I notice I am really sensitive to the spice, [[ginger]]. I don't have an allergic reaction or anything, it's just that the tiniest amounts make my food seem really hot. I don't seem to be any more sensitive to capsaicin than others, but that's known to have a variable response (or at least a resistance that builds up). Since I usually consume dried ginger, the active ingredient is [[shogaol]], I believe. So, is there a variable "heat" response to this ingredient ? [[User:StuRat|StuRat]] ([[User talk:StuRat|talk]]) 06:12, 25 March 2015 (UTC)
I notice I am really sensitive to the spice, [[ginger]]. I don't have an allergic reaction or anything, it's just that the tiniest amounts make my food seem really hot. I don't seem to be any more sensitive to capsaicin than others, but that's known to have a variable response (or at least a resistance that builds up). Since I usually consume dried ginger, the active ingredient is [[shogaol]], I believe. So, is there a variable "heat" response to this ingredient ? [[User:StuRat|StuRat]] ([[User talk:StuRat|talk]]) 06:12, 25 March 2015 (UTC)
:Shogaol and capsaicin both bind predominantly to the TRPV1 receptor, which gives you the 'heat' effect, so it's hard to explain why you would be more very sensitive to one, but not the other. Must be secondary binging to other receptors, or a different component in ginger. [[Special:Contributions/131.251.254.154|131.251.254.154]] ([[User talk:131.251.254.154|talk]]) 11:08, 25 March 2015 (UTC)
== Geoid: how far from the center of earth does gravity point? ==
== Geoid: how far from the center of earth does gravity point? ==
Revisionasof11:08,25March2015
Welcome to the science section of the Wikipedia reference desk.
The best answers address the question directly, and back up facts with wikilinks and links to sources. Do not edit others' comments and do not give any medical or legal advice.
What does "view posterior" mean, as in this image?(Attention: not for squeamish people.) Is it A: Given that it depicts a hollow structure, one might think it shows the posterior part; in other words, the view is towards the posterior side. Or, to the contrary, B: The view is from the posterior side. Thanks! — Sebastian01:20, 21 March 2015 (UTC)[reply]
That view is toward the anterior. The tongue is anterior (and superior) to the epiglottis, and we are looking at the posterior surface of the epiglottis with the tongue beyond. -- Scray (talk) 03:40, 21 March 2015 (UTC)[reply]
How does the density of a real mixture change as a function of composition? For simplicity, assume a two component mixture. This means to take into account volume participation and other effects. Does it change exponentially, logarithmically, polynomially, etc.? Plasmic Physics (talk) 09:28, 21 March 2015 (UTC)[reply]
The chart at Walvisbaai#Climate says that the city's highest average high temperatures come during the Northern summer, while its highest average low temperatures come during the Southern summer, i.e. mid-year days have a lot wider range between high and low than end-of-year days do. Why does its weather vary like this? It's profoundly arid, receiving less than 1cm of rain in an average year, so it's not the result of monsoons or other rain-related effect. Nyttend (talk) 12:44, 21 March 2015 (UTC)[reply]
But it's still an interesting question. The place is almost as far south as the Tropic of Capricorn. Why then are the extreme high temperatures for May, June, July, and August markedly higher than for any other months, when most of those months are colder by average high temperature than the rest of the year? --65.94.50.15 (talk) 23:39, 21 March 2015 (UTC)[reply]
the effects of irregular charge in an atom
Hi there,
The science acknowledge the existence of particles which have non-integer amount of electric charge.
I wonder, how would an atom behave if I replaced an electron with such a particle?
16:07, 21 March 2015 (UTC)~ — Preceding unsigned comment added by Exx8 (talk • contribs)
The particles of which I am aware that have fractional charge are quarks. I am not aware of quarks being observable for sufficiently "long" periods of time to be observable as components of atoms. Robert McClenon (talk) 18:41, 21 March 2015 (UTC)[reply]
Hi guys, I wanted to know, if quarks are fundamental particles, how come they emit an electron or a positron when they decay? that doesn't make sense to me, the decay mechanism makes it look like if quarks are made of electrons, positrons, neutrinos and antineutrinos, etc. Can someone of you explain this to me in a simple manner, since I don't know anything about physics? Thanks so much in advance.
(I'm not a physics expert, so I might be wrong, or explaining unclearly...) Are you sure they're emitting electrons/positrons? Isn't the usual decay mode for the quarks (other than the stable up and down) through absorbing/emitting W bosons?
Regardless, just because a particle is emitted at decay doesn't mean that the progenitor particle somehow "contains" the emitted one. A neutron doesn't somehow "contain" a proton, electron, and antineutrino. Instead what happens is that one of its constituent down quarks changeflavour via the weak interaction, emitting a W and turning into an up quark: this is the only possible decay mode for the neutron that conservesbaryon number. Double sharp (talk) 17:21, 21 March 2015 (UTC)[reply]
Mmm, no, I'm not sure at all about quarks emitting electrons and positrons, What you say makes a lot of sense, thanks a lot for pointing me in the right direction, I'm going to read about weak interaction and probably I will understand about the flavour changing process of quarks. Thanks again for for your great reply.
Richard Feynman explains precisely this problem in this video. Quarks don't contain electrons any more than your body contains the words that you speak; they're just produced on the spot. --Bowlhover (talk) 00:08, 22 March 2015 (UTC)[reply]
Force affect
Hello,
I would like to know the scientific effect of the following:
A metre, from ‘A’ to ‘B’ is the size of a luggage holder, situated on the left hand side of a Bus, in between the two left tires, holding people along, inside. When it continues its journey, it accelerates, when it is braking, it is trying to stop, of course when it accelerates hard or brakes hard, any vehicle will have a greater effect on people as well on Luggage.
Q:
1)Where should you put your luggage, in point A or B?
I think point 'B'. I also had a massive argument with someone as they thought point 'A' is where the bag wouldn't get squashed... Who's right?
You don't specify where your points A and B are. Nor have you defined the "best" that you seek in placing the bag. Safety in the event of a crash? The contents not being crushed in normal motoring?
But let us assume A is the front and B the back of the luggage compartment with respect to the direction of travel. In short, there are likely to be larger forces exterted when braking (or crashing) than accelerating, and so I would tend to place stuff at point A such that it can be resisted by the structure directly in front of it, rather than at point B where it might, just, fly forwards and impact point A. --Tagishsimon(talk)21:03, 21 March 2015 (UTC)[reply]
@Tagishsimon: Hello, a Bus would have less accelerating and more/less braking power than a Car. Point A is suitable for Cars, but for Buses shouldn't it be point B? There would be/are other luggage's if kept starting from/at point A up to where my luggage would be... When the braking affect (hard or crashing) takes place of the Bus, isn't it less likely get squashed at point B. And when it accelerates, it would definitely take off slowly which won't squeeze the bag at point B...
I still don't understand how it works. Let say I have some bitcoins. I then use it to buy an item A from someone. If I give him the bitcoin first, what's going to stop him from not giving me item A? Vice versa, he can send me the item first and what going to stop me not giving him the bitcoin?
Second question, I know all bitcoin data are public and can be accessed by anyone, but it's pretty massive. Where do they store all of that data?146.151.91.70 (talk) 23:19, 21 March 2015 (UTC)[reply]
First question: nothing, and nothing. Bitcoin is more prone to fraud than regular money, not less.
Second question: It is not stored in a central repository. Every user of Bitcoin downloads all of the public data before they can verify transactions. --Bowlhover (talk) 00:03, 22 March 2015 (UTC)[reply]
The obvious approach is to use a trusted escrow (trusted because they've followed through (if the transacting parties have) thousands of times before). --Tardis (talk) 03:04, 22 March 2015 (UTC)[reply]
Well, sort of. Let's say you give a shopkeeper $20 in person. Nothing is stopping him from denying that you've paid him. If you pay for something with a credit card, there's nothing to say that a merchant can't just lie and say they already gave you the product in question when they never did. There's no honor among thieves, and bitcoin is no more or less susceptible to humans lying than any other medium of exchange. If someone is going to lie, it is inconsequential which means of payment they lie about. --Jayron3223:30, 22 March 2015 (UTC)[reply]
Not sure I entirely agree. Yes people can lie, but Bitcoin has some differences which can be an advantages and disadvantages depending on your POV. For example, my impression is credit card companies tend to be fairly generous with their anti fraud policies. If it comes to a shopkeeper vs a customer and the shopkeeper says they gave the product and you say you never received in, the absence of good evidence (e.g. security tapes or at least other witnessess) they'll probably side with the customer and reverse the transaction. At the very least, it's possible to get your money back, without the shopkeeper really being able to do anything about it. Even with something like a bank transaction, if it's within a country there is some added security in that these aren't generally designed to be anonymous. If I buy something of Trademe in NZ and don't receive anything, if I paid with Bitcoin and it turns out the person's details are all fake, my ability to track them down is likely to be limited. They might not even live in NZ, perhaps they're actually from Alaska. If I pay by bank transaction as most people do, the person's identity could probably be determined. Perhaps they used fake ID to open accounts, although they will probably at least be on CCTV. While it's possible to open an account remotely, so there is also a chance they never stepped foot in NZ, for various reasons this is far less likely. Of course the usefulness of this is likely be limited, unless it was a very large sum of money but still.... Probably cash or cash equivalents have the most similarities with Bitcoin. Nil Einne (talk) 12:09, 23 March 2015 (UTC)[reply]
Alaskan? As in the Alaskan Mafia? Alaskans don't even have internet service, given the vicious mass penguin-attacks that wi-fi signals cause. μηδείς (talk) 16:54, 23 March 2015 (UTC)[reply]
Hi, I just wanted to know if the electron is really an elementary particle. The article about the quasiparticle called "holon" states:
"The electron can always be theoretically considered as a bound state of the three, with the spinon carrying the spin of the electron, the orbiton carrying the orbital location and the holon carrying the charge, but in certain conditions they can become deconfined and behave as independent particles."
So, if the electron can be theoretically considered a bound state of these three particles (or quasiparticles) doesn't that mean that the electron is not elementary? — Preceding unsigned comment added by 181.90.0.9 (talk) 23:23, 21 March 2015 (UTC)[reply]
These quasiparticle states only exist within solids. They aren't really properties of the electron, but rather an emergent phenomenon that occurs when electrons and atoms are bound together in a lattice. Certain propagating excitations of the system behave as if they were particles of pure charge, or pure spin, etc.; however, those excitations only exist within the solid. In free space, such particles don't exist. The current understanding of physics is that an electron is an elementary particle with no component parts. Dragons flight (talk) 03:07, 22 March 2015 (UTC)[reply]
Hi Dragons flight, thanks for your reply. I've just read that some researchers managed to split an electron into 2 separate particles, the holon and the spinon, also, doesn't quarks also only exist within solids and get anihilated when in free space? But that doesn't stop quarks from being called a fundamental particle. --181.90.0.9 (talk) 06:52, 22 March 2015 (UTC)[reply]
To give an analogy consider the difference between fish in the ocean and waves in the ocean. I can pick up a fish and remove it from the ocean and it still exists, but how would one pick up an ocean wave and remove it from the ocean? It doesn't make sense, ocean waves are fundamentally a property of the medium in which they occur and can't be separated from it. In the same way, quasiparticles are fundamentally a property of the medium in which they occur and could never be separated from it. If someone says that electrons can be split into holons and spinon, they are at best oversimplifiying, since those effects only exist in the context of a sea of electrons within a solid to begin with. We can use particle accelerators to identify the individual quarks within isolated protons, and many particle interactions depend on the fact those constituents exist. There is no such substructure observed within isolated electrons. The whole notion of "quasiparticles" as opposed to real "particles" is that quasiparticles are a kind of fake particle-like behavior that happens in certain conditions when very large numbers particles are brought together but which can't be present when individual particles are isolated. Dragons flight (talk) 18:00, 22 March 2015 (UTC)[reply]
The classic example of a quasiparticle is the phonon, which is perhaps the best known. It's important to remember that concepts like wave-particle duality are not confined to any one frame of reference, size scale, or physical system. For any system with wave-like behavior, there will always be a particle-like behavior that goes along with it and visa-versa. Whether or not these behaviors are physically meaningful (that is, whether the calculations of them describe some observable behavior) differs from system to system, but fundementally, all these "quasiparticles" are are useful models for explaining behavior in some system. --Jayron3223:27, 22 March 2015 (UTC)[reply]
It appears to be a coolant, I remember a friend of mine (who had a rear projection tv) telling me about the same thing. It may serve two purposes, cooling and the use of the purity of the liquid for its optical refractive index and optical coupling ability. Old style laser systems used purified water for the optical coupling medium between the laser rod and the mirror system, water was used as the cheapest heat transfer medium. Read-write-services (talk) 22:31, 22 March 2015 (UTC)[reply]
March 22
Iridium satellites - flash, etc
There were to be two bright Iridium flares tonight (-6 and -8 magnitude) in about the same part of the sky, 90 seconds apart. See
this
and
this. I tried to photograph them. There were some thin clouds blockins a lot of the stars, but I did see the second one through the clouds. I missed the flares in the photograph, but I'm wondering if I got the trails. I couldn't see many stars so I got what I thought was Jupiter in the frame, but I think I was aiming too far to the left.
