Wikipedia:Reference desk/Archives/Science/2011 January 5

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January 5

Jet engines

How do jet engines provide thrust? The mass of the hot air leaving the rear must be the same as the mass of the cold air entering the front, so its not throwing away mass apart from the small amount of fuel used. Thanks 92.28.251.68 (talk) 01:03, 5 January 2011 (UTC)

See Jet engine#General physical principles. It doesn't really have much to do with the mass of the air. Dismas|^(talk) 01:05, 5 January 2011 (UTC)

Simply, here is what happens. Large amounts of air are sucked into the engine. It is then compressed. At this point, fuel is injected. It bursts into flames, causing a small explosion. It is pressure, not mass, that drives the engine. --T H F S W (T · C · E) 02:40, 5 January 2011 (UTC)

I disagree with THFSW. The rate at which the mass of air leaves the engine is almost the same as the rate at which it enters the engine. However, the rate at which momentum leaves the engine is many times greater than the rate at which momentum enters the engine. This is because the exhaust gases leave through a propelling nozzle specially sized so the speed of the exhaust is many times greater than the speed at which air enters the engine. Newton's 2nd Law of Motion can be expressed as the resultant force on a body is equal to the rate of change of the body's momentum. In the case of a jet engine, the rate of change of momentum of the air passing through the engine is very great and is equal to the thrust on the engine. See Turbojet#Net thrust.

If the propelling nozzle was inappropriately sized so that the exhaust left the engine at the same speed as air entered the engine there would be no surplus of momentum and no thrust on the engine. The pressure of the exhaust gas as it exits the propelling nozzle is usually about atmospheric, whereas the pressure of the air as it enters the engine is usually a bit higher than atmospheric so the pressure difference across the engine contributes drag, not thrust. That is why I disagree with THFSW. Dolphin (t) 02:57, 5 January 2011 (UTC)

(edit conflict) Our Airbreathing jet engine article says: "An airbreathing jet engine (or ducted jet engine) is a jet engine that has an inlet duct that admits air for the combustion of fuel in the air stream which forms a jet of hot gases used for propulsion. So thrust is provided by what is in effect a directionally controlled explosion (which our article says "is a rapid increase in volume and release of energy in an extreme manner, usually with the generation of high temperatures and the release of gases"). WikiDao ☯ 03:23, 5 January 2011 (UTC)

Yes, but if we view the entire engine as a black-box system (as the OP clearly does), the net effect that relatively slow air is ingested at the front of the engine and exits it at a much higher speed. The difference in speeds, times the mass of the air that streams through, is the thrust. The nozzle design serves to ensure that the exhaust jet is about at ambient pressure when it leaves the engine, such that as much as possible of the combustion energy contributes to thrust. –Henning Makholm (talk) 03:57, 5 January 2011 (UTC)

I'm sorry, but by "ambient pressure" do you mean to say that there is no jet blast? WikiDao ☯ 04:30, 5 January 2011 (UTC)

Jet blast is caused by the air's velocity. I think Makholm is saying that as much of the pressure is converted into velocity as possible before it leaves the engine. (After all, it doesn't help the airplane if the air expands 'sideways', which it could do if it were at high presure, but not constrained.) APL (talk) 05:27, 5 January 2011 (UTC)

Well, what I asked myself as a kid, and sometimes still do, is the following: A jet engine is basically a pipe with two sets of fans, one forcing air in at the front, one in the back, being driven by the exhaust and used to power the front fan. In between the two is the combustion chamber, where fuel is injected and burned, so that the gas rapidly heats and expands, causing an increase in pressure that escapes to the rear, providing thrust. Why is the pressurized hot gas not escaping both ways, front and back, thereby of course cutting off its own air supply and fizzle? I suspect it has something to do with both the shape of the "pipe", and the relative size and configuration of the fans, but I never found a satisfactory solution. It's even worse with ram jets, of course. Is is simply that the overpressure from the air speed is larger than the pressure in the burning chamber, and hence the hot gas has to escape to the back? --Stephan Schulz (talk) 13:53, 5 January 2011 (UTC)

Why is the pressurized hot gas not escaping both ways, front and back...? Before takeoff the fans have to start rotating in the right direction. This can be ensured by designing the intake and exhaust fans to have different efficiencies in competing as windmills. As long as they keep rotating, the jet engine produces thrust. Thrust is lost in the case of an abnormal Compressor stall. Ramjets are no good for takeoff because they don't work at all at zero air speed. Cuddlyable3 (talk) 16:29, 5 January 2011 (UTC)

I think the answer to Stephan's question is in the turbojet article: "As the mixture burns its temperature increases dramatically, but the pressure actually decreases a few percent. ... Some pressure drop is required, as it is the reason why the expanding gases travel out the rear of the engine rather than out the front." So the front of the turbine sees a slightly lower pressure than the rear of the compressor. But the temperature is higher at the turbine, so the volume of gas per time unit is larger through the turbine than through the compressor. Therefore the turbine does not need to extract all of the energy from the gas in order to drive the compressor; there is some left for thrust. –Henning Makholm (talk) 17:51, 5 January 2011 (UTC)

