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January 23

air compressor

I want to know that what is two-stage air copressor? and how i done it's intercooling and thermal analysis? —Preceding unsigned comment added by Munish malik (talk • contribs) 00:24, 23 January 2009 (UTC)[reply]

See our article titled Gas compressor, expecially the section with the name Staged compression. Cheers! --Jayron32.talk.contribs 02:42, 23 January 2009 (UTC)[reply]

how do explosions destroy things

will ... i looked about the meaning of explosions , and it was a rapid chang in volume , but i need to understand whats the mechanisem of destruction effect ... how did it destroy objects within the range . —Preceding unsigned comment added by Mjaafreh2008 (talk • contribs) 01:28, 23 January 2009 (UTC)[reply]

Shock wave? Also, even a simple "rapid change in volume" means there's a lot of force pushing out in all directions...push against "something" hard enough and it will break. DMacks (talk) 01:48, 23 January 2009 (UTC)[reply]

Overpressure says that (for example) C4 explosive produces 10 psi (pounds per square inch) of "overpressure" (although it doesn't say how much C4 or in how much volume?!?) - but let's run with that number. Suppose you have a brick wall that's 20' long by 10' high. That's around 30,000 square inches - and at 10psi of pressure, the explosion would exert a third of a million pounds of pressure onto the wall! It's really not hard to imagine why that would destroy the wall! SteveBaker (talk) 03:03, 23 January 2009 (UTC)[reply]

Another way to look at it: The forces exerted by the explosion are not applied equally for all parts of the exposed object – typically having the greatest effect on the near exposed surface of the object. That causes internal stresses (compression, tensile, sheer, or torque) within the object which may exceed its strength. The result is that the object distorts or breaks up. Those same forces may then accelerate different parts to different velocities resulting in a scattering of the remains.

In contrast, if the forces were applied equally to the entire object, down to the atomic level inside and out, the object would remain intact because there would be no internal stresses. Theoretically, the object could survive near-infinite acceleration in such cases. Examples would include gravitational and possibly electrical and magnetically induced acceleration. For example, the earth's gravity exerts an enormous amount of force on the ISS, but the station remains intact because the force is applied uniformly. If an equal force were applied using a thruster pack connected to one module of the station, the resulting stresses would likely cause structural failure. -- Tcncv (talk) 05:15, 23 January 2009 (UTC)[reply]

And note that if it is easier for the object in question to maintain its structure and to just move, it'll do that rather than be destroyed, even if the explosive force is huge. (See, for example, Project Orion (nuclear propulsion)) --98.217.14.211 (talk) 11:18, 23 January 2009 (UTC)[reply]

Viewing memories from a cat's brain?

Did I dream this? I remember reading, a few years ago, an article about a scientific study that was able to somehow find images from the memory of the cat, somehow encoded into its brain cells. It was accompanied by a few blurry pictures of a man that were said to be from the cat's brain. Knowing what I know now about how memory works, this seems somewhat unlikely, but I'd like to find the article again. Does anyone know about it? Was it a hoax? 99.245.92.47 (talk) 01:52, 23 January 2009 (UTC)[reply]

This is the closest I could find on a quick search. I'm pretty sure it's not the article you were after, but will (purr-haps) jog your memory? Cycle~ (talk) 02:36, 23 January 2009 (UTC)[reply]

That's by measuring the neural activity in realtime - that can only work at the instant the cat sees the image. There is no way we know enough to extract a memory of an image - that would require decoding the entire storage mechanism - and we are a tremendous way away from being able to do that. It was either a hoax or you are remembering the research on seeing the images in realtime. SteveBaker (talk) 02:55, 23 January 2009 (UTC)[reply]

I would not be surprised if something like this were possible far in the future, but it seems beyond the capabilities of today's technology. I note that there was a recent report of a brain scan technique which could brain imaging to accurately detect which letter of the alphabet a person was looking at, and iirc they hypothesized that the technique might also allow determining which letter the subject was thinking of. Edison (talk) 05:25, 23 January 2009 (UTC)[reply]

Yes - but that's still "What you are thinking about right now" - and that's a VERY different matter than "Extracting images from memories". It's pretty clear that our memories of scenes from the past do not consist of a bunch of pixels stored away like a photo in your computer. Instead we have a bunch of associations. We have a picture on the wall at home that I "remember" - there is a river with some canal boats and a pub in the background. I can "picture" that painting clearly in my mind - but I can't tell you the colors of the boats or the number of windows on the front of the pub. That's because I'm only remembering the concept of "classic village pub", "river" and "canal boats" - I don't have anywhere in my brain the actual image stored. Hence, in order to reconstruct that image, the computer would have to be able to extract from my memory not just the memory of the painting - but also my general concept of what a village pub typically looks like. That would require decoding more or less the whole of the brain function - and THEN being able to reconstruct the pathways by which we remember things. Worse still - if the person remembering that photo was an expert on 18th century architecture - he wouldn't be remembering a "village pub" - he'd be remembering a particular style of architecture and construction. An expert on canal boats would be remember a particular boat manufacturer and a specific model number they produced. A horticulturalist might remember the species of tree off to the left of the pub. Someone who's been there will also remember the flavor of the beer they serve there and details of the pub interior that aren't even IN the painting. No two people will remember that picture in the same way. We know that memories are stored in a diffuse 'holistic' manner so nothing short of scanning the connections between every neuron in the brain would enable us to do this. So this is AMAZINGLY beyond our current understanding - let alone the technology to measure what we'd need. Worse still - the number of neural connections in even a cat's brain are vastly too great to store in the available RAM + disk space of even a pretty large super-computer. So even if we knew what to do - we don't have the storage space to process it. It's going to be 30 to 40 years of "Moores law" improvements before we have a computer that has the power to do this - I'm betting a hundred years before we know enough about brain organisation to do it. This is science-fiction technology. So our OP is DEFINITELY mistaken. The only question is how/why. SteveBaker (talk) 14:21, 23 January 2009 (UTC)[reply]

