Wikipedia:Reference desk/Archives/Science/2008 August 14

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August 14

Issue of quantum

mooved from the language desk Julia Rossi (talk) 00:24, 14 August 2008 (UTC)
moved back to the language desk here Wikipedia:Reference desk/Language#issue of quantum DuncanHill (talk) 01:06, 14 August 2008 (UTC)

Since this has been moved forth-and-back, I'll fill in the space with a different question/comment. We often see in the popular media that something or other represents a "quantum leap", i.e. it is a major advance. In the physics interpretation, a quantum is the smallest possible change. Does anyone else note this dichotomy? Franamax (talk) 02:27, 14 August 2008 (UTC)

Yep have noticed -- suggest is mis-used because while it means "very small" people use it to mean radical change better served by "paradigm shift" or something about "giant step for..." etc. when trying to impress. Quantum unfortunately became a buzzword when quantum as theory made a big impression on the public (media) mind. One small sacrifice for accuracy... one big leap for rhetoric? Julia Rossi (talk) 02:57, 14 August 2008 (UTC)

Now there's a good point! If the choices are "paradigm shift" or worse yet, "tipping point", maybe I should convince myself that I'm happy with "quantum leap". Ughh, I do dislike such choices... :( Franamax (talk) 03:03, 14 August 2008 (UTC)

I can't imagine that a speaker would look too closely at the meaning of what they say, intending only to sound important and blind them with science catch phrases. A comparison is when science popularisers use football fields to talk about the universe. It's the dumb or dumber option that gets you, isn't it. Julia Rossi (talk) 03:17, 14 August 2008 (UTC)

Btw, "leap" would be enough in itself really, I can't imagine a little leap. Julia Rossi (talk) 03:19, 14 August 2008 (UTC)

This too-often-heard complaint about the expression "quantum leap" or "quantum jump" is not a good point. First, a quantum leap or jump is not necessarily the smallest possible transition. For example, for an electron in shell 1 of an atom the smallest possible transition is to shell 2, but it could also jump directly to shell 3 or 4 or 5, or for that matter, shell 1,729. Second and more important, the complaint amounts to missing the point of the metaphor, which is that a quantum leap or jump involves an instantaneous transition between states without passing through any intermediate states. The size of the transition is irrelevant. If the price of oil is $100 one week and $130 the next, this represents a change so rapid that it seems as if $101, $102, $103, $104, etc. had been directly skipped over -- as they might be if the price was quantized and intermediate values were not possible. --Anonymous, 03:30 UTC, August 14, 2008.

(e.c.) To me, "quantum leap" signifies an abrupt change from one "state" to another, as opposed to a gradual change through a series of intermediate states. I think of this as an analogy to an electron jumping from one orbital level to another without going through any intermediate state. Wanderer57 (talk) 03:39, 14 August 2008 (UTC)

I'll buy both those positions, if I interpret them correctly, you're saying that a quantum is an indivisible amount. But that can apply to anything: I make bicycles, last year I painted them all black. This year I paint them all blue. We changed over the paint line on January 1st, no bicycles were painted with an intermediate colour. Thus I have achieved a "quantum leap" in bicycle design. I used to put cream in my coffee before carrying it over to the couch. Now I carry the coffe mug and the cream over, then put the cream in (less spillage). There was no overlap in my methods. Thus a "quantum leap" in coffee/cream handling. Doesn't the term then become meaningless? Franamax (talk) 03:51, 14 August 2008 (UTC)

It doesn't sound like we're saying the phrase isn't meaningless. DMacks (talk) 05:00, 14 August 2008 (UTC)

I pour the cream and sugar into the coffee but never stir - because every morning when I wake up, the first thing I want to be sure of is that the laws of statistical mechanics which dictate brownian motion and convection are still applicable - and this is sufficient to reassure me that we still live in a causal universe and I am safe for another day. Well, that is the second thing I need to do, after I check Wikipedia Reference Desk. Nimur (talk) 15:13, 14 August 2008 (UTC)

Just want to point out that "quantum leap" is a phrase from the Bohr model of the atom, not from modern quantum theory, which replaced the Bohr model circa 1925. So aside from being inappropriately applied, the term is also 80 years out of date. -- BenRG (talk) 11:18, 14 August 2008 (UTC)

Re Franamax's point. To me, the change in bicycle color would not "qualfy" as a quantum leap. It's not a dramatic enough change. There were lots of blue bikes around before your new models came out. If you introduce a new coating next year that makes your bicycles invisible and therefore less likely to be stolen, that might more reasonably be called a quantum leap.