Chemical formulæ

What are the rules to identify how to properly write the chemical formula? — Preceding unsigned comment added by Sci-math-tech (talk • contribs) 08:46, 5 January 2011 (UTC)

title added by CS Miller (talk) 08:56, 5 January 2011 (UTC)

Revised title so as not to reflect one rule exclusively. Cuddlyable3 (talk) 15:59, 5 January 2011 (UTC)

Basically, there must be the same number of individual atoms on each side of the equation. If need be, you must have several of each molecule. For example

2 ( NH₃ ) + 3 ( O₂ ) → N₂ + 3 ( H₂O )

Here 2 molecules of ammonia, each containing 1 atom of nitrogen and 3 of hydrogen, react with 3 molecules of oxygen (each 2 atoms of oxygen) to form 1 molecule of nitrogen (2 atoms) and 3 molecules of water (each 2 hydrogen and 1 oxygen). CS Miller (talk) 08:56, 5 January 2011 (UTC)

How to write the formula of a given chemical? Or how to write the formula of a chemical reaction? Perhaps chemical formula or stoichiometry will be helpful to you. 206.116.252.164 (talk) 09:54, 5 January 2011 (UTC)

Formulae for chemical reactions should be in terms of element atoms using their standard abbreviations, as shown in the article Periodic table. Cuddlyable3 (talk) 16:01, 5 January 2011 (UTC)

How did the Big Bang escape its Schwartzschild radius?

When all the mass of the universe was once concentrated in a very small space, why didn't it collapse into a black hole? 93.132.167.177 (talk) 12:02, 5 January 2011 (UTC)

The initial outward exapansion of the big bang must have overwhelmed gravitational collapse. -Plasmic Physics (talk) 13:04, 5 January 2011 (UTC)

Not really my field of expertise, but the nature of the Big Bang's singularity is that it's the point where the math -- specifically, the math of general relativity (and therefore gravity) -- breaks down. GR doesn't claim to explain the initial moments of the BB, and speculation is that the forces involved in the Planck epoch simply may not be the ones we recognize today. As such, it may be that gravity didn't exist (or function) then as we understand it now. — Lomn 14:30, 5 January 2011 (UTC)

To avoid singularities or effects near singularities, look at the time 1 second after the BB. The mass cannot have progressed more than 2 light seconds and that should be much smaller than the Schwartzschild radius. 93.132.167.177 (talk) 15:05, 5 January 2011 (UTC)

The term "Schwarzschild [sic!] radius" doesn't mean an awful lot in that context. Matter had a very high density 1 sec after the "Big Bang", but it filled the entire space homogeneously. Hence every particle was attracted equally in all directions, so effectively, it wasn't attracted at all. Expressed differently, there was no centre toward which the matter could have collapsed. Still, the effect of gravitation was there, of course, it slowed down the expansion. With sufficient matter (and no dark energy) it could have slowed down the expansion and the Universe could have recollapsed. As we know it hasn't done so yet, and as far as we know, it will not in the future. As a general rule I suggest never to start reasoning at the Big Bang — that's a singularity, or in any case a condition that we know little to nothing about, you cannot derive anything from that. Always start your reasoning at the present time, then go backwards. --Wrongfilter (talk) 15:42, 5 January 2011 (UTC)

(edit conflict) The point is that, in the time arbitrarily close to the big bang, our physics breaks down. Things like the mathematics used to explain black holes and event horizons and Schwartzchild radius and all that stuff is based on how the universe works today. We don't have effective tools to look at the big bang itself, so attempting to extrapolate a concept like Schwartzchild radius to the big bang doesn't hold up. --Jayron32 15:44, 5 January 2011 (UTC)

Because it was the inside not the outside of a black hole expanding? Cuddlyable3 (talk) 15:50, 5 January 2011 (UTC)

So then are we to assume that the laws of science we have today are invalid at some point arbitrarily close to the big bang? How long after the BB do we have to wait for the laws of science to become valid? Googlemeister (talk) 16:36, 5 January 2011 (UTC)

No bright line cut off, but physics from the end of the inflationary epoch, at about 10^-32 seconds after the Big Bang, and onwards is well understood. The details of cosmic inflation, from about 10^-36 seconds aBB to 10^-32 seconds aBB, are somewhat speculative, and to go back earlier than 10^-36 seconds aBB you need a Grand Unified Theory, for which we don't currently have an agreed model. Gandalf61 (talk) 16:49, 5 January 2011 (UTC)

(edit conflict) See Lomn's post above; we really don't know since General Relativity literally doesn't work under the conditions present during the very short time periods after the Big Bang. See also Timeline_of_the_Big_Bang#Planck_epoch and Planck epoch. There's lots of propositions, many of the conflicting, about what this time might have worked like. But we have nothing firm or reliable to work from. So, your statement, which seems a bit sarcastic, is actually spot on. Particularly relevent is the statement at Big_Bang#Timeline_of_the_Big_Bang, which says "How closely we can extrapolate towards the singularity is debated—certainly not earlier than the Planck epoch." There is some stuff we are pretty sure about; say everything from the inflationary epoch onward. Some stuff, like the Planck epoch, we're confidently clueless, and the stuff in between is progressively fuzzier the further we go back. --Jayron32 16:56, 5 January 2011 (UTC)

