Wikipedia:Reference desk/Archives/Science/2015 January 24

= January 24 =

Vaccination
Why don't vaccines always protect against their targeted infections? — Preceding unsigned comment added by 199.119.235.190 (talk) 07:27, 24 January 2015 (UTC)


 * Well sometime the immune system does not develop a response to it. Perhaps the combinations available do not match any of the substances in the vaccine.  Or perhaps the infection is a mutant. Graeme Bartlett (talk) 08:05, 24 January 2015 (UTC)


 * Some good info and refs at Vaccine. SemanticMantis (talk) 18:23, 24 January 2015 (UTC)


 * If you are asking about the ineffectiveness of the flu vaccine this year, that's because there are many strains of flu, and they typically only make the vaccine capable of fighting what they predict will be the 3 worst strains that year. Of course, predicting which strains will be the worst is tricky, especially considering that not providing any vaccine for a particular strain will make it worse.  They might consider increasing the number of strains they cover.  (I wonder how far down the list they would have had to go to have covered the current worst strain.) StuRat (talk) 15:37, 26 January 2015 (UTC)


 * It's also the case that there isn't really one specific kind of virus out there because they mutate so easily.  So even if the vaccine manufacturers guess the right strain 100% correctly, the vaccine may mimic only some of the properties of the live virus that is the dominant mutation within that strain - making it only partially effective.   Recent announcements said that even though they didn't precisely nail the exact strain of virus this year, the vaccine is still giving some protection against the most common flu virus.   Of course successful vaccinations result in an evolutionary pressure on the virus that will result in it mutating so as to make the vaccine less effective - so there can't ever be 100% success. SteveBaker (talk) 15:53, 27 January 2015 (UTC)

M1 Abrams
Someone claimed on YouTube that M1 Abrams` engine requires weekly maintenance, is that true ? 92.253.1.159 (talk) 12:14, 24 January 2015 (UTC)


 * First of all, we are talking about the Army here, and the Army has their own way of doing things. Secondly, tanks might be operating in dusty environments, so cleaning the air filter could easily be the weekly engine maintenance. Lastly, this engine probably uses roller bearings (as opposed to air bearings), which require oil, just like a regular car engine.50.43.56.168 (talk) 12:25, 24 January 2015 (UTC)


 * "...the time between AGT1500 depot-level overhauls has declined from 2,000 operating hours for a brand-new engine to 700 hours for one returned to stock from the depot." I suspect that there is a programme of lesser maintenance which is performed in the field. This critical report says (see "Chart C: Actual Use Data") that for an average of 0.6 Maintenance Hours per Mile (MH/M) for the M1 and 0.41 MH/M for the M1A1 results in 88% of tanks being available for the M1 or 82% for the M1A1. So basically, for every hour that you operate the tank, you need to spend about half an hour maintaining it; and even then, only 9 out of 10 tanks are going to work. Alansplodge (talk) 12:52, 24 January 2015 (UTC)
 * Actually the ratio is much worse, it's over half an hour of maintenance per mile travelled, not per hour operated. Roger (Dodger67) (talk) 15:04, 25 January 2015 (UTC)


 * We discussed M1 Abrams' combat-time and maintenance regimen in November 2013. A variety of reliable references indicate that these vehicles require lots of maintenance, and in exchange for that extraordinary preventative effort, they provide extraordinary combat capabilities.  Nimur (talk) 02:18, 25 January 2015 (UTC)
 * Well maybe. The Leopard 2 is just as fast and has a better operating range. The Challenger 2 is slower but is said by some to be better protected. They both use ordinary diesel engines. Alansplodge (talk) 23:25, 25 January 2015 (UTC)

How much lead is dangerous?
I'm looking for ways to change the surface of the keys on my keyboard and someone suggests that a drop of hot solder would embed itself in the key and be relatively permanent. This suggestion is immediately followed by someone warning of THE DANGERS OF LEAD POISONING. My gut reaction is nonsense. I've done some soldering with lead-based solder and while I might be crazy, I ain't dead yet. One time exposure is one thing, but repeated exposure, day after day, year after year is another. So now I'm wondering how much lead could you handle safely? Or is skin contact with room temperature solder a non-issue? 50.43.56.168 (talk) 12:15, 24 January 2015 (UTC)
 * Lead poisoning is a good start. Jarkeld (talk) 12:23, 24 January 2015 (UTC)
 * Lead poisoning Jarkeld (talk) 12:27, 24 January 2015 (UTC)


 * The urban rise and fall of air lead (Pb) and the latent surge and retreat of societal violence (August 2012) says: "Our findings along with others predict that prevention of children's lead exposure from lead dust now will realize numerous societal benefits two decades into the future, including lower rates of aggravated assault." Alansplodge (talk) 15:12, 24 January 2015 (UTC)


 * Why do it that way though? Couldn't you (for example) drill a small hole and use a hot glue gun to fill it full of polymer - or use some Polycaprolactone (sold in many craft stores under the name "polymorph" or "shapelock") which softens in 80 degC water and can be reshaped over your keycap to make any shape or surface texture you could imagine - as it cools to room temperature, it hardens and leaves a nice surface.  Just making a small dimple in the surface using a dremel would seem much preferable to messing with solder. SteveBaker (talk) 17:54, 24 January 2015 (UTC)