Are Iridium satellite trails solid, or do they have small flashes periodically? Along the bottom is something making regular pulses. I didn't see any airplanes. Over to the bottom left there are two, in parallel, with one flashing twice as often as the other. I can't identify the stars well enough to tell if this matches the paths of the Iridium satellites. Can anyone tell? Bubba73You talkin' to me?02:02, 22 March 2015 (UTC)[reply]
The trails of an Iridium flash (I've seen several) is solid, because it's caused by a single reflection event. It's also brighter than Jupiter (as those magnitude figures demonstrate), and lasts no more than a few seconds, which doesn't seem consistent with your photo.
A rapidly spinning satellite could give a pulsed trail something like this, but most satellites don't spin rapidly because they wouldn't be able to function: nearly all the ones I've seen while using astronomical telescopes or with the naked eye (it's surprising how many there are if you look out for them) have a steady appearance that fades out slowly and smoothly when they pass into the Earth's shadow.
This looks to me very much like aircraft lights – they may have been too faint to see with the naked eye at the time. {The poster formerly known as 87.81.230.195} 2.218.13.204 (talk) 03:39, 22 March 2015 (UTC)[reply]
I know I missed the flares - I saw one of them visually and they were out of the frame. I was wondering if the path of dots was the satellite before or after the flash. It must have been an airplane I didn't see. Bubba73You talkin' to me?04:06, 22 March 2015 (UTC)[reply]
And if these were regular running lights, they would be streaks because of the time exposure. Therefore, if they are on an airplane they must be white strobes. Do any airplanes have such lights? Bubba73You talkin' to me?16:21, 22 March 2015 (UTC)[reply]
I used Stellarium to show last night, and I changed my location from just my city to my actual location to within a fraction of a minute, but it didn't show the Iridium satellites. Maybe there is an option I don't know about. Bubba73You talkin' to me?17:08, 22 March 2015 (UTC)[reply]
Put your mouse over the bottom menu; there should be a stylised satellite icon to the left of the time-control cluster. This turns on satellite highlighting, and the short-cut key is Ctrl-Z. To pan to a satellite, open the Config menu (F2), select the Plug-ins tab, and the Satellites from the list on the left of the dialog. Ensure that "Load at Start up" is checked. If not check it, and then restart Stellarium. (After restarting if needed), press the Configure button, and a second dialog will open. Select the Satellites tab, and scroll down the list to the Iridium satellites. Double click on a satellite's name to pan to it, and then follow it as it moves. Iridium flare doesn't seem to be emulated. HTH LongHairedFop (talk) 21:45, 22 March 2015 (UTC)[reply]
Thanks, fascinating. Having the Iridium names in very dark blue on a black background isn't a good choice. I got it to follow Iridium 4, which is the one Heaven's Above said it was. It was following it, but it wasn't up at the right time. I checked my location and date/time, and it still didn't match. Then something went wrong and it quit following I4. I put the time and date in and watched the sky and it showed other satellites going by, but not the Iridiums. I'll try again the next time a clear night and an I-flare coincide. Bubba73You talkin' to me?03:09, 23 March 2015 (UTC)[reply]
Life expectancy pie chart
I would think our current life expectancy could be divided up into a pie chart, starting with a certain number of years our hunter/gatherer ancestors had, then add in some years for water and sewage treatment, modern food processing (such as pasteurization of milk), vaccines, emergency medical treatment, etc. So, is there a chart somewhere that attempts to break down how many years each of these technologies adds to our lives ? StuRat (talk) 03:48, 22 March 2015 (UTC)[reply]
For something in the ballpark see Spiegelhalter's work on microlives and related literature. However, my understanding is that all this work is valid only for small perturbations of one factor at a time around the mean while other factors are kept constant. So one cannot really add these numbers up to say something akin to current life expectancy = base life expectancy + 10 years dues to sanitation + 5 years due to vaccines etc, and in fact I'll be highly skeptical of the scientific validity of any "popular" source presenting such a chart (even the idea of microlives is "intended for popular rather than scientific consumption"). To make an analogy: we can surely measure the marginal change in a computer's performance (however one measure it) of adding 1GB of RAM vs faster hard-drive vs speedier processor, but cannot conclude from that that computer performance is 50% due to processor + 30% due to RAM etc, since those factors are neither independent nor additive without the small-perturbation assumption.
But then again, maybe someone has put in the rigorous thought and work required to come up with such a chart and some refdesk responder will find it, but beware of clickbait! Abecedare (talk) 04:32, 22 March 2015 (UTC)[reply]
These stats need to be careful. Most actuarial tables list median remaining years based on current age. For example, the average remaining years on a newborn may be 72 years. The remaining years of a 60 year old might be 25 years. It would not be surprising if 50 year old hunter/gathers had a median remaining lifespan the same as today. Infant mortality is the largest difference between disparities in lifespan. The human lifespan has not improved much, to wit, the oldest humans seem to be a rather constan age. --DHeyward (talk) 08:10, 22 March 2015 (UTC)[reply]
Yeah: I'd expect that if you managed to survive to 50 in those times, your odds of a couple more decades should be about the same as they are now. Keyword "if", of course. Today that is no longer a keyword, however, and as a result life expectancy shoots up without maximum lifespan getting quite as much higher). Double sharp (talk) 15:05, 22 March 2015 (UTC)[reply]
NB I did actually spend some time searching for a relevant WP article on life expectancy (at birth and at age 10, or something like that). I've seen it before, and I'm sure it's around somewhere, so if someone could dig that up, I'd really appreciate it myself. IBE (talk) 09:56, 22 March 2015 (UTC)[reply]
The pie chart makes perfect sense, given the way the question is phrased. It seems clear to me that Stu wants a chart that starts with a hunter-gatherer lifespan, and assumes that various developments, historically sequenced, always add to that, as if independently. This seems to involve no assumption of independence as a medical/statistical fact, because it looks rather like a whim of the OP, for interest's sake. Then it makes sense. IBE (talk) 16:46, 22 March 2015 (UTC)[reply]
I see what you're saying, though that would only be useful if all the items were positive or you only included positive items; the pie chart could not show any decreases. That, in turn, would make all the other numbers suspect. Just as a theoretical, let's say that the LE of a hunter-gatherer was 30 years and the switch to simple agriculture dropped the LE to 25, how would that be shown? And if the changes to food preparation (say, the shift to "safe" beer from "unsafe" water) then boosted it back to 32, would it show as an addition of 2 or of 7? Done as a line graph, all that would show correctly and would allow for easy comparisons with other populations (which you'd really need to do because the effects of these things were hardly homogenous across the world). Matt Deres (talk) 16:49, 23 March 2015 (UTC)[reply]
If my assumption that hunter-gatherers had the lowest lifespan is incorrect, then start with early agricultural society, or whoever did have the lowest lifespan. And temporary drops in lifespan, as due to the Black Plague, need not be noted, only items which contribute to lifespan now. StuRat (talk) 17:16, 24 March 2015 (UTC)[reply]
Windshield wipers
Why is it that windshield wipers are included for rear windows when they are close to or actually vertically oriented (as on SUVs) but not when they are slanted (as on passenger cars). It would seem to me that accumulation of precipitation is less likely when the window has close to an infinite slope. Thanks! DRosenbach(Talk | Contribs)13:51, 22 March 2015 (UTC)[reply]
See these results for a simple search for 'rear windshield wipers sedan'. [1][2][3][4][5][6][7]. There are a lot more if you aren't satisfied. Perhaps part of the confusion comes from the examples given. AFAIK and as per our article, windscreen wiper#Rear wipers, many hatchbacks and station wagons do have rear window wipers nowadays. As Greglocock has hinted at, of passengers cars it's mostly commonly sedan (automobile) that don't. Nil Einne (talk) 14:17, 22 March 2015 (UTC)[reply]
(ec) I believe [warning -- this is unfounded speculation] that it ends up having to do with aerodynamics.
A sedan's rear window is, yes, more horizontal, and nominally catches more rain falling down. But (a) raindrops are at least transparent, and anyway (b) the slipstream of air tends to carry them away fast enough that vision out the rear window isn't badly impeded.
For an SUV or something with a more vertical rear window, on the other hand, there's much less of a slipstream to carry the water away, instead there's an eddy where stuff can collect. And the "stuff" that collects includes not only rainwater, but dust and other road gunk. So the rear window gets filthy pretty fast.
I find that when I'm driving something with a vertical rear window, I need the rear window washer just about as much as the rear window wiper. --Steve Summit (talk) 14:30, 22 March 2015 (UTC)[reply]
Yes, it has to do with the non-aerodynamic shape of those vehicles, which causes turbulent flow of air rather than laminar flow. This in turn causes dust to get caught in those eddy currents and slam into the rear window and get stuck, whereas an aerodynamic sedan has a nice aerodynamic laminar flow and dust hits at a shallow angle and is immediately blown back off. If you watch the rear window of an SUV while driving it during rain, you may even see the rain pile up in the middle of the window, since it's weight is countered by the updraft. StuRat (talk) 01:20, 23 March 2015 (UTC)[reply]
I suspect you can't measure the angular momentum of the entire universe, because would would need a frame of reference outside the universe to compare any possible rotation to, and we don't have such a frame of reference, because if we did it would automatically be part of the universe too! So I think the answer is that the question isn't meaningful, though I'm hoping that a real expert (disclosure: I'm not really a physicist) will come along and check my reasoning! RomanSpa (talk) 15:53, 22 March 2015 (UTC)[reply]
If the universe were rotating in a normal, three-dimensional sense, we would expect there to be an axis of rotation, where distant stars would experience no unusual force relative to us, and an equator, where we or they or both are being pushed away by "centrifugal force". I think. Of course, rotating relative to what is another question, one which makes the situation harder to grasp.