Here's the story Edison's referring to — Matt Eason ^{(Talk  Contribs)} 01:35, 25 January 2009 (UTC)[reply]

It was more recent than 1999, and definitely talked about memories, but thanks. 99.245.92.47 (talk) 05:51, 23 January 2009 (UTC)[reply]

Your are either mistaken - or you've been hoaxed. It's not remotely possible with todays technology. SteveBaker (talk) 14:21, 23 January 2009 (UTC)[reply]

They did a study on cats dreaming where they removed the block that keeps cats (and us) from moving their muscles in response to the dreamed reality. They found cats dream about hunting and stalking. But that's not memories either, they might have mentioned those in the report, though. 76.97.245.5 (talk) 09:18, 23 January 2009 (UTC)[reply]

Hmmm - if they'd used dogs they could have saved themselves a lot of trouble! I've owned many dogs over the years and you can EASILY tell that this is what they are dreaming about because that suppression of muscle action during REM sleep isn't as complete as it is with humans (and perhaps cats). Dogs sleep a lot - so you have plenty of chance to watch! When you see their eyes start to move under their eyelids - you know they are entering REM sleep. Then within seconds you can see the tips of their paws twitching as they walk in their dreams - you see the feet first twitch in a 1-2-3-4 pattern like they are walking - then you see them start to twitch in pairs like they are running - you can see their bodies heaving like they are breathing fast (but they really aren't) - then you will see the jaws moving and the cheeks puffing slightly in and out and you may even hear very soft barking (more like little 'yip' sounds). On some occasions, I've seen the dog's running pattern stop and chewing actions happen in the jaw. It's totally compelling - my dog dreams of running and chasing things - sometimes of catching and eating them. No question about it. Cats on the other hand...who knows? SteveBaker (talk) 14:18, 23 January 2009 (UTC)[reply]

Yeah, cats do that too. I don't know why they felt the need to operate on some cats' brains to see this when I've had countless cats go through the same thing curled up on my lap. They run, they creep, they pounce, they bat prey, they eat it. All in twitching paws and jaw, sometimes with little smackings of the mouth. 79.66.105.133 (talk) 12:53, 25 January 2009 (UTC)[reply]

Do female athletes and sportswomen, especially in 'masculine' sports, have more testosterone than average women?

I would expect so but is there any good scientific work on the subject? Just seeing today's featured article made me think about it.--Ib.nib op.cit. (talk) 06:31, 23 January 2009 (UTC)[reply]

Because they say (citation needed) that oestrogen is produced by fat, and that testosterno is produced by muscle. Is this true?--Ib.nib op.cit. (talk) 06:34, 23 January 2009 (UTC)[reply]

Here is a nice review of the scientific literature: Athletes’ Testosterone Levels by Sports Team: An Exploratory Analysis. Note also that during the Cold War, East Germany decided to administer about 60 times the natural level of testosterone per day to their female athletes (under the direction of Manfred Ewald). The result was striking. Although a relatively small country, the GDR excelled in the Olympic medal tables during the late 70s and 80s. Their female athletes and swimmers did particularly well. Rockpocket 07:09, 23 January 2009 (UTC)[reply]

It's important to distinguish between the questions, "do women who have more testosterone tend to end up in 'masculine' sports?", and "do women in masculine sports tend to end up with more testosterone?". --Sean 12:46, 23 January 2009 (UTC)[reply]

One of the problems that the sports officials have had is determining whether some of their "female" competitors are truly female (See: Santhi Soundarajan)- to that end they have defined limits to the amount of testosterone that they consider normally "female" and a few women have been outraged to discover that they are excluded from female sports because the officials consider them to be "men" by virtue of their body chemistry [1]. So I think the answer is likely to be a cautious "yes". Cautious because of the definition of a "masculine" sport...weight lifting? shot putt?...yeah - sure. Is rifle shooting "masculine"?...if so then I need to change my answer and say "no". SteveBaker (talk) 13:48, 23 January 2009 (UTC)[reply]

In the Physical exercise article you see that one of the consequences of exercise is increased testosterone production. So yes, female athletes will have more testosterone than other females, but male athletes also have more testosterone than other males due to the amount of exercise they do. Anythingapplied (talk) 15:00, 23 January 2009 (UTC)[reply]

Neanderthal cranial size

What is the current consensus concerning the cranial capacity of H. neanderthalensis? Was it smaller, about the same, or larger than H. s. sapiens? Viriditas (talk) 09:27, 23 January 2009 (UTC)[reply]

We have an article: Cranial capacity which contains a table of cranial capacities:

Orangutans: 275–500 cc
Chimpanzees: 275–500 cc
Gorillas: 340–752 cc
Humans: 1100–1700 cc
Neanderthals: 1200–1700 cc

So more or less the same. Of course we have FAR more human skulls to measure - so we know where the outliers are in the data set (ie we've seen incredibly small and incredibly large skulls) - but the relatively limited number of Neanderthal skulls that we've found could easily mean that we simply have not yet found any of the larger or smaller-brained individuals yet. That suggests to me that the 1200 to 1700 range is probably narrower than it really was. That's why I'd tend to say "larger".

We should be careful about what we take away from those numbers though. Larger skull capacity doesn't necessarily equate to greater intelligence. Neanderthals also had much larger nasal cavities than modern humans - so they may have had a very acute sense of smell - that being the case, they may have devoted more of their brain to processing, analysing and remembering olfactory sensations. So even with a larger brain than us - they may not have been as smart. Also, while Neanderthals were typically shorter than modern humans - they would have been taller than the humans that were when Neanderthals were common - and their body's were generally chunkier. (Our Neanderthal article somewhat scathingly compares them to modern Americans & Canadians!) But our Brain to body mass ratio and Encephalization articles point out that a rough measure of intelligence can be obtained by comparing the ratio of body mass to brain size between species. So even with slightly bigger brains, if their bodies were significantly larger than the humans of their era then perhaps we have to assume that they were not quite so smart. (Did I just imply that Americans and Canadians are stupid?)

