, so it has low brisance.  In contrast, RDX is a high explosive that detonates, producing a supersonic shock wave which would likely rupture the gun barrel.  Red Act (talk) 09:17, 23 October 2011 (UTC)

Molecular bonding
What does the MO diagram for monomeric beryllium hydride look like? Is there sp hybridisation? This my idea:

__ __ __ 3×2σ*                                 |  __ __ __  3×2σ*

__     1σ*                                     |  __ __ __  3×2σ

↑↓ __ __ 3×2σ = 2pz1(Be) + 1s1(H2)   |      ↑↓       1σ* = 2s1(Be) + 1s1(H2)

↑↓      1σ = 2s1(Be) + 1s1(H1)        |      ↑↓       1σ = 2s1(Be) + 1s1(H1)

↑↓      n = 1s2(Be)                         |      ↑↓       n = 1s2(Be)

If this is true, then why is there no sp mixing to allow for the diberyllium molecule? Plasmic Physics (talk) 09:15, 23 October 2011 (UTC)
 * Well, one thing is not to merge hybridization and MO, two very separate approaches. MO is more straightforward.  Start with symmetrical H1s+H1s and antisymmetric H1s-H1s linear combinations.  Match these with atomic orbitals on Be.  The two symmetry-suitable atomic orbitals on Be are 2s and the 2pz (z defining the molecular axis). Add 4 valence electrons and you are there.--Smokefoot (talk) 12:23, 23 October 2011 (UTC)

How is it possible to separate the two theories. Hybridisation explains the two different MO diagrams above. Left: with sp hybridisation, the 1σ* MO is unoccupied, because it is higher in energy than the 2σ MOs. Right: without hybridisation, the 1σ* MO is occupied, because it is lower in energy than the 2σ MOs. I'm not sure what you mean by your combinations, the s orbitals are all spherical and therefore symmetrical. Plasmic Physics (talk) 14:04, 23 October 2011 (UTC)
 * When building molecular-orbital electronic diagrams, it's common to consider the symmetry with respect to the combinations--symmetry-adapted linear combinations of atomic orbitals--not each just each AO's symmetry itself or hybridization on individual atoms first. The antisymmetric H1s-H1s combination is the case where the two H 1s AO are of opposite phase compared to each other. There is a node between them, and the +z and –z directions from the Be center have opposite phase. That's the same symmetry pattern as the Be 2pz. So "Be 2pz has same symmetry as the antisymmetric 1Hs-1Hs" and therefore is a good combination for a MO. Likewise, the 1Hs+1Hs combination is the case where the two H 1s AO are of the same phase, which means "no node between them", so using the Be2s continues that pattern of having no node in the middle of the molecule, and therefore is another good combination for a MO. DMacks (talk) 14:22, 23 October 2011 (UTC)

OK, then which MO diagram is correct, if neither, what is it? Initially, I chose left. Plasmic Physics (talk) 22:49, 23 October 2011 (UTC)
 * Neither scheme looks correct. I would avoid the term sigma bond (a term mainly from hybridization approach).  You already know that there are 2e in the 1s level of Be.  We do not include those core electrons and core orbital in our scheme, by convention.  So that leaves 4e for the scheme. You put 2e in an MO that is Be 2s combined with the (symmetric) H2 SALC and another pair in the Be 2pz combined with the (unsymmetric) H2 SALC.  Both of these bonding MO's have cylindrical symmetry, so you could call them sigma, but better would be the irreducible representation from group theory.  As DMacks mentioned, a key step that takes some serious pondering to understand, is the factoring the symmetry equivalent H atoms (always 1s AOs) into symmetry adapted linear combinations.  Once you understand SALC's, you can do CH4, H2O, NH3.  Each bonding scheme harbors its own surprise relative to the hybridization approach, which can be misleading (e.g. that all eight bonding electrons are equivalent in methane or that water has "rabbit ears" for its lone pairs, etc).  There are a finite number of SALCs, practically speaking, which is why teachers can recite them quickly. --Smokefoot (talk) 17:13, 24 October 2011 (UTC)


 * People have already said some really good stuff. One article not mentioned is Linear combination of atomic orbitals.  That provides a decent outline of using MO theory qualitatively, as is being done here.  -- Jayron  32  17:59, 24 October 2011 (UTC)

So, the mistake in the diagram is that both bonding orbials should be degenerate? Plasmic Physics (talk) 22:21, 24 October 2011 (UTC)

__   2σ*

__ __ 2×1σ*

__   2σ

↑↓ ↑↓ 2×1σ = 2s1(Be) + 2pz1(Be) + 1s1(H1) + 1s1(H2)

↑↓   n = 1s2(Be)

I have not heard of group theory or SALCs before in any of my chemistry papers. Plasmic Physics (talk) 10:48, 25 October 2011 (UTC)

Hello? Aanybody there, or am I just talking with myself again?

