Wikipedia:Reference desk/Archives/Science/2013 April 24

= April 24 =

Magnetohydrodynamic drive
Can't magnetohydrodynamic drive be used in jet engines. If it requires high voltage of current couldn't it provided with battery power for a passenger jet liner like Air Bus. If we can achieve this we can drastically reduce cost of air travel and passenger risk at time of crashes. — Preceding unsigned comment added by Mihinduep (talk • contribs) 01:49, 24 April 2013 (UTC)


 * The reason why hydrocarbon fuels are used in transportation, especially air transportation, is that the energy stored per kg of fuel is vastly greater than the energy you can store per kg of battery weight. Toy aircraft that are battery powered work because their mass is very low for the wing area (if you reduce the size of something, say by 50% in length, weight goes down 0.5 cubed, ie drops 88% but are only goes down 75%, so the wing loading has improved 88/75 ie 17%, and they only fly for a few minutes between charges anyway.) As a general rule, if you are wondering "why don't they ...." on any commercial subject, there's a darn good reason - and it's usually pretty obvious if you look. Wickwack 120.145.21.97 (talk) 02:07, 24 April 2013 (UTC)
 * Wickwack: I see what you are trying to prove, but your math is way off. You don't compare 88 to 75, you compare what's left of wing area and weight: 12.5% vs. 25%. The result is not even close to 17%, it's a factor of two... - ¡Ouch! (hurt me / more pain) 09:33, 25 April 2013 (UTC)
 * Correct - I don't know how I made such a silly error. Wickwack 121.221.28.17 (talk) 00:36, 29 April 2013 (UTC)


 * Even if you had batteries capable of supplying the power needed, a magnetohydrodynamic drive wouldn't be any use - they only work with electrically-conducting fluids. AndyTheGrump (talk) 02:32, 24 April 2013 (UTC)
 * Actually, they can work with gasses not normally regarded as conducting - they just need to be ionised (which absorbs energy of course). Wickwack 121.215.143.151 (talk) 13:04, 24 April 2013 (UTC)
 * And also, ionizing air requires extremely high local temperatures (on the order of 3000 C or so) -- which means that an MHD drive engine using atmospheric air would only be capable of running for a few minutes before burning out. 24.23.196.85 (talk) 22:53, 24 April 2013 (UTC)
 * Any system capable of heating air to 3000 C would hardly need a magnetohydrodynamic drive to create thrust - one could simply use the ramjet principle, in the way that the nuclear powered Project Pluto system did. Assuming you could find something to build it from... AndyTheGrump (talk) 23:12, 24 April 2013 (UTC)

Robot like thing for children learning by movement
About 2 or 3 years ago (I think), I came across a robot/automaton like things for kids. It wasn't humanoid but instead I think generally moved like a caterpillar or worm, you put the parts together (but I could be remembering wrong and there may have been other things it could do). A key fact I do remember is you programmed or trained it by movement. So if you moved it a certain way, it would learn to do that. I also remember it was quite expensive, I think a decent kit of stuff cost like US$500 or so. I believe it was from the US. I wanted to read about it again, twice now, but both times have failed to find it. Does anyone know what I'm referring to? Cheers Nil Einne (talk) 03:04, 24 April 2013 (UTC)


 * I bet you could build that with Lego Mindstorms...and for less than $500. SteveBaker (talk) 13:23, 24 April 2013 (UTC)

Maximum Burning Temperature
Where does this concept come from? I've only heard about it in the context of 9/11 conspiracy pages (in the sense, jet fuel has a Maximum Burning Temperature of x °, steel need 2* x ° to melt). Can't you reach any temperature if you keep adding heat and have a good oven to keep it from dissipating? OsmanRF34 (talk) 13:06, 24 April 2013 (UTC)


 * (ec)I googled [melting point of steel] and the first thing that came up was this, which demolishes the 9/11 conspiracists by pointing out that steel is sufficiently weakened by heat well below the jet fuel burning temperature. ←Baseball Bugs What's up, Doc? carrots→ 13:25, 24 April 2013 (UTC)
 * It might come from a misunderstanding of ignition temperatures for materials as fixed like melting point and boiling point are fixed temperatures. But fire is far more complex than that. Rmhermen (talk) 14:58, 24 April 2013 (UTC)


