Wikipedia:Reference desk/Archives/Science/2011 September 19

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September 19

Surface charge on a conductor

So if there is a net charge in a solid spherical conductor let's say, we know that the charge will flow to the surface and arrange themselves to minimize its electrostatic energy (as far apart from each other as possible). But what happens to its one and two dimensional analogs? For a very super thin circular conducting disk, would the charge all go to the perimeter (the circumference)? And for a very thin needle, does the charge go to the ends? If not then what happens? 67.40.134.8 (talk) 02:23, 19 September 2011 (UTC)

I'd say no, because those things cannot be true 2- or 1-D analogs. In the real world the disk, for example, actually still has two large faces, and even if it were one atom thick it would still effectively have one large face, over which the electrons or holes would distribute. (If it were so thin that its thickness could not accommodate even one layer of electrons, the same would be true of its edge). However, due to the geometry of the edge or ends there would, I suggest, be higher concentrations of charge there, which is what we observe in, for example, St Elmo's fire or on lightning conductors. 90.193.78.7 (talk) 11:35, 19 September 2011 (UTC)

{edit conflict) In the case of the 3d charge at the surface the field inside the sphere is zero (see shell theorem) - because the field is zero everywhere inside the sphere this means that there is no net force on charges inside a spherical surface of charge.. This actually means that you could have a charge distribtion in the solid spherical case of charge on the surface and a percentage of the charge in a spherical shell inside the sphere. A third band of charge is not possible since one of the bands would lie outside the inner band and experience a net force - causing the charge to move to merge with the surface charge..

(This means that your initial assumption of edge charge on a spherical body may not be absolutely true) Sorry about that - just spotted my error - ignore

Did that make sense so far (you probably need to be familiar with "shell theorem" or maybe Gauss' law for gravity as applied to electric charge..?)

As I remember it's a specific property of the spherical case that the field inside is zero. This is definately not true within the 1D (needle) case, and as I recall not true inside the 2D (disc) case either.

Therefor it seems quite possible that you can get equilibriums where the charge is not all at the edge (I ignore the cases where some charge is at the absolute centre).

To be certain in the 2D you would need the field potential inside a symmetrical "flat ring" of charge .. only with this information can you be certain - if it is zero everywhere inside then the case is the same as the 3D case, if not then other equilibrium arrangements will apply.

Can you/ have you got that potential due to a ring of charge?

For the 1D case I can propose at least one case - charge at the edge, charge in the centre, and a pair of charges neither at the edge or in the middle held in equilibrium by the centre and edge charges.. - this is a possibility different from the 3d sphere case. I can't say if there are other cases too.Imgaril (talk) 11:50, 19 September 2011 (UTC)

Just for the record it looks like calculating the field potential inside the 2D ring is fairly tricky (for a non-maths degree holder) and I couldn't find the answer easily searching - if you need to pursue this for the 2D case you might be better off asking at the maths desk..Imgaril (talk) 12:19, 19 September 2011 (UTC)

apologies if all that just muddied the water by the way - the answer in at least the 1d case is that its not the same as for the 3d case - I'll leave a note on the maths desk about this too

You may find this of interest: Lightning rod#Should a lightning rod have a point?. --Tango (talk) 12:02, 19 September 2011 (UTC)

That charges end up at the surface of a conductor doesn't follow from "minimization of electrostatic energy". It follows from that and Coulomb's law, but that means that the intuitive idea that "the chages are repelling each other so they will move as far apart as possible and therefore they end up on the surface" is incorrect. Replace the 1/r^2 law by an 1/r^3 law and you'll find that the charges will reside in the bulk, not on the surface.

What does always happen in electrostatics (Coulomb's law or no Coulomb's law), is that in equilibrium, the electric field has to be zero inside the conductor. If not, then forces would act on charges and they would therefore move around. Applying Gauss' law (which is equivalent to Coulomb's law) then yields the result that charges reside on the surface of a conductor.

In case of a needle, the charges don't go to the point of the needle. What does happen is that a electric field near the needle point is large. You can see this intuitively as follows. The electric field inside the needle is zero, this means that the electric potential in the needle is constant. Now, the electric field jumps if you move from inside the needle to outside the needle because of the surface charges, but the potential changes in a continuous way.