But whatever the difference - it would probably be fairly small and there would certainly have been some Neanderthals who were smarter than some Humans - and vice-versa.

SteveBaker (talk) 13:39, 23 January 2009 (UTC)[reply]

That last bit is an easy claim to make. Clarityfiend (talk) 00:13, 24 January 2009 (UTC)[reply]

Neanderthals (who should probably be considered to be human, but a different subspecies from modern humans) survived under harsh conditions for hundreds of thousands of years. Their tenure might even surpass that of mmodern humans, when all results are in. Not really so stupid as some modern humans like to think. Much of the criticism of Neanderthals sounds like early 20th century "racial superiority" screed of eugenicists about the supposed superiority of some groups of humans over others. Edison (talk) 00:00, 24 January 2009 (UTC)[reply]

The problem is - if they were smarter and stronger - how come they were out-performed by the cromagnons? It's not a matter of 'racism' - it's a matter of scientific enquiry. In the changing environment of that era - they must have had some flaw (compared to modern humans) - it's just a question of what that flaw was. SteveBaker (talk) 00:17, 24 January 2009 (UTC)[reply]

It's possible that their flaw was just a lack of luck, of course. Algebraist 00:20, 24 January 2009 (UTC)[reply]

One theory suggests that the cro-magnons (modern humans) may have out competed the Neanderthals because the environment changed in Paleolithic Europe from dense forests to wide grasslands due to the change in climate.[2] The modern humans were pre adapted to a grassland environment because they migrated to Europe from Africa. The Neanderthals hunting strategies, tools and physical characteristics were better suited to a dense forest habitat. The Neanderthals were ambush hunters while as modern humans were better suited for perusing game across long distances which may have given modern humans a tremendous advantage over the Neanderthals (moderen human's physical build made them better runners[3] and their weapons and hunting stratiges were better adapted to a grassland enviorment). Neanderthals presumably used heavy spears to stab prey while as modern humans used lighter spears to throw at animals from a distance which also was more useful in a grass land environment. Also because of their large stocky builds Neanderthals may have required more calories than modern humans (Neanderthals may have required as much as 5,000 calories a day while as modern humans only require 2,000 or 3,000 calories a day)[4] and because their hunting methods became inefficient with the changed landscape they probably did not get the energy they needed and starved to death.--Apollonius 1236 (talk) 13:39, 24 January 2009 (UTC)[reply]

Thanks. Can anyone recommend the most recent articles or research on the subject? I added this statement to the Neanderthal article: "Neanderthal cranial capacity is thought to have been as large or larger than modern humans, indicating that their brain size may have been the same or greater." Is this accurate? Based on what I've read, I think it is. Recently, User:Tibbets74 added this statement: "however, a 1993 analysis of 118 hominid crania concluded that the cranial capacity of H.s. neandertal averaged 1412cc while that of fossil modern H.s. sapiens averaged 1487cc."[5] I temporarily moved it to a footnote because referring to a study from 1993 raised a red flag for me. Obviously, there's been new research since then and I found it strange to refer to a single paper (from 16 years ago) in the lead section. Any ideas? Or is this discussion best suited for the article talk page? Viriditas (talk) 04:02, 24 January 2009 (UTC)[reply]

OK - we've switched from a RefDesk question to a question about what Wikipedia should or should not state...those are not the same thing! No - a 1993 peer-reviewed paper trumps what we said here. The article should contain Tibbet74's comment and not yours because you have no references. Measurement of skull volumes is not likely to have changed over 16 years so his/her reference is an excellent one despite it's age. (The RefDesk uses references - it doesn't create them!). But (a) a sample of only 118 skulls is not many and (b) it's NOT true to say that brain size matches cranial capacity and (c) a difference of 75cc in a skull of around 1400cc is likely to be close to the limits of experimental error and (d) the range of sizes in one species is something like 1200 to 1700 - so while a 75cc increase might indicate an average brain size - it tells you nothing about one individual Neanderthal versus one individual human. However, a fact is a fact - and Tibbet74's fact is a good one. If you need to dispute it then you need references of your own...and this debate belongs on the Talk: page of the article...not here. SteveBaker (talk) 14:52, 24 January 2009 (UTC)[reply]

Thanks, but I think you misunderstood (and assumed too much from) what I wrote. I know how the RefDesk works, and my edit to the Wikipedia article was made 23 minutes before I asked my question, not after. I never said I didn't have references (I do and I still need to add them) and Tibbet's material is still in the article as a note. What I did request was a) a summary of the current consensus on the matter, and b) any recent, important articles on the subject. As for facts, depending on the field or subject, they tend to change over time, as I'm sure you are aware, so citing a paper from 1993 when there are more recent papers on the subject is a red flag. Also, the lead section isn't the best place to introduce new information, which is why I moved it to a note and made a minor change to what was previously in the lead. Viriditas (talk) 08:33, 25 January 2009 (UTC)[reply]

Brain size does not nessecarily correspond to intelligence. ~A H 1^(TCU) 18:08, 25 January 2009 (UTC)[reply]

spiral bevel gears

Can the teeth of spiral bevl gears can be forged directly without any machining? —Preceding unsigned comment added by 203.90.64.100 (talk) 10:57, 23 January 2009 (UTC)[reply]

First, what size? Microscopic gears can be produced by building them up layer by layer or laser cutting. Small ceramic gears can be baked from powdered metal. As for larger metal gears the question would come to: Would that be feasible and what properties would you gain in the process? AFAIK machining is a cost efficient way to produce the desired precision of pitch and regularity. 76.97.245.5 (talk) 16:06, 23 January 2009 (UTC)[reply]

Electronics

I'm having trouble trying to understand something to do with electricity/electronics. In a circuit current is said to take the path of least resistance. So what I don't understand is why parallel circuits work: wouldn't the current just flow through the path which has the least resistance? --212.120.247.244 (talk) 14:07, 23 January 2009 (UTC)[reply]