I agree with Wrongfilter and Cuddlyable3. There is no mystery here at all. As our Schwarzschild metric article says: "The Schwarzschild metric is a solution of Einstein's field equations in empty space, meaning that it is valid only outside the gravitating body". But at (or soon after) the Big Bang there was no "outside" - the whole universe was very homogenous and at approximately the same density, hence the assumptions behind the Schwarzschild metric and Schwarzschild radius do not hold. Gandalf61 (talk) 16:41, 5 January 2011 (UTC)

So what you are saying is that there could be more of a structure inside a black hole but the commonly believed singularity within which all the mass is concentrated? Is this just an ad hoc assumption or do you have any reference? 93.132.167.177 (talk) 18:22, 5 January 2011 (UTC)

Actually the answer is we don't know. The singularity is the simplest explanation, and works with current theories, but it is literally impossible to do any experiments, tests, or anything else to study it or confirm it. See Cosmic censorship hypothesis. --Jayron32 19:32, 5 January 2011 (UTC)

Another way to look at this is to think about a black hole. Nothing can escape because even light is sucked back in, and nothing can exceed the speed of light. But in the Big Bang nothing escaped - it is all still inside the "hole". And in the sense in which the universe does "expand", it can expand faster than the speed of light. This is an expansion of space, rather than a motion of everything through space. Wnt (talk) 19:37, 5 January 2011 (UTC)

I feel many of you got me wrong. I'm not speculating about singularities or extreme and unknown physics around them. As from the article, one second after BB is pretty a lot of time for physics to adjust to what we see now. The other point is that we (the universe) might be inside of a black hole but from what I have heard about black holes, everything inside is attracted to the central singularity (or the very proximity in case of unknown effects) in a finite (and, as I remember, small) amount of proper time. So either there is something substantially wrong with my understanding of Big Bang, and/or with my understanding of black holes, or I am missing a clue to conciliate what looks like a contradiction. As my original question is a quite simple and obvious one and I never heard that anyone asked that before I can only guess that the answer is so simple that no one who knows it bothers to tell. 93.132.167.177 (talk) 21:45, 5 January 2011 (UTC)

Where do people get the idea from that there might be a connection between the Big Bang and black holes? From what I know there is none. What is that even supposed to mean, "the universe might be inside of a black hole"? Mind, I don't blame you, this comes up in forums (from laypersons) again and again. --Wrongfilter (talk) 22:38, 5 January 2011 (UTC)

Well, that's easy enough: big mass concentrated in a small volume. But, ok, if you people here feel that everything outside your encyclopaedic universe is a crackpots thought ... well thank you all for your contributions so far but don't waste your time on that any more. 93.132.167.177 (talk) 23:24, 5 January 2011 (UTC)

There have been some pretty bad answers above. There is nothing mysterious about Big Bang plus one second. All the normal rules of physics apply. The problem is actually with the understanding of how black holes form. If you have an isolated mass that is more compact than a Schwarzschild radius then it will inevitably collapse into a black hole. However the "isolated" part is important. If you view gravity in terms of the "rubber sheet" analogy, then the isolated mass creates a depression so deep that nothing (not even light) has enough energy to climb out of. But what happens if you sprinkle mass uniformly across the sheet? Every part of the sheet is pressed down but no part forms a localized depression. Nothing has to expend energy climbing out of a hole, because there is nothing to identify a "hole" or an "outside", everything is the same. If you assume that the universe was uniformly filled with mass and has no boundary, then the Schwarzschild solution just doesn't apply. Predicting a collapse from the Schwarzschild radius relies on the ability to distinguish between a very massive region and an approximately massless region around it. In a homogeneous universe that contrast just doesn't exist. (The universe actually could still collapse into a singularity. This is the possible future that we refer to as the Big Crunch, but the mechanics of how this occurs are more nuanced than the simple Schwarzschild considerations.) Dragons flight (talk) 06:08, 6 January 2011 (UTC)

The rubber sheet model isn't a good model of GR, but yes, it's important to understand that the early expansion of the universe (including inflation!) is modeled by general relativity and not by some unknown pre-GR theory of gravity. There are theorems establishing certain conditions under which a black hole singularity must form, and the big bang doesn't satisfy those conditions. The conditions are more complicated than just a lot of mass concentrated in a small volume. A big bang universe that's above the cosmological "critical density" does satisfy the conditions for black hole collapse, and does (re)collapse (the big crunch).