Key is stuck in lock; how to remove anyhow?
When my sister came home at about 4:30, she woke me up from the noise of trying to get the key out of the lock. I looked, and the key very much appeared stuck. I tried turning it, and it seems like I was just bending the metal of the key, and she told me not to mess with it until the morning.

I have a bad feeling for what would happen if I reported this to the landlady, so how do I remove a defective key on my own?

By the way, this was the same key that she got from Menards when she got mine duplicated; apparently Menards has poor quality service. Or did my sister just choose the cheapest/least-reliable type of key?

I hope to get this solved soon before they find out. Also, if I hire a locksmith, will they remove the mess from the keyhole, or will they be forced to change the cylinders and get me a new set of keys? (If they change the cylinders, the maintenance people would need a new set of keys too, so I hope it doesn't come to that.) Thanks, --70.179.174.63 (talk) 09:57, 23 October 2011 (UTC)


 * Can you hold the key with needle nose pliers? It seems the best way to remove it, if it's at least protruding a little. Nail clippers will probably work too. Otherwise, try to put some machine oil into the lock with a needle and pull the key a little bit out with the same needle. 2.138.250.205 (talk) 10:28, 23 October 2011 (UTC)


 * If it is a Yale night latch, then if you remove the latch (inside part), then the barrel (its attached to a plate behind the latch), you can then push the stub of the key with a sewing needle. For thin-door double-sided locks, then you can normally push a needle through them as well, if the key hasn't been turned. CS Miller (talk) 11:18, 23 October 2011 (UTC)


 * Incidentally, this does just sometimes happen. I had a key get stuck in a lock that was just impossible to get out. I ended up just replacing the lock. It wasn't very hard and not very expensive. We somehow got the locksmith to make it so our old keys worked in the new door. --Mr.98 (talk) 13:14, 23 October 2011 (UTC)

No guarantee, but it often works to squirt some WD-40 into the slot around the key and jiggle it back and forth for a while until the WD-40 works its way into the mechanism. (The two universal solutions are WD-40 and duct tape. When two things move indpendently and you need them to move together, use duct tape.  When two things move together and you need them to move independently, use WD-40.) Looie496 (talk) 14:24, 23 October 2011 (UTC)


 * Locks are easy to remove. Why not just take it apart, take out the key, and see what the problem is. It is likely that one of the pins bent or something got in the lock and jammed the pins. -- k a i n a w &trade; 14:31, 23 October 2011 (UTC)


 * A locksmith or hardware store can create a new cylinder keyed to match the old one, so the old keys work. Rekeying a lock should cost under $8 (US) if you do the work of removing and installing, in addition to the cost of the new lock. Edison (talk) 19:33, 23 October 2011 (UTC)


 * Try WD-40. Really.  It's easy and cheap and often works in cases that seem hopeless. Looie496 (talk) 23:00, 23 October 2011 (UTC)

Center of mass of a van
Is there a practical way for me to locate the center of mass of an old Ford Econoline van?

I have access to ordinary tools, but not to anything hard-core, like a lift or a winch. I ask because I'm thinking that's the best place to install a mast, if I want to turn the van into a sailing craft. So my second question is: Am I correct, that the center of mass would be the best place to install a sail mast?

Thanks in advance for any help. -GTBacchus(talk) 20:12, 23 October 2011 (UTC)
 * Good luck! I assume you have seen the top gear episode. With regards to your second question I think that most boats I have sailed on have the mainmast somewhat forwards of the centre of mass. In fact I think the most important thing is where the centre of effort ends up in relation to the centre of lateral resistance (this website explains it a lot better than I could, it's written from a model yachting perspective but the principles are the same). The idea is that you want just a little weather helm, as then the boat will luff up and depower itself automatically in a gust. You certainly don't want Lee helm as that can lead to repeated uncontrolled gybes if you get overpowered. In terms of determining the centre of lateral resistance, it is usually approximated as the geometrical centre of the below waterline hull (by the way, you may want to add some sort of keel, centerboard or even a leeboard to the van, which you will need to take account of in the calculations, as otherwise you will go predominantly sideways at anything tighter than a beam reach). The centre of effort is estimated as the geometrical centre of the sail plan. You probably don't need to be too precise about this, a sailing van will probably handle like a recalcitrant hippopotamus however well it is designed. P.S. That all being said you do of course want to make sure that the mast is not so far off the COM that it will cause the van to tip over in the water. Equisetum (talk&#124;contributions) 22:18, 23 October 2011 (UTC)
 * I don't watch television, so I don't know what you mean by the "Top Gear" episode. Did they outfit a car with a sail? Regarding handling and tipping, I forgot to mention - I imagine outfitting the van with (wheeled) pontoons on either side, like a land-catamaran. Otherwise, tipping to the side would be an inevitability, I think. This would probably only be practical to use somewhere very flat, like the Nevada desert or something. Thanks for the link and the info! :D -GTBacchus(talk) 23:19, 23 October 2011 (UTC)
 * Oh, you want to make a land yacht, not a boat! (far to sensible for us Brits), I'm not sure how much my advice applies in this case as the steering/lateral movement is somewhat different. Watch the clips below for a look at what happens if you try to make real boats out of land vehicles! Equisetum (talk&#124;contributions) 09:37, 24 October 2011 (UTC)
 * Ah, I see that my original question wasn't clear on this point. By "sailing craft", I just meant something powered by a sail. I'd definitely stay away from water. I guess I might start with models, and see what I can learn that way. A Ford land-yacht with pontoons and a mast would probably go over quite well at Burning Man or some similar venue. -GTBacchus(talk) 17:21, 24 October 2011 (UTC)
 * Hello my good man - couple of links for the Top Gear segments in question - first failed attempt is at and the second somewhat more successful one is at . Enjoy old chap! Quintessential British Gentleman (talk) 00:26, 24 October 2011 (UTC)
 * Thank you! I'll watch these when I get home where my computer has sound. :) -GTBacchus(talk) 17:21, 24 October 2011 (UTC)