Everything that moves is said to take that path. The "resistance" in "path of least resistance" is the general word meaning opposition to movement. The "resistance" in electronics is a special case of the word, borrowed rather than make up a new one, a well-defined term meaning something like "opposition to the gross movement of electrons in non-reactive media". There is another, more specific term, "impedance", that actually applies more accurately to electronics and is the word best thought of when thinking about these things. Bottom line, electrons do not take the path of least resistance, they take all paths available in inverse proportion to the impedance of those paths. --Milkbreath (talk) 14:42, 23 January 2009 (UTC)[reply]

How about if you connect a superconductor and a non-superconducting wire in parallel? Will the non-superconductor get truly no current whatsoever? Not even due to some quantum mechanical random process thingamagic? 88.114.222.252 (talk) 14:50, 23 January 2009 (UTC)[reply]

Let's do the math. Ohm's law says I=E/R. We can't use that, because division by 0 is undefined (boy, am I glad this isn't the math desk—they'd eat me alive). So let's look for the voltage drop across the superconducting leg: E=IR, the voltage drop equals the current through the leg times zero, zero. With zero voltage drop across the parallel array, there can be no current through any resistive leg, because current through that leg would cause voltage drop, and there has to be a potential difference for there to be current. So, right, no current. I'll hand this off now to anybody who feels qualified to do that voodoo that the quantum guys do so well. --Milkbreath (talk) 15:23, 23 January 2009 (UTC)[reply]

It's like a traffic jam. There are two routes from A to B. Route #1 is slightly shorter (less 'resistance') than route #2. So all of the cars start heading down route #1...but this causes a traffic jam - so now, smart motorists will take route #2 because even though it's a bit longer, it'll get them from A to B faster because there is no traffic jam...this relieves the traffic on route #1 - so now people go that way because it's faster. In a perfect world where everyone behaved rationally and everyone had 'perfect information' - the two routes would carry traffic in inverse proportion to their 'resistances' so that everyone gets there in the best possible time. That's what happens with electrical resistance - which is why the current splits in inverse proportion to the resistances of the two paths. The "path of least resistance" thing is a kind of rule of thumb - it's not physically what happens. MOST of the electrons take the easy route - and if there is a significant disparity then it'll SEEM like they all took the path of least resistance. So if you wire up a short length of copper wire (with a resistance of say 0.001 ohms) in parallel with a ten megaohm resistor - then almost all of the current flows down the wire (1/0.001 versus 1/10000000 - so a billionth of the electricity struggles through the resistor and all of the rest takes the copper and your science teacher can boldly tell you that the electricity took the path of least resistance). If you take a 100" length of copper wire and a 101" length of copper wire and put those in parallel - then their resistance is so similar that the voltage will travel down both at almost exactly the same rates. SteveBaker (talk) 14:56, 23 January 2009 (UTC)[reply]

I like to think of it as pipes (conductors) and water (electrons), where a small diameter pipe is like a wire with more resistance. So, if you have two ways for water to drain out of a lake, one being a large pipe and one being a small pipe, water will drain down both pipes, but more will go down the large pipe (which is like the wire with least resistance). StuRat (talk) 15:39, 23 January 2009 (UTC)[reply]

The "path of least resistance" thing is a rule of thumb; but in order to be true it would imply that, ahead of time, each electron knows before it enters a wire which wire it should go down to travel down the "path of least resistance". In reality, the first electrons through will go down both wires at exactly the same probability, until the electrons start to "back up" to the junction, at which point further electrons will still choose their path randomly, but will now not choose 50/50 but will choose by the ratio of the resistance between the wires. --Jayron32.talk.contribs 17:30, 23 January 2009 (UTC)[reply]

I think a philosophical approach must be taken here. In the superconductive branch R is not zero. It simply does not exist. It cannot exist in a superconductor. Since R does not exist, the formula I=E/R cannot be applied - one of the terms is missing. – GlowWorm —Preceding unsigned comment added by 98.17.34.148 (talk) 22:14, 23 January 2009 (UTC)[reply]

IR=E looks less mathematically impossible: Ix0=0 is true for any I. And that's correct...you can load an essentially arbitrary amount of current into a superconductor. An algebraist would say "dividing both sides of IR=E by R is not appropriate if R could be zero" precisely because it would lead to a limitation not present in the original equation...if you're gonna do that, you have to keep separate track of that side-effect of the manipulation. DMacks (talk) 23:19, 23 January 2009 (UTC)[reply]

If someone tells you that - you simply have to point out that the "original" form of the equation could have been IR=E and transforming it into I=E/R could have been the mathematically invalid step. So which form of the equation is the 'real' one? I'm going with the convenient one! SteveBaker (talk) 01:50, 24 January 2009 (UTC)[reply]

Superconductors have no resistance, but there appears to be a limit to how much current they can carry. If one exceeds its critical current, at a given temperature and magnetic field "quench." This quenching property can be used as a fault current limiter [6]. Are there superconductors immune to this limit, or are there ways of operating them (extremely low temperature?) that avoid quenching? Seems doubtful. Maybe someone has studied the phenomenon. It is a real problem in superconduction magnet windings, where quenching results in the stored energy becoming heat and often destroying the winding. Edison (talk) 23:55, 23 January 2009 (UTC)[reply]

Another thing to keep in mind (though I now see it's already been mentioned) is that superconductors have zero resistance, not necessarily zero impedance. That is, current flowing through a superconductor does not lose any energy to dissipation. It can still lose energy to, for example, radiation, and it should be possible for there to be a transient potential difference across it. So to answer 88.114.222.252 is that sure, the non-superconductor can get some (transient) current. --Trovatore (talk) 03:31, 24 January 2009 (UTC)[reply]

What Trovatore says makes sense; the impossibility of perpetual motion and the inviobility of the second law of thermodynamics pretty much guarantees that all systems will "bleed" energy in some form. While a superconductor may lack resistance in the traditional sense, what it does not lack is the fact that there is some friction-analog in the system. If nothing else, electrons in motion generate a magnetic field, and this magnetic field will interact with other availible magnetic fields (even, like, the earth's own magnetic field) such that over time, the magnetic fields in the super conductor will interact these other magnetic fields, gradually slowing down the flow of electricity. It may well be a VERY gradual slow down, but its still real, and it still allows the system to obey the laws of physics. --Jayron32.talk.contribs 04:47, 24 January 2009 (UTC)[reply]

Well, it's not going to slow down because of a static magnetic field. You have to have a changing field to change the current in a superconducting loop. I don't offhand see any reason it wouldn't be as likely to increase the current as to decrease it.