The total angular momentum of the observable universe, i.e. that part of the universe that we can see, is usually assumed to be zero (or near enough as to make no difference). This is in keeping with the cosmological principle. One doesn't have to be outside the universe to measure the angular momentum. In principle one can measure the angular momentum relative to our location, though in practice it isn't an easy thing to do, and as far as I know everyone who has tried to do it directly has gotten values consistent with zero. This paper [8] argues that sensible space-time metrics require global rotation less than 1 revolution per 200 billion trillion years. At that rate, the total angular momentum would be ~1082 J-s. More recently [9], observational studies of galaxies suggested there might be a slight deficit of clockwise vs. counterclockwise rotation, which would be consistent with a small (but non-zero) global angular momentum. However, I would say the jury is still out on whether the universe has any net rotation or not. Dragons flight (talk) 18:32, 22 March 2015 (UTC)[reply]
Found two interesting blog posts on Michael Longo's work. [10][11] He came up with a statistically significant effect, but some people think it could be observer bias. Now why that bias is different in the southern hemisphere than the northern... I dunno, maybe they grew up watching their toilets swirl in opposite directions. :) Wnt (talk) 19:08, 22 March 2015 (UTC)[reply]
Doesn't the universe have to have some angular momentum ? After all, the rotation of the planets around the Sun comes from the rotation of the protoplanetary disk, which in turn comes from the rotation of the galaxy (possibly with some intermediary rotations in the local arms of the Milky Way). The galactic rotation then comes from the rotation of the Local Group, and eventually it all must come from the rotation of the universe, right ? That is, a system with absolutely no rotation shouldn't be able to change to one with rotation, without an external force acting on it, which I suppose is another possibility. StuRat (talk) 23:07, 22 March 2015 (UTC)[reply]
Not necessarily. If two discs spin with opposite polarity, they would have a net angular momentum of zero. It just requires that every spin has a matched spin in the opposite direction, relative to when they started spinning. That's all that is required for conservation of angular momentum. --Jayron3223:22, 22 March 2015 (UTC)[reply]
True, but is a universe that starts with different parts spinning in different directions as simple to explain as a universe that starts with a single spin ? StuRat (talk) 01:10, 23 March 2015 (UTC)[reply]
If the universe starts with any inhomgeneities at all, which it must since galaxies and stars did form via gravitational collapse (i.e. structure formation), then it follows that you expect small random fluctuations in angular momentum which would get amplified as gas clouds collapse. Asking why any primordial inhomogeneities exist is a tricky thing, but it is no more tricky in the angular momentum case than in the mass density case. Dragons flight (talk) 02:07, 23 March 2015 (UTC)[reply]
I asked this same question a few years ago, could the expansion of the universe be caused by its complex rotation in a higher dimension. The answer (at the math desk, I believe) was that a non-solid shperical body would expand in all three dimensions if rotated properly in some higher dimension, although I don't remember the details. The problem with this, as I see it, is that it wouldn't explain inflation or the current increasing rate of expansion. μηδείς (talk) 22:11, 23 March 2015 (UTC)[reply]
See white hole. One idea is that a black hole is a white hole, and the matter just comes out really slowly. It becomes less and less obvious to me that anything ever "really" enters a black hole at all... A "white hole" that looks like a garbage dump for unknown aliens tossing their apple cores and radioactive waste into some black hole close to them... has yet to be discovered. Wnt (talk) 16:13, 22 March 2015 (UTC)[reply]
Gravitational fields are generally steady and do not fluctuate. However, if a local gravitational field were suddenly to change, would gravity waves propagate out from the location. If so, of what form are these waves and at what speed do they travel? If at 'c', would they be electromagnetic waves? --109.146.20.31 (talk) 15:41, 22 March 2015 (UTC)[reply]
In case of any sudden change of gravity field the gravity waves will be emitted. They will propagate with the speed equal to that of light. No, the are not electromagnetic waves. Ruslik_Zero18:21, 22 March 2015 (UTC)[reply]
The correct answer is, we don't know. Gravitational waves are predicted by theory which is well-proven in other ways, but we have yet to actually observe them happening. It is still possible that Newton was correct and that gravity acts immediately across distance. See gravitational wave for details. GoldenRing (talk) 03:58, 24 March 2015 (UTC)[reply]
Human teeth only stay relatively sharp because western civilisations tend to eat softer food these days. If you started trying to grind hard grain with your teeth, they would eventually wear down.--109.146.20.31 (talk) 16:27, 22 March 2015 (UTC)[reply]
I doubt it's the primary cause. By analogy with a food-cutting kitchen knife, incisors and some other teeth should become blunt in a couple of years (which seemingly doesn't occur). Brandmeistertalk18:39, 22 March 2015 (UTC)[reply]
The food cutting knife is a bad analogy. Knives are made from steel which is not only softer than tooth enamel, but it is more malleable. Tooth enamel is more like glass than steel. I don't believe teeth "self sharpen" like bird beaks or something. Human teeth I believe essentally don't grow after they are done, if you "blunt" your teeth, that's it, they'll be blunt, they won't ever "sharpen" them selves, as anyone with a chipped tooth will testify. Vespine (talk) 22:35, 22 March 2015 (UTC)[reply]
I'm not saying that human teeth work this way, but note that it is possible for something to self-sharpen without growing. This can be done by having a hard, sharp "blade" or "point" in the center, protected by softer materials on one or more sides. The softer materials wear away more quickly, revealing more of the blade or point. Think of a pencil, where the wood protects the lead and is then removed to expose more lead, when you sharpen the pencil. Even a piece of sandpaper can thus sharpen a pencil, and chewing could do the same to teeth with this design. And, as in the pencil, teeth are eventually worn down to beyond the point of sharpening. (Of course, a pencil lead is quite soft, so the analogy ends there.) StuRat (talk) 22:56, 22 March 2015 (UTC)[reply]
You may be interested in this paper which tries to explain why human front teeth met edge-to-edge until about 200 years ago and then changed to the overbite which is normal today. The introduction of the table fork seems to be the answer. [12]Alansplodge (talk) 16:21, 23 March 2015 (UTC)[reply]
I would weigh in to say that teeth definitely wear other teeth away, and even more so, does porcelain from prosthetic crowns wear opposing enamel. Because posterior teeth (premolars and molars) are generally rubbing against each other on their occlusal surfaces, they tend to reduce the cusps until the biting surface becomes flat (as can be seen in bruxers, those who grind), while anterior teeth (incisors and canines) do not meet edge to edge in what we would term a physiologic occlusal scheme (biting edge to edge is termed a malocclusion, type 3 occlusion to be specific -- see the edge-to-edge bite of the pic at the top of the page on bruxism). To summarize, as a dentist, I never considered how and why teeth remain sharp. I can tell you that I personally have an edge-to-edge bite (it is my grimace gracing the top of the bruxism page) and my incisal edges are therefor worn down from their original forms, and yet I'm able to rip into and chew my food with no appreciable deficiency. Maybe it is, as 109.146 suggests above, because we eat soft foods. DRosenbach(Talk | Contribs)15:42, 24 March 2015 (UTC)[reply]
Time it takes for a TV to turn on
Back in my day, it took quite a few seconds for a TV to come on - because it takes time for tubes to get hot and start working. But it takes our solid-state LED TV more than 10 seconds to come on - why? Bubba73You talkin' to me?16:23, 22 March 2015 (UTC)[reply]
With our TV the screen comes on in about 2 seconds, but it's something like 10 seconds before all the controls work so you can change channels, zoom to allow for a different shape picture, etc. Evidently the TV has an embedded computer that handles these things and it takes some time to boot and get all the processes running. --65.94.50.15 (talk) 18:14, 22 March 2015 (UTC)[reply]
If you are controlling your TV through a digital cable box you will notice quite a delay over on-air transmission. That delay was one of the reasons my parents got rid of cable. μηδείς (talk) 19:09, 22 March 2015 (UTC)[reply]
Yes, we seem to have gone backwards in this regard. Originally you had to wait for the tubes to warm up, but you would get the sound almost immediately and then the cathode ray tube would slowly get brighter. Still using CRTs, some then added an "instant on" feature that would burn some energy to keep the tube warm all the time, in standby mode. But these days, electronic TVs seem to need quite a while to "boot". Even worse, some give you no indication they are on until the boot process has completed, causing people to hit the on/off button repeatedly waiting for something to happen. StuRat (talk) 22:45, 22 March 2015 (UTC)[reply]
Not to criticize anyone, but I can't help reflecting on what it says about Western culture that we are discussing the fact that it takes a few more seconds to turn on our television. But that would take the thread into the realm of WP:Reference desk/Humanities, so I'll stop there. ―Mandruss☎22:58, 22 March 2015 (UTC)[reply]
Vacuum tube TVs took quite a few seconds for the picture and sound to work. A book I once read on the FBI in the time of J. Edgar Hoover said that he called a tech in and asked that his TV be fixed. The tech checked the TV and found everything to be working fine, and asked what the problem was. The answer was that Hoover did not want to wait for the picture and sound to come on, The tech modified the set so that it was always on, and when the on-off switch was operated it merely gated the already-present picture and sound to appear, at a considerable waste of electricity and tube life. Later,coincidentally, the "Instant-on" feature was added to most TVs, also at a considerable waste of electricity. Today, even with solid state electronics, the sets consume some power when supposedly turned off, as a portion of the electronics are kept energized. This is sometimes called "vampire power" o Standby power. In the 1950's when a radio or TV was turned off, it generally drew zero power. The switch actually interrupted power flow. Edison (talk) 01:44, 23 March 2015 (UTC)[reply]
Hey, that electricity wasn't wasted—it was performing a useful function by enabling the TV to come on instantly! By the way, in my experience in the days of vacuum tubes the time to warm up was about 30 seconds; I'd call that more than "quite a few". --65.94.50.15 (talk) 04:33, 23 March 2015 (UTC)[reply]
An LED TV contains a computer inside. The computer has to boot up in the same way as a desktop or laptop computer. It is optimized to boot as quickly as possible, but it still takes some time. Looie496 (talk) 14:24, 23 March 2015 (UTC)[reply]
Also of note, many modern HDTVs have noticeable "input lag" which older CRTs did not. Besides the "boot-up" time issue with computers, there's also a signal-processing time, older CRTs had much faster response times: This article explains some of it, some video game systems have lag-time compensation built in to them to account for this problem, but it can still screw up multiplayer games. --Jayron3214:29, 23 March 2015 (UTC)[reply]
A television's DTV DSP is very similar to a GPU (graphics processor) in that it can have a complex graphics pipeline, perhaps including multiple full frame buffers. Each frame buffer necessarily delays output by one full frame; a state-of-the-art DTV DSP might actually have 48 or 96 frames (1 to 2 seconds or more!) of buffer - if we include the incoming analog signal, mixed signal, decompressor/decoder group of pictures, and the rendering pipeline, the video lag can easily manifest in the range of several seconds. This is a severe usability regression; but arguably is intended to improve your experience. For the majority of the time you watch the video, you aren't transitioning (starting or stopping playback or changing channels); and while your video is in steady state, it is arguably playing at a higher quality because of these digital features.
However, in the imperfect world we live in, the folks who make DTV DSPs are severely constrained by cost, talent, and intellectual property. Few consumers want to pay big bucks to put a faster CPU in their television's embedded processor. Few people want to pay big bucks to patent-trolls and large corporate intellectual-property owners - especially when license terms may include royalties per view. Imagine if each time you changed channels - it was nearly instantaneous but also deducted a few cents from your account! Instead, television DSP engineers often must choose bargain-basement, royalty-free or low-cost alternatives to standard technologies. Next time you read a press release about a "royalty free" codec like VP9, read it with a skeptical eye - somehow somebody is paying for the technology, either up front (e.g. in the sale price of the electronic device/computer/TV) or per-use (e.g. out of your monthly television or internet service subscription fee) - or perhaps, as in some cases, the technology company bills the movie studio up-front, just in case the movie or TV show is ever distributed in a proprietary digital form at any time in the future! Regarding the talented engineers who work on DTV systems: which qualified engineers want to go work in a low-margin commodity industry dominated by companies without name-recognition, especially when the major designers and manufacturers of these systems lay off their entire engineering staffevery couple of years? Invariably, the intellectual property owners will acquire a bunch of young kids from a hot new video engineering start-up, bilk the patent system, sell a lot of televisions, and repeat again when the next marketing buzz-word catches on. Anybody with experience and talent at optimizing a very complex graphical computer system can get a better job in a more stable industry.
So: there is a sort of a regression on the user-interface speed: but this is Moore's law playing out in reverse of its normal pattern: the massive economic feedback loop is actually driving lower performance, even though the DTV processors are getting more features, higher transistor count, perform more megaflops and have more RAM. The money in the business dictates the user's perceived performance. When consumers start basing their television purchase-decisions on boot time, this problem will start to be addressed.