 * The answer to the last question is at Adiabatic flame temperature. Long story short, no, you can't reach any temperature, even without heat loss to the environment, because the best you can do is transfer all of the energy of the molecular bonds into the kinetic energy of the combustion products. As you pile in more energy, you are also distributing it over more mass, so the temperature doesn't tend to infinity. However, as the other posters have said, you don't need either an adiabatic flame or a puddle of molten steel to make a building collapse. --Heron (talk) 20:00, 24 April 2013 (UTC)

Gulf stream
Is there a Gulf stream or some stream similar that goes from Ireland to Canada (or the U.S.) that ships might take advantage of?LordGorval (talk) 13:38, 24 April 2013 (UTC)
 * Not exactly, but sort-of. Check out Ocean current. ←Baseball Bugs What's up, Doc? carrots→ 13:43, 24 April 2013 (UTC)
 * O.K. Thank you. Now I see the "N. Atlantic Drift" and if a ship (or sail boat) went on this from Ireland to the Canary Islands and then to the West Indias, about how far would that be?LordGorval (talk) 13:53, 24 April 2013 (UTC)
 * In other words, is it possible to travel on a small sail boat from Ireland to the U.S.? How long would that take? Miles?? Which stream would help the sail boat the most on such a crazy voyage?LordGorval (talk) 14:38, 24 April 2013 (UTC)
 * Not only is it possible, it may have happened over 1000 years ago - see the Brendan Voyage. --TammyMoet (talk) 15:19, 24 April 2013 (UTC)
 * I have doubts about establishing possibility using voyages that probably never happened. Anyway, Columbus actually followed that route on each of his voyages, except starting from Spain rather than Ireland.  See Voyages of Christopher Columbus.  Generally speaking, though, winds are a more important factor than currents. Looie496 (talk) 16:08, 24 April 2013 (UTC)
 * You could design a vessel that would have ocean currents as the biggest factor. A submarine, for example, would be carried around by the current but would be unaffected by the wind.  SteveBaker (talk) 18:36, 24 April 2013 (UTC)
 * See Tim Severin; in 1976 he sailed across the atlantic in a leather boat.-- Gilderien Chat&#124;List of good deeds 18:40, 24 April 2013 (UTC)


 * Indeed: and Tim Severin did that journey to prove that Brendan could have made his journey all that time ago. --TammyMoet (talk) 19:08, 24 April 2013 (UTC)

I see Voyages of Christopher Columbus. Unless I missed it, it doesn't show the distance (one way) from Spain to West Indies on his 1st, 2nd, and 3rd trips. Does someone have an estimate? LordGorval (talk) 19:03, 24 April 2013 (UTC)


 * Off course it is possible. The sea faring people of Southeast Asia traveled in small boats to far away places including Hawaii, New Zealand, and Madagascar. Dauto (talk) 19:07, 24 April 2013 (UTC)


 * You're looking for Triangular trade. Indeed, it worked... though it would have been better if it hadn't. Wnt (talk) 04:55, 25 April 2013 (UTC)
 * Better for who? ←Baseball Bugs What's up, Doc? carrots→ 11:14, 25 April 2013 (UTC)
 * Better for the slaves, but not so good for the British and American economy. 24.23.196.85 (talk) 00:36, 26 April 2013 (UTC)

medicine
Over use of a nasal spray cause what? — Preceding unsigned comment added by Titunsam (talk • contribs) 14:33, 24 April 2013 (UTC)
 * There are a variety of different medications that can be delivered via nasal spray. What the potential side effects are would depend on the specific medication.  Red Act (talk) 14:59, 24 April 2013 (UTC)
 * There's a start about the topic at nasal spray. Gross over use would cause drowning I'd have thought ;-) Dmcq (talk) 15:47, 24 April 2013 (UTC)


 * Think think the OP is probably referring to rebound congestion. This isn't a very good source but it explains it simply. Nasal Sprays Can Bring on Vicious Cycle--Aspro (talk) 18:24, 24 April 2013 (UTC)


