Wikipedia:Reference desk/Archives/Science/2012 March 24

= March 24 =

Reactivity of the lanthanides
Why is europium the most reactive lanthanide? How fast does each lanthanide corrode? Which lanthanides would corrode in air at 25 °C and 1 atm? What about the actinides? Double sharp (talk) 10:29, 24 March 2012 (UTC)
 * All the lanthanides corrode in air. Europium has the highest atomic volume of them, and is the one with the most stable divalent ion. It only donates two electrons to the conduction band and and has a larger core, and thus lower binding force. Graeme Bartlett (talk) 12:10, 24 March 2012 (UTC)

Nuclear drones
Assuming the "nuclear drones" discussed here are for real (at least, in the sense that they work out on paper, and are indeed nuclear), what's the likely power source? An airborne reactor? A nuclear rocket? A nuclear ramjet? Some kind of high-output, low-weight RTG? --Mr.98 (talk) 16:23, 24 March 2012 (UTC)


 * A nuclear rocket's operation time would still be limited by the amount of liquid hydrogen on board. The high velocities a ramjet requires to even start would pose problems, though I suppose they would be acceptable for a U-2 replacement. Whatever they chose they would have favored a high power-to-weight ratio and a low cost. My understanding is that RTGs are bad in those categories, so that would leave reactors, or something totally else. --145.94.77.43 (talk) 17:51, 24 March 2012 (UTC)


 * Nuclear reactors are big, heavy, need to be very carefully controlled to be safe, last years (not months, as described in the article) and generate far more power than a UAV would need. I'm also not sure what you would use as a coolant while in mid-air for several months. I think RTGs are far more likely. RTGs aren't exactly cheap, but they are pretty low-weight. There is a table here with the weights of the ones used in space probes - they can get down to a few kilograms (and the space ones usually need to last a lot longer than the UAV ones would need to). I would be very surprised if the proposals in question were to use anything other than RTGs. An RTG would work perfectly and would be far easier, cheaper and safer than any other nuclear technology (at least those that a publicly known, although I doubt there are any secret ones that would work better than an RTG either). --Tango (talk) 18:25, 24 March 2012 (UTC)


 * I had myself assumed RTGs, but the ones we have listed there don't seem like they put out enough electricity for their weight requirements. Of course, the ones on the list are not tailored for that kind of output... --Mr.98 (talk) 18:53, 24 March 2012 (UTC)


 * Currently, I think that the most efficient way to turn isotope heat ( which requires less heavy shielding) in to motive power is to use an Advanced Stirling Radioisotope Generator. However, a practical power unit would be required to produce about a Kilowatt of propulsion for every Kilogram of mass/weight. This it can't do. Also, the political kicker is two fold: the best isotope to use  is Pu238 which the US has to now buys off the Russians. Second: who wants to run the risk one of these aerial contrivances coming to earth and coming in to the possession of people that don't exactly like American's (hard to believe I know but not everybody likes them – nor Europeans for that matter).--Aspro (talk) 19:36, 24 March 2012 (UTC)


 * If the goal is weeks / months rather than years, then you could get a lot more bang for your buck by using more highly radioactive and lighter-weight substances than plutonium. A low-mass relatively short-lived substance like Beryllium-7 (56 day half-life) has an baseline energy output of about 1 MW / kg, not including the mass of shielding, heat exchangers, and other material required to make it useful.  It certainly seems feasible that one could design an RTG for moderate duration aircraft use, but you'd have to think carefully about the materials used, and what isotopes could be created and concentrated relatively easily.  Dragons flight (talk) 21:12, 24 March 2012 (UTC)


 * Agreed on the shorter half-life, which would also limit the ability of it to be used by an enemy who captures it. For cooling they could use the passing air.  As for shielding, they could likely reduce that by putting it far from the important bits (weapons, surveillance, communication, and navigation system).  Perhaps they could put the nukes in the tail or at the wing tips.  The radiation leaking out might make it possible to design a radiation seeking missile to shoot them down, though.  They would also need to ship them to their deployment points with shielding (or ship the "fuel" separately in it's own shielding).


 * To say it's politics stopping the project seems rather an understatement, though. This seems unwise in many ways.  Aside from those mentioned, the difficulty in resupplying the "fuel" to deployment points during a war must be considered, as you can't stockpile something with such a short half-life.  It seems to me that programming the UAVs to ride thermals and adding solar panels might extend their capability without the increased risks.  StuRat (talk) 21:32, 24 March 2012 (UTC)


 * Could something with a short half-life be produced in good quantities? It can't be a byproduct of the mining industry. --145.94.77.43 (talk) 21:42, 24 March 2012 (UTC)


 * As long as you use an alpha- or beta-emitter rather than a gamma-emitter, there won't be any significant radiation leakage (you would also need to avoid any decay products that give off a lot of gamma radiation, though). You would need to produce the radioactive material in a reactor, which is where the material for RTGs usually comes from anyway. --Tango (talk) 21:49, 24 March 2012 (UTC)


 * And, since you really don't want to set up a breeder reactor in hostile territory, you then have the problem of needing to constantly deliver the products from the reactor to the battlefield. StuRat (talk) 08:27, 25 March 2012 (UTC)


 * If the drone is designed to fly for weeks or months without refueling, you could just fly it to the warzone from bases well behind friendly lines, avoiding the delivery issue entirely. As for flying it there safely, if you intend for it to be in the air for that long, then either it's a stealth drone that should have no trouble, or you intend to fight an enemy with limited AA capability (low-tech terrorists/guerrillas). So flying it say from the continental US where your fuel production facility is located shouldn't be an issue for any sensible use of such a drone. Someguy1221 (talk) 08:38, 25 March 2012 (UTC)


 * Now you're talking about flying nuclear materials over friendly nations. They wouldn't be too happy about this, as UAVs will occasionally crash all on their own, spreading radiation all over the crash site.  And the extended range would be significantly shortened, if it takes a week to fly into position, and another week to fly back to refuel. StuRat (talk) 08:49, 25 March 2012 (UTC)


 * OK, sensible is a relative term here :) Someguy1221 (talk) 08:59, 25 March 2012 (UTC)


 * Spacecraft with RTGs can crash in friendly nations too, but they are still used. If you're useful an isotope with a short half-life, then the contamination would be short-lived too (unless there are problematic decay products, I suppose). --Tango (talk) 02:16, 26 March 2012 (UTC)


 * True, but we are talking dozens of spacecraft versus potentially thousands of UAVs, and they spend millions on each of those spacecraft to ensure a safe launch, and I believe they typically try to launch over water to minimize damage in a crash. Also, there may not be any viable alternative for a spacecraft headed to the outside of the solar system, as conventional fuel or batteries wouldn't last, and sunlight is too weak for solar panels there. StuRat (talk) 03:04, 26 March 2012 (UTC)
 * The contamination would be short lived no matter the half-life, since the authorities would come in and dig the whole crash site out of the ground and dispose of it. If the crash site happened to be my back yard, I would prefer it to be a material with a longer half-life, since then the intensity of the radiation would be lower (but as mentioned further up, that's not conducive to high power output to weight ratios). 203.27.72.5 (talk) 21:12, 28 March 2012 (UTC)

Kelvin–Stokes theorem
Can you give me a source to read about what it means "intuitively"? I mean I wanna have an intuitive understanding of it so that it'd seem obvious to me... the article didn't work...--Irrational number (talk) 19:12, 24 March 2012 (UTC)


 * Intuitively speaking, I think that for you to be able to phrase  such a question in this way, suggest so me,  that you already know the answer. --Aspro (talk) 19:40, 24 March 2012 (UTC)
 * I'm not a native speaker so... forgive me for my my phrasing :D but I guess you git the point...--Irrational number (talk) 20:12, 24 March 2012 (UTC)


 * [[Image:Stokes-patch.png|150px|right]]Did you look at Stokes' theorem? I think the image there is a good intuitive picture. In two dimensions, the curl of a vector field at a point is the integral of the field around a tiny loop at that point. When you take the sum of all of those loop integrals, the parts where the loops touch cancel out, leaving only the part on the boundary. It's a little more complicated in three dimensions—is that the case you don't understand? -- BenRG (talk) 23:20, 24 March 2012 (UTC)
 * I did read it, but I think you said it better than the article did, I think I get it now...--Irrational number (talk) 06:28, 25 March 2012 (UTC)

Substances more reactive than pure fluorine
Are there any elements or compounds that are more reactive than pure fluorine gas? Whoop whoop pull up Bitching Betty 21:58, 24 March 2012 (UTC)


 * Elements: no, compounds: yes. Plasmic Physics (talk) 22:06, 24 March 2012 (UTC)


 * Reactive with what ? Sodium is rather reactive with water.  Not sure if it's more reactive, though. StuRat (talk) 22:10, 24 March 2012 (UTC)


 * Don't know if this is what you're looking for. I personally don't know of anything more reactive than Francium. I certainly would not have allowed it into my lab -even if some was was ever available... --Aspro (talk) 22:16, 24 March 2012 (UTC)
 * Lol...Whoop thinks Fluorine is highly reactive and then Apsro says maybe Francium is more reactive. Look where those two are on the periodic table and remember that "electronegativity increases on passing from left to right along a period, and decreases on descending a group". Reactivity is relative, and fluorine would theoretically react very strongly with francium to produce a very stable francium fluoride (probably similar to caesium fluoride). I say theoretically because all isotopes of Fr are highly unstable and decay with a maximum half-life of 20mins, which is the real reason that Aspro shouldn't have it wandering around his lab. Caesium is chemically very similar to Fr, and I have a bit of that in my lab. The chemical reactivity of it just isn't that scary. 203.27.72.5 (talk) 20:49, 28 March 2012 (UTC)
 * Liquid_rocket_propellants has be tried for rocket propulsion and about as reactive as is practicable to archive but even then -not something one should try at home – unless of cause, your baby-sitter is distracted because she's  invited her boyfriend round; then you're free and at liberty to try anything...--Aspro (talk) 22:22, 24 March 2012 (UTC)


 * Here's an entertaining blog about violently reactive compounds . Whenever I feel my day job is boring I just remind myself that at least I'm not an experimental chemist cooking up FOOF all day.Anonymous.translator (talk) 22:53, 24 March 2012 (UTC)
 * I liked the bit about ". . .not unless I'm at least a mile away, two miles if I'm downwind." Sounds like the type of experiment that should only be performed in up-town Manhattan. --Aspro (talk) 23:10, 24 March 2012 (UTC)
 * More specifically Dowisetrepla.Anonymous.translator (talk) 23:27, 24 March 2012 (UTC)
 * Oh Yes!!!  Do they have a fan-forum on which we can post  how they can home-brew   some of  this stuff – in Very Large Quantities? --Aspro (talk) 23:47, 24 March 2012 (UTC)
 * "Things I Won't Work With"; my favourite chem blog! 84.197.178.75 (talk) 13:36, 25 March 2012 (UTC)
 * There are also unstable things that would be more reactive, eg monatomic fluorine, or atomic oxygen or C4+ plasma. These may be stable when dillute, but react when meeting anything else.  And I note you already are aware of helium hydride. Graeme Bartlett (talk) 03:32, 26 March 2012 (UTC)

You know how a worm can sometimes re-grow into two whole ones if you cut it in half?

 * a) What are the 'rules' for whether this works or not?
 * b) How many pieces can a worm be chopped into and still have each piece grow into a complete worm?
 * c) Could you cut one in half right down the middle (i.e. from head to back end) and have both halves live?

and related q.


 * d) Is there any complex animal that can regrow its entire body from a single removed cell?

(don't worry, I'm not going to try it on real worms, just interested, as it was something my son asked me about today)

Thanks. --95.148.105.119 (talk) 23:57, 24 March 2012 (UTC)


 * Please refer to our article on Earthworm.Anonymous.translator (talk) 00:04, 25 March 2012 (UTC)
 * And some flatworms, such as Planaria, can regenerate parts more readily than your average angleworm. (Their abilites in this regard used to be the subject of standard demonstrations in biology classes.) It really depends on what sort of worm you have in mind. Deor (talk) 01:10, 25 March 2012 (UTC)
 * A university I know once tried to replicate as a freshman science class experiment the slicing in half of flatworms to see if the head or tail retained a learned behavior, but their experience was that generally both halves died. It is not as easy as it might seem to get a bisected planarian to regenerate. Edison (talk) 03:23, 25 March 2012 (UTC)


 * Starfish, cut in half with the cut passing through the middle, will regenerate into two independent animals. Rather than a brain, per se, starfish have a ring of neural material that circles the central mouth - whether a cut segment can regenerate into a viable organism depends on whether that segment has enough of that neural ring. For most starfish that number is about 40%; for a few (e.g. Linckia) that number may be as low as 20% (meaning you could get 4 or 5 viable starfish from cutting up one individual). This works in part because a starfish is, in a way, a community of limbs rather than a centrally managed individual like ourselves; that neural ring serves for basic coordination (mostly stuff like "you go limp now, I'll pull"). Far further along the line of being a community than an individual are sponges. Sponges are anatomically fairly complex (they're not just a big pile of cells all the same as one another) and capable of some organised behaviour - but render a sponge through a sieve and the cells revert to being individuals, before forming into new sponge colonies.  So sponges are on the very cusp between being a "city" of cells and a coherent individual. -- Finlay McWalterჷTalk 12:59, 25 March 2012 (UTC)