Wikipedia:Reference desk/Archives/Science/2016 October 28

= October 28 =

Why did old TV screens dwindle to a tiny dot when switched off?
And why was this dot in the centre of the screen specifically? Equinox ◑ 13:35, 28 October 2016 (UTC)


 * Because the scanning coils no longer scanned. The the few electrons (cathode rays) being produced by the still hot emitter (gun), thus took a straight path to the centre of the screen.--Aspro (talk) 13:59, 28 October 2016 (UTC)


 * Some relevant diagrams at cathode ray tube. SemanticMantis (talk) 14:33, 28 October 2016 (UTC)


 * A CRT screen works by creating an electron beam pointed toward the center of the screen, which is then deflected by electromagnetics to create each dot. The electromagnets turn off right away when the TV turns off, and the residual electron stream thus hits the center. StuRat (talk) 16:24, 28 October 2016 (UTC)


 * Not quite instantly. The picture could be seen visibly shrinking to a dot, and the dot might wander around slightly for a bit until the scanning waveforms died out completely.  By the way, the picture on CRTs was not made up of dots (I presume StuRat is implying pixels), rather, it is made up of lines and each line carries analogue information. SpinningSpark 21:45, 28 October 2016 (UTC)
 * A colour CRT was made of dots. The spots still scanned as lines, but there is a dot mask - literally a metal sheet with holes in - just behind the front glass. The beams then hit spots of coloured phosphors on the back of the glass. The dot mask is needed to stop the closely spaced beams from the three electron guns, one for each colour, from overlapping and blurring the colours. Andy Dingley (talk) 20:53, 30 October 2016 (UTC)


 * I am not sure, but I think a cathode ray tube television is just an oscilloscope being fed a fancy signal. Certainly there are some elaborations like using magnetic rather than electrostatic deflection to allow a larger screen, and the colored phosphors laid out in a repeating pattern, but the basic idea seems there. Wnt (talk) 22:39, 28 October 2016 (UTC)


 * Not exactly "just an oscilloscope being fed a fancy signal". Based on my limited experience with oscilloscopes, my understanding is that the intensity of the electron beam in those is constant.  In a CRT TV, the intensity of the beam is what's varied to produce bright or dark areas on the screen. --69.159.60.36 (talk) 10:38, 29 October 2016 (UTC)
 * A friend of my uncle's converted a 'scope to a tiny TV for an aged relative. All the best: Rich Farmbrough, 23:46, 2 November 2016 (UTC).


 * The electrons are drawn towards the screen by the high positive charge on an anode with a hole in its center for the electrons to go through. The high voltage is generated by a voltage multiplier and without some active way of discharging it will continue to hold its charge for some time. It will discharge eventually through a high resistance but in the meantime there will be a spot on the screen as the cathode will stay warm for a while. Making a mechanism to quickly discharge the anode when the power is turned off is a non-trivial task because of the high voltage. Dmcq (talk) 23:23, 28 October 2016 (UTC)

Just as an interesting tangent to this question: while everyone knows that computers used CRTs to display text and sometimes graphics for many years, some of the earliest computers (in the late 1940s) had a completely different use for them: they were the main memory of the computer. The idea was that when the electron beam is pointed at a particular part of the glass and then turned off, it takes a while for the charge on that part of the glass to dissipate. You can then tell whether there's a charge there by pointing the beam at it again and seeing how it behaves. If there is, that's a 1-bit and you recharge the spot using the beam. If not, it's a 0-bit and you don't recharge it. A single CRT might have a 32x32 grid of positions, enabling it to store 1,024 bits, or as we would now say, 128 bytes. See Williams tube. Modern main memories are a bit larger! --69.159.60.36 (talk) 10:46, 29 October 2016 (UTC)