In response to Mandruss scathing critique of Western Civilization, Steve Jobs famously said something along the lines (not verbatim) that "improving the boot time on the Macintosh would save lives. A 10-second improvement added up to many lifetimes over the millions of users booting their computers multiple times a day." I'm sure there's a better source with a verbatim quote. μηδείς (talk) 21:05, 23 March 2015 (UTC)[reply]
Steve Jobs was a smart guy. If he said that, I'm sure it was tongue-in-cheek. (BTW, I can do "scathing", and that weren't it.) ―Mandruss☎21:09, 23 March 2015 (UTC)[reply]
I can think of more direct ways reduced boot time could save lives. In an emergency, if you need to boot your iPhone to call 911, then seconds could literally mean lives. And with people on VoIP, the same can be true of booting their home PC. (I now have VoIP from the phone company, and have to wait about 5 seconds for it to boot each time I lift the handset.) StuRat (talk) 21:21, 23 March 2015 (UTC)[reply]
My comment was about time to turn on a television, not about boot times in general. I apologize for missing Medeis's non sequitur. ―Mandruss☎21:40, 23 March 2015 (UTC)[reply]
TV boot time could also cost lives, if the sky looks strange so you boot up your TV to check the weather, only to find out a tornado is headed your way too late to take cover. StuRat (talk) 21:51, 23 March 2015 (UTC)[reply]
The quote was certainly not tongue in cheek by Jobs, it was reported in the press, and it was meant to inspire his company to perform better by making a very valid point. Every time 230,000,000 people totally waste 10 seconds that's about the equivalent of an entire 72 year lifespan lost. The concept of man-hour is not invalid, and those 10 seconds represent hundreds of "man-lives" wasted a year. The same thing applies to waiting for the TV to warm up. It's why cars have radios. Of course, knowing my old HP running Windows 95 would take three minutes to boot, it was a good opportunity to roll a joint and light up. But had I wasted that time passively it would have been like contributing to the stillbirth of several thousand people a year. It's the same reason you should always carry a good novel on the train or to the doctor's office. μηδείς (talk) 21:58, 23 March 2015 (UTC)[reply]
Mandruss, I did not insult you or resort to ad hominem--your argument is not with me, and it's not to me you concede. Here's the exact quote of Jobs at wikiquote, where it is sourced: "If it could save a person’s life, could you find a way to save ten seconds off the boot time? If there were five million people using the Mac, and it took ten seconds extra to turn it on every day, that added up to three hundred million or so hours per year people would save, which was the equivalent of at least one hundred lifetimes saved per year."μηδείς (talk) 00:58, 24 March 2015 (UTC)[reply]
That 10 seconds waiting for a device to boot is not like 10 seconds of non-existence. That time could be spent planning what to do, grabbing a cup of coffee, stretching, etc. StuRat (talk) 02:55, 24 March 2015 (UTC)[reply]
Jobs didn't say lives, he said the equivalent of lifetimes spent, time as you spend it during your life. And I also mentioned the point that I would use the waste time to do something else, but I would rather not have to. Who here is in favor of longer boot times for electrical appliances? Raise your hands. μηδείς (talk) 21:52, 24 March 2015 (UTC)[reply]
The LCD computer monitors I've used go blank for a second or more even when changing video resolutions, so it's not a boot time or a warm-up time. It's not a seconds-long video pipeline, since the screen responds much more quickly than that when I move the mouse cursor. It happens even when the connection is digital (DVI), so it's not re-syncing to a new refresh rate. -- BenRG (talk) 06:22, 24 March 2015 (UTC)[reply]
Are you able to investigate the root-cause further? What device is generating the video output? Who makes your HDMI PHY? What software drives the PHY, what software runs on the video processor, and what software implements the display driver on the host computer? What company designs and manufacturers these parts, and what company writes the software for them? Is the issue isolated to HDMI or to all video outputs? Does it manifest on all display devices, or only certain LCD monitors?
In all seriousness, these are questions that we can actually answer... but we probably can't fix the symptom: it would be prohibitively expensive to buy the necessary (proprietary) specifications, tools, and software required to fix this. For example: try to figure out where to sign the necessary paperwork to get access to the official HDMI specification: How do you license HDMI 2.0? (I did mention, earlier, that intellectual property can be a severe hurdle for manufacturers - for an independent hobbyist, this is a non-starter! It costs $15000 US to gain access to the forum from whence you may ask questions about licensing fees and specification access levels. Pause for a moment to contemplate the value of a free resource like Wikipedia, in context: consider the open market price for access to a person who knows important information?)
On a hunch, BenRG's symptom sounds like many manifest items: peripheral device discovery is probably handled very slowly by the host operating system (it is a rare event to plug a device in, so it is probable that the software is designed such that the polling loop is low-rate, or the management daemon is launched only on demand, or the hardware interrupt is low priority); and the device itself - including the port management peripheral and perhaps even the cable - may require firmware load, firmware boot, and physical layer calibration after boot.
Where to start? Well, if you're a Windows user, start reading the developer documentation at MSDN's DDK webpage; if you're an Apple user, start reading IOKit Fundamentals... if you're a Linux user, O'Reilly's Linux Driver Development text is a good starting point. Optimizing performance on hardware peripherals is a lot messier than optimizing pure software: much more of the hard work is spent researching where the performance problem even manifests, and often, there is no solution available (except to change the hardware). Nimur (talk) 14:30, 24 March 2015 (UTC)[reply]
On a related note: as several people above have already invoked Apple history without citing any sources... I happened to be involved in a discussion about device latency on a different mailing list - with my friend. He worked on the ASR33 circuit and software - before I was alive! That sent me off to read the Red Book (1978 edition) to review the device driver software (about thirty lines of code, page ~118) for the circuit that Woz built. It is stunning how very few parts and how very little software used to be involved in the manufacture of computer peripherals. Today, it is very normal for a peripheral device to have many dozen CPUs with separate software environments running on several hundred million transistors. Your HDMI cable probably has more compute horsepower than the Apple II. Faster computers, in themselves, do not make for less user-latency. Nimur (talk) 14:38, 24 March 2015 (UTC)[reply]
March 23
Using a multimeter
I got one multimeter like this: http://image.made-in-china.com/2f0j00NMbtZHvdwquF/YX-1000A-Analog-Multimeter.jpg, but unhappily no instructions where by it. There were two cables with it (red and black). I'm aware this is one bottom of the market device, and have no interest in connecting it to AC, nor I believe it was developed for it.
There are instructions online, but they seems to be more about fancy devices, digital and all. Can someone explain a couple of basic functions of it? Like checking a 1.5V battery, and other random stuff that can be performer with stuff you find at home (batteries, broken electronics, light bulb)
These are the possible setting:
Ω (X10, X100, X1K)? I suppose this is resistance.
DCmA (0.5 50 250)
DCV (10 50 250 1000)
ACV (10 50 250 100)
There is also a wheel at the left side that cannot be seen on the picture. --YX-1000A (talk) 00:28, 23 March 2015 (UTC)[reply]
I believe you would use one of the DCV settings to measure voltage on a DC device, like a battery. I believe the number there would be the maximum on the scale, so, with a 1.5V battery you would want the 10V setting.
Also note that it's possible to damage the reader or device by using the wrong setting, so be sure you figure it all out ahead of time, rather than experimenting. StuRat (talk) 01:05, 23 March 2015 (UTC)[reply]
(after edit conflict) For DC mA, DC V, and AC V, the number associated with each range setting is the maximum value corresponding to a full-scale deflection of the needle. There are several graduated lines on the face of the meter, each is for a different type of measurement. From farthest from the base of the needle inward: (outermost) resistance, (next inward) DC voltage or current (depending on range setting), (next inward) AC voltage. I think the innermost scale is for using the meter as a VU meter, but I haven't used an analog multimeter for that purpose, so I'm not 100% sure.
For the resistance measurement ranges, the number associated with the range setting is a multiplier. Say you're measuring the resistance of something, the setting is "X10" and the reading is 4. The resistance is 4 x 10 = 40 ohms. The dial on the left of the meter is for calibration when using the resistance measurement settings. After you choose the range, say "X10" or "X100", you touch the two measurement probes together. The needle should deflect all the way to the right (or very close). The resistance should read 0 at that point. You adjust the dial so that the meter indeed reads 0. When you switch to a different resistance measurement range, you will need to fuss with the dial again. There's a mirror-like line right below the resistance scale. It's there to help you read measurements, if you see the needle aligned with its mirror image, you're reading the meter from the correct position.
Note that, to measure resistance, the meter needs a battery - one AA cell. See this YouTube video for how to install the battery (and set up the resistance measurement). Tevildo (talk) 01:27, 23 March 2015 (UTC)[reply]
Despite being a basic model, your multimeter can measure AC. Note that the measured value is supposed to be the RMS value of the voltage, based on the assumption that the waveform is sinusoidal. A meter like that will NOT measure the true RMS value of arbitrary AC waveform.
I think a proper battery tester tests batteries under load. Your multimeter doesn't have a setting for testing batteries, so it's not well suited for testing batteries (as a voltmeter it doesn't put much load on the battery under test). Fresh 1.5V batteries have a voltage higher than the nominal value (something like 1.65V in my experience, if I remember it right). If the measured voltage is below 1.5V, it's at least partially spent. It may still work for some time if the device consumes very little power (like a calculator or a clock). Generally I'd throw away anything that measures below 1.4V. You can use the resistance measurement setting as a continuity tester, to see if an incandescent bulb is burnt or not. --173.49.16.112 (talk) 03:29, 23 March 2015 (UTC)[reply]
I mostly use my multimeter in the resistance-measuring mode to perform continuity tests, particularly on circuits that aren't working, to find out which circuit element contains the discontinuity! It is also useful for checking circuit elements before inserting them in a circuit, to confirm they are working. The resistance-measuring mode is the one that requires a AA cell. Dolphin(t)06:21, 23 March 2015 (UTC)[reply]
It's a long time since I've seen a 'wheel' on a meter, it is used to zero the resistance measurement when you short the two leads together. Most of the above advice is not misleading, tho I expect it'll need a 9V battery for resistance. It is an extremely cheap and nasty meter, but perfectly useful, and probably quite accurate. Incidentally note that the current capacity is 1/4 A DC, essentially useless.Greglocock (talk) 06:50, 23 March 2015 (UTC)[reply]
I agree a 9V battery is probably more likely for the resistance measurement. Either way though, it's generally a good idea to turn it off the resistance measurement mode when not in use as that will potentially run down the battery. Nil Einne (talk) 11:29, 23 March 2015 (UTC)[reply]
Old TV Picture "Flips" for Several Minutes After Startup
The above question about older TVs reminds me of something I experienced in the early 1980s. We had a television housed in a large wooden structure that sat on the floor. It was BIG! After a few years of ownership, I noticed that when you turned it on, the picture would flip continuously for several minutes. By "flip", I mean that it looked like the channel was changing, although it stayed on the same channel. The "flip time" started out very fast (say once per 1/2 second), then would slow down to once every several seconds, and finally would disappear entirely after several minutes. I found that I could cut down on the time it took to stabilize by turning the TV off and on repeatedly. I hadn't thought about this in a long time, but the above question jogged my memory, and now I'm curious: does anyone know what would cause something like this? Thanks! OldTimeNESter (talk) 15:42, 23 March 2015 (UTC)[reply]
If the image is rotating, with a black bar between the top and bottom, possibly with pulsating white dots in the bar, then that means the horizontal hold is slightly out, and you are seeing the horizontal blanking interval information. Non-CPU controlled CRTs can do this, especially at power-up as the timing circuit is temperature sensitive. LongHairedFop (talk) 16:26, 23 March 2015 (UTC)[reply]
It is most likely the flyback transformer failing. They don't tend to fail immediately. They fail slowly. You lose hold (the picture scrolls up and down on the screen). You then being to lose throw (the picture gets squashed). When I used to repair televisions (shortly after we ate the last dinosaur), it is about 80% replacing the power supply and 20% replacing flyback transformers. 209.149.113.207 (talk) 18:57, 23 March 2015 (UTC)[reply]
I was going to say the same thing, except that it was always the "vertical hold" going out on mine. (A "console TV" is the name of a TV that looks like a piece of furniture, BTW: [13].) StuRat (talk) 19:00, 23 March 2015 (UTC)[reply]
Yes, this is vertical hold. It controls how long the TV takes to do its vertical scan and has to be set close to the actual rate that the frames arrive at, so the TV can synchronize by using the blanking interval. Because the analog electronics that determined the time weren't entirely reliable and could perform differently while warming up, a control was provided so you could compensate. With the analog TVs we had, it was likely to need adjustment from time to time.
Horizontal hold controlled the time to scan one line of the image, and with the analog TVs we had, in practice you never needed to adjust it. I think this was because the TV was good at syncrhonizing even if it was set wrong, because if you turned the knob far enough that it couldn't synchronize, the picture immediately went very bad. The rectangular image would "tear" into a narrow parallelogram shape that would repeat as needed to fill the height of the screen, maybe 10 times. --65.94.50.15 (talk) 20:33, 23 March 2015 (UTC)[reply]
Thanks for the answers, everyone. I don't know if the TV had a vertical hold adjustment, although it probably did (I think this was standard). We threw it away a few months later, and it would be funny if it was something simple like that.