sleep
why do we sleep? — Preceding unsigned comment added by 203.112.82.128 (talk) 21:05, 23 October 2011 (UTC)
 * Sleep's functions aren't really well understood, but for some of the proposed theories, see Sleep. Red Act (talk) 21:13, 23 October 2011 (UTC)
 * And for what happens if you can't sleep, which might imply why you need to sleep, read about fatal familial insomnia. ←Baseball Bugs What's up, Doc? carrots→ 23:33, 23 October 2011 (UTC)
 * It has to be a very good idea, because it seems to exist in one form or another in almost every species. Imagine Reason (talk) 12:14, 24 October 2011 (UTC)
 * See also melatonin, circadian rhythm, sleep deprivation, non-24 hour sleep phase syndrome and polyphasic sleep. ~ AH1 (discuss!) 14:58, 24 October 2011 (UTC)

3 CRT questions
1) Are there any qualitative differences between the CRTs in televisions and those in computer monitors?

2) I'm aware that CROs use electrostatic deflection rather than magnetic, whilst TVs and computer monitors use magnetic deflection. I'm aware of the merits of using electrostatic deflection, so why don't televisions and computer monitors use electrostatic deflection also?

3) Is there any "principal" limit to the resolution of a monochrome CRT display? And if not, why isn't there a market for cheap, ultra-high resolution monochrome displays?--Leon (talk) 23:24, 23 October 2011 (UTC)

A1 According to the page on Cathode ray tube, there does not seem to be a significant difference between those used in TVs and those used in monitors, althogh the inductance of the monitor deflection coils may need to be lower to allow faster scanning than the 62us (or thaereabouts) per line as in Tvs.

A2 TVs and computer monitors dont use electrostatic deflection because, I believe, this method has difficulty in scanning over a wide angle to produce a large picture as required in TV sets and monitors

A3 I cant see any limit to the resolution of a monochrome CRT apart fron the video bandwidth limitation in driving the grid of the tube and the granularity of the phosphor. --92.25.104.152 (talk) 13:52, 24 October 2011 (UTC) Oh yeah, I suppose everyone wants color now?--92.25.104.152 (talk) 13:58, 24 October 2011 (UTC)
 * A magnetic field will cause deflection perpendicular to the field, so the deflection doesn't change the distance between electron and magnet. It's more or less a linear relation between the magnetic field and the angle of deflection. With electrostatic deflection, the small distance between electron and electrode at maximum deflection would make it very sensitive to small fluctuations. Like trying to deflect a metal ball rolling from a slope with a strong magnet without the ball getting too close and stick to it. Metal ball in a magnetic field is (more or less) the equivalent of an electron in an electric field. DS Belgium (talk) 01:38, 25 October 2011 (UTC)

Seismic wave polarisation


For S-waves, what is the difference between horizontal and vertical polarisation? I mean, I understand polarisation in general, but apparently SV waves are able to be reflected/refracted to P waves, and SH waves are not. I'm not sure why the difference isn't just arbitrary. Why doesn't it just depend on the orientation of your coordinate system? --130.216.172.64 (talk) 23:38, 23 October 2011 (UTC)
 * I'm not an expert, but it seems to me that this has to do with the wave's orientation relative to the Earth's surface. 67.169.177.176 (talk) 00:21, 24 October 2011 (UTC)


 * The waves are reflecting and refracting off of discontinuities on the earth. Those discontinuities are caused by temperature and pressure gradients which are vertical. The discontinuity surfaces, being perpendicular to those gradients, are horizontal. Those planes set the orientation of the coordinate system. The oscillation direction for a SH-wave is horizontal and will be on the plane of discontinuity. On the other hand, the oscillation direction for a SV-wave is not horizontal (it's not vertical either unless the wave is propagating horizontally). That means that those waves interact differently with the discontinuity surface. The SV-waves hit the surface like a drumstick hits a drum, creating sound waves (p-waves). The SH-waves skim the surface and create no sound waves. Dauto (talk) 05:12, 24 October 2011 (UTC)