Perpetual motion machines ~~"of the first kind"~~ "of the third kind", the ones that just keep moving but you can't extract any useful work from them, are not a priori impossible -- consider an electron orbiting an atom (assuming there's no such thing as proton decay). --Trovatore (talk) 04:52, 24 January 2009 (UTC)[reply]

Think about it this way. In a circuit, the conducting wires' resistance is so low that there's barely any difference in the different paths of a parallel circuit. Assume that the resistance is zero (which of course isn't the case), and you'll see why 2 x 0 and 4 x 0 both = 0. That little bit of extra distance really doesn't make much of a difference. Unless, of course, there's a resistor lined onto the conducting wire for every centimetre of wire, then there'd be a difference. ~A H 1^(TCU) 16:03, 24 January 2009 (UTC)[reply]

I'm not sure what you're responding to here. In a superconductor, according to currently accepted theory, the resistance really is exactly zero.

But, if I've analyzed this correctly, that doesn't mean that if you put a superconductor parallel to a normal conductor and then apply voltage, you'll get zero current in the normal conductor. The reason is that the DC laws do not apply perfectly except to entirely unchanging systems (of which there aren't any).

When you turn on the voltage, a current starts to flow in the superconductor, which generates a magnetic field, which stores energy. That means you have to supply the energy to create the magnetic field. Therefore it doesn't happen instantly. In the mean time there is a potential difference across the normal conductor, and therefore also some current. --Trovatore (talk) 22:55, 24 January 2009 (UTC)[reply]

Just in case no one has mentioned it above (I cant be bothered to read it all) I should mention the Current splitting rule--GreenSpigot (talk) 04:14, 25 January 2009 (UTC)[reply]

OOoh look we have an article on this. Its called Current divider--GreenSpigot (talk) 04:17, 25 January 2009 (UTC)[reply]

microbiology

what i meant by streptolysin test,where is it applied and which is the procedure of carrying it out? —Preceding unsigned comment added by 196.202.206.37 (talk) 14:48, 23 January 2009 (UTC)[reply]

Do you mean Antistreptolysin O titre? If so, follow the references at the bottom of that article. SteveBaker (talk) 14:59, 23 January 2009 (UTC)[reply]

Question about the hologram universe article in New Scientist issue 2691

I love this article - fantastic stuff [[7]]

But it made me wonder: if 'the amount of information papering the outside must match the number of bits contained inside the volume of the universe', do the theorists believe that includes all the information implicit in everything we do? - do the bits include changes across time in our physical universe? if so, are the bits also changing across time, or are the changes themselves also encoded in the bits?

Thanks Adambrowne666 (talk) 21:21, 23 January 2009 (UTC)[reply]

I'm not sure I understand your question. I've read the article, and it's rather vague, and seems to try to explain everything in layman terms, which probably add more confusion than it resolves. I'm sure the original authors have a solid mathematical description of their theory. Our article about the Holographic principle has some more information about this topic. baszoetekouw (talk) 21:53, 23 January 2009 (UTC)[reply]

In "holographic" theories, there is generally a one-to-one correspondence between states on the boundary and states on the inside. In other words, things on the inside change if and only if things on the boundary also change. Dragons flight (talk) 22:10, 23 January 2009 (UTC)[reply]

Thanks, Dragons - sorry for not explaining myself, Baszoetekouw; will check out the article. Adambrowne666 (talk) 03:55, 25 January 2009 (UTC)[reply]

What does "space is a vacuum" mean?

Does space have atoms? Or just nothingness? It sure would be fun to capture a jar of that! When space bends from nearby mass, what exactly is it that's bending!? This has always confused me. Why does your blood boil in space? --THE WORLD'S MOST CURIOUS MAN (talk) 23:16, 23 January 2009 (UTC)[reply]

We have a detailed Outer space article, which includes a whole section about the environment (i.e., "what is in there?"). Outer space is a definite and fairly simple thing--be careful not to confuse it with the more advanced idea of spacetime. DMacks (talk) 23:26, 23 January 2009 (UTC)[reply]

Every atom has a gravitational field that extends to infinity. Therefore a light beam cannot be outside gravitational fields. In the case posited, the atoms of the glass jar will affect the light except where two atoms cancel each other's gravitational field because they are equidistant on each side of a point position in the light. More distant atoms will augment the gravitional field of the jar atoms. – GlowWom. —Preceding unsigned comment added by 98.17.34.148 (talk) 23:44, 23 January 2009 (UTC)[reply]

So many questions! Fortunately - for today only - the RefDesk has a five-for-the-price-of-one deal!

Does space have atoms? Or just nothingness? - yes, there are atoms - but a LOT less than here on the surface of the earth. As you move out deeper and deeper into space - the number of atoms in every cubic meter gets less and less. In orbit, near the earth, there are still quite a lot of atoms - enought to gradually slow satellites down so that they have to be boosted into higher orbits once in a while. Out near Pluto - there is a lot less. Halfway between our star and the next one - there is almost nothing - but still there will be the odd atom here and there. In the deep voids between galaxies - less even than that.
It sure would be fun to capture a jar of that! - not really. You can get the same effect (a totally empty jar) with a decent vacuum pump down here on earth. But it would have to be quite a sturdy jar not to collapse from the pressure of the earth's atmosphere pushing in on it without any air inside pushing back the other way.
When space bends from nearby mass, what exactly is it that's bending!? - space ITSELF is bending...the actual nothingness bends...
This has always confused me. - you and just about everyone else! The things that go on in the universe frequently only make sense when you look at the mathematics. Most of the time, you have to simply turn off your common sense and trust to the numbers.
Why does your blood boil in space? - it's not exactly true that it does. Liquids behave strangely - they both freeze AND boil at the same time. Water left alone in the vacuum freezes AND evaporates from the surface. It freezes because it's cold - it boils because there is no air pressure to oppose the molecules flying off the surface. When a human is exposed to space, the fluids take a while to cool down to freeze - and your skin applies some tension to prevent it from immediately boiling. It's generally believed that humans could survive a brief exposure to the vacuum. We have an article about that Vacuum#Effects_on_humans_and_animals and Human adaptation to space.