(209.149.113.207) you know, when I repaired Mac Classics, it was about the same percentage of power supply vs. flyback transformer. Usually, the monitor wouldn't show an image, though. I always liked replacing the transformer, because our repair manual told us to put one hand behind your back when you pry off the anode cap, so that if you accidentally complete a circuit, the high voltage won't go through your heart. Now, the transformer had a bleeder resistor, which in normal operation would drain it of charge; still, this was highly-skilled work for $6 an hour! OldTimeNESter (talk) 12:09, 24 March 2015 (UTC)[reply]
I have an odd historical question. I remember that in some houses built in the United States in the 1950s and 1960s, it was common for the lights in the bathroom to be fluorescent lamps, which would flicker for some number of seconds and then come on strong. When one finished using the bathroom, one would normally turn off the light. Why was a fluorescent lamp used? This would have seemed to make no sense in terms of energy use, since turning on a fluorescent lamp requires a large energy use. (I realize that there wasn't a whole lot of consideration given to energy conservation at the time.) It would have seemed to make more sense for the home builder to provide fluorescent lamps in the kitchen or living room, that could be turned on in the morning and turned off at night. Does anyone have a thought as to why this was done? Robert McClenon (talk) 17:19, 23 March 2015 (UTC)[reply]
Just for clarification, are you using the term 'bathroom' in its literal sense - a room with a bath in it - or as seems common in the U.S., as a euphemism for a room with a toilet that may or may not also have a bath in it? I ask, because I've seen a great deal of confusion resulting from this, and we might as well get it straight from the start.
In answer to the question, assuming that you actually mean 'a room with a bath in it', there are safety issues with light fittings in such contexts - specifically the electrocution risk inherent in accessing a light fitting from a bathtub which might be wet (or even filled) and thus liable to be effectively earthed. Florescent lamps are often fitted on walls, and can thus be sited well away from the bath itself. Generally, fitting incandescent lamps to walls is avoided because they provide an intense localised light source, which one doesn't generally want to be facing directly when e.g. using a sink. AndyTheGrump (talk) 17:31, 23 March 2015 (UTC)[reply]
It was a room with a toilet and a hand sink in it that my parents were visiting with me. I would have gone into it to use the toilet. Would it matter whether there was a tub also? I don't know whether the rooms that I am remembering were what are known in the United States as full bathrooms or as half bathrooms. I don't understand why it matters from a safety viewpoint. Either way, there was running water, and the fixture would have been away from the water. Maybe the reason was to avoid having the intense local light source of an incandescent lamp; if so, it illustrates the lack of energy conservation common sense in the 1950s and 1960s. I still think that the use of a fluorescent lamp would have made sense in the kitchen to avoid the bright light of an incandescent lamp. There were other ways to diffuse the light from an incandescent lamp, such as various sorts of lampshades. Robert McClenon (talk) 17:50, 23 March 2015 (UTC)[reply]
As far as the U.S. National Electrical Code is concerned, the presence of a bath certainly matters - see here for illustrations of what can be placed where [14]. As for what the electrical codes required in the 1950s or '60s, I've no idea, but they may be relevant. AndyTheGrump (talk) 19:14, 23 March 2015 (UTC)[reply]
In a room where a mirror is used (e.g. an American bathroom), you want as much light as possible. It is used to check your appearance in the mirror before you leave. At the time, a single fluorescent light produced a lot of light across a wide spectrum of wavelengths. An incandescent bulb produced less light and often tended to be narrower in the spectrum (I always noticed they were yellow). So, imagine looking in the mirror. You think you have on black pants and a red shirt. You walk outside in the sunlight and notice you are wearing brown pants and a purple shirt. It would have been better to have extra light in the bathroom. Extra note: My experience is that men tend to not notice themselves in the mirror while women do. So, you get a completely different response to this type of question from men (who will talk about wattage and money) and women (who will talk about use of the light). 209.149.113.207 (talk) 18:25, 23 March 2015 (UTC)[reply]
Male here, but will talk about the quality of the light, not the wattage. Fluorescent lights tend to produce harsh blue-white light. (They do have softer variations, too.) You really can't get that harsh blue-white light from an incandescent light unless it's blindingly bright. So, then, why would a woman want a harsh light ? Well, if putting on make-up prior to going someplace with harsh fluorescent lighting, you would want matching light. This could show, for example, where a vein is visible through the skin, at least until covered with a bit more rouge. StuRat (talk) 18:39, 23 March 2015 (UTC)[reply]
I was told that for these kinds of presentations its best to read your report, write a PowerPoint then use the slides as prompts rather than have a separate set of notes. I was told that using a separate set of notes would just confuse you since you wrote the report and should know it well. Is this true? 194.66.246.6 (talk) 18:14, 23 March 2015 (UTC)[reply]
It depends on the audience. There is no definition of "best" practice. I've presented at places where powerpoint presentations were banned. I've presented at places where you are required to print your notes, numbered, each on a separate card for the entire audience. I prefer to have slides in the background (not many - just a few) and give a presentation without a podium of any kind. I want to walk around. I want to walk up to the audience. In my opinion, that is best. For others, it is strange, unusual, and very scary. 209.149.113.207 (talk) 18:18, 23 March 2015 (UTC)[reply]
An oral presentation isn't the time to give detailed numeric values, those can be included in a separate handout. The oral presentation is to "get the gist across". So, a few key facts, written on the slides and then repeated by you, is a good way to go. However, you do run the risk of getting somebody in the audience who asks for a level of detail inappropriate for that format. You may then refer them to your detailed handout, and keep a copy handy for yourself so you can point out the page number, etc. I'd avoid actually reading the details, though, as that will tend to bore the rest of the audience and throw off the schedule. StuRat (talk) 18:52, 23 March 2015 (UTC)[reply]
thanks but I more meant for your own preparation. Is it a good idea to write notes for yourself? I was told it's better not to otherwise it just seems too scripted or you go into too much detail or get confused. 82.132.219.217 (talk) 18:58, 23 March 2015 (UTC)[reply]
Can you tell us 1) the size of the audience 2) the broad field of science 3) whether this is for school 4) if so what level and in what country? 5) How long is the talk? There are lots of ways give talks, but recommended methods will change depending on audience, level, and lots of other context. For example, I've given 15 minute talks to large audiences of scientists at professional meetings, and I've also presented to 10 people around a table for 2 hours. The techniques and best practices are very different. SemanticMantis (talk) 19:03, 23 March 2015 (UTC)[reply]
That's not very long. 30 slides at 30 seconds each should do it. You should be able to put 30 seconds worth of spoken material onto each slide, eliminating the need to resort to notes. I suggest that the handout contain copies of those slides, along with additional details, if needed. That way, if the projector dies, or they can't see it, they can still follow along. Be sure to number the pages, so you can direct them to a particular page. And try to rehearse on an actual slide projector, so the time to flip slides, adjust the focus, etc., can all be taken into account. StuRat (talk) 19:25, 23 March 2015 (UTC)[reply]
30 slides for 15 minutes is not recommended by any sources I've found. Having been to hundreds of these talks and prepared dozens myself, my WP:OR is that 1 per minute is good, and minimal text per slide is good. SemanticMantis (talk) 20:22, 23 March 2015 (UTC)[reply]
Well, at 1 slide per minute with minimal text, that won't be enough to fill the minute, so then you are back to either memorizing additional material or using notes. I was trying to avoid that. StuRat (talk) 21:27, 23 March 2015 (UTC)[reply]
What do you mean, it "won't be enough to fill the minute"? Surely you're not suggesting that the presenter read the slides. That's the classic boring Powerpoint presentation. I've found that one slide for every 45 to 60 seconds is about right (i.e., up to 20 slides for 15 minutes). Short Brigade Harvester Boris (talk) 00:06, 24 March 2015 (UTC)[reply]
Well, not word for word, but pretty much, yes. If he has a chart showing 10 whatchamajiggers for item A and 31 for item B, he could throw in "We were amazed that item B has over 3 times as many !". StuRat (talk) 03:00, 24 March 2015 (UTC)[reply]
I don't think there is any one answer. I've given many scientific presentations over the years and almost never write any notes. The key facts are included on the slides, and I rely on memory for other things. By contrast, my wife (who is also a scientist) writes incredibly detailed notes when preparing presentations. That is what works for her. Either way, I would suggest that it is very important to practice your talk repeatedly. By practicing the details of what you are going to say you can work on getting the flow right and making it all sound natural and polished. Whether you choose to prepare notes or not, I'd say that practice is the best way to get comfortable with what you are going to say, build confidence, and make it sound good. Good luck. Dragons flight (talk) 19:44, 23 March 2015 (UTC)[reply]
Here are selection of suggestions for how to prepare a scientific presentation [15][16][17][18]. Some mention having a set of notes, some don't. Use of separate notes is ultimately a personal preference. If you're afraid you'll get lost without them, use notes. If you are concerned that you will appear distracted, write notes and don't use them. Powerpoint has a "presentation mode" that allows you to view notes on each slide while it is up, though they are not shown to the audience [19]. SemanticMantis (talk) 20:21, 23 March 2015 (UTC)[reply]
Better yet, become familiar enough with your material that you don't need notes. Don't worry if you forget some little detail.
Having done literally hundreds of these, here's my OR. 30 seconds a slide is definitely far to quick. My personal rule is about 1.25 slides/minute, but I'm a fairly fast presenter. 15 minutes as a rule of thumb means 12 minutes of talking with 3 minutes of questions, so I would normally go for about 15 slides. NEVER more than 5 lines of text on a slide, and try and have a relevant image on most of them to keep the attention. Don't listen to Sturat who seems to be suggesting you just read out your slides, that's the single biggest mistake you could make. Don't write out notes, but know you subject well enough that the text on your slides just prompt you what to say. On every results slide, put a one line take-home message about what it's showing, then repeat those in your conclusions in the end. Drive your message home! Judicious use of animation (only some appear/disappear, no flying about)can make a more complicated slide a lot easier to follow. Anyway, just my 2p. Fgf10 (talk) 08:04, 24 March 2015 (UTC)[reply]
I agree on not more than 5 lines per slide, in fact, I might reduce that a bit. But as for not "reading the slides", I've got to disagree there. Some people may not be able to see, although the handout with a copy of the slides will help there. Of course, you don't have to read each slide verbatim, instead you should summarize it. One of the most annoying things is when the presenter puts up a slide and says "This leads us to this conclusion !", and I can't see what the slide says before it changes. The presenter should orally present all the relevant info, with the slides as a supplement, not the entire presentation itself.
As for the number of slides per minute, I like the style where there's a list of items, but only one is highlighted, and the presenter only talks about that one. Then, the next slide has the next item on the list highlighted, and he then talks about that item. So, each slide is on the screen for a shorter period, this way. (Note that the handout need only contain the list once.) StuRat (talk) 17:26, 24 March 2015 (UTC)[reply]
If people can't see, you haven't made your font large enough. No reading of slides, capital sin number one, which is essentially what you're saying anyway, "instead you should summarize [sic] it". All you need on a slide are the main points or figure and a take-home message. The focus should be on the speaker, not the slides. Though with many poor speakers it's on the slides anyway, which will then generally be poor as well. I would also strongly advise against the 'highlighted list' type of presentation, as it's far more confusing to the listener than a properly designed presentation. Fgf10 (talk) 18:18, 24 March 2015 (UTC)[reply]
As for people not being able to see, they might be behind somebody taller, might have forgotten their glasses, the projector may be blurry, etc. So you shouldn't count on people reading your slides.
Regarding highlighting, that's a tried and true method of calling attention to an item. What's your objection to it ? The alternative is to use a pointer, but that's not going to be as visible from the back of the room. StuRat (talk) 05:55, 25 March 2015 (UTC)[reply]
Where do the electrons go to?