What then matters is how much charge must flow to the point of the needle to make its ptential the same as on the rest of the needle. Of course, the smaller an object, the less charge you need to raise its potential. That the electric field does get large near the point, can be seen as follows. Instrad of the needle, consider a big charged sphere connected to a thin wire. The total charge on that wire will then be very small. Now, you can write the potential V(r) at position r as the summation of Qi/(r-ri) over all charges Qi located at positions ri (this summation can be replaced by an integral over the surfaces involving surface charge densities). Then because there are hardly any charges on the wire, the potential V(r) is practically the same as what it would be if there were no wir cnnected o the sphere, except very close to the wire (small q means that q/(r-ri) will become large only if r becomes close to r1).

A bit away from the wire, the potential is practically the same as what it would be if the wire weren't there. Suppose the wire extends very far for the sphere. If you approach the wire, the potential will at first hardly increase and only at the last moment will the potential increase to the value it has inside the wire (which is the same as the potential of the sphere). So, compared to approaching the sphere, you get the same increase in potential if you approach it from infinity, but in case of the wire it now all happens at the last moment. The rate of potential increase is the maginitude of the electric field, and that is thus much larger near the wire. Count Iblis (talk) 16:11, 19 September 2011 (UTC)

I don't see what's wrong with saying that the charges move to minimize electrostatic energy, and that ends with them all on the surface. It may be false in a fictitious world with a 1/r³ force law, but it's true in the real world. And with a 1/r force law you will end up with a nonzero field in the bulk at equilibrium, unless I'm missing something.

It's qualitatively true that the charges move to be "as far apart as possible"; it's just not clear what that entails. When the charges are all on the surface they'll be much closer to their nearest neighbors than if they were distributed through the bulk, so it's not obviously better. It comes down to the nature of the force. If it falls off quickly then nearer neighbors are more important and the charges might prefer to be in the bulk. If it falls off slowly then the distance to faraway charges matters and it might be better to be on the edge.

As far as I know, the explicit answer to the original question is that in d dimensions with a 1/r^k force law, the particles will end up on the surface if and only if k ≤ d−1. So they don't go to the edge of the disc or the ends of the needle (in 3 dimensions) -- BenRG (talk) 23:29, 21 September 2011 (UTC)

Arctic Survival, Part Deux

Not sure if this is really a science question per se, it's mostly a follow-up from above. I know that Alaska requires pilots to carry survival gear, and I'm guessing that they're far from alone on that, but what is included in that survival gear, and what of that would be present on a commercial airliner? I'm guessing that you could have a pretty good weekend in Dallas Vegas with all of it, but what's typically in it? SDY (talk) 07:07, 19 September 2011 (UTC)

See survival kit, which will give you some idea.--Shantavira|^{feed me} 07:19, 19 September 2011 (UTC)

I don't know if jetliners have a survival kit, but if yes, what for? People rarely survive a jetliner accident. Quest09 (talk) 09:21, 19 September 2011 (UTC)

I believe they usually do. It doesn't cost much to do so, so the gain doesn't need to be much to make it worthwhile - that gain may be mostly PR. Of course, people used to say similar things about life jackets. People said they were just their for show because nobody ever survived a water landing in a commercial passenger jet. People don't say that anymore, since there was an extremely successful water landing on the Hudson recently (extremely successful as in everyone escaped with only a few minor injuries - the plane was obviously a write-off). It is extremely unlikely that a survival kit on a plane will ever get used, since accidents usually happen at take-off or landing (when rescuers are very nearby) and those than happen in mid-flight are usually due to some kind of catastrophic failure that results in everyone being killed on impact. It is possible that circumstances could occur in which it would be useful, though. --Tango (talk) 11:56, 19 September 2011 (UTC)

I don't remember if any one has to use a life jacket to be rescued. I think they were rescued by boats. Quest09 (talk) 13:10, 19 September 2011 (UTC)

In that case yes, but one can't always have their water landing be in the center of a large city with dozens of boats around. Googlemeister (talk) 13:35, 19 September 2011 (UTC)

No, you can't have your water landing the way you like it most. On the Hudson River, without victims, and with TV cameras filming you is fine. However, the most common outcome is that if something happens at take-off or landing, rescue would be easy, as above; and on mid-flight, rescue would b difficult, but you have even less reason to expect survivors. Are life jackets just a question or PR? Quest09 (talk) 18:27, 19 September 2011 (UTC)