 * The OP did say 'diminish' : (intransitive) To disappear gradually. wiktionary even though this takes just a few milliseconds. It is a matter of proportionality. The scanning coils need energy to scan and one can see this if the supply voltage falls – the picture gets smaller, (30 odd years ago in Brazil one could by primitive looking electromechanical voltage regulators, to keep the TV picture the same size as the mains voltage went up and down). Same thing happens to the electron beam but instead of having to illuminate the whole screen, the residue electron  only illuminate a small area. Good thing too. If all that energy required to illuminate the whole screen just hit the centre of the screen without being spread out across the whole screen,  they burn away the phosphorus coating. I used to come across this sometimes when someone had tried to 'fix' a telly  without out ensuring the raster circuits were working first. It left the TV with a dim spot in the centre of the screen and as the c.r.t. was the most expensive thermionic vale in the whole set, it rendered it only good for spare parts. Talking of oscilloscopes, the TV has  a built in oscilloscope. It is the screen itself. By looking at (preferably) the test card transmissions one could tell what needed doing. I think it was seeing how a magnet placed upon the screen and deflect that little dot that got me interested as a kid in repairing TV in the first place  – together of course with the extra pocket money it started earning me. So, the energy to keep the oscillators and raster scan going, dies pretty quickly but the hot  'gun'  still emits enough electron, with  at a high enough potential between cathode and anode to excite the phosphorus at the centre of the screen. Final, the CRT acted as big condenser (capacitor). This was evident even if it was removed and on the work bench if  one placed one's fingers on the wrong places before discharging the charge through a high resistance and received a big belt from it. Hence, their early use as computer memory  tubes. @SpinningSpark. Colour television screens have a mask behind them  so to only allow one RGB picture element (PEL) to be illuminated at a time. Take  a magnifier glass and look a (say) a Sony Trinitron and they are all little distinct PEL's, so me thinks StuRat is right in that context.--Aspro (talk) 12:00, 29 October 2016 (UTC)

Reasons for and severity of the dangers of leaving mobile phones on during flights
I have repeatedly been told to shut off the wireless functionality of mobile phones during flights, but do not know why. Why is it hazardous, and how hazardous is it?--Leon (talk) 15:44, 28 October 2016 (UTC)


 * We have an article Mobile phones on aircraft, not a very good one but at least it leaves a trail of links to follow if you are interested. Long story short, it could interfere with various avionic systems, including communications and GPS, but there is no instance where wireless was proven to be at fault. Tigraan Click here to contact me 15:49, 28 October 2016 (UTC)


 * There is the danger from cell phone batteries catching fire, but that isn't removed by turning them off. Also, the in-flight entertainment system, often pushed as a "safer" and profitable replacement for passenger's own electronic devices, has been blamed for at least one incident: "The IFE system was implicated in the crash of Swissair Flight 111 in 1998". StuRat (talk) 16:36, 28 October 2016 (UTC)


 * Without this rule, you would have many passengers picking up their phones to call home near landing time. Now, mobile phones will increase transmission power if needed, and that's certainly likely to happen when calling from inside a plane. Then imagine 200 people switching on their phones, each phone would transmit at a power of the order of 1 watt, this RF power of all the phones collectively would cause massive interference inside the plane affecting the phone users, so they would keep their phones switched on for longer trying to get in touch with whoever they are calling. So, even if you start with a handful of phone users at any one given time, their mutual interference would cause more simultaneous phone users as the phone calls take a lot longer to be completed. You could thus easily get a total RF power of a few hundred Watts inside the plane.