If you power a light with an electrical generator and disconnect the light, what happens to the electrons that were powering the light?--Fend 83 (talk) 18:54, 23 March 2015 (UTC)[reply]
If the generator is running with no load connected, then it will maintain an electric charge on each of its contacts. With a DC generator, one contact will remain positive and the other negative—just as happens with a battery. No current flows, so the electrons aren't moving. With an AC generator (sometimes also called an alternator), each contact will alternate between positive and negative in opposite phase. In that case the electrons just flow back and forth along the path from one contact to the other through the generator. --65.94.50.15 (talk) 20:39, 23 March 2015 (UTC)[reply]
I just had this explained to me differently in the case of AC electricity. Apparently in an open (non-working) circuit one wire from the alternator at the generating station is live, with a potential alternating flow. The other wire, which goes to ground--not all the way back to the generating station--is by itself dead. The live wire's potential current originates at the alternator located at the power generator, where the alternator too is grounded. When the circuit is closed at the consumption end, the potential AC power then flows from the alternator, down the live line, through the bulb, or whatever, into the "dead" (but now live) line which is grounded near the consumer, with the earth itself completing the circuit. For this reason the live wire alone cannot electrocute you unless you yourself are grounded, or you also touch the grounded "dead" wire and the complete your circuit. μηδείς (talk) 00:42, 24 March 2015 (UTC)[reply]
You're assuming a remote generating station with the ground forming one side of the circuit; I was assuming a local generator, as the question seemed to imply that. --65.94.50.15 (talk) 04:56, 24 March 2015 (UTC)[reply]
Better never assume that you are not grounded when in presence of an electrical wire. Generators even remote will not abdicate their bites. --Askedonty (talk) 10:29, 24 March 2015 (UTC)[reply]
I was specifically addressing the fact that at the household end, the two lines you see in the cord are not both live by themselves. One line carries the AC into the house, t is always live. If you touch it and then are grounded the electricity will flow though you. (IP 64 had said that "each contact will alternate between positive and negative") but this is not precise--you cannot be shocked by the dea line unless you touch the live line. You can be shocked by the live line regardless of the dead line, as long as you are gronuded
If you touch the dead line but nothing else, nothing will happen. The second line only carries a current if it is (1) grounded, and (2) the circuit is connected between the to lines with a switch, bulb, human body, or other conductor. I was not interested in the details of the upstream activity like distribution lines and how plants deal with a varying consumption load. μηδείς (talk) 21:46, 24 March 2015 (UTC)[reply]
Connecting the circuit between the live wire and the dead with your body would only be dangerous if there were a large floating ground, able to accomodate the flux of the live line, to "accept the electrons". With absolutely no grounding of any sort at all there should still be no current, correct?
Electrons power things when they move. When you disconnect the light they just stop where they are, so nothing happens to them, they just don't move anymore. (This is simplified a bit, if you want more details ask. In particular be aware that electrons move very little when powering things.) Ariel. (talk) 00:57, 24 March 2015 (UTC)[reply]
Fred (to expand a bit on what Ariel said), the bulb is not powered by an excess of electrons, but by a flow of electrons, which constitutes electricity. So if you suddenly disconnect a bulb from its power source there are no "excess" electrons in the bulb (or, in the generator) that have to go anywhere. Relatedly, an electrical generator does not produce electric charge or extra electrons; it produces an electric potential difference that causes electrons in the wire and the bulb to flow, which powers the bulb. Let us know if that answers your question. Abecedare (talk) 01:02, 24 March 2015 (UTC)[reply]
(EC) I think the confusion, for me at least, comes from the fact that VOLTAGE is not CURRENT, electrons flow with CURRENT not VOLTAGE, so even though the voltage is changing in AC, if there is no CURRENT, the electrons aren't "flowing". Voltage is the POTENTIAL difference. So I believe the comment by 65.94.50.15 In that case the electrons just flow back and forth along the path from one contact to the other through the generator. is incorrect. I THINK, please correct me if I'm wrong. Vespine (talk) 01:13, 24 March 2015 (UTC)[reply]
When the voltage at a point changes, that means the number of electrons there is changing, which means they are going somewhere and that's a current. In an AC system the voltage at any one point is changing continuously. So yes, electrons are flowing—just not as many of them as there would be if the light bulb was still there. --65.94.50.15 (talk)
I have the impression that the hydraulic analogy is misplaced here. If you are pumping water, and the pipes are not connected to anything, the water will run loose at the end. However, I don't believe the electrons will "fall" at the end of a cable that's not connected to anything. 14:36, 24 March 2015 (UTC)~ — Preceding unsigned comment added by Senteni (talk • contribs)
Picture it as a pipe with a cap on the end, then. Just as a bare wire would not lose electrons right away, a capped pipe would not spill water. If you keep pumping more water into the pipe, however, pressure builds up until a weakness is exploited, and the pipe springs a leak, shooting a jet of water out into the room. Likewise, in a wire, electrons can continue to build up, generating a voltage (voltage = electron pressure) until a high enough voltage builds up to overcome the resistance of the air, at which point a jet of electricity (AKA a spark) shoots out. Analogy works perfectly. --Jayron3214:40, 24 March 2015 (UTC)[reply]
Even better: imagine a sort of "balloon" cap at the end of the water pipe. This models the capacitance of an unterminated wire. If you pump water into the pipe, it will collect in the balloon. The balloon will stretch under pressure - storing water.
Analogously, if you have a copper wire and you apply voltage to it, electrons will accumulate (in the wire itself! But - perhaps slightly concentrated at the unterminated end). As electric current flows into the capacitor, the wire begins storing up electrons, but as they accumulate, the voltage keeps increasing. If you applied DC current forever, eventually, your "balloon" will "fill up" - the wire can not handle an infinite amount of excess charge. At that point, the wire will spark. This is called dielectric breakdown, and it's analogous to a balloon popping under extreme pressure. The material simply cannot contain so much stored energy, and it "breaks."
In the case of alternating current, the direction of electron flow keeps changing - so it's similar to filling a balloon, and then reversing the water flow to drain it again, and then repeating.
... just to clarify Jayron's and Nimur's analogy, the build up of pressure to the point of dielectric breakdown of air will occur only if charge is constantly added as in a Van de Graaff generator. It will not happen with a fixed voltage supply. Dbfirs23:06, 24 March 2015 (UTC)[reply]
Right: the idealization I described was that of a constant current, current source power supply. Most real, general-purpose power supplies that you will encounter behave more similarly to a constant voltagevoltage source power supply; but in reality, a real power supply is never capable of delivering perfectly constant current or voltage when connected to a pathological load. Electronics engineers sometimes use test-equipment that can very nearly model an ideal power supply for some range of operations. Nimur (talk) 02:38, 25 March 2015 (UTC)[reply]
How do humans generate electricity, how do they store it, where does it go to after being used?
The proton pump in the mitochondria is the first electrical energy storage, but it is transferred to other locations via chemical energy and a range of other ion pumps, channels, cotransporters, antiporters etc.
When electrical energy stored in a plasma membrane or any capacitor is discharged, such as by letting ions through a channel, it's basically gone, leaving behind only the waste heat one would expect from that amount of energy (which is puny and I doubt anyone has actually measured it, except maybe in an electric eel or something, but that's just a guess). Wnt (talk) 20:50, 23 March 2015 (UTC)[reply]
I made a redirect for activation potential > action potential. You can measure the total human electrochemical waste heat quite easily, it's basically what raises endothermal body temperature above ambient temperature, but I won't go into the details. μηδείς (talk) 21:00, 23 March 2015 (UTC)[reply]
Explain the following observations as fully as you can: (a)The mass spectrum of a certain element consist of a number of closely-spaced lines.The intensity of one of these lines decreases over a period of time as two new lines simultaneously appear.One of the new lines corresponds to a relative mass of 4. (b)When 1cm³ of concentrated (10 mol\dm³) hydrochloric acid is added to 1dm³ of water the pH of the latter decreases by approximately 5 units,but when 1cm³ of concentrated hydrochloric acid is added to 1dm³ of an aqueous solution containing both ammonia and ammonium chloride the decrease in pH is very small. — Preceding unsigned comment added by 188.164.1.183 (talk) 21:00, 23 March 2015 (UTC)[reply]
Welcome to the Wikipedia Reference Desk. Your question appears to be a homework question. I apologize if this is a misinterpretation, but it is our aim here not to do people's homework for them, but to merely aid them in doing it themselves. Letting someone else do your homework does not help you learn nearly as much as doing it yourself. Please attempt to solve the problem or answer the question yourself first. If you need help with a specific part of your homework, feel free to tell us where you are stuck and ask for help. If you need help grasping the concept of a problem, by all means let us know. DMacks (talk) 21:28, 23 March 2015 (UTC)[reply]
The OP states "turning on a fluorescent lamp requires a large energy use". I found this thought odd so I googled "do fluorescent lights use more energy to turn on" (sans quotes) and I got a lot of hits from seemingly reliable sources that this is a myth. So my question is: what gave rise to this misconception? 196.213.35.146 (talk) 13:19, 24 March 2015 (UTC)[reply]
To my knowledge, it has never been the case that turning on a fluorescent lamp required that much of an energy spike. If you had a massive factory with thousands of bulbs, I am sure you would notice a major spike. In a home with 1 or 2 bulbs, it isn't much of anything. The catch is that old bulbs broke down a little every time they were turned on. So, turning them off and on shortened their life. It was also a bit rough on the ballast. Personally, I replaced ballasts more than bulbs when I worked in a theater in which the bulbs were turned off at the start of every movie and turned on at the end. 209.149.113.207 (talk) 14:08, 24 March 2015 (UTC)[reply]
Star life cycles
Stellar evolution of low-mass (left cycle) and high-mass (right cycle) stars, with examples in italics
Hi all! After drawing this graphic, I wondered if I got it right, particularly whether red dwarfs form directly from nebulae or from protostars. Next, where do brown dwarfs fit in — do they start from protostars and end as white dwarfs? Thanks, cmɢʟee⎆τaʟκ13:32, 24 March 2015 (UTC)[reply]
I am trying to find out what color will you get if you place a red filter over a red object and have searched the internet trying to find out to no avail. My question comes because I was told that if you place a red filter over a red object it will appear white for one red cancels the other. Boxy22 (talk) 09:35, 24 March 2015 (UTC)
There's some annoying imprecision in the language about filters, as explained here. Usually a phrase like "red filter" is interpreted to mean a filter that passes red light, and so a red object through a red filter is red. But sometimes, especially with "UV filter", it can mean one that blocks that color. In that rare instance, which I'd tend to think of as a misuse of the term, a red object hypothetically could appear entirely black because all its red light is blocked. However... no filter is absolute, the shoulders are often very wide, and since we're speaking of the intersection of what is filtered out by reflecting off the red object and what passes through the red filter (which includes some red light) I think what you see could be red, orange, yellow, even white ... it's hard to say because you're multiplying two entire absorption curves and seeing your eyes' overall determination of the dominant colors of what is left. Wnt (talk) 16:22, 24 March 2015 (UTC)[reply]
I would think that white and blue would be the colours you would not expect to see, but I agree with Wnt's comments about imprecise language and filters that can be surprisingly wide. In most cases, you will see red because red light is reflected from the object and is passed by the filter. Cancelling might happen if the filter is red-blocking (looks cyan) and just happens to block the exact extra red light reflected by the object, but even then your eye is unlikely to interpret the result as white, more probably a muddy grey. Where does refraction come in? Perhaps you are confusing filters with prisms where you can split up light then recombine it to get white. Dbfirs16:53, 24 March 2015 (UTC)[reply]
It is an incorrectlty-formatted citation that had existed for about eight years! Regrettably, I don't recognize the source, so I can't independently verify it. The name shows up in references for a few related articles, e.g. Brown dwarf, but it's a ton of work to track down a hypothetical ...book/article/webpage ... from one name alone. We can more easily find alternate sources. If I were going to look for an alternate replacement source, I'd pull out my copy of de Pater & Lissauer: Planetary Sciences, which has great content on stellar evolution. Perhaps later today I can re-source that article. Nimur (talk) 15:44, 24 March 2015 (UTC)[reply]
White dwarf / Chandrasekhar limit questions
All this star talk reminds me that there are still essentials of white dwarf and Chandrasekhar limit that I don't understand. It is clear enough to me that there is electron degeneracy pressure because electrons can't have the same quantum state (whyever that is, the Pauli exclusion principle as observed). The tighter you press plasma together, the faster the electrons have to move. At a certain point, they move so fast that instead the pressure drives them to electron capture. But...