Probably not. Ditching an aircraft (a water "landing", really more of a controlled crash) is often easier than trying to set it down on land, especially if there are control failures or the landing gear doesn't work. If you ran into a Gimli Glider or similar situation at sea where you're forced to land well short of your destination, you don't automatically lose all the passengers. Single engine naval aircraft (e.g. the F-35C) are pretty rare for exactly that reason - if you lose your one engine, you're screwed. The Canadian air force specifically went to the two engine CF-18 instead of the single engine F-16 the US uses, precisely because losing an engine in the Great White North is a big deal. SDY (talk) 22:20, 19 September 2011 (UTC)

FYI, according to the article "Ditching", the average survival rate for an emergency water landing is about 75%. This is of course highly dependent on the location and manner of the crash. 67.169.177.176 (talk) 00:44, 20 September 2011 (UTC)

Just because they were rescued by boats doesn't mean they didn't need life jackets. Our own article says "Evacuees, some wearing life-vests, waited for rescue on the partly submerged slides, knee-deep in icy river water. Others stood on the wings or, fearing an explosion, swam away from the plane." but it's not clear if there would have been enough space on the wings and slides for all passengers who wouldn't have wanted to enter the water without lifejackets. Remember plenty of people can't swim or even keep themselves afloat in water for long (and it was very cold and I guess it was fresh water too) so even if it only took 2 minutes for the boats to come there's no guarantee they would have survived, particularly in the panicked situation of that sort. Definitely from a rescuer POV even if they could have saved everyone without life jackets I'm sure they were happier they didn't have to worry (much) about trying to rescue people drowning because they couldn't stay afloat. In any case, this isn't the only water landing. There is incidentally a risk of people opening their life jackets prematurely, if you listen to the instructions they give before takeoff they always say to inflate one side just before or as you jump out and the other side after. I watched a MayDay episode once which mentioned some people died because they trapped when they opened their lifejackets before they reached the doors and then as the plane was already partly full of water they were taken to the surface and couldn't get out. (I believe it is Ethiopian Airlines Flight 961 which is mentioned in the water ditching subsection.) Nil Einne (talk) 01:17, 20 September 2011 (UTC)

Life jackets also make extra floating wreckage to find if everything goes terribly wrong, even if they were not of much help to the passengers on that plane. Googlemeister (talk) 13:09, 20 September 2011 (UTC)

You might like to look at Alaskan Survival Equipment Requirements, and Lets Fly Alaska Aviation Survival Kit, also Alaskan & Canadian Survival Kit Regulations. Alansplodge (talk) 12:38, 19 September 2011 (UTC)

That doesn't apply to jet-pilots. Quest09 (talk) 13:10, 19 September 2011 (UTC)

I was answering the question: "I know that Alaska requires pilots to carry survival gear... but what is included in that survival gear?" Airliners is a seperate issue - I couldn't find much on that I'm afraid. Alansplodge (talk) 18:41, 19 September 2011 (UTC)

Fuel/Air ratio

I have split this question into its own section. Nimur (talk) 21:20, 19 September 2011 (UTC)

Why multi cylinder engines need high fuel air ratios than single cylinder for same throttle opening — Preceding unsigned comment added by 173.254.204.203 (talk) 20:37, 19 September 2011 (UTC)

I know of a couple of considerations regarding multi-cylinder engines and fuel/air ratio. First, balancing the fuel/air ratio between cylinders in a multi-cylinder engine is an issue, there are various tricks to acomplish this, but I imagine in some applications they just up the the mixture to avoid problems. Second, multi-cylinder engines have slightly poorer combustion chamber cooling and at high loads require a richer mixture to avoid knocking. Neither of these issues leads add up to a huge difference and only make a difference under high load, under low load the mixture will be very near 14.7:1 for all gasoline engines. There may be other issues that I don't know about though. --Daniel 00:50, 20 September 2011 (UTC)

Thanks for above answer but as per some books.since fuel is having higher inertia.so the fuel air ratio while distribution into many cylinders gets disturbed.thats why we give a richer mixture for same throttle opening in comparision of single cylinder engines.i am talking according to H.B. gupta .automobile engineering book present in google book.please comment on this argument presented by book author. 203.197.246.3 (talk) —Preceding undated comment added 09:57, 20 September 2011 (UTC).

