Wikipedia:Reference desk/Archives/Science/2016 September 2

= September 2 =

Sonic boom
Is it possible, in principle, to put out a fire by deliberately booming it (by a similar mechanism as blowing out an oil lamp)? Note that I am not asking whether this is a practical way of putting out a fire (obviously it is not), but whether it's possible in principle. 2601:646:8E01:7E0B:F88D:DE34:7772:8E5B (talk) 00:09, 2 September 2016 (UTC)
 * This looks promising. -- Jayron 32 00:54, 2 September 2016 (UTC)


 * Not caused by aircraft, but it did remind me of blast waves for extinguishing oil well fires. See that article and also extinguishing efforts of the Kuwaiti oil fires and blowout control expertise. ---Sluzzelin talk  01:31, 2 September 2016 (UTC)

Concorde jet
What was the V(ne) of the Concorde jet (in terms of IAS)? 2601:646:8E01:7E0B:F88D:DE34:7772:8E5B (talk) 00:12, 2 September 2016 (UTC)
 * Concorde did not have a published VNE, as its performance at high speeds was limited by heating rather than aerodynamics, so the Mach number rather than the IAS was the critical parameter. MMO for Concorde was 2.04, and the overspeed warning would trigger at an IAS of 6 kt above MMO.   See this forum thread for a copy of the flight envelope. Tevildo (talk) 09:48, 2 September 2016 (UTC)
 * So, nobody knows whether it could break Mach 1 at low altitude, F-111 style? 2601:646:8E01:7E0B:F88D:DE34:7772:8E5B (talk) 09:51, 2 September 2016 (UTC)
 * Looking at the flight envelope displayed on the link given by Tevildo, the Concorde would be outside the envelope if it went above Mach 1 below 30±1 K feet. So while it possible (but not likely) could do it, no sane pilot would attempt it. WegianWarrior (talk) 11:36, 2 September 2016 (UTC)


 * Small aeroplanes are limited by Vne, the never exceed speed (IAS); and large aeroplanes, including Concorde, are limited by Vmo, the maximum operating limit speed (IAS). Vne and Vmo are very similar and we can interpret the original question as being about Vmo.
 * Any high –speed aircraft will also be limited by Mmo, the maximum operating Mach number. The maximum speed of such an aircraft, including Concorde, at low altitudes will be its Vmo, and at high altitudes its Mmo will be more limiting. The relationship between the two limits is not simple but the pilots merely have to ensure the needle pointing to current indicated airspeed does not get so high it reaches the maximum speed, usually indicated by a second needle coloured red and white and called the “barbers pole”. See Airspeed indicator.
 * Limitations applied to Concorde by the US Federal Aviation Administration are visible in Type Certificate Data Sheet A45EU. (Go to http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgMakeModel.nsf/MainFrame?OpenFrameset and search for A45EU.) TCDS A45EU does not reveal precisely what Vmo and Mmo are for Concorde. Instead, on page 3 of 9 it states “As presented in the speed and Mach number of the approved AFM.”) Dolphin  ( t ) 11:44, 2 September 2016 (UTC)
 * Thanks, User:WegianWarrior! 2601:646:8E01:7E0B:F88D:DE34:7772:8E5B (talk) 22:17, 2 September 2016 (UTC)

Supernova compared to a hydrogen bomb pressed against your eyeball?
Randall Munroe in one of his "what-if" dissertations (what-if.xkcd.com/73/) makes the claim that a supernova, seen from as far away as the Sun is from the Earth, is brighter than the detonation of a hydrogen bomb pressed against your eyeball, by nine orders of magnitude. I've found this quoted several other places, but never discussed or disputed and I haven't found an original reference. It seems wrong to me, especially the last qualification. Please check my logic: The sun (which is already "as far away as the sun is from the earth") has an absolute magnitude of 4.83 (en.wikipedia.org/wiki/Sun). Call it 5 for ease of calculation. Supernovae vary a bit, but call it -20. So a supernova is brighter than the sun by 25 stellar magnitudes. 5 (stellar) magnitudes = 100x or 2 orders of magnitude so 25 magnitudes = 10 orders of magnitude. So a supernova is brighter than the sun by 10 orders of magnitude. Subtract the 9 orders of magnitude in the scenario means that the hydrogen bomb is only ten times brighter than the sun. That seems low. Is Munroe wrong, or did I miss something in my calculations? 216.167.245.21 (talk) 11:50, 2 September 2016 (UTC)