SteveBaker (talk) 00:04, 24 January 2009 (UTC)[reply]

It doesn't freeze because it's 'cold' exactly; it freezes because it is evaporating. Space isn't exactly cold in any meaningful sense. 79.66.105.133 (talk) 12:48, 25 January 2009 (UTC)[reply]

Good answers, but your answer to #2 is not quite right...interstellar space is much emptier than any vacuum achievable at the earth's surface(according to this Tufts page). It would certainly be interesting to some scientists to study the "nothingness" out there in more detail.-Running On Brains 01:54, 24 January 2009 (UTC)[reply]

That's an interesting page and it accords well with our Vacuum#Quality article. However, us humans can employ more ingenious tricks than nature itself, things like getters, and can make an ultra high vacuum that's pretty competitive with most regions of space. Maybe you're thinking of intergalactic space being much emptier, as Steve noted. Franamax (talk) 02:37, 24 January 2009 (UTC)[reply]

Well, yes - indeed. Worse still - even if you got a super-hard vacuum, a few microseconds later, some silicon and oxygen atoms shed from the glass jar itself would contaminate your pure vacuum. Yeah - indeed - you don't get a pure vacuum here on earth.

The actual numbers are rather revealing:

Best artificial vacuum: 10⁹-10⁴ molecules per cm³
On the Moon: 4x10⁵ molecules per cm³
Interplanetary space 10 molecules per cm³
Interstellar space 1 molecule per cm³
Intergalactic space 10^-6 molecules per cm³ (1 molecule per cubic meter!)

So we can maybe get down to ten thousand molecules per cm³ - which is a better vacuum than the surface of the moon...but nowhere near as good as in the space between planets, between stars or between galaxies. SteveBaker (talk) 03:30, 24 January 2009 (UTC)[reply]

What about vacuum energy? ~A H 1^(TCU) 03:22, 24 January 2009 (UTC)[reply]

The source to a lot of these facts are from the book Modern Vacuum Physics by Austin Chambers which is can be read online at google books here —Preceding unsigned comment added by Anythingapplied (talk • contribs) 07:08, 24 January 2009 (UTC)[reply]

Your blood doesn't boil in outer space. At least, not unless you get a cut. It won't freeze unless it boils, as the boiling is what causes it to cool enough to do so. See freeze drying for why this would happen. It would make you saliva and the water on your eyes boil. — DanielLC 22:49, 25 January 2009 (UTC)[reply]

You don't need a particularly sturdy container to contain ultra high vacuum. Most any walled glass vessel will do. The pressure difference across the wall is almost the same whether the vessel contains rough- or ultrahigh- vacuum. ike9898 (talk) 20:33, 26 January 2009 (UTC)[reply]

Energy stored in air, compressed by supersonic object

There is an object, say a flat square, that is accelerated to supersonic speeds. This will create a shock wave behind the object. But also, many waves will all crunch together directly in front of the object and build up an area with high pressure. What is the energy of these waves in front of the object?

This might be hard to figure out without any specific numbers, but you can just assume whatever is reasonable, because I was just wondering whether the energy is enough to knock over a person. --Yanwen (talk) 23:55, 23 January 2009 (UTC)[reply]

That energy is what creates a sonic boom. Where is this person standing? If they are directly in front of the object, then the object itself will knock them over... --Tango (talk) 00:03, 24 January 2009 (UTC)[reply]

Right, the plane or whatever will probably run into the person and kill him because he is in the direct path of the shockwave (and supersonic airplane). But I just need a rough approximation of the energy of the shockwave in Joules or something.--Yanwen (talk) 22:08, 25 January 2009 (UTC)[reply]

Also, as an analogue, I've heard that an unsuitable plane flying into a sonic boom (passing the sound barrier) would be like passing into a brick wall. ~A H 1^(TCU) 03:19, 24 January 2009 (UTC)[reply]

There must be some factors which limit the energy in a sonic boom, or else a plane traveling at the speed of sound would have a wave in front that would build up without end. StuRat (talk) 06:41, 24 January 2009 (UTC)[reply]

Well, the wave spreads out, that's why you can hear it. --Tango (talk) 22:10, 24 January 2009 (UTC)[reply]

Most of the sound waves aren't aimed straight forward, so those you can hear. But, the portion of the waves headed straight forward should theoretically "collect" at the leading tip of an aircraft going exactly mach 1. StuRat (talk) 15:11, 25 January 2009 (UTC)[reply]

Conveniently that portion is 0, since "straight forward" is zero solid angle. Moreover, any air that is being carried with the plane (as would have to happen if there was any collection going on) can now carry energy away and dissipate it, because its bulk speed is that of the plane and it can propagate sound relative to the plane at its sound speed. As you get farther in front of the plane, the bulk speed drops (toward the ambient air's speed) — and the sound speed changes in some more complicated way — until a point is reached where the sum falls to the plane's speed and the disturbance can just keep its lead. The energy added to that disturbance by the oncoming air is dissipated around the aircraft by the intermediate zone which is capable of doing so. --Tardis (talk) 16:58, 26 January 2009 (UTC)[reply]

So you're saying there's a dead spot, straight in front of a plane, where it's absolutely silent ? I find that difficult to believe. StuRat (talk) 21:20, 27 January 2009 (UTC)[reply]

Well, at any non-trivial distance in front it will be silent, because the plane is supersonic. Otherwise, I'm not sure what you mean — are you talking about the leading edge of the disturbance? There's a shock wave there which is (effectively) discontinuous, and there is presumably noise behind it (because information can propagate up to it from behind) and no noise in front of it (where nothing has happened yet). In the shock itself (which is actually at least a few atoms wide) concepts like "sound" don't really apply because the gas does not act like a fluid on that length scale. There will probably be vibrations of some sort in it that decay as you move forward through the shock. --Tardis (talk) 04:54, 28 January 2009 (UTC)[reply]

Chaos Theory. Is this a croc, or what?