That applies only to engines with an intake manifold and not to engines with one carburetor venturi or fuel injector per cylinder. Cuddlyable3 (talk) 16:38, 20 September 2011 (UTC)

Airplanes

Hi. Who was responsible for the invention of airplanes?70.82.113.90 (talk) 21:54, 19 September 2011 (UTC)

Well, many people contributed, but the Wright Brothers are usually given the credit for developing the first one actually capable of flying. StuRat (talk) 21:57, 19 September 2011 (UTC)

The Wright Flyer III was the first practical airplane, but it's far from the first flying machine. Flyer I is recognized as the first powered, controlled, heavier than air human flight, but there are quite a few qualifiers in that "first," and there's a question of whether "invent" implies the idea, which the Wright Brothers didn't come up with, they just made it work. Santos-Dumont would contest their claim anyway, but the original question isn't specific enough to give a rock-solid answer. First flying machine gives a whole bunch of claims to consider. I'm guessing that John Stringfellow (first powered) or George Cayley (first controlled) are good candidates to the original question, though the Wrights get credit for getting "controlled" and "powered" on the same device, which makes the thing actually worth considering instead of just a curiosity. SDY (talk) 22:10, 19 September 2011 (UTC)

Flyer I was not, technically speaking, an airplane (unable to leave ground effect...) Whoop whoop pull up ^{Bitching Betty | Averted crashes} 22:22, 20 September 2011 (UTC)

But the Flyers 2 and 3 certainly were. (Which means that the Wrights still beat Santos-Dumont by at least two years.)67.169.177.176 (talk) 03:54, 21 September 2011 (UTC)

George Cayleyj's gliders were not fully controlled -- it was Otto Lilienthal who first figured out how to control a flying machine during turns, and it was not until the Wright Brothers came up with wing warping that an adequate control system came into existence. Likewise, Stringfellow's machine might have been powered but could not actually fly (insufficient power-to-weight ratio), which makes either the Wright Brothers, Karl Jatho or Gustave Whitehead the first to make a powered flight. (Note that Whitehead's machine, although adequately powered, was not fully controlled, and Jatho's had no controls at all). 67.169.177.176 (talk) 00:39, 20 September 2011 (UTC)

This is similar to the question recently about whether Edison invented the light bulb. He didn't literally invent the light bulb. But he did invent the first practical light bulb. And the Wrights developed the first practical airplane. ←Baseball Bugs ^{What's up, Doc?} carrots→ 22:21, 19 September 2011 (UTC)

Only if you learn about it from American sources. British schoolchildren learn that Joseph Swan invented the first practical lightbulb before Edison, although Edison got the American patents first and loadly claimed in marketing that he invented it first, and they agreed to share the credit and sell Ediswan lightbulbs. No doubt French children learn something else again. 86.164.76.231 (talk) 10:43, 20 September 2011 (UTC)

I know Russian children learn that the lightbulb was invented by Lodygin (whoever he was...) But then again, in Russia they even used to claim until quite recently that Mozhaisky was the one who invented the airplane, even though his steam-powered contraption could do no better than to hop off the ground after running down a ramp and immediately stall out upon leaving the ground. 67.169.177.176 (talk) 03:52, 21 September 2011 (UTC)

See, this is all why I prefer the James Burke perspective on technological and scientific change. We tend to think of history in terms of the Great Man theory, and scientific progress is no different: We assume that scientific and technological progress happens when a lone genius invents some novel idea out of whole cloth and changes the world. We are stuck on looking at "the man" as a scientific hero, and so we have those heroes: Newton, Einstein, Edison, Jobs & Wozniak, etc. ignoring the fact that scientific progress happens not in giant leaps, but in tiny incremental changes. People like Einstein and Edison and the Wright Brothers get the "credit" because they have good marketing more than anything else. What the so-called Great Inventors and Great Minds have in common is more publicity than anything. There are inventions and ideas which do cause major paradigm shifts; but these things are always the product of many years of incremental change, and the final "credit" goes to the guy who was there when it happened; it is quite likely that if the Wright Brothers didn't get there first, someone else would have within a few months or so, given the sheer number of people working in the field. --Jayron32 02:52, 21 September 2011 (UTC)

Make that three years or so. 67.169.177.176 (talk) 03:58, 21 September 2011 (UTC)