 * Your idea sounds very dangerous. From what I learned when I was in school, lead is a cumulative poison. I remember reading somewhere that there is no safe exposure level for it. If you have handled lead, you'd know that it's a soft metal. You could actually draw on paper with it! (Not that you should try it, I'm just making a point about how easily lead can transfer to things it come in contact with.) Having it on your keyboard means that your hands can easily pick it up and transfer it to other things that you touch and consume, including food. --108.36.120.196 (talk) 22:26, 24 January 2015 (UTC)


 * Why not just use lead-free solder? Richerman    (talk) 23:27, 24 January 2015 (UTC)


 * The danger of lead aside, there's also the question of whether the plastic the key caps are made of can withstand the temperature of molten solder. --108.36.120.196 (talk) 23:46, 24 January 2015 (UTC)

I had looked at the Lead Poisoning article but it is very long and I didn't really want to learn all about this. The section on pathophysiology contained this little bit: "...however inorganic lead found in paint, food, and most lead-containing consumer products is only minimally absorbed through the skin." That makes it sound like minimal contact entails minimal risk, for some value of minimal. So, thank you, Jarkeld, for the link. The rest of you sound like a bunch of PC ninnies. 50.43.56.168 (talk) 14:14, 25 January 2015 (UTC)


 * Absorption through the skin is only one route lead could get into your body. Like I said in my earlier reply, if you have lead on your keyboard, it can rub off onto your fingers and get transferred the food someone eats, through direct or indirect contact. This can happen if you eat finger snacks while using the keyboard. OK, let's say you are careful not to touch food while using the keyboard. You may still transfer lead to the coffee mug that you drink from while using the keyboard. Next time you have cookies and drink coffee from that cup, your fingers can pick the lead and transfer it to the cookies. Even if you're careful not to do any of those, what about family members and visitors? Can you trust them not to do something that would eventually get lead onto their food? Why risk contaminating your environment with lead just for a customized keyboard? Being prudent is not silly, and has nothing to do with political correctness. --108.36.120.196 (talk) 15:11, 25 January 2015 (UTC)


 * Traditionally kids would play with lead figurines in a house with walls of lead paint and lead dust from motor vehicles wafting in the window and eat pretty freely and no obvious fatalities... still, it's clear that at some point the lead may add up and can cause measurable IQ reduction, etc. even if they don't chew on it.  A keyboard is an ultra common contact for fingers and might therefore be problematic.  I assume that just touching the end of the soldering iron (hmmm, maybe you better decontaminate ;) ) to the keyboard would cause a noticeable imperfection to be felt anyway.  Or for a visual impact, surely there are a hundred kinds of good paint to use (not lead paint :) )  Erm, anyway, none of us are supposed to be giving you advice, but the set of references we can cite on the chronic health effects of typing on a keyboard with a lead key is just not gonna be good. :) Wnt (talk) 16:22, 25 January 2015 (UTC)


 * You are right, being prudent is not silly. But there is prudence and there is ignorant, screaming fear of the unknown. I am trying to find the line between these two, or more like what can you do with lead, if anything, that does not entail a real risk.
 * Also, I hadn't considered the bit about transfer. That could be a real problem.
 * As for actually modifying the keys, yes, there are any number of ways to accomplish that. The lead droplets just piqued my curiosity. And the way it was explained was to heat the solder with the iron and let a molten droplet fall on the key. It would melt its way into the keyboard leaving a little bump. The big advantage of this technique is that it is quick - no waiting for your glue or paint to dry. Never mind having to round up all the necessary bits.50.43.56.168 (talk) 19:38, 27 January 2015 (UTC)

Prestressed concrete transfer equation
In a prestressed concrete beam, how do you find the characteristic strength of the concrete at the time of transfer of the prestress? Im not sure what information I need. — Preceding unsigned comment added by 194.66.246.92 (talk) 18:31, 24 January 2015 (UTC)

Magnetic pick up for electric guitar
Is it, or is it not, a variable reluctance transducer. If not, what is it?--86.190.220.121 (talk) 19:08, 24 January 2015 (UTC)
 * No. And they vary; see Pickup (music technology) and then come back here for more. --Tagishsimon (talk) 19:39, 24 January 2015 (UTC)
 * Ok looked at that page: no info at all on how magnetic pickups actually work. So what in principle is the difference between a variable reluctance setup and an induced voltage caused by a magnetised string?--86.157.136.156 (talk) 19:30, 25 January 2015 (UTC)
 * Hmm. Paraphrasing the second paragraph of the Variable reluctance sensor article we would have:
 * "As the [strings] pass by the face of the magnet, the amount of magnetic flux passing through the magnet and consequently the coil varies. When the [string] is close to the sensor, the flux is at a maximum. When the [string] is further away, the flux drops off. The moving [string] results in a time-varying flux that induces a proportional voltage in the coil. Subsequent electronics are then used to [amplify] this signal to get a [stronger] waveform that can be more readily [connected to a speaker]."
 * To me this seems to very closely match how a typical electric guitar pickup works. Short Brigade Harvester Boris (talk) 21:12, 25 January 2015 (UTC)
 * See also Magnetic reluctance, which lists "Variable reluctance magnetic pickups" under "Applications". A standard guitar pickup is indeed a variable-reluctance device. Tevildo (talk) 22:36, 25 January 2015 (UTC)