 * A phone that is transmitting at some specific frequency that is subjected to huge levels of RF interference will start to produce intermodulation signals. To transmit a signal at some frequency, an oscillator produces a signal at the desired frequency, which is then amplified. But if you subject the device to an RF interference, then this RF signal will mix with the current that is used to amplify the signal from the oscillator, giving rise to spurious signals at different frequencies (at the sums/differences of the original frequencies). Each phone will thus end up transmitting at many different frequencies, it is then possible that the plane's communication or GPS systems will be affected. That would only require a small fraction of the total power of a few hundred watts to be converted to signals at frequencies close to those used by the plane. Count Iblis (talk) 18:01, 28 October 2016 (UTC)
 * Sorry, this is a pure speculation. There is no evidence that phones can produce any unusual frequencies because some other phones work nearby. The highly non-linear parametric processes, that you refer to, require such strong electromagnetic fields that phones (together with the plane) will likely melt before this parametric generation becomes significant. Please, stick to verifiable facts. Ruslik_ Zero 19:27, 28 October 2016 (UTC)
 * Well yes there is evidence, see this paper for instance. Specifically, the research found intermodulation products from mobile phones in the DME and GPS bands, two important aviation navigation aids.  Intermodulation due to transmitters receiving an interfering signal through their transmission antenna is a well known and common phenomenon amongst radio engineers. SpinningSpark 21:19, 28 October 2016 (UTC)

There's another issue besides any possible hazards to the airplane, which is possible problems with the cellular system. Depending on the altitude, a single phone might try to connect simultaneously with a large number of cell towers, and the system might not be able to handle that. --69.159.60.36 (talk) 10:50, 29 October 2016 (UTC)


 * Disregard the answers above. Youtube has the right one: Why can't you use phones on airplanes. Hofhof (talk) 14:06, 29 October 2016 (UTC)

Question about plane waves
On the "Plane waves" page it states that the argument of the wave equation (ω t − k ⋅ r) can be obtained by using the "scalar product" as shown below. But I'm wondering, where did the negative sign come from? Since this section references the four-vectors of special relativity, I think this assumes the use of the standard metric signature (+---) but I don't see that stated and I want to be sure. Also, why is the argument (ω t − k ⋅ r) always subtracted? I think this is because if it were positive (ω t + k ⋅ r) the wave would travel backwards, but this is also not stated so I'm not sure. Thanks.

Regarding:

In special relativity, one can utilize an even more compact expression by using four-vectors: The four-position R = ( c t, r ) The four-wavevector K = ( ω c, k ) The scalar product K ⋅ R = ω t − k ⋅ r — Preceding unsigned comment added by Dugclaws (talk • contribs) 17:16, 28 October 2016 (UTC)
 * Yes, correct, the expression is for a forward travelling wave if k is in the same direction as r. For the version of the equations which use a scalar k rather than a vector, a plus sign would indicate a wave travelling backwards.  This kind of notation is used for analyzing transmission lines for instance, where the wave is restricted to travelling in only one of two directions.  However, when using a vector k a backward travelling wave can be represented by pointing k in the opposite direction to r (or it can be pointed in any other direction at all).  There is thus no need to have two different forms of the equation if vectors are used. SpinningSpark 21:35, 28 October 2016 (UTC)