There is essentially no randomness. Everything unfolds in a certain order, or else it wouldn't have happened that way. Every single thing is the absolute result of an absolute action. Correct?--Sunburned Baby (talk) 23:46, 23 January 2009 (UTC)[reply]

Moved this question here from the Misc. desk. -- Captain Disdain (talk) 00:21, 24 January 2009 (UTC)[reply]

Sorry, there is randomness. See determinism and quantum indeterminacy. Chaos theory is a little different: it's the idea that even if there *wasn't* any randomness, a complex system is so sensitive to its starting conditions that you can't predict the outcome anyway. --Sean 00:38, 24 January 2009 (UTC)[reply]

No - Sunburned Baby is not correct. If anything, the complete opposite is the case! True, fundamental randomness is everywhere.

Heisenberg's uncertainty principle guarantees that you can't know exactly where a particle is and what speed it's going at - hence randomness at the lowest levels of existence.
Schrodinger's equation is the fundamental description of a particle - it says that (for example) an electron is essentially a "probability cloud" a fuzzy blob of uncertainty.
Quantum theory - particles and antiparticle popping into existence from literally nowhere - more randomness.
At the level of atoms - an atom of uranium 232 has a half-life of around 70 years. This means that if you have a big pile of the stuff - half of it will have decayed down to something else after 70 years. But if you have just one atom - there is a 50/50 chance of it breaking down before 70 years are up. But you don't know when it'll do that. The event of the atom emitting a neutron is completely and utterly random.
Even at the 'macro' level Chaos theory says that no matter how precisely you measure some kinds of events - you can't do so precisely enough to predict what comes next. Take my favorite example. Place a large sheet of paper on a table. Put two magnets a few inches apart onto the paper - mark their positions - call one of them "RED" and the other "BLUE". On some kind of very rigid stand, tie a pendulum with a magnetic 'bob' on the end so it hangs somewhere between the two magnets. You'll need two pencils - one red, the other blue. Now - hold the pendulum over the paper and note the point beneath it. Let the pendulum swing around - it's swing will be disturbed by the two magnets - but eventually - the bob will come to rest hovering over one or other of them. Mark the point where you released the pendulum from with either a red or a blue dot depending on where it ended up. Do this a LOT of times. What you find is that there are some 'release points' where the pendulum always ends up over the red magnet - and others where it ends up over the blue. If you do the experiment enough times - you'll see there are large, solid areas of red marks and large solid areas of blue marks. But there are some areas that are striped or checkered alternately red/blue/red/blue. There are areas where those regions get very small and it becomes impossible to start the pendulum from the EXACT position where you released it before so the red and blue gets all mixed up. But the equation for the motion of the pendulum is well known - it's very simple physics - so we can program a computer to simulate the pendulum's motion. But even when you calculate where the regions of red and blue are in the simplest system with no air resistance and no friction and a perfectly flexible bit of string and idealised perfect magnets - the 'map' of red and blue starting points is a fractal...like the mandelbrot set. So the closer you look at the picture, the more detail it reveals. In some areas of your table - moving the starting point of the pendulum by even a millionth of the width of an atom is enough to change the outcome. Since quantum theory and the uncertainty principle apply to the pendulum bob - this experiment "amplifies" quantum uncertainty to the point where it matters.
It goes on...Hawking radiation from black holes is random. Catastrophy theory induces fundamental randomness. Godel's theorem says there are fundamental pieces of mathematics that can neither be proven true or false.
There are 'for real' infinities and zeroes in the universe - and when you plug those into sane, normal equations - the answer often comes out "undefined" - so some things are fundamentally unpredictable.

The universe is a total mess! Your idea of the clockwork universe is about as far from the truth as it's possible to be. This upsets a lot of people - Einstein spent almost all of his productive life trying to 'fix' this. But sadly - the computer you're sitting in front of right now wouldn't work if the universe worked 'mechanically' without randomness.

SteveBaker (talk) 01:36, 24 January 2009 (UTC)[reply]

Your examples are somewhat incomplete. Schrodinger and Heisenberg don't, by themselves, introduce randomness. They replace our concepts of deterministic point masses, with wave functions that still obey deterministic rules. Rather it is the process of measurement that introduces randomness by forcing the wave function to choose amongst multiple states in a probabilistic manner. It is only that process of wavefunction collapse that is random. The rest of the time, wave functions evolve according to rules that are just as deterministic as the rest of physics (even though the objects they describe act in ways that are hard to understand). In fact, wavefunction collapse might also be just an illusion, in which case, all of quantum mechanics is deterministic, albeit in a way that does not correspond to the classical notions of how matter should behave. Dragons flight (talk) 02:19, 24 January 2009 (UTC)[reply]

On another note, this is one of the areas where I have problems. You say that the randomness becomes apparent when a measurement is made, and looking at that article (through the forest of tags), it mentions "observers" and "measuring apparatus". This implies that physicists have wisely confined themselves to describing only their particular experiment and acknowledged that if they hadn't written down the results, the results might not have actually happened. But that plus a dollar buys me a cup of coffee. If a bird flies through the photon stream, is it an observer? Is its feather a measuring apparatus? What about a rock? What if the cat was late-period pregnant when it went into the box? Does it have a born/unborn kitten? We have pretty solid evidence that kittens are not quantum states, so when that kitten gets born, does it become a measuring apparatus? I can follow the line of reasoning, but it does seem to me that it leads pretty quickly to "stop asking" as the next response. Either that or "many worlds" which to me is mis-stated, it should be "incredibly huge number of worlds within the first two nanoseconds". This stuff has bothered me for a long time. :) Franamax (talk) 03:01, 24 January 2009 (UTC)[reply]

It's worth reading Neal Stephenson's latest book (it's fiction - and oddly compelling) - his idea of 'state space' is another interesting take on many worlds - it takes things yet one step further than classic many-worlds. SteveBaker (talk) 03:36, 24 January 2009 (UTC)[reply]

(ec) I see nothing "sad" about this. I'm kind of mystified why people would want the world to be deterministic. What would be the point of anything, then?