Meh. There were many people in the months both before after the very first Wright Flyer in 1903 who had similar success in "flying" a plane (Richard Pearse, Karl Jatho, Horatio Frederick Phillips, etc.) and none of them is any less impressive than the December, 1903 flights by the Wright Brothers. The Wrights simply had better publicity. What Alberto Santos-Dumont did had some good publicity as well, but his was yet another incremental design change over other designs. Had he not done what he did, any of a number of other engineers, through similar trial-and-error, would have arrived at a similar design as well. If the Wrights had never made their flight, we'd still have the same state of modern aviation we have today; the credit would have changed but the ultimate historical events would have occured anyways. Heck, pick anyone of the random other people I cited above; the Wright's credit would have gone to them instead, and we'd be exactly where we are today. --Jayron32 04:13, 21 September 2011 (UTC)

Pearse did not actually fly, but only made a couple of short, practically uncontrolled hops a few yards long, each of those hops ending with a crash into shrubbery. (Is that what you call "impressive"?) Jatho actually made a short flight before the Wrights did, but it was much shorter (200 ft. distance) than the Wrights' flight (852 ft. distance) and was likewise uncontrolled; he then gave up all attempts to further develop his contraption. And Phillips did not make his first flight until 1907, a half year after Santos-Dumont and almost three and a half years after the Wrights! 67.169.177.176 (talk) 04:26, 21 September 2011 (UTC)

Sometimes there are setbacks. Check out this early version of a Batman-like cape/parachute:[1] ←Baseball Bugs ^{What's up, Doc?} carrots→ 06:26, 21 September 2011 (UTC)

Wow, this gives a whole new meaning to the term "crash dummy"... (Not to disrespect his tragic death, but I mean, couldn't he stick to dropping dummies from the tower until he had at least a reasonable chance of survival?) 67.169.177.176 (talk) 06:35, 21 September 2011 (UTC)

Sir George Cayley invented the concept. --Swedmann (talk) 01:23, 23 September 2011 (UTC)

Right, the concept of fixed-wing (as opposed to flapping-wing or lighter-than-air) aircraft using airfoils to create lift. I never said otherwise. However, he did not realize the need for roll control (Lilienthal was the first to do so, and the Wrights were the first to actually achieve it), and of course he didn't put any powerplant on his gliders (for the simple reason that none was available). Which means that his machines were neither powered nor fully controlled, and therefore do not count as airplanes. 67.169.177.176 (talk) 05:59, 23 September 2011 (UTC)

Feces

What is the average buoyancy and density of a regular feces? 75.6.243.251 (talk) 23:51, 19 September 2011 (UTC)

What type of shit are we talking about here? Sir William Matthew Flinders Petrie | ^{Say Shalom!} 23:57, 19 September 2011 (UTC)

Type 3 on the link you gave me. 75.6.243.251 (talk) 00:18, 20 September 2011 (UTC)

The density is about that of water (which makes sense, as it is about 75% water). The average is somewhat greater than water (sorry, I don't have a number): most poop sinks. There has been great debate among the, uh, well, among certain segments of the population, about whether sinking or floating poop is indicative of good or bad health. Hardcore vegetarians and especially vegans have been known to claim that poop in a healthy person should float. Despite these claims, it is generally difficult to get floating poop through diet alone (and, needless to say, I don't think there are any real convincing studies that show that floating poop is indicative of health). The factors that can cause poop to be less dense are

1. Higher Fat Content, and
2. More integrated gas

While some variation in feces is normal, it is true that certain diseases can cause Malabsorption, which can lead to greasy, floating stool. Other chronic, abnormal bowel movements can be indicative of other problems - see the last link I have provided. If you are concerned about your poop, you should seek advice from a medical professional. [2], [3], [4] [5] Buddy431 (talk) 06:31, 20 September 2011 (UTC)

I thought you might be interested in this: Bristol_Stool_Scale --TammyMoet (talk) 08:13, 20 September 2011 (UTC)

That link was the first supplied. Dismas|^(talk) 08:27, 20 September 2011 (UTC)

Yeah, I already saw that, and don't need two links. 75.6.243.251 (talk) 23:52, 21 September 2011 (UTC)

Could you possibly find me the exact number for density and buoyancy? I'm not seeking medical advice; I read the series of notices at the top already. 75.6.243.251 (talk) 23:52, 21 September 2011 (UTC)