Largest fragrant flower
What is the largest fragrant flower in the world (in terms of diameter, flower weight or both)? I mean with pleasant smell, unlike Rafflesia, etc. Thanks in advance. --93.174.25.12 (talk) 17:39, 28 October 2016 (UTC)
 * Besides Rafflesia, Amorphophallus titanum also has huge flowers (I'm pretty sure the name means "giant floppy dick", which brings me no end of giggles), but also apparently stinks of decomposing flesh. Reading through a few "listicles" of large and unusual flowers suggests the largest "non-rotting-dick-smelling flower" in the world may be the common sunflower, or Helianthus annuus; some of the domesticated breeds have very large flowers, this image search here turns up sunflowers whose central pod, not including the outer ray petals, exceed 16 inches (40 centimeters).  It, of course, depends on how one defines a "flower", the term in common speech differs greatly from the botanical definition.  See flower and Plant reproductive morphology for some of the background.  -- Jayron 32 18:15, 28 October 2016 (UTC)
 * The sunflower, however, does not smell, not to mention it's not fragrant. 93.174.25.12 (talk) 19:10, 28 October 2016 (UTC)
 * I can smell them. -- Jayron 32</b> 21:01, 28 October 2016 (UTC)
 * I think "giant misshapen penis" is probably a better translation to modern English. A-morphos - without clear shape. SemanticMantis (talk) 20:08, 28 October 2016 (UTC)
 * Except amorphous doesn't mean misshapen, just "does not hold a regular shape" Hence, floppy.  -- Jayron <b style="color:#090">32</b> 21:07, 28 October 2016 (UTC)
 * Ok, you are free to translate however you like, I was merely offering an alternative. A. titanum are not floppy, by the way, they are rather firm in fact, due to turgor pressure. Perhaps "unshapen" is a better word for 'amorphus', but translation is always a bit subjective. SemanticMantis (talk) 21:34, 28 October 2016 (UTC)
 * I think a more relevant point is both wiktionary which says "without form, shapeless, deformed" and our article "without form, misshapen" seem to imply misshapen or deformed is a resonable (I won't say better) translation. Whether this is correct or not I cannot say. And I assume we are talking about translation here, if the word is from the Ancient Greek ἄμορφος ‎(ámorphos) rather than the English amorphous, we potentially should be going by the Ancient Greek rather than the modern English meaning which may not be entirely the same (i.e. a form of false friends). Although scientific names can be complicated, as people may be thinking of the English word, look for a Latin, Greek or whatever alternative etc rather than studying the meaning of the Greek or Latin word that well. Then again, Amorphophallus was first scientifically described and I assume named in 1878 by an Italian botanist. Nil Einne (talk) 09:31, 29 October 2016 (UTC)
 * No, obviously, it's glassy rather than crystalline. —Tamfang (talk) 20:17, 29 October 2016 (UTC)


 * Also yes, to clarify, sun flowers have rather large inflorescences, but rather small flowers. The Asteraceae and a few other families an especially uncommon/derived inflorescence, called a Pseudanthium. SemanticMantis (talk) 20:59, 28 October 2016 (UTC)
 * When I stated "it depends on how one defines a "flower", the term in common speech differs greatly from the botanical definition" what I meant by that was "it depends on how one defines a "flower", the term in common speech differs greatly from the botanical definition." Just in case that was unclear. — Preceding unsigned comment added by Jayron32 (talk • contribs) 21:03, 28 October 2016 (UTC)
 * And the concept of nested quotation marks continues its slow death. —Tamfang (talk) 20:19, 29 October 2016 (UTC)


 * All flowers smell pleasant, that's why they do it. It's just a question of audience.  To a bluebottle, rafflesia is delightful. Andy Dingley (talk) 19:19, 28 October 2016 (UTC)
 * Some water lilies are about the size of that sunflower (um no my imagination getting the better of me Victoria cruziana) grow to about 25cm and they do have a light pleasant scent. Plus I think one can get a lotus growing to about the same size as a water lily flower. Dmcq (talk) 19:21, 28 October 2016 (UTC)


 * Individual Cattleya warscewiczii orchid flowers can reach 11" across, and there are usually several (up to 10) of them per spike (inflorescence). Several Cattleya hybrids are also in the same ballpark of size. By weight - I need to look up the literature, but Stanhopea tigrina flowers are quite massive (albeit short-lived). Several other orchids (e.g. Angraecum sesquipedale) have flower parts that are very long, and have a pleasant smell, but I'm not sure that's what the OP is asking. Dr Dima (talk) 23:32, 28 October 2016 (UTC)
 * note to self: I need to fix a bunch of our orchid pages, they are ridiculously lacking in content. I'll try to do this over the weekend, unless life happens. Dr Dima (talk) 23:36, 28 October 2016 (UTC)
 * OK, I fixed our Cattleya warscewiczii article. On to the next one... Dr Dima (talk) 05:11, 30 October 2016 (UTC)


 * How about Magnolia grandiflora? The article says the flowers are up to 12 inches in diameter, and I know they are fragrant. -Arch dude (talk) 16:29, 29 October 2016 (UTC)

Largest inflorescence (branched) is the talipot palm which does have an odor. The Titan arum is the largest unbranched inflorescence. Rafflesia arnoldii is the largest single flower.  Eve rgr een Fir  (talk) 17:14, 29 October 2016 (UTC)