1. What says how much faster an electron has to move than its neighbors, or at how much of an angle relative to them, in order to get its own quantum state? (Fermi sea is less than informative for me, at least...)
2. When the Chandrasekhar limit is reached, that indicates that electron capture has become a feasible process. Is this determined by kinetics (how close you have to jam electrons and protons before they meet often enough to become neutrons) or thermodynamics (the electrons at this point are moving just fast enough to make neutrons lower energy than proton + relativistic electron)?
3. Above I assume it's protons that interact, based only on the name "neutron star", but presumably white dwarfs should be largely helium. Is it actually helium nuclei that capture the electrons?
4. Our article Chandrasekhar limit references the Tolman–Oppenheimer–Volkoff limit where neutron degeneracy pressure fails as sort of the next step on, but our article on neutron stars has a cute figure with a number of intermediate levels between white dwarf matter and quark-gluon plasma as one looks deeper and deeper into the neutron star. Oddly, there are no "just pure neutrons" layers listed, which makes me wonder if the TOV limit is just an approximation, something that doesn't purely happen? Are there more precise limits for explaining when each type of nucleus in the white dwarf breaks down by electron capture, or something?
A question what says how much faster an electron has to move than its neighbors, or at how much of an angle relative to them? can only be answered indirectly in the framework of quantuum mechanics. Indeed, the way the quantum mechanics is prsently understood, the electrons are (a) indistinguishable and (b) can not be described by an exact value of the velocity vector in any spatiotemporally local sense. We can, however, make an oversimplified mental picture of electrons that (i) do not interact electrostatically with each-other and with the ions, and (ii) are confined to some hypothetical unit volume with impenetrable walls surrounding it. Electrons under such hypothetical conditions can assume single-electron states that look like resonator modes of that unit volume (that is, standing waves), two per mode because they are spin 1/2 fermions. At relatively low temperatures, single-electron modes will be filled from the ground up to the Fermi energy. The full (many-electron) state will be a fully anti-symmetric combination of all the single-electron states. If you do the math right, you will find that the extensive quantities (internal energy, entropy) in this model indeed come out extensive, that is, depend linearly on the volume for a given free electron density. Pressure, therefore, will depend on the free electron density but not on the (hypothetical) unit volume. Good! When free electron have a temperature, and the temperature - in energy units - is much lower than the Fermi energy, the most energetic occupied single-electron free electron states will have energy close to the Fermi energy. As you increase the temperature, chances of higher-energy free electron states to be occupied will increase as well. Does this help? --Dr Dima (talk) 17:30, 24 March 2015 (UTC)[reply]
(1) I don't know if you can understand fermion degeneracy in the particle picture. If you think of the electrons as waves, then the waves have to be orthogonal in spacetime. Classically, the Nyquist rate is analogous, I think. The number of independent samples you can encode in a signal with a maximum frequency is the same as the number of fermions you can pack into that space with a maximum energy (in the absence of any forces). (Edit: retracting my answers to 2-4 because I'm not knowledgeable about this.) -- BenRG (talk) 20:08, 24 March 2015 (UTC)[reply]
(1) As others have said, each electron needs to have its own quantum mode. In three dimensions, each mode can be described by three integer wave numbers, such that the energy of each particle is:
and where the integer indices are unique for each electron. For a system with a very large number of particles, this implies that some of the particles must have very large indices and be extremely energetic. In a degenerate Fermi gas, the most energetic particle will have energy approximately equal to the Fermi energy:
Which depends only on the particle number density.
(2-3) These questions are premised on a mistake. The Chandrasekhar limit doesn't exist because of some other process taking over. It exists because there is a maximum mass sustainable by electron degeneracy period. Think of it this way, a star's radius can be described as a function of it's mass, . As you add mass, the gravity increases, which increases the central density. For a normal star, sustained by nuclear fusion, increasing the central density will increase the rate of fusion, make the star hotter, and cause its radius to increase. Hence, most stars exhibit for some index x, typically between 0.5 and 1. However, in a white dwarf, the electron degeneracy pressure doesn't increase very quickly as mass increases. The end result is that radius decreases with increasing mass. It turns out that the combination of the relativistic electron degeneracy pressure and the Lane–Emden equation (governing pressure-density relationships in gravitational objects) predicts that for a finite mass the radius the white dwarf goes to zero, i.e. . If white dwarfs were an ideal Fermi gas, they would collapse into a black hole at the Chandrasekhar limit. Of course, they aren't ideal Fermi gases, but rather real stars made of atoms. The result is that as the collapse begins the increased density and temperature reaches helium fusion temperatures and the star detonates as a type Ia supernova.
(4) White dwarfs never become neutron stars because the supernova detonation intervenes. Neutron stars are heavily enriched in neutrons compared to ordinary matter (e.g. >10 times as many neutrons as protons), but they aren't pure neutron objects. A dynamic equilibrium exists between neutrons and protons / electrons depending on pressure and temperature. Dragons flight (talk) 21:47, 24 March 2015 (UTC)[reply]
Thanks for some great answers! I'll start unpacking @Dragons flight:.
(1) The difference in energy seems recognizable as the classic 1/2 m v2, where v is the momentum divided by the mass of the electron. That gives the 1/2m part; the sum of squares is vector addition for momenta in three dimensions; and the remaining part, the quantization, is Planck's constant h (unreduced) divided by the length - assuming that V is a cube, that is. I assume the cubical star is something of an approximation. :) This is the same as the momentum of a photon with a wavelength equal to the length of the cube. (But the angular momentum of a photon measured along its axis is h/ 2 pi rather than a length) Going a bit "out there", the subjective feeling I'm getting from this is that for each quantum state you have to string some distinct number of h's of momentum/angular momentum around some sort of loop to define it. Pushing the electrons together shortens the loop, and so the amount of momentum at each part of the loop increases, i.e. the pressure and observed momentum increases.
(2) This part is where I go off the rails. I've seen the phrasing that degeneracy pressure 'can't sustain' more than a certain force, yet the math from (1) shows an energy that clearly goes to infinity as V goes to zero. It doesn't make sense to me that you can press something together and create infinite energy; that's why I'm thinking some other process has to interfere (and can find sufficient energy to do so).
(3-4) It makes sense that the helium nuclei simply undergo fusion and set off the supernova; yet in a neutron star itself, the outer layers are something like white dwarf material, so it should be possible to find a continuous gradient... I think. The article is a bit vague about whether the outer layers are all iron nuclei or hydrogen and helium; actually I find myself wondering if there could be a long period of ongoing fusion near the surface of the neutron star. But I suppose it makes sense that at least the bottom layer of nuclei has to be iron, and so the iron nuclei ought to be what (if there is gradually accretion on top of it) would have to give way to capture the electrons? In which case... they aren't iron nuclei anymore, and should undergo more nuclear reactions. Hmmm. I wonder just how complicated this ecosystem really is! Wnt (talk) 02:44, 25 March 2015 (UTC)[reply]
Star Trek and Science
Dear Ladies and Gentlemen.
It has come to my attention that many people claim, that the tv shows of the franchise Star Trek are supposed to be scientifically accurate. Even famous scientists like Neil deGrasse Tyson has touted the shows for their "scientific literacy". I have never seen Star Trek until very recently (I am from a different culture, Star Trek isn't as popular here as it is in the United States, in the UK or in Germany) but grew up with the books of various hard science fiction authors like Stanislaw Lem, Arthur C. Clarke and Herbert Werner Franke. Given that I am currently studying physics, it seems to me, that some of the Star Trek shows lack even some very basic knowledge about science. There are episodes, where the crew of Captain Picard encounters planets, that are colder (Theta 116 has a surface temperature of -291 °C. -291 °C is below absolute zero (-273.15 °C)) than the lowest possible temperatures and older than our universe (the same planet's age is estimated to be 7.2*10^10 or 72 billion earth years old; far older than the universe itself). I like Star Trek, but to me, it does not seem more scientifically accurate than Farscape, Stargate or Firefly (like most soft sci-fi products). So why do so many people make rather strange comments about the representation of actual science in these shows? I apologize for not being perfect in English, I started recently with the study of your beautiful language.
The difference is the time period. Star Trek is from the late 1960's, and in the time frame "sci-fi" shows were notoriously inaccurate. So, Star Trek was a step up from those earlier attempts. Of course, any sci-fi show set in space will have to make compromises, whether to keep from boring the audience with only sublight speed or keeping on budget by having 1 g of gravity in most every scene.
I did notice a reduction in the level of scientific realism from the original series to Next Gen, though. For example, in one original episode (The Tholian Web), Kirk is caught in an interdimensional rift, coming in and out of phase with the our universe. Next Gen did a similar episode, but there instead of being in a space suit with an air supply and floating freely about, the person so affected could still breath the air and walk on the floor, which made no sense whatsoever. StuRat (talk) 17:39, 24 March 2015 (UTC)[reply]
From the Next Generation series onwards they tried to get things as scientifically accurate as they could within the confines of telling a good story. For instance, having been told that a transporter couldn't work because of Heisenberg's uncertainty principle they introduced a "Heisenberg compensator". [20] Nobody knows what it does but it sounds good and gets round the problem. When Leonard Nimoy died it said on the NASA website that many of their scientists were originally inspired by Star Trek to go into their chosen field. Richerman(talk)18:37, 24 March 2015 (UTC)[reply]
I strongly disagree! To take *just* the transporter system: The entire point of Heisenberg's uncertainty principle is that it's a fundamental property of nature - you can't conceivably "compensate" for it. The transporter violates conservation of momentum, conservation of mass/energy, it implies more data bandwidth between source and destination than is reasonable, not to mention the requirement to reassemble large objects without causing the air to move out of the way around them and without having any equipment at the far end. The range, capability and capricious nature of transporters is poorly maintained from one episode to the next...and the crew conveniently forget to use it in situations where it would be exceedingly useful. In STTNG, the ship has 'site to site' transport capabilities - and yet everyone still has to go to the "transporter room". Why do they even need a transporter room? Transporters frequently function in situations where getting even a simple voice radio link to work fails! The idea to transport bombs or other weapons on board other ships is rarely used. The shuttle craft have transporters - but they are never used to get them out of trouble on the surface of planets and such. We're told that the transporters are also able to 'filter' disease carrying organisms as aliens are beamed aboard - but never to cure people who carry those diseases while on board. On occasion they transport people aboard while selectively NOT transporting their weapons. That degree of selectivity would allow an enormous number of industrial processes within the ship to be carried out with them...but no sign of that either. We're told (in STTNG) that transporters are the safest form of transport devised by man - yet there are dozens of transporter screwups in just one starship over a matter of just a few years. It's evidently possible to reconstitute a human from copies of their recordings in the transporter "pattern buffer". Why are the crew not routinely recorded in this way before going on dangerous missions?
And that's just the transporters.
It's true that many scientists were inspired by StarTrek - but that's true for any number of other sci-fi books, TV and movies.