 * Monroe refers to energy delivered, not luminosity. So I'll take a quick look at the energies involved.  A type 1a supernova releases about 1 J.  At 1 AU, that is an energy density of about 4 J/m^2.  A large hydrogen bomb (10 Mt) releases about 4 J.  The human eye is about 2 cm across, so the hypothetical hydrogen bomb exploding at your eyeball would seem to delivers roughly 8 J/m^2.  Of course, an actual hydrogen bomb has some bulk to it, so the detonation won't actually happen at the surface of your eye.  If the core is say 1 m from your eye, then the energy density at the eye would be about 3 J/m^2.  So for my choice of parameters, it does seem correct that a supernova at the distance of the sun is delivering more energy than a hydrogen bomb pressed against your eyeball, but maybe not the nine orders of magnitude stated in Monroe's post.  Dragons flight (talk) 12:23, 2 September 2016 (UTC)
 * I think its Randall Munroe, not the guy with the doctrine. --Stephan Schulz (talk) 12:30, 2 September 2016 (UTC)
 * You're quite correct. I've corrected spelling in the original note. Apologies to Mr. Munroe (or the guy with the doctrine as the case may be).
 * The original source was this post, but Randall did not provide a source or calculation. Someguy1221 (talk) 17:27, 2 September 2016 (UTC)


 * I suppose Randall Munroe is wrong, as many times before. Maybe this was commented at xkcd sucks, like many other slips of Munroe. Llaanngg (talk) 22:47, 2 September 2016 (UTC)

Question relates to UK law and electrical regulations.
I own my house, it needs re-wiring because the "consumer unit" is from the 1960s and the wiring probably is too. The energy company keep telling me when they come to read the meter. But I don't want to pay for the whole house to be re-wired when I live on my own and almost never turn the lights on or use most of the sockets. I basically just live in one room downstairs and the rest of the house is storage for of my late parents stuff and I never go into those rooms. I only need two sockets; one for the computer and one for the microwave. So I am wondering if I can simply have a new consumer unit put in, disconnect the existing light and socket cables but just leave them in the walls unconnected, and instead have the guy put a couple of plugs by the consumer unit to use directly. That way he wouldn't have to go around my whole house (I'm a very private person) ripping out cables, digging holes in walls, and testing sockets and fittings. He would not need to work anywhere except the under-stairs cupboard. It should cost a lot less as well. Would that be legal and acceptable per the electrical regulations? Or to put it another way, am I required by UK law to have working lights and sockets in every room, even if I don't want them? Thanks for your help. — Preceding unsigned comment added by ElectricianQuestion (talk • contribs) 14:04, 2 September 2016 (UTC)


 * Wikipedia does not answer requests for legal or professional advice. You will need to seek out an appropriately licensed electrician, contractor, lawyer, or similar professional in your jurisdiction.  You might also try contacting the local council responsible for enforcing building codes, as they will presumably know what is allowed and may be willing to advise you for free.  Dragons flight (talk) 14:31, 2 September 2016 (UTC)


 * This - http://electrical.theiet.org/building-regulations/part-p/faqs.cfm - looks to be a fairly good introduction to the rules in the UK, and has links to the detailed regulations. Wymspen (talk) 16:00, 2 September 2016 (UTC)