If the term quantum leap is used for changes such as paint color, or how you prepare your coffee, I agree the term becomes meaningless. Wanderer57 (talk) 13:57, 14 August 2008 (UTC)

plum tree bark

Are there any poisons in plum tree bark? —Preceding unsigned comment added by 210.55.43.55 (talk) 06:49, 14 August 2008 (UTC)

Dr. Duke's Phytochemical and Ethnobotanical Databases list only several flavonoids in the wood so there is some probability that there is nothing really bad in the bark. If you're thinking about cyanides as per amygdaline, that is only in the seeds, according to Dr Duke. Oh, and that's only concerning humans. Animals/microorganisms may react differently. So please be more specific. --Ayacop (talk) 09:06, 14 August 2008 (UTC)

'pop' sound 11 seconds after a shooting star

Unfortunately I missed the Perseids this year, and I wasn't in a quiet area anyway. When I was a teen-ager, though, I remember standing outside our house watching the perseids with my father and my brother, and we noticed that approximately 11 seconds after each one, we heard a distinct 'pop' sound. It sounded a little like a shotgun being fired several miles away. None of us could think of an explanation for this, so we wrote to Patrick Moore, and he replied saying that it was not possible. I'm not talking about this kind of sound, it was very different. Can anyone think what this might have been? My father was a physics teacher, and my brother and I are both fairly scientific in our ways of thinking, so I really don't think that it was a psychological phenomenon or just a naïve mistake. — PhilHibbs | talk 09:07, 14 August 2008 (UTC)

Could it have been a shotgun being fired several miles away? --Ayacop (talk) 09:13, 14 August 2008 (UTC)

The same time delay after each one? I guess some crackpot could have been trying to shoot down the alien invaders. Seems unlikely though. — PhilHibbs | talk 09:45, 14 August 2008 (UTC)

How sure are you of the time interval? Hearing sounds produced by meteors is not unusual, but 11 seconds translates to a distance of only 3.8 km if 350 m/s is used for the speed of sound. If I remember correctly, the average time interval is more than a minute, corresponding to a distance of >21 km.

Also, did you hear a sound after every meteor? If not, what was the dimmest meteor that you heard? --Bowlhover (talk) 09:54, 14 August 2008 (UTC)

Pretty sure. We timed it by counting at first, I think one of us used a wristwatch. And yes, once we first noticed it we heard the sound after every one we saw. — PhilHibbs | talk 10:09, 14 August 2008 (UTC)

I know 11 seconds makes no sense at all for an object that burns up in the mesosphere, which is 50km up, a distance that the meteoroid should travel in around one or two seconds if it doesn't burn up. Unless, of course, they are making it through the atmosphere to an altitude of around 3.8km and then exploding. — PhilHibbs | talk 10:21, 14 August 2008 (UTC)

I call foul - this is all bogus.

Sound moves faster at higher altitudes and at higher temperatures. 380 ms^-1 is roughly the speed of sound AT SEA LEVEL. - so they could be higher than 3.8km and the sound would still reach you in 11 seconds...but it's not enough to double the altitude we estimated already. These are hot, supersonic objects - maybe there is something odd going on because of that? But it still sounds wrong to me. With the denser meteor showers, meteors come in at a rate of more than one per minute - with a more realistic interval of one minute between meteor explosion and the sound arriving at your ears, it's much more likely that the sound you were hearing was from the PREVIOUS meteor than the one you were actually looking at. But that would make an absolutely solid 11 seconds unlikely - meteors don't arrive that regularly.

Worse than that - even if these meteors were exploding at an unexpectedly low altitude and AMAZINGLY close to your location, they are all different sizes, shapes and masses - there is no way they'd all blow up at the exact same altitude - you'd get a WIDE spread of times, not something as repeatable and precise as 11 seconds - you'd be saying "Oh, somewhere between 10 and 20 seconds". So this makes zero sense.

I strongly doubt that the sound of a meteor "exploding" would be loud enough (they don't so much "explode" as gradually boil away anyway...that's why you get the long trails) - I think you'd have to be hearing sonic booms as they hurtled past. That means that the point at which the meteor started to glow visibly (when you start your timing) would not correspond to the moment when the sound barrier was broken - a cold, dark meteor could create a sonic boom long before it started to glow and be visible. Hence the time when you started your measurement is not related to the moment when the sound was produced anyway - so all bets are off.

The times are both too short and FAR too repeatable. I suspect "observer error".

SteveBaker (talk) 22:05, 14 August 2008 (UTC)

I'm not offended. I'm just some guy on the internet to you. I assure you that I'm not making this up, any "observer error", if that's what this is, is genuine. — PhilHibbs | talk 21:30, 27 August 2008 (UTC)

Steve? O_O -hydnjo talk 00:44, 15 August 2008 (UTC)

Hold on, sound moves faster at higher altitudes? Below the tropopause, the temperature usually decreases as altitude increases. 350 m/s should be too fast and 3.8 km too high, not the other way round.

I agree that the OP was probably suffering from confirmation bias. This type of bias has nothing to do with insanity or unscientific thinking; the only requirement is for an observer to believe his or her theory.