Factor of 2 in photosynthesis equation
Our photosynthesis article gives the following equations:
 * 2n CO2 + 2n DH2 + photons → 2(CH2O)n + 2n DO (General equation)
 * 2n CO2 + 4n H2O + photons → 2(CH2O)n + 2n O2 + 2n H2O (Oxygenic photosynthesis)
 * 2n CO2 + 2n H2O + photons → 2(CH2O)n + 2n O2 (Oxygenic photosynthesis, simplified)

What is the purpose of carrying the extra factor of 2? Why not simply write the following?
 * n CO2 + n H2O + photons → (CH2O)n + n O2

Is there something inherently "two by two" in photosynthesis? -- ToE 23:18, 24 January 2015 (UTC)
 * I believe that is because the reactions involved are Lewis acid-base reactions, which depend on the transfer of electron pairs. -- Jayron 32 23:51, 24 January 2015 (UTC)
 * The answer to this is not immediately obvious to me. I am more familiar with a non-doubled version (especially for a "general formula") and see it is in sources like .   The electron pairs are at a pretty low level of this - the figure in Photosynthesis illustrates an electron pair being promoted for each half of an O2 produced, and this formula lists two O2s produced. Wnt (talk) 14:07, 25 January 2015 (UTC)

&, I brought this up at Talk:Photosynthesis, and then digging around a bit more in the article's history I found a potential source for the 2. Earlier versions of the article had an initial equation along the lines of:
 * 2n CO2 + 2n DH2 + photons → 2(CH2O)n + 2n D + n O2

where the 2 is necessary. Setting D to O gives:
 * 2n CO2 + 2n H2O + photons → 2(CH2O)n + 2n O2

unless you cancel the factor of 2 at the same time. I don't know what generality is lost by our use of the "DO" in the first equation, but another editor has already stripped out the factor of 2. -- ToE 23:37, 29 January 2015 (UTC)

If the birth rate is less than 2.0 birth/woman, is the population decreasing?
Is it? --Senteni (talk) 23:40, 24 January 2015 (UTC)
 * Yup. In fact, it's higher than 2.0 births/woman.  Because of various issues, it's actually a bit higher than that.  According to Sub-replacement fertility, the birth-rate necessary to maintain exactly zero population growth is more like 2.33 births/woman worldwide.  It varies by country based on things like child mortality, but even in the developed world it would still be about 2.1.  Read the article I just linked for more information.  -- Jayron 32 23:46, 24 January 2015 (UTC)
 * Note however, as the article explains, a country with a sub-replacement fertility may not have a decreasing population for reasons such as immigration and also population momentum and increases in life expectancy in the short term. Although our article does suggest even immigration may not be enough to counter another effect of sub-replacement fertility, population ageing. (Conversely I think a country could have a total fertility rate above sub-replacement fertility but still have a decreasing population due to emmigration and other factors.) Nil Einne (talk) 00:35, 25 January 2015 (UTC)


 * Not necessarily. The population will start to decrease eventually if that birth rate is maintained, but it might not happen for a long time, if there is a large excess of young people in relation to old people.  (This is the population momentum factor that Nil Einne mentioned.  I'm repeating because it is actually extremely important in real life.  Typically population does not begin to decrease until decades after the birth rate falls below 2.0.) Looie496 (talk) 16:59, 25 January 2015 (UTC)


 * The number is clearly around 2.0 - but bear in mind that you might (for whatever reason) produce more girl children than boys - which would theoretically allow the population to continue to grow with a birth rate of 2.0 or less. This doesn't appear to be the case, however; Human sex ratio says that averaged across the world, the ratio is currently 101 men for every 100 women, so a birth rate of 2.0 would likely make for a theoretical decline.  But of course, not all children live long enough reproduce - which is why the practical self-sustaining birth rate has to be a little over 2.0.  But if you imagine a society where boys are more valued by their parents than girls (eg India & China) - where it is not at all uncommon for parents to abort girls before they are born...then in such cases, 2.0 births per woman would result in a substantial population decrease as fewer than half of the population would be women. SteveBaker (talk) 00:22, 26 January 2015 (UTC)

The normal figure is "net reproduction rate" which is the number of female children a woman will have on average. In fact, even without counting sex-selective abortions, slightly over half of all infants are male, thus the total number of children needed is slightly over 2. Amazingly enough, the NRR is underpopulated developed areas tends to rise (viz. post WW II Europe) and to decline where the population density appears to be too high in such areas. Collect (talk) 15:23, 26 January 2015 (UTC)


 * The answer will also depend on how many of the women are parthenogenetic. μηδείς (talk) 20:16, 26 January 2015 (UTC)


 * Also depends on whether children favor male or female. --DHeyward (talk) 20:37, 26 January 2015 (UTC)