 * You can do whatever you like. You will then be responsible for what you did. UK regulation is based on two levels of paperwork: we have no "licensed contractors" in such a sense, instead the regulations are based on a "competent person" doing work and some work (basically work for other people) requiring to be done by a NICEIC registered organisation under "Part P", which is a tax ruling more than a certification or licensing scheme. There is no formal standard of skills certification required. The contractor may allocate their workers to do tasks quite freely (and will take the blame if they get it wrong), but they're not restricted to checking training certificates.
 * You need to have a "competent person" do this work. That can be you. It's your call. This isn't rocket surgery.
 * There are at least three useful sources of information you can look at now. One is an "On-site guide", a precis of the (long and unreadable) full wiring regulations, intended for electricians to keep in the van. 15-20 quid, any electrical wholesaler, essential. Another source is Usenet and the old [news:uk.d-i-y uk.d-i-y] newsgroup, now still available through http://groups.google.co.uk/group/uk.d-i-y  an excellent resource and well inclined towards this sort of project.   Also the UK DIY wiki is useful: http://wiki.diyfaq.org.uk/index.php/Category:Electrical
 * One thing you will need is some test gear, which needs to be calibrated under a current certificate for commercial use and should at least still be accurately in calibration for your own use. Some of these pieces of test gear are obscure, can't be emulated with a multimeter (RCD testers, earth loop impedance, megger) and really are needed for some commissioning tests. They can be hired though. It's worth getting a megger though - they're generally useful things.
 * I have a similar situation with our house. So I replaced my own consumer unit and I'm gradually re-wiring from that point downstream. I currently have four consumer units: the old and new in parallel (circuits gradually moved across as each is done) and two more new ones in outbuildings.  It's quite easy to swap a CU: first ask the electricity supplier to install an isolating switch after the meter (£40 or so, sometimes free), connect this into a 'Henley block' and then run the two CUs from the Henley.  Provided you meet modern good practice for RCDs in the CU, you gain most of the safety benefits without needing a full rewire. An insulation test, visible inspection and general condition inspection is still needed first - if you have grey PVC cable without rodents, damage or leaking insulation, there is no pressing need to replace it "just because".  Any work on lighting circuits would probably be tricky because they won't be earthed yet and you should rewire with full earthing - also avoid the "shared neutrals" problem.
 * You can also rewire rooms quickly and easily (although in an ugly way) by using 20mm surface mounted round plastic conduit (how my workshops are done). Works fine, less trouble than not having lighting. Wiring is quick and easy, burying wiring under plaster in a UK house - that's where the long hard work goes.
 * So to answer your basic question, there's no way I'd do that. Too awkward, too weird (hard to sell in the future). But I might well rewire in conduit quickly and cheaply - you can always do it again later, neatly (this is a good way to do total refurbs in derelict houses you have to live in at the same time).   Andy Dingley (talk) 16:12, 2 September 2016 (UTC)


 * I won't try to address all the questions posed by, I will just point out a misconception. The post takes the attitude that wiring won't be a problem unless you use electrical devices in the room that the wiring serves. This isn't true. A wide variety of problems, such as rodents eating the insulation of wiring (I've seen the results with my own two eyes), can cause a fire even if no electrical devices are operated on that circuit. Jc3s5h (talk) 16:27, 2 September 2016 (UTC)


 * I think you are the one with the misconception, from not reading my post properly? How can a rat biting through a cable that is disconnected from the consumer unit cause a fire? There is no power supplied to the cables because they are not connected to the consumer unit. Unless the rat is causing this fire from the friction of its teeth or somehow causing a chemical fire from its urine, I don't see how a fire can possibly start in these circumstances. ElectricianQuestion (talk) 16:40, 2 September 2016 (UTC)


 * Because electrical wires commonly contain a positive and negative wire, and the rodent could chew away the insulation between them, bringing them into contact. This closes the circuit, allowing electricity to flow between them.  If it's a poor connection, it would be likely to get hot, and set something on fire, like the electrocuted rodent's fur.  If the rodent made a nest there from fabric/carpet he found, you could have quite a bit of flammable material in the area.  Perhaps enough to act as kindling and set the wooden house frame on fire, especially if dry. StuRat (talk) 18:32, 3 September 2016 (UTC)

What diverts airflow between the nose and mouth?
Given that you can easily breath (more or less solely) through your nose with your mouth open, I assume that there must be some form of valve that selects which one you breathe through. However, I can't seem to find anything that says what does this. SphericalShape (talk) 22:41, 2 September 2016 (UTC)
 * Don't quote me on this because I can't find a source (yet), but I think it's the back of the tongue pressing against the soft palate that performs this function. See the picture at right. clpo13(talk) 22:46, 2 September 2016 (UTC)
 * I'm not sure that tells the whole story -- I do feel my tongue wanting to rise to the soft palate when switching to nasal breathing when my mouth is open, but I'm still able to breath through my nose when I make sure that my tongue isn't in contact with the palate.
 * Thanks for the link to soft palate, though! I didn't see a whole lot of explicit mentions to this during a brief look, but the muscles within it seem to be responsible for partitioning the airways. It seems counterintuitive to me that something at the top, rather than bottom, of the oral cavity is capable of closing off air to the mouth, but I suppose that it solves my question if my understanding is correct. (Sleep and Breathing in Infants and Young Children, Soft palate and oronasal breathing in humans, and Neurology of Breathing were the things I found, for anyone interested.) SphericalShape (talk) 00:32, 3 September 2016 (UTC)
 * The flap of tissue at the tip of the soft palate is the uvula, it can partially close off the nasal cavity from the pharynx. -- Jayron 32 00:56, 3 September 2016 (UTC)
 * clpo13 is correct, I just did this in front of a mirror and could observe the tongue lifting to make a seal with the soft palate above, causing me to breathe solely through my nose even with my mouth open. RegistryKey(RegEdit) 07:38, 3 September 2016 (UTC)