On the other hand, QM's challenge to realism (see for example Aspect experiment) is rather more disturbing. I just recently saw in the article on hidden variable theories that Edward Nelson has a hidden variable interpretation that is still indeterministic, but saves realism (though not, I guess, locality).

On a side note, chaos theory is definitely not a croc. Whether (at least as espoused by some of its exponents) it is a crock is a subtler question. --Trovatore (talk) 02:24, 24 January 2009 (UTC)[reply]

I think OP just had it the wrong way around Crock is chaos. Everything unfolds in a certain order and you still end up with a perfectly chaotic mess in the kitchen. :-) 76.97.245.5 (talk) 03:11, 24 January 2009 (UTC)[reply]

Observer bias? ~A H 1^(TCU) 03:16, 24 January 2009 (UTC)[reply]

(after ec)The OP might like to consider Richard Feynman's theory of sum over histories as an explanation of what is perceived to be "the way things happen". —Preceding unsigned comment added by TammyMoet (talk • contribs) 15:25, 24 January 2009 (UTC)[reply]

will mabye the world is big and complex for us ... will not evry thing could be rulled by mathmatical formula , but i belive that even a particle of sand flaying in the desert is going to some where for a reason ...no thing is happining as a coincidence ... if you read the quraan (islam book) you will find articles about earth ... sky ... oceans ... moon ... sun ... it talked about moon been split in half ... about univers explosion in the future ... about stage of human growing in the mother billy ... about sun and moon is moving ... about that the earth is loosing an amount from its mass with time ... it talked about evry thing ... evry thing will happen for areason ... evry single tiny small thing . —Preceding unsigned comment added by Mjaafreh2008 (talk • contribs) 21:43, 24 January 2009 (UTC)[reply]

Well - sadly, it doesn't matter that you feel like that - or that some book of religious mythology says otherwise - but the science and the mathematics is VERY clear. Quantum theory is all about fundamental randomness - and quantum theory is ESSENTIAL for things like flash memory chips. Chaos theory is right there in the Mandelbrot set.

To deny fundamental randomness is to deny that the very computer that you are reading this post on can work. So - if the Qur'an says that there is no randomness in the universe then you have to face up to this question: EITHER:

The Qur'an is correct and the computer you are using can't possibly work. ...OR...
The Qur'an is in error and the computer you are using works just fine.

Since you are still reading this post - you should probably assume (2). Failing that - you need to take the question off to some bunch of religious nutjobs because science is VERY clear on this point.

There is no fundamental "reason" why a particular atom of uranium should choose some exact moment to spit out a neutron and decay. That's a fundamental scientific truth that you can't dodge. Nuclear power stations and atom bombs wouldn't work the way they evidently do if there was a 'reason' why the neutron picks a particular moment to do it's thing.

It's arguable that the entire universe couldn't exist if there wasn't some kind of fundamental randomness. When the big bang happened, the universe was PERFECTLY symmetrical because it was a zero sized dot. As it expanded, all forces would be pulling precisely equally in all directions - all energy and mass would have to be completely uniformly distributed. You need the tiniest wiggle in there somewhere - the tiniest disparity from mathematically perfect symmetry - or else there is no way for large scale features to ever appear. So if there was no randomness - there would be no way for anything other than a perfectly smooth uniform universe to come from that. Randomness makes atoms and everything else up to and including galaxies.

I know that giving up on the idea of a clockwork universe is tough - great scientists have fought to find other explanations for almost 100 years now...and as yet, there is no alternative theory that really explains what Quantum theory explains.

Now - having said that - the randomness averages out once there are more than a few bazillion atoms so at the 'macro' scale where humans experience life, things do seem to be completely regular and predictable - but we can provoke that fundamental randomness and with the right situation (the two-magnets-and-a-pendulum experiment - or the "butterfly effect" in weather prediction - or with a geiger counter) you can make those microscopic randomnesses become very evident. That's how we know it's true.

SteveBaker (talk) 01:43, 25 January 2009 (UTC)[reply]

Schroedinger's cat and Quantum theory--GreenSpigot (talk) 04:21, 25 January 2009 (UTC)[reply]

Many things we see in the quantum world seems random. Electrons not being where they're supposed to be (Heisenberg's uncertainty principle), nuclei decaying for no apparent reason, and chaos theory etc. But what if it's just our observations that are uncertain, but the universe itself is completely deterministic. This "perceived" randomness is not actually there. Just because we can never determine both the exact position of an electron and it's momentum at the same time doesn't mean that it doesn't have an exact position or momentum. Even when the decay of a nucleus is apparently random, there has to be a reason that it decayed at that specific point in time, we just don't know the reason. As for the magnetic pendulum, the only thing preventing us from determining the outcome is the precision and accuracy (or lack thereof) of our measurements. The angle of release is an exact number; but our perception of the angle of release, however, is not exact and cannot be exact. If you release the pendulum from exactly the same spot every time, you will get the same result every time. If you don't, then that means your measurements are not accurate, not because the universe is random. (I think I might be talking about Hidden variable theory, which has been disproved by Bell's theorem, but I think there is a difference.)--Yanwen (talk) 21:30, 26 January 2009 (UTC)[reply]