 * There are worse complications than that...recall the Zycam controversy when hundreds of people permanently lost their sense of smell. That was a homeopathic remedy delivered as a nasal spray - the non-spray version of it was perfectly safe (although, being homeopathic, it doesn't work worth a damn to reduce your cold symptoms) - but when you sprayed this relatively innocuous stuff up your nose, the consequences were pretty severe! SteveBaker (talk) 18:34, 24 April 2013 (UTC)

Light and gravity
If light is partially made of particles, and those are affected by gravity, then why doesn't light go faster than "light speed" when headed down because of gravity? — Preceding unsigned comment added by GurkhaGherkin (talk • contribs) 18:40, 24 April 2013 (UTC)


 * According to the theory of relativity, massless particles always travel precisely at the speed of light. Their energy can however change, despite traveling at the same speed. Massive particles when approaching the speed of light will approximately behave like this. E.g. the protons in the LHC will be accelerated to an energy of a few TeV which is thousands of times the proton mass. The protons then travel at practically the speed of light, but you can still add more and more energy to the proton beam without that having much effect on the speed.


 * A dumbed down analogy is boiling water. The temperature is very close to the boiling point. If you turn on the heat more and more the water will boil more and more intensely, however the temperature will hardly increase. Count Iblis (talk) 18:48, 24 April 2013 (UTC)

Thanks. I am not Jimbo Wales (talk) 18:59, 24 April 2013 (UTC)

Vertical bright lines from flashlights
Why is it that when in a movie someone points a flashlight directly or almost directly at the camera, bright vertical lines appear extending from the center of the flashlight bulb to the top and bottom of the screen?

For an example see "Day of the Moon" starting at approximately 17:46. Whoop whoop pull up Bitching Betty 20:22, 24 April 2013 (UTC)


 * There are two possibilities...well, three actually:
 * In "the real world", those kinds of lines can be formed due to refraction of light through your eyelashes - and that happens when you look at a very bright light in an otherwise dim scene. That feel of staring into a bright light is absent when you look at a TV screen - which never gets bright enough to reproduce that effect...so sometimes, the effect is added as a special effect in order to produce the feeling of looking at a very bright light in the audience.  This is my best bet as to what's going on here.
 * Lens flare is another common thing you see in poor optical systems - which is also sometimes added as a special effect to make something look very bright. There is also a specific kind of flare that comes about when using an "anamorphic" lens to take wide-screen images with a standard 3:4 aspect ratio sensor or film that produces a single line artifact...although the ones I've seen have tended to be horizontal lines.
 * Some older digital cameras generate bright, perfectly vertical lines that are exactly one pixel wide when the image is too bright for them to handle. I'm not quite sure why.  I'd hope that Dr.Who episodes would be filmed with better cameras - so that's probably not the reason.  I used to have a 10 year-old Canon camera that did this all the time when you get a glint of sunlight reflecting off of a shiney car...very annoying!
 * SteveBaker (talk) 21:12, 24 April 2013 (UTC)
 * Steve - your #3 item is called "CCD smear" and it is because old CCDs transferred charge (in the analog domain) column-wise to a sense-amp. (The read-out logic piped data through other pixels using analog charge transfer, before it was finally digitized at the bottom of the sensor).  One single saturated pixel (over-exposed by light) would therefore cross through every other pixel in the column; this saturated every diode in the entire column.  It was the bane of many a CCD camera engineer - very ugly, very visible, and represented total data loss so it was difficult to digitally correct or "bleep."  CMOS sensors (in newer-technology cameras) do not work this way; and so there is no column-wise saturation.  CMOS sensors therefore have different characteristic artifacts.  Here is a nice cartoon from Hamamatsu, Interline Transfer CCD architecture, written by an Olympus Corp. engineer.  Nimur (talk) 21:48, 24 April 2013 (UTC)
 * My reading of the lens flare article (and personal experience) leads me to believe that lens flare can also occur in the human eye -- no camera necessary. SemanticMantis (talk) 21:21, 24 April 2013 (UTC)
 * Looking at the pictures, it's almost certainly lens flare, Whoop whoop pull up Bitching Betty 21:57, 24 April 2013 (UTC)
 * It's not #1 - when that effect occurs, the lines are as wide as the light source. In the scene with Amy Pond's flashlight, the lines are much narrower than the light source. Whoop whoop pull up Bitching Betty 21:33, 24 April 2013 (UTC)