There is a close connection between the big bang and black holes. The big bang resembles a time-reversed version of a (highly symmetric) black hole collapse. You can surround the whole visible universe by a Schwarzschild vacuum, putting everything "inside a black hole", without contradicting observations. -- BenRG (talk) 00:18, 8 January 2011 (UTC)

The point of the original question is that during the very early universe you can identify quite small volumes (even smaller than the size of the visible universe at that time) such that if taken in isolation they have sufficient mass to individually form black holes. Yes, the universe as a whole might be viewed as living inside a black hole, but that doesn't answer the original question of why primordial massive regions didn't collapse. The answer is that if the mass is homogeneous rather than isolated then the total implied curvature is insufficient to form an event horizon around those local massive regions. Dragons flight (talk) 14:43, 9 January 2011 (UTC)

The above discussion confuses me in one respect: it was my understanding that there was no matter up to and including the inflationary epoch (and a bit beyond) -- i.e., that the universe was "space" and only space. Did I mistakenly believe that I'd repeatedly read that? 63.17.67.25 (talk) 03:59, 8 January 2011 (UTC)

It's not what the what the inflationary hypothesis ordinarily says. During the inflationary period, space was certainly not empty. What you may have read is that the elementary particles of our current universe could not exist during inflation. But there would have been something else there instead -- weird and wonderful high-energy stuff that nobody can explain in detail given today's knowledge. The inflationary epoch ended when the weird stuff eventually "crystallized" into our current set of quarks, leptons and so forth. –Henning Makholm (talk) 04:14, 8 January 2011 (UTC)

I don't have all or even most of what I've read on hand. But here's one example (similar to others) from what I do have: "During this brief but critical period the universe was but a vacuum.... [T]he cosmic vacuum remain[ed] empty even after falling below the temperature at which particle production ordinarily would take place." -- "Coming of Age in the Milky Way" by T. Ferris, pp. 358-59. 63.17.67.25 (talk) 04:35, 8 January 2011 (UTC)

The universe during the inflationary epoch is often modeled as a "Lambda vacuum". This has the word "vacuum" in it, and it's perfectly fine to think of it as a vacuum, and many physicists do think of it as a vacuum; but that doesn't mean quite what you might think. The GR field equation (ignoring some uninteresting constants) looks like G + Λg = T, where G is the "Einstein tensor" describing spacetime curvature, Λ is the "cosmological constant", g is the "metric tensor" describing... well... spacetime curvature again, I suppose, and T is the "stress-energy tensor" describing everything else (the other forces and the fermions). Inflation is described by a model with T zero and Λ nonzero. T=0 makes it a vacuum, by definition. However, you could just as well define T' = T − Λg and write the equation as G = T', with T' nonzero. T' includes the stuff that was in T plus something else represented by −Λg, and the extra thing turns out to behave like a spin-0 particle (=field) with a nonzero value everywhere (like the Higgs field). In point of fact, you need a particle with some kind of time-varying behavior in order for inflation to eventually end (Λ can't really be a constant), so this view is more accurate that the cosmological-constant view. Inflation ends when the background field decays into a mess of weird and wonderful high-energy stuff, which eventually turns into the low-energy particles that we're made of. -- BenRG (talk) 05:57, 8 January 2011 (UTC)

63.17.67.25 - I think you are confusing "matter" and "mass". There was no matter before the inflationary epoch (in the sense that the elementary particles that make up what we currently think of as matter did not exist) but there was certainly plenty of mass (or, if you prefer, plenty of energy). Gandalf61 (talk) 06:49, 8 January 2011 (UTC)

grueblers equation

relation between number of pairs and number of links as given by the L=2P-4.is it also named as the grueblers equation ..? there is one equation for degrees of freedom by gruebler.. is this also ... please help me .. —Preceding unsigned comment added by 59.93.130.246 (talk) 14:23, 5 January 2011 (UTC)

Your question is not at all clear to me, but you might find the information at Linkage_(mechanical)#Theory useful. AndrewWTaylor (talk) 14:45, 5 January 2011 (UTC)

I am asking that besides popularly known(F=3(n-1)-2f) can we name (l=2p-4,applies to kinematic chains) also as gruebler's equation. very simply can we call L=2p-4 as gruebler's equation. —Preceding unsigned comment added by 59.93.130.246 (talk) 14:59, 5 January 2011 (UTC)

Your popularly known equation is Kutzbach-Gruebler's equation. Wikipedia doesn't have an article about Arnulf Grübler but his Curriculum Vitae is online with an e-mail address so you could try asking his opinion. Cuddlyable3 (talk) 15:46, 5 January 2011 (UTC)

Color of food and nutritional value

Is the color of food related to its nutritional value? A certain Gillian_McKeith claims it is, however, she is a mixture of crackpot theories and common sense advice, so I don't know if she's right on this point.Quest09 (talk) 15:23, 5 January 2011 (UTC)

Certainly not in the modern world. Many foods have colors added to them that don't effect the nutritional value. Even some fruits and veggies have been selectively bred so that they don't have the same colors as their wild counterparts! (Carrots weren't originally orange, for example.) APL (talk) 15:34, 5 January 2011 (UTC)

It depends a bit on what you mean by "color". There's a rough rule of thumb that "white foods" tend to have less nutritional value (that is, they have less vitamins and minerals and fiber per calorie) than more colored foods; so one can consider "white bread" to be less nutritional on that metric than whole-grain bread, and "white rice" as less nutritional than brown rice. Also, some nutrients are known for bright colors; lycopene is associated with tomatoes and other red foods, beta carotene with orange carrots, the so-called "green-leafy vegetables" like spinach and collards carry more nutritional value than the less strongly colored ones like iceberg lettuce. However, it only goes so far; white-meat chicken is lower in fat than say, more brightly colored steak. Using color as a metric to help remember which foods are more nutritious and healthier for you may be helpful, but it probably isn't the color itself which is the key factor. After all, if it were, wouldn't artificially colored foods be just as healthy as brightly colored vegetables? --Jayron32 15:39, 5 January 2011 (UTC)

I suppose that she meant raw food. Processed food can certainly have (almost?) any color you want.Quest09 (talk) 15:44, 5 January 2011 (UTC)

Well in the same vein as APL above, with modern biotechnology raw food probably can almost as well if you really want to. Nil Einne (talk) 16:10, 5 January 2011 (UTC)

Yes, you both are right on this. Food color can be manipulated. Salmon from fish-factories, for example, has its color due to what it was feed to. It's appealing salmon color is not that prevalent in nature. Quest09 (talk) 17:21, 5 January 2011 (UTC)

I'd never heard of the food-colour strategy in such a broad sense as that. I've only heard that rule of thumb applied to fruits, particularly berries. Our article on berries touches on this, but notes that the effect may not be that great. That being said, it's hard to argue with any rule of thumb that encourages eating a diversity of foods; that will almost always turn out to be more nutritious than eating a more limited diet. Matt Deres (talk) 16:06, 6 January 2011 (UTC)

Keep in mind that Gillian McKeith has a really poor understanding of science. She claims ... (quote) Chlorophyll is "high in oxygen". And the darker leaves on plants are good for you, she explains, because they contain "chlorophyll - the 'blood' of the plant - which will really oxygenate your blood." [[1]] -- JSBillings 20:12, 6 January 2011 (UTC)

Nutritional doesn't necessarly mean beneficial. Foods like ice cream, deep fried nacho-cheese flavored tortilla chips, and butter are all very nutrient-dense foods; the problem is the nutrients they are almost exclusively comprised of are fat and carbohydrates.

A six month old orange.

This is a bit of an odd question, but here goes. I keep dried fruit on my desk. Well, sometimes I don't eat an orange, and instead of rotting, it goes hard and shrivels. I have a three year old satsuma that is like a ping-pong ball now.

I had an orange. I let it dry for six months (the secret is to keep turning it, otherwise it rots). I colleague wanted to see what it was like, cut in half. So, I cut it in half, and to my surprise it was still juicy inside.

Here's the question. I was dared to drink the juice, and I did. It tasted a bit funky, but it didn't kill me. What has happened to the juice in the six months? Is it toxic? Is it safe to drink?

Thanks in advance,

Zzubnik (talk) 15:46, 5 January 2011 (UTC)

Just hazarding a guess based on the funky taste, but it might be possible that the juice inside the orange fermented. Note, though, that this is sheer speculation, and I'm not claiming that this is necessarily the correct answer to what happened. Ks0stm ^(T•C•G) 16:01, 5 January 2011 (UTC)

(edit conflict)If the juice inside was protected by the peel (which I suspect it would have been) so that nothing could get inside to ferment it, I suspect it wouldn't be all that bad for you. Without yeasts or bacteria to turn the juice into alcohol or vinegar, I don't see that much could happen. There may be internal reactions within the juice that slowly change it, but keeping out air completely is a common method of preservation. Well-sealed and properly handled bottles of wine can last decades and still be drinkable. They do change over time, but they remain perfectly fine. Wine that is exposed to air goes rotten to the point of inedibility in much shorter time periods. The trick is, keeping it so the peel remains intact; even microscopic holes in the peel could let in cultures which could spoil the orange. So, I would probably count yourself "lucky" it wasn't bad; you could run the exact same experiment again and get a very different result. --Jayron32 16:03, 5 January 2011 (UTC)

We seem to be thinking the same means with opposite ends...I was thinking the sealing of the peel would block oxygen from getting in, creating the anaerobic environment needed for fermentation...but given my lack of knowledge in the area, I'll defer to your answer. Ks0stm ^(T•C•G) 16:06, 5 January 2011 (UTC)

Well, if it is so sealed that oxygen cannot get in, then bacteria and yeast, many orders of larger than oxygen molecules, would ALSO not be able to get in. Furthermore, if there were bacteria present in an oxygen-poor environment, it would be REALLY bad, anaerobic bacteria are particularly nasty, bacteria that cause botulism are anaerobic. Fermentation is also an anaerobic process, see Fermentation (biochemistry), but it too usually requires cultures to be carefully controlled; you want the right yeasts and bacteria to be present, random stuff from the environment can be a hit-or-miss prospect when it comes to getting "good" fermentation versus a nasty mess. But in general, fermentation cannot go on without some little bacteria or yeast to do it. --Jayron32 16:12, 5 January 2011 (UTC)

But plants have an innate immune system. This (probably) stops the Clostridium botulinum from entering the orange whilst it is developing; after that, as long as the skin is intact, the bacterium can't enter it. CS Miller (talk) 18:46, 5 January 2011 (UTC)

That's kinda, exactly, my point. --Jayron32 19:25, 5 January 2011 (UTC)

Indeed it was. I should have read your earlier reply better before posting. CS Miller (talk) 23:51, 5 January 2011 (UTC)

An important factor here is that the oranges you buy in the store are invariably coated with wax. Unwaxed citrus fruit will dry out within a few days. The peel itself has strong antibiotic properties, unless it is punctured, and the wax helps even more. Looie496 (talk) 18:48, 5 January 2011 (UTC)

I don't think they are coated with wax, at least not here, in Detroit. I know the feel of wax on my hands, which I get when touching waxed fruit, and I don't get that feeling when peeling an orange. StuRat (talk) 19:27, 5 January 2011 (UTC)

Also, I'd expect the orange to be dessicated, so that the juice is rather concentrated. Did it taste like that ? StuRat (talk) 19:28, 5 January 2011 (UTC)

Hi guys, and thanks for all the replies! To answer the question, it did taste concentrated, but it also had a different taste that I didn't recognize. I presume this is just what happens to orange juice over time if it doesn't spill. Like with wine, the flavour changes over time. Zzubnik (talk) 08:43, 6 January 2011 (UTC)

Interesting. And I suddenly feel much better after having just eaten a two week old Clementine! 10draftsdeep (talk) 14:41, 6 January 2011 (UTC)

Dog Knot

What is the puprose of the knot at the base of a dogs erect penis?

See here. --Sean 17:47, 5 January 2011 (UTC)

I think you mean see here. Cuddlyable3 (talk) 22:01, 5 January 2011 (UTC)

Nope. --Sean 22:34, 5 January 2011 (UTC)

Dark side

Does Uranus ahave a dark side? — Preceding unsigned comment added by BenYorkie (talk • contribs) 17:17, 5 January 2011 (UTC)

Yes, the one where the sun don't shine. See night. --Sean 17:44, 5 January 2011 (UTC)

No. All parts of Uranus receive direct sunlight at some point, so there is no permanent "dark side". However, due to Uranus's unusual axial tilt and 84-year orbit, some parts of the planet can spend extended periods in the dark. See Uranus#Orbit and rotation, where it states: "Each pole gets around 42 years of continuous sunlight, followed by 42 years of darkness". Karenjc 18:03, 5 January 2011 (UTC)

It's exactly analogous to the Midnight sun in the polar regions on Earth. The only difference is that Uranus has much larger polar regions (due to the extreme tilt), comprising nearly the entire planet, and that the Uranian year is much longer than a terrestrial year. Buddy431 (talk) 01:45, 6 January 2011 (UTC)

Okay, we've had "yes" and "no"; I'll add that the sunlight at that distance is so weak — under 1/300 of the amount we get here — that you could say it's all dark. --Anonymous, 19:01 UTC, January 5, 2010.

Eh, that'd be plenty of light for reading (it's substantially more than a full moon on Earth), so I don't think "all dark" holds up. — Lomn 19:34, 5 January 2011 (UTC)

Depends on how many clouds you have above you, since as a gas giant there really isn't a surface you can stand on, thus the point chosen is pretty arbitrary. Googlemeister (talk) 19:36, 5 January 2011 (UTC)

Wine Grape variety abreviations

Is there a universal Vitis vinifera grape variety abreviation list? —Preceding unsigned comment added by 200.6.100.224 (talk) 19:13, 5 January 2011 (UTC)

See List of grape varieties. --Jayron32 19:23, 5 January 2011 (UTC)

The OP is asking for a list of abbreviations - which are not in our article. Some of them are in this list, but it is not exhaustive. SpinningSpark 02:50, 6 January 2011 (UTC)

Soap making

I have been making my own bar soap .what chemicals can i add to the soap to improve the quality ? —Preceding unsigned comment added by 41.221.209.6 (talk) 19:57, 5 January 2011 (UTC)

Soap#Traditional_soap_making has an overview of homebrew soap making, as well as some ingredients and methods to alter the finished product. --Jayron32 20:00, 5 January 2011 (UTC)

I would like to recommend you look at glycerine soap which can be made by unskilled children without toxic or corrosive chemicals. But you should really ask Lisa Chouinard here because she does more fantastic stuff with soap than anyone I know. 71.198.176.22 (talk) 16:55, 8 January 2011 (UTC)

Miracle Mineral Supplement

Where can I get this? --98.221.179.18 (talk) 21:13, 5 January 2011 (UTC)

You don't want to. Seriously. Read the article. You have no use for it. --Jayron32 21:17, 5 January 2011 (UTC)

The claims made on the "official website" clearly fall within the realm of jurisdiction of the FDA - and in fact, has drawn legal and regulatory attention. The product is being illegally marketed as a drug without passing regulatory and safety requirements, according to the F.D.A. - so not only this substance highly toxic and hazardous, it is also against the law to sell or purchase it for medical use in the United States. Nimur (talk) 21:30, 5 January 2011 (UTC)

It's a total fraud, i did a lot of investigating into this because a close relative of mine got their hands on some about a year ago. This is a site which has some good information about this fraud.Vespine (talk) 21:48, 5 January 2011 (UTC)

(edit conflict)I was looking forward to getting some chlorine dioxide and/or sodium chlorite (for chemistry experiments). At least to get some pictures for Wikipedia. Of course I wouldn't drink it any more than I would drink bleach! --98.221.179.18 (talk) 21:49, 5 January 2011 (UTC)

(ec)(ec) The book by Miracle Mineral Solution's promoter Jim Humble states "I have provided complete details on how to make the solution in your kitchen, and to buy most of the ingredients off the shelf". Sale of the MMS concoction for any medical purpose whatever is judged illegal and dangerous. Its ingredient Sodium chlorite is a toxic oxidant of which 1 gram causes sickness and 10-15 grams death. The only positive medical advice we can give is that it is healthy to take reasonable doses of the compound dihydrogen monoxide in pure form. Cuddlyable3 (talk) 21:51, 5 January 2011 (UTC)

The book's author also links to this excellent example of a Medicare scam - a phony "cancer screening test kit." "While we do not work directly with private insurers, HMOs, or PPO plans, we can provide you with a receipt which you may be able to use to get reimbursement from your provider." Sadly, they probably have been successfully billing the Government for numerous sham tests. Let's hope the Feds swoop in on the whole operation quickly. Nimur (talk) 21:53, 5 January 2011 (UTC)

If you are after sodium chlorite fir experiments, just get some water purification tablets from a camping store. Vespine (talk) 22:08, 5 January 2011 (UTC)

Searching "Miracle Mineral Solution" in Google [2] will net many places willing to ship the solution to you for a hefty price. Some of the places even appear to have brick and mortar stores as well. As a random example, "Moon's Light Magic" appears willing to ship you the stuff, and also appears to have a physical store in Roselle, Illinois. If you could be more specific in where you are, we could help you find a store that's closer. Buddy431 (talk) 23:40, 5 January 2011 (UTC)

Under 3 dollars, of course, or I wouldn't want it. That would probably rule out everything. --Chemicalinterest (talk) 23:44, 5 January 2011 (UTC)

I think Buddy431's reply is inappropriate for the science ref desk! Regardless of what the op claims he wants it for, this IS a question of medical claims and dangerously false ones at that. I strongly disagree with promoting or even linking to sites which sell this snake oil. Vespine (talk) 00:27, 6 January 2011 (UTC)

Why do you have a prejudice against a sodium chlorite solution? --Chemicalinterest (talk) 11:42, 6 January 2011 (UTC)

It's in deciding what is and isn't appropriate treatment that we are dispensing medical advice. Providing a link to a specific treatment that is asked for, whatever we think about the appropriateness of it, is not medical advice. Buddy431 (talk) 01:42, 6 January 2011 (UTC)

I've looked too deeply into this fraud to agree, spend a few minutes googling Jim Humble and if you are not flabbergasted by the blatant fraud and disinformation this guy spreads I'll be very surprised. Have a look at the google site i linked above. If you had said: "you can get homeopathy from here" you could argue that you're just giving information and leaving the choice to the individual, we could have an argument about scientific proof and what harm it can do and whatever, but MMS is a FRAUD, full stop, and it has been proven to be dangerous and is now illegal in several countries. It's definitely not just harmless woo woo. What you are doing is not just "giving information"; by linking sites involved in this scam you are in effect promoting for them, whether you do it inadvertently or not is beside the point IMHO. I still believe it's still inappropriate for the science ref desk. Vespine (talk) 02:05, 6 January 2011 (UTC)

The OP clearly says he wants it for chemical experiments, not as medicine. If he's lying, and takes the "supplement" as medicine despite all the warnings here, then he's an idiot and deserves whatever consequences result. --140.180.26.37 (talk) 07:48, 6 January 2011 (UTC)

Please note that User:Chemicalinterest is the same as User:98.221.179.18 (I memorized my IP). The only reason I am interested in this product is because it contains sodium chlorite; if I can find out how to make sodium chlorite from sodium chlorate without the explosive chlorine dioxide intermediate, then I will not ask for this product. --Chemicalinterest (talk) 11:43, 6 January 2011 (UTC)

Formula pronunciation

This formula appears in our proton-proton chain reaction article:

1 1H

+

1 1H

→

2 1D

+

e+

+

ν e

+

0.42 MeV

Can someone tell me how that would be pronounced in English? Thanks. --Sean 22:33, 5 January 2011 (UTC)

One hydrogen atom fuses with another hydrogen atom, resulting in an atom of deuterium, a positron, an electron neutrino, and some extra energy. I mean, that's one way to do it. You could get more or less technical about it if you wanted to, e.g. describing the hydrogen as "hydrogen-one" and making clear how much energy there is (.42 million electron volts). You don't have to call deuterium anything other than deuterium (there is only one kind, by definition—I don't know why the formula links to "Deuterium-2", because deuterium has, by definition, a mass of 2). --Mr.98 (talk) 22:49, 5 January 2011 (UTC)

I think it is a priori ambiguous whether the notation ¹
₁H
stands for neutral atoms or just the nuclei. Your suggestion picks the wrong interpretation (because then charge is not conserved across the reaction). –Henning Makholm (talk) 22:56, 5 January 2011 (UTC)

OK, that makes sense. I think specifying it as a bare proton probably is indeed most straightforward. --Mr.98 (talk) 23:24, 5 January 2011 (UTC)

(ec) "A proton and a proton become a deuterium nucleus, plus a positron, plus an electron neutrino, plus zero point four two megaelectronvolts"? –Henning Makholm (talk) 22:51, 5 January 2011 (UTC)

Going fully wordy: Hydrogen having 1 proton and one nucleon combines with hydrogen having 1 proton and one nucleon resulting in hydrogen having 1 proton and 2 nucleons (aka deuterium) plus a positively charged electron (aka positron) plus a neutrino of the electron type (aka electron neutrino) plus point four two Mega electron volts of energy. A lot of the more common particles have shorthand names, and ²
₁H
and ²
₁D
are the same thing - so you can write and say it either way. Ariel. (talk) 23:22, 5 January 2011 (UTC)

But that's not how you would actually pronounce the notation, is it? –Henning Makholm (talk) 00:20, 6 January 2011 (UTC)

Not for this, but by listing all the parameters it can help the OP generalize to other chains. Ariel. (talk) 00:53, 6 January 2011 (UTC)

I would be cautious about a potential ambiguity in that phrasing — the statement "hydrogen having 1 proton and one nucleon" can be readily and incorrectly misread as meaning that hydrogen nuclei contain one proton and one (other) nucleon. It might be clearer to go with something like "hydrogen, having 1 nucleon (a proton)..." or something in that vein. TenOfAllTrades(talk) 16:41, 6 January 2011 (UTC)

I'm not a physicist, so this is not an answer, but I think what the OP wants is a literal reading, as you would say when reading off the equation. Something like "one-hydrogen-one plus one-hydrogen-one yields one-deuterium-two plus electron-positive plus electron-neutrino plus zero-point-four-two em ee vee". But that's almost surely wrong - for example, I'm not sure exactly what order a physicist should read off the numbers or whether anyone would really say "electron-positive" rather than "positron" even when writing it on a blackboard. Wnt (talk) 18:51, 6 January 2011 (UTC)

Thanks to all. I was interested in both the minimal-ambiguity and colloquial readings, and they've been covered well, I think. --Sean 21:54, 6 January 2011 (UTC)

Plant food

I boil water on the stove as a method of humidifying my home. After a while of adding water and having it boil down, I end up with mineral-rich water that I need to dump to prevent it from depositing minerals on the pot. I was wondering, if, after I let it cool, of course, this would make good plant food. That is, would the mix of minerals in tap water be appropriate for your typical house plant ? StuRat (talk) 23:31, 5 January 2011 (UTC)

It's mostly calcium and magnesium (hard water), and they are listed as secondary macronutrients for plants. This and this don't mention any upper limit for them, but I didn't check any other sources. Ariel. (talk) 23:41, 5 January 2011 (UTC)

Edit: If you have a water softener then it's actually sodium and would be pretty bad for plants. Ariel. (talk) 00:04, 6 January 2011 (UTC)

Tap water composition depends a great deal on where you live. If your water company or municipality provides information about the tap water quality and composition, then you have a pretty good idea about what's left in your pot after boiling the water down. It is unlikely there will be a lot of potassium, phosphorus, and nitrogen, which is what plants need most; more likely you'll find a lot of calcium, magnesium, sodium, iron, chloride, and (bi)carbonate. Calcium carbonate is used to make soil more alkaline, but many houseplants actually prefer acidic soil. Sodium chloride is table salt, you definitely don't want that in your plants (see Soil salinity and salting the earth). --Dr Dima (talk) 00:11, 6 January 2011 (UTC)

You know, I don't think I've ever really thought about this before, but wouldn't it take kind of a lot of salt to make a large plot of land infertile? And salt used to be fairly expensive. The salting the earth article doesn't seem to provide any account of whether the method was actually effective, as opposed to symbolic. --Trovatore (talk) 06:06, 6 January 2011 (UTC)

I've long wondered if there was a chance that such stories didn't refer to NaCl but something else like copper sulfate that might have been available to the ancients from mining, or borax from desert evaporite deposits. But I've never seen the possibility mentioned let alone evaluated skeptically. Wnt (talk) 10:53, 6 January 2011 (UTC)

Salinity tolerance varies greatly among plants. For some orchids, a regular (not boiled-down) tap-water is often too salty. At the other extreme, there are quite a few plants that can tolerate ocean-water salinity level. Also, it matters a lot how you water the plants. If you remove the excess water from the saucer under the flowerpot every time, some of the salt is washed out. If, on the other hand, you let the plant "drink" all of its water every time, and never empty the saucer under the flowerpot, then the salt accumulates in the soil, and may kill the plant eventually. There are tables on the internet (e.g. here and here that quantify the salt tolerance of various plants. The tables use either units of concentration (ppm) or units of specific conductance (dS/m). A rule of thumb -- not exact! -- is 1 dS/m = 600 ppm. This roughly equals 0.6 gram of table salt per one liter water. In other words, if a quarter-gallon flowerpot has 6 grams of salt in it, you will have soil salinity of about 10 dS/m, which would kill some plants and stunt the growth of most others. --Dr Dima (talk) 21:01, 6 January 2011 (UTC)