One more point: if the sound you heard was due to an actual physical effect, you should have occasionally heard the sound without seeing a meteor. After all, some meteors were blocked by obstructions and some were outside your field of view. Did you ever hear a "pop" without seeing its source? --Bowlhover (talk) 03:57, 15 August 2008 (UTC)

We didn't notice any other pops, but we might have been talking or just not specifically listening like we were immediately after seeing a shooting star. — PhilHibbs | talk 21:30, 27 August 2008 (UTC)

Well, sound slows down at around 10,000 to 40,000 feet - then begins to speed up again closer to the ground - it's a battle between air pressure and temperature and the two change at different rates. But it's irrelevent. The difference isn't enough to make this theory fly. It's gotta be observer error. SteveBaker (talk) 07:31, 16 August 2008 (UTC)

I've always thought that since air is almost an ideal gas, the effect of air pressure on the speed of sound is negligible. The speed of sound article seems to agree. Are you sure that sound slows down from 40 000 ft to 10 000 ft? --Bowlhover (talk) 09:56, 16 August 2008 (UTC)

Look further down speed of sound - there is a table of speed versus altitude...although I think you may be right about the effect of pressure. SteveBaker (talk) 16:04, 16 August 2008 (UTC)

Another thought. Do you live in an 'underprivileged', 'rural' area? If so, have you considered the possibility that some whiskey-enhanced redneck was sat on his roof and trying to shoot down meteors with his 12-gauge shootin' iron? --Kurt Shaped Box (talk) 00:52, 15 August 2008 (UTC)

That's not probable because different observers see different meteors and different numbers of meteors. The length of time one has been outside, eye sensitivity, viewing direction, obstructions like trees, and other factors make it extremely unlikely for two people to see the exact same meteors. --Bowlhover (talk) 03:57, 15 August 2008 (UTC)

Just to add an alternative hypothesis, did your father place emphasis when you were young on your curiosity developing your powers of discovery and analysis and/or was he fond of practical jokes? Franamax (talk) 23:31, 16 August 2008 (UTC)

We do play jokes on each other, but he wouldn't have taken it to the extent of writing to Patrick Moore with the observation. That would have been grossly unethical and entirely out-of-character. Also he had no opportunity to rig up something to fool us, as we had just returned from a caravan holiday and arrived home at about midnight. — PhilHibbs | talk 21:30, 27 August 2008 (UTC)

Kidney output not in article

What is the normal healthy human kidney output in ml/hr. Not home work I just want to know. Its not in article. —Preceding unsigned comment added by 79.76.203.9 (talk) 10:52, 14 August 2008 (UTC)

Output depends on input. Roughly, the normal range is 1 to 2 liters per day. If we take the average 1.5 L, and divide by 24 hours/day, we get about 63 ml per hour. That's obviously both kidneys. If you want, you can half that for the value of one kidney's output. You may be interested in oliguria, which contains other values relating to the question. - Nunh-huh 10:58, 14 August 2008 (UTC)

Im not looking for medical advice but was just wondering if a total kidney output of 91 ml/hr is considered normal in a healthy human. —Preceding unsigned comment added by 79.76.203.9 (talk) 15:07, 14 August 2008 (UTC)

More than the amount given above could be easily due to ingestion of diuretic food plants like asparagus or rice (there are a many more of them). --Ayacop (talk) 16:06, 14 August 2008 (UTC)

Or by drinking a lot of water. 91 ml/hr = 2184 ml/day is on the high end of normal. Polyuria (which you should look at) has a somewhat arbitrary cut-off of 2500 ml/day. In general, the concern would be for low urine output, not for high output; if high output is noted a urinalysis (to check for urine glucose or protein) will be more useful than quantifying the amount of urine. If you've noticed you're peeing more frequently than you used to, it's probably something to mention to your doctor and let him or her deal with it. - Nunh-huh 18:37, 14 August 2008 (UTC)

According to someone who is not my sister, who is not a doctor, who I never even talked to about this: the kidneys first make sure the body system has enough water for its needs (balancing fluid electrolyte level, etc.) and then excrete water; 70 ml/hr is standard (good estimate Nunh-huh); 91 ml/hr is fine (assuming you are drinking a matching amount of water). Basically, the kidneys respond to the blood composition, if there's more water than needed, they pump it out. The more water you drink, the more they will pass. If you drink a lot of water at once or consume a diuretic, kidney output will increase. This of course excludes pathological conditions, and any specific concerns should be discussed with your own doctor. Franamax (talk) 23:25, 16 August 2008 (UTC)

CD player -- most common type of fault

What is the most common type of fault associated with CD players that causes them to refuse to read a disc. Is it faulty transport (tracking) or the other electronics? —Preceding unsigned comment added by 79.76.203.9 (talk) 10:57, 14 August 2008 (UTC)

In my experience dirty lenses cause far more read failures than anything else. ~ mazca ^{t | c} