Wikipedia:Reference desk/Archives/Science/2013 July 21

= July 21 =

Lightning and outdoor sport
1. Although it is widely recommended not to seek for shelter during a storm below a tree, wouldn't it be advisable to stay more or less near a tree (or something high)? That seems wiser than an open field. The lightning follows the shortest path, and that won't be me, but the tree. Additionally, does hiding in a woods somehow protect you against lightning for the same reason? 2. Couldn't really thick soles serve as protection, in the same way that a car's tires protects us? 3. What is the highest point that still gets hit by lightning? Is it possible for lightning to strike above the clouds? — Preceding unsigned comment added by 34and34 (talk • contribs) 00:35, 21 July 2013 (UTC)


 * The reason people are sometimes advised not to shelter close to trees is because a lightning strike can cause the tree to explode - see Exploding tree. -- Finlay McWalterჷTalk 00:40, 21 July 2013 (UTC)


 * On the "really thick soles" point, see Dielectric strength. The typical voltage of a lightning bolt is several hundred million volts, which can easily get through a centimetre or so of rubber. Tevildo (talk) 00:48, 21 July 2013 (UTC)


 * To clarify, the car's _tyres_ don't protect the occupants - the car's body forms a Faraday cage, which keeps the lightining outside the car. Tevildo (talk) 00:49, 21 July 2013 (UTC)


 * This website describes the crouch method. Lightning can strike cloud-to-cloud and objects flying through them, and there are recently confirmed phenomena like sprites that occur above them. μηδείς (talk) 01:36, 21 July 2013 (UTC)


 * If you're in danger of getting hit with a sprite, crouching is unlikely to help you. Matt Deres (talk) 02:22, 21 July 2013 (UTC)


 * Odd it's not actually mentioned in the lightning article. See Sprite (lightning). μηδείς (talk) 02:44, 21 July 2013 (UTC)


 * If you were touching the tree and it were energized to a high voltage, your body would provide an alternative conducting path to the tree trunk, and a fatal current might pass through you. If you were standing near the tree, and it were energized, there would be a voltage gradient along the surface of the ground, so that if one foot were closer to the tree than the other, there could be hundreds or thousands of volts difference between where the two feet were.  The same hazards hold if a high voltage wire is touching some object or ground. A similar potential difference could be there if you were touching another person. If a powerline falls on a car, and you step out onto the ground while touching the car, you are likely to be killed. The same holds if it falls on a metal fence and you touch the fence.

And how did Supersaurus protect itself during thunderstorms? Count Iblis (talk) 15:52, 21 July 2013 (UTC)
 * By sticking close, but not too close, to an even bigger supersaurus. ←Baseball Bugs What's up, Doc? carrots→ 17:09, 21 July 2013 (UTC)


 * They did feed among large coniferous forests, so they may have used the crouch method. μηδείς (talk) 18:13, 21 July 2013 (UTC)


 * Lightning strikes are just not that common. Probably they didn't protect themselves and a very few of them died as a result. SteveBaker (talk) 21:13, 21 July 2013 (UTC)


 * That's 100 strikes per second year-round worldwide, and a 1/6250 chance of being stricken once in a human lifetime in America. μηδείς (talk) 21:21, 21 July 2013 (UTC)


 * 1 in 6 if you suck at statistics... - ¡Ouch! (hurt me / more pain) 12:40, 22 July 2013 (UTC)


 * Yes, I agree that some distance from a tree is probably better than just standing out in an open field. Just what that distance is would be an open question, though.  I wonder how far away from a tree people have been affected by lightning which struck that tree.  I'd suspect that the formula one would come up with for the ideal distance to stand from a tree would have a constant distance, to which you would add an adjustment factor based on the height of the tree.  (For one thing, you certainly don't want to be so close it could fall on you.) StuRat (talk) 19:24, 22 July 2013 (UTC)


 * I've heard that 8-12 yards is usually the best distance -- at that distance, the tree acts as a lightning rod, while the danger from splinters or from collateral streamers is minimal. 24.23.196.85 (talk) 04:05, 23 July 2013 (UTC)


 * The most elegant and cogent explanation I've heard for how lightning moves is (amazingly) an XKCD "what if" entry: Today's topic: Lightning. It explains that a good way to visualize how lightning travels is to imagine a 60 meter sphere centered on the tip of the lightning bolt...eh...just read it.  SteveBaker (talk) 02:59, 23 July 2013 (UTC)


 * I was lucky to not die last week when lightning struck near enough my front door to make sparks fly from my doorknob with a cartoonish buzzing sound (immediately followed by one of those insanely loud thunderclaps). The nearest tree was about 70 feet away, and the nearest field about 30. The door was three away from me, and I was petting a long-haired cat.
 * You can draw your own conclusions. What I took from it is Mother Earth is cruel and unusual. Period. InedibleHulk (talk) 03:01, 24 July 2013 (UTC)


 * Guess I should mention I was inside my own house. InedibleHulk (talk) 03:02, 24 July 2013 (UTC)
 * Hint #1. That was no lightning. Hint #2. You just made me wish I had a cat with that kind of static buildup. XD - ¡Ouch! (hurt me / more pain) 06:27, 25 July 2013 (UTC)
 * That was exactly what I had in mind when I said "draw your own conclusions". Seriously, though, that cat may have saved my life. If I hadn't stopped to pet her or for as long, I'd have been holding that door or under the bolt. Unless my body being there made the lightning strike somewhere else. Which probably would have been the case, from what little I know about electricity. InedibleHulk (talk) 23:29, 25 July 2013 (UTC)

Insulating patches in otherwise-complete Faraday cages
In the section immediately above, the passenger compartment in an automobile is described as a Faraday cage (note: I don't think that article mentions insulators). Whereas a classic Faraday cage is composed of a continuous sheath of conductor (with gaps smaller than the wavelength of the electromagnetic radiation in question), an automobile has a substantial proportion surface composed of glass (windows), which I would consider insulator. So: what proportion (in terms of surface area) of a Faraday cage can be insulator? Does a light bulb (mostly insulator) serve as a Faraday cage? Apologies if this is obvious, but I don't completely trust intuition when it comes to physics. -- Scray (talk) 01:21, 21 July 2013 (UTC)
 * A lightning bolt is not characterized by any particular wavelength so the protection of a discontinuous Faraday cage depends on it not having an aperture larger than the distance from the aperture to a person inside. The link shows a man with faith in a flimsy Farady cage.. Lightning bolts care little for insulators. DreadRed (talk) 09:52, 21 July 2013 (UTC)


 * The definition of a Faraday cage is quite loose and depends on the application. If you're shielding against omnidirectional microwave radiation then of course any hole larger than the wavelength will admit energy. If you're shielding against lightning, you're not really interested in the high-frequency components (which exist, but don't contain much energy). For example, this study (see p3) says that the current in an average lightning strike is down to 1% at 60 kHz, so most of the energy is well below that frequency. The high-frequency stuff may interfere with your radio but won't fry you. At 60 kHz the wavelength is 5 km. At these scales, any nearby conducting path that doesn't go through you is sufficient to protect you from the electric field. --Heron (talk) 10:08, 21 July 2013 (UTC)


 * That seems a bit large. I certainly wouldn't feel safe from lightning if I was in a cage with 5 km holes between the bars. StuRat (talk) 19:28, 22 July 2013 (UTC)


 * Lightning is low frequency because the current path is big - a few km long. When the conducting path is completed it takes several microseconds for the collapse of the electric field (travelling at the speed of light (300 m/μs)) to work its way along the whole path and for the current to grow, and some tens of μs for the whole thing to discharge and for the current to decay.


 * Electromagnetic radiation can propagate freely through an aperture half a wavelength wide (see perhaps waveguide (electromagnetism)), which on the basis of Heron's argument would mean a maximum acceptable hole width of 2.5 km. However (1) propagation through the hole does not instantly stop at the cut-off frequency, but rolls off to lower frequencies (we ought to have a page on Bethe aperture theory but haven't). Perhaps more importantly (2) even a static (zero frequency) electric field is not reduced to exactly zero over the plane of the aperture, but 'fringes' ('bulges') through the hole (see perhaps near field or evanescent field), so you would still need to stand a km back from the hole. Examine the viewing screen on the door of a microwave oven, and you will notice that (1) although the wavelength is ~122 mm, the holes are only couple of mm across, and (2) there is a glass panel that stops you putting your hand right up against the holes. --catslash (talk) 23:22, 23 July 2013 (UTC)

Why are deciduous trees light green ("spring green") in the spring?
We all "know" that deciduous trees are light green ("spring green") in the spring and that the green darkens through the summer. I can't find any documentation or explanation of the effect.

My hypothesis is that we're seeing a rough reversal reverse of the fall foliage effect. The yellow colors must come in first together with some chlorophyll and then more chlorophyll is produced resulting in a darker green as the season progresses. Apparently the chlorophyll is withdrawn first in the fall so we see pure yellow / orange / red leaves but it returns together with the yellow pigments in the spring so that we don't simply see fall foliage in reverse in the spring. — Preceding unsigned comment added by Doppler13 (talk • contribs) 03:15, 21 July 2013 (UTC)
 * Or more poetically "Nature’s first green is gold," Rmhermen (talk) 12:49, 21 July 2013 (UTC)


 * Yes increased chlorophyll production, usually limited by nitrogen uptake. See . Overly dark leaves can be a symptom of many diseases, including a surfeit of nitrogen uptake.  Spinning  Spark  17:49, 21 July 2013 (UTC)


 * I'm thinking that the chlorophyll relatively untainted by the other pigments would be the brighter green, and as the synthesis of the reds and yellows catches up to the chlorophyll, that's what darkens the leaves? Gzuckier (talk) 08:31, 22 July 2013 (UTC)


 * Another effect might just be the new leaves being thinner, so more light shows through from the back, making them appear lighter. StuRat (talk) 19:14, 22 July 2013 (UTC)
 * There's definitely a color difference, though. looking at bushes in my yard, you can tell this spring's growth as definitely lighter, both deciduous and evergreen; on a similar note, I can scan for weeds in the lawn pretty quickly, since they are a brighter green, although that could be contaminated by native color of the weeds, but my belief is that's more because they tend to be growing quicker than the grass. Gzuckier (talk) 16:01, 23 July 2013 (UTC)

Main Battle Tank
Can T-90 withstand hits from rpg-7 and spg-9 on the sides, can T-90 resist these weapons without reactive armor ? — Preceding unsigned comment added by Tank Designer (talk • contribs) 10:44, 21 July 2013 (UTC) I am sorry because maybe these questions are not suitable here but I do not like forums. — Preceding unsigned comment added by Tank Designer (talk • contribs) 10:46, 21 July 2013 (UTC)


 * I think your questions are suitable here. They are after all reference questions, asking for the endurance of T-90 when the battle tank is contacted with rpg-7 and spg-9 on the sides. As a matter of fact, Wikipedia does have an article on T-90. It is a Russian third-generation main battle tank. rpg-7 and spg-9 are war weapons. Out of curiosity, why are you narrowing your search to those specific weapons? I am just curious, because I would have asked "Can a T-90 withstand a 10-foot-tall boulder?" Sneazy (talk) 00:09, 22 July 2013 (UTC)
 * The question is very likely because the RPG-7 and SPG-9 are the weapons that the Syrian opposition are trying to use to stand up to T-90 tanks.  Spinning Spark  01:09, 22 July 2013 (UTC)
 * According to the article, "During combat operations in Dagestan, there were witness accounts of one T-90 sustaining seven hits from RPGs, and remaining in action" -- which is all the answer that you could ask for! 24.23.196.85 (talk) 04:30, 22 July 2013 (UTC)
 * Actually, it's not "all the answer". It all depends on where a weapon hit the tank and if you're after a mobility kill, mission kill or catastrophic kill. Both the RPG-7 and SPG-9 will be able to affect a mobility kill against a T-90 if the warhead strikes the running gear. Both will also be able to affect a mission kill if the warhead hits the main gun of the T-90. Neither is likely to be able to affect a catastrophic kill however, unless the warhead enters a open hatch or something similar. WegianWarrior (talk) 09:19, 22 July 2013 (UTC)

Well thank you but I think that the ammo of T-90 is exposed because you know that the side amour of T-90 is very thin, what about the rubber side skirts are they effective against these weapons Tank Designer (talk) 09:50, 22 July 2013 (UTC) ? — Preceding unsigned comment added by Tank Designer (talk • contribs) 09:49, 22 July 2013 (UTC)
 * It should be obviouse that you need bigger weapons as battle tanks are mainly build to fight other battle tanks. If you check M1 Abrams, Merkava or Leopard 1 you find they have rather large and additional very high tech main guns using similar very high tech shells way bigger than these "old" rpgs. You could also compare these rpgs with newer, more advanced and much bigger anti-tank weapons like the BGM-71 TOW to estimate these old russian rpgs probably wont be very effective. --Kharon (talk) 17:55, 22 July 2013 (UTC)

There is no doubt that those big antitank weapons and tank guns can penetrate the sides of T-90 -of course any other main battle tank- but the problem is the wrong way of storing the ammo of the tank which allows for a catastrophic accident when any simple penetration occurs through the sides of the tank .Tank Designer (talk) 02:11, 23 July 2013 (UTC)--Tank Designer (talk) 02:11, 23 July 2013 (UTC)
 * Careful with "other battle tanks". MBTs are meant to destroy other heavy armor too, including heavy APCs, self-propelled artillery, etc. The duel with an enemy MBT, however, is the discipline that really puts its capabilities to the test.
 * The Abrams M1A1 had a lot of trouble destroying another disabled Abrams (possibly the HA variety with DU mesh armor) when it attacked its weak spot using shells far more destructive than those fired by T-90s and other Warsaw Pact artillery. IIRC, they fired two LRP shells at its rear flank to little result and decided to salvage the remains instead of firing more shells. Anecdotic, but it does challenge the "there is no doubt" aspect. - ¡Ouch! (hurt me / more pain) 06:55, 23 July 2013 (UTC)

What is Osmazome?
Mrs Beeton frequently uses the term "osmazome" in Household Management, in which she defines it as "that part of the meat which gives flavour and perfume to the stock." Currently, Osmazome redirects to Umami (which isn't inconsistent with Beeton's definition), but the term is not mentioned in that article, and the earliest use of "Umami" is from 1906. If there are any references to support this redirect, or for a more appropriate one (Glutamic acid (flavor), perhaps?), I think they should be included. Does anyone have any? Tevildo (talk) 18:43, 21 July 2013 (UTC)
 * Molecular gastronomy indicates that the term "osmazome" was possibly introduced into the lexicon in the early 19th century by Marie-Antoine Carême. It is unclear (and probably unlikely) that Carême knew exactly what "osmazome" was aside from "that meaty flavor", that is, essentially Umami.  -- Jayron  32  18:57, 21 July 2013 (UTC)
 * More This link gives a citation to a 1913 edition of the Webster's Revised Unabridged Dictionary which indicates that even by 1913, the term was considered archaic. -- Jayron  32  18:59, 21 July 2013 (UTC)
 * Thanks for the information! I'll see if I can track down which of Carême's publications contains the word. Tevildo (talk)


 * (ec) Here's what the OED says: "The name given by Thénard to that part of the aqueous extract of meat which is soluble in alcohol and contains those constituents of the flesh which determine its taste and smell." It includes a quote from 1813 that states, in part, "According to Dr. Thomson, it is very doubtful if osmazome be anything but fibrin, slightly altered by solution in water." I have no idea if the fibrin mentioned in that quotation is the same as what's called fibrin today, but I present it for what its worth. Deor (talk) 19:40, 21 July 2013 (UTC)
 * Oh, and in case you're interested, the word is a coinage based on Greek ὀσμή, "smell", and ζωμός, "broth" or "sauce". Deor (talk) 19:50, 21 July 2013 (UTC)
 * Thanks again. fr:Extrait de viande contains the relevant citations - perhaps our Meat extract article is the best target, with an appropriate translation of the French text. Tevildo (talk) 20:39, 21 July 2013 (UTC)
 * No, I think Umami is fine. "Those constituents of the flesh which determine its taste and smell" is how I would describe the concept of Umami.  To wit, from the Wikipedia article Umami, "It can be described as a pleasant "brothy" or "meaty" taste"  which is a pretty close match to the OED citation.  -- Jayron  32  20:43, 21 July 2013 (UTC)
 * Umami is often presented as a sort of core flavor type, that can be applied to many foods. Osmazome seems specific to meat, and at least one definition gets at solution/solutes, neither of which are part of Umami. I'd support a redirect to meat extract, though perhaps a disambig to osmazome_(flavor) and osmazome_(extract) (with redirects to umami and meat extract) would be the best of both worlds. SemanticMantis (talk) 23:13, 21 July 2013 (UTC)
 * I would agree with SemanticMantis here - it seems that osmazome:umami::haemoglobin:red, the difference being that osmazome can't be isolated as a separate chemical. (Or can it?  Is it glutamic acid?)  Perhaps WP:RFD is the next place to go. Tevildo (talk) 20:41, 22 July 2013 (UTC)

Operating characteristics of induction motors on modified sine wave
I am looking for references on operating characteristics of induction motors when operated on modified sinewave in the region of 50 to 60 Hz for the article Power_inverter. Modified sinewave in this context is a square wave with a period of off cycle at zero crossing. I would like to know the variation in characteristics as a function of the duty cycle of this off period as well. The characteristics I am seeking are starting torque, slip rate, running efficiency at varying loads and starting current.

These references look interesting from preview, but I do not have access to the full thing.

books.google.com/books?id=U-BsAAAAIAAJ

books.google.com/books?id=LLo3FbUdYBsC

Cantaloupe2 (talk) 22:18, 21 July 2013 (UTC)


 * Are you looking for sources, or free online sources? This paper "Analysis of the impact of pulse-width modulated inverter voltage waveforms on AC induction motors" seems to have what you want but you will need to find someone with access to IEEE Xplore if you don't want to pay for it.  A friendly student perhaps?  Is this related to Wikipedia article improvement in any way?  Spinning  Spark  01:24, 22 July 2013 (UTC)