It's safe to say that there is very, very little scientifically accurate stuff in StarTrek. It's rarely even self-consistent from one episode to the next. HOWEVER, that doesn't prevent the genre from being able to produce some entertaining stories. SteveBaker (talk) 19:10, 24 March 2015 (UTC)[reply]
Also tangentally related are Clarke's three laws, especially the third law. Clarke's Third Law the sine qua non for science fiction writing, and is what separates good, readable science fiction from the sort of "Moby Dick in Space" in the sense of being a massive technical work with some minor story going on in the background. Really good science fiction doesn't try to explain or even "get right" its violations of the laws of physics as we know them. The basic application of Clarke's Third Law for good writing is: get right what your audience would notice if it were wrong, and the rest isn't important to get right". That is, any technology which would allow faster-than-light travel would have to be "magic" under the current understanding of physics, so: let it be magic. Maybe you throw a MacGuffin into your work like "dilithium crystals" or "warp drive" or something like that, but as soon as you try to make the magic "real", you lose your audience (read any of the criticisms of midichlorians in the Star Wars universe to see what happens when you treat the Third Law with disrespect). Some authors do deal creatively with the "faster than light" problem in more realistic ways than Star Trek. For example the EnderversebyOrson Scott Card deals with the problems of relativistic aging due to the twin paradox and time dilation in FTL travel. But he still has to play the "it's magic" game when it comes to other aspects; for example in the Enderverse, instantaneous communication between distant points in space is still possible, and FTL travel still occurs, both of which require the author to just ignore some fundamental rule of physics. The balance must be made between "writing an engaging story set in a futuristic world" and "not getting the physics so bad that it distracts from the writing". A good way to avoid the latter is to ignore discussions of physics in general. The audience will forget that your story is impossible so long as you don't actually try to actively prove to them it is... --Jayron3219:23, 24 March 2015 (UTC)[reply]
Star Trek (original series), like lots of "science fiction" wasn't originally about science so much as about exploring the human condition, in a setting unfettered by present society, politics, and technology. So Star Trek could talk about things like socialism, communism ([21]), inter-racial and multicultural relationships (etc. [22]), that would have been much harder to do on TV at the time with a contemporary setting. Lem does a bit of exploring the human condition too, but Lem of course also knew a lot of math and science. Think of it another way: has anyone ever criticized Kurt Vonnegut for lack of scientific realism in his works? Not really, because although they have elements of Sci-fi, Vonnegut's novels aren't really even about science. SemanticMantis (talk) 19:28, 24 March 2015 (UTC)[reply]
Well, to flip it back on you, name any well regarded science fiction work which isn't fundamentally about "exploring the human condition". It's true that the level of realism with regards to the laws of physics is often what separates hard science fiction from soft science fiction, but ultimately, even the "hardest" science fiction in the world is still fiction, and still deals with issues of character development and plot arcs. You mention Lem as someone knowing a lot of "math and science", but even he had to leave as unexplained such issues as FTL travel and instantaneous communication, and has a lot of unexplained "Magical" phenomena in his works. You can't say Solaris isn't a work about the human condition; it's a novel which uses a science fiction setting to explore an aspect of the human condition in unique and novel ways, but it still does that. Without that, it wouldn't be fiction. --Jayron3219:46, 24 March 2015 (UTC)[reply]
Can you point to a source where Tyson (or anyone) is quoted as saying that Star Trek is scientifically accurate? It seems unlikely he'd say that, since, well, it obviously isn't. People write books called "The Physics of Star Trek" because most people are not willing to buy books about physics unless they have "Star Trek" or "Einstein" in the title. They make tenuous connections between real science and Star Trek because they don't know how else to reach the lay audience that they want to educate. -- BenRG (talk) 20:36, 24 March 2015 (UTC)[reply]
(To BenRG's first question, can you point to a source ... saying that Star Trek is scientifically accurate?): Quite the opposite! A famous piece of Star Trek memorabilia is "The Writer's Guide", (widely believed to be) written by Gene Roddenberry. Unofficial copies are available online, and the most official copy is presently kept in the Houghton Library at Harvard University. In this very famous document, Gene Roddenberry (Creator of Star Trek) specifically warns his writers not to fixate on scientific accuracy, but to focus on good story-telling and character development.
(If you'd like to engage in hearty debate about whether the writers adhered to Roddenberry's advice - or whether Roddenberry himself backtracked on his own admonitions when he produced the later series and movies... there's a vibrant community of internet folks who would love to argue minutia with you. Consider perusing the official list of Star Trek fan sites to locate a forum).
See here for the view of one of the NASA scientists and a discussion about the science. To sum up he says "The real science is an effort to be faithful to humanity's greatest achievements, and the fanciful science is the playing field for a game that expands the mind as it entertains. The Star Trek series are the only science fiction series crafted with such respect for real science and intelligent writing. That's why it's the only science fiction series that many scientists watch regularly... like me." Richerman(talk)09:14, 25 March 2015 (UTC)[reply]
From electricity to electromagnetic wave and back
If I heat the filament of a light bulb with light, would electricity start flowing through the attached cables? That is, it would be the reverse of powering the light bulb to emit light. Instead of transforming electricity into light, we would be transforming heat into electricity.Fend 83 (talk) 18:34, 24 March 2015 (UTC)[reply]
Sometimes yes, and sometimes no. In your specific example, no, you would not. Heating the filament would produced stochastic motion of the atoms in the filament, and that motion and energy would not then generate an electric potential; no potential, no electromotive force, no EMF, no electricity. However, some electrical processes are reversible. Consider two examples, which are basically the same device used in reverse. Consider first a speaker and a microphone. What is the difference between them? Actually, basically nothing. A speaker is a device which vibrates in response to electric current fluctuations. If you send an AC current to a speaker, you get a sound from the speaker. If you do the reverse: if you take a speaker and shout into it, you generate an AC current in the wires attached to the speaker. That's all a microphone is: a speaker you shout in. Secondly, consider a motor and a generator. A motor is a device which spins in response to an electric current. If, however, you take a motor and drive it by hand, you've just made a generator: you can detect a current in the wires attached to the motor if you physically spin the motor. Why do these systems work that way, where the lightbulb doesn't? It has to do with the specific way they are constructed. Both a motor and a speaker work by means of a moving magnet. In the case of a speaker, fluctuations in the electric current in the wire causes an electromagnet (technically a solenoid) to vibrate back and forth; this vibration is amplified by the speaker cone, generating a sound. In the case of the motor, the motor spins because a fluctuating magnetic field (generated in this case by a commutator) caused by the electric current causes the magnet attached to the axle of the motor to spin. In each of these cases, the process is reversible because moving electrons generates a magnetic field, AND moving magnetic fields cause electrons to move. That's where electromagnetism comes in as a concept: The two processes are two sides of the same coin; you don't get electricity (moving electric charge) without generating a magnetic field in response. And if you move a magnetic field, you generate an electric current in response. So, reversible electric devices would be ones where the transducer is one that relies on magnetism to transfer its work. Processes that rely on heat to transfer their work are fundamentally non-reversible because heat is always lost as entropy. And entropy cannot be recovered as usable energy. The second law of thermodynamics is a mean bitch that way. However, processes that don't depend on heat are usually in some way reversible. --Jayron3218:53, 24 March 2015 (UTC)[reply]
I should also note that it is possible, however, to generate electricity by shining light on things. Once we take heat out of the situation, there are some materials that generate electricity in response to shining light on them, which is the reverse process of using electricity to generate light (though not the exact reverse process that happens in an incandescent light bulb). See photoelectric effect, the explanation of which was one of Einstein's Annus Mirabilis papers of 1905. However, the photoelectric effect does not depend on heating anything. --Jayron3219:01, 24 March 2015 (UTC)[reply]
Hypothetical heart control
The medical sources I've looked into say a person fades out after cardiac arrest in some 8–12 seconds (and "rather longer if the patient is recumbent"). In theory would it mean that if a particular person were able to control his heart, he would be able to stand or seat seemingly well for those 8-12 seconds (or at least, say, 5 seconds) with stopped heart before restarting it? Brandmeistertalk19:31, 24 March 2015 (UTC)[reply]
The heartbeat is controlled by the Autonomic nervous system which, by definition, is not under conscious control, except tangentially (for example, being able to increase one's heart rate by imagining a stressful situation, for example). I am not aware of anyone who is actually able to control their own heart to the point where they can stop it and start it on command, and given the number of people who have lived in the history of history, that's a pretty good experimental sample for saying it isn't likely to be possible. --Jayron3219:39, 24 March 2015 (UTC)[reply]
I know, that's why I wrote "in theory", despite several claimants like Guy Bavli or Cristian Gog. For the sake of argument, let's assume some ultra-rare mutation similar to dysautonomiaorOndine's curse, which enables a person to control the heartbeat until feeling of impending fadeout. Brandmeistertalk20:05, 24 March 2015 (UTC)[reply]
Per Medeis, if you're going to invent a fictional magical power, you get to decide how it manifests itself. We have nothing to say on the matter in this forum, where our role is to provide you with references to further research factual questions you may have. Questions where we speculate about the effects of magical superpowers are not what we do here. --Jayron3200:27, 25 March 2015 (UTC)[reply]
Indeed this is starting to sound like how much wood would a woodchuck chuck... it's not truly magical of course, since there's nothing impossible about such a "power", but certainly there's no reason to evolve it, and so we can't really predict the form it would take with any accuracy. As a rule, though, cardiac myocytes want to beat; they'll beat in a tissue culture dish; so they should keep contracting on their own if whatever mechanism is interrupted by unconsciousness. Fibrillation is the main worry. But of course fibrillation isn't really an intended mechanism and it's no great stretch to suppose the person has a good cardiac pacemaker that prevents death by this means, at least until it advances the plot to start dealing with it. So basically you need to mumble mumble inhibitory synapses mumble neurotransmitters mumble Eastern mystics mumble and you're good to go. :) Wnt (talk) 02:52, 25 March 2015 (UTC)[reply]
Section on electron mobility in the article about Indium-Gallium-Arsenide semiconductors (specifically related to Gain-Bandwidth product)
I posted three questions on the article's talk page a few days ago, but I haven't gotten any response. Talk:Indium_gallium_arsenide Could someone knowledgeable about semiconductor physics or electron mobility please look it over?
Also, I'm wondering if maybe that particular section should be included in the specific article about Gain-Bandwidth products, since it deals with math that is generally applicable to more than just InGaAs semiconductors? 97.84.96.60 (talk) 21:45, 24 March 2015 (UTC)[reply]
If I don't get a response to either this comment or my comment on the talk page sometime tonight, I'm going to at least correct the formatting on the relevant equations, as I laid out on the talk page. 97.84.96.60 (talk) 04:31, 25 March 2015 (UTC)[reply]
The equation doesn't look correct to me... at the very least, that equation is being quoted out of context, and in my opinion, it's just flat wrong. A source is provided: Photonics Essentials. Anyone have that book to check what the original authors meant? Nimur (talk) 04:54, 25 March 2015 (UTC)[reply]
I'm not sure what the SOP is here, so I removed the material in question and added a note about it on the talk page. It can be added back in after this gets sorted out. 97.84.96.60 (talk) 09:54, 25 March 2015 (UTC)[reply]
March 25
Variable ginger sensitivity ?
I notice I am really sensitive to the spice, ginger. I don't have an allergic reaction or anything, it's just that the tiniest amounts make my food seem really hot. I don't seem to be any more sensitive to capsaicin than others, but that's known to have a variable response (or at least a resistance that builds up). Since I usually consume dried ginger, the active ingredient is shogaol, I believe. So, is there a variable "heat" response to this ingredient ? StuRat (talk) 06:12, 25 March 2015 (UTC)[reply]
Shogaol and capsaicin both bind predominantly to the TRPV1 receptor, which gives you the 'heat' effect, so it's hard to explain why you would be more very sensitive to one, but not the other. Must be secondary binging to other receptors, or a different component in ginger. 131.251.254.154 (talk) 11:08, 25 March 2015 (UTC)[reply]
Geoid: how far from the center of earth does gravity point?
A vacuum is the absence of matter; I don't know what you mean by a gas vacuum or a liquid vacuum. You might find it useful to see the list of examples listed in the vacuum article. Deep space is generally much more empty than any artificial vacuum.--Shantavira|feed me09:22, 25 March 2015 (UTC)[reply]
Resolved
Please do your own homework.
such that the energy of each particle is:
. As you add mass, the gravity increases, which increases the central density. For a normal star, sustained by nuclear fusion, increasing the central density will increase the rate of fusion, make the star hotter, and cause its radius to increase. Hence, most stars exhibit 
for some index x, typically between 0.5 and 1. However, in a white dwarf, the electron degeneracy pressure doesn't increase very quickly as mass increases. The end result is that radius decreases with increasing mass. It turns out that the combination of the relativistic electron degeneracy pressure and the Lane–Emden equation (governing pressure-density relationships in gravitational objects) predicts that for a finite mass the radius the white dwarf goes to zero, i.e. 
. If white dwarfs were an ideal Fermi gas, they would collapse into a black hole at the Chandrasekhar limit. Of course, they aren't ideal Fermi gases, but rather real stars made of atoms. The result is that as the collapse begins the increased density and temperature reaches helium fusion temperatures and the star detonates as a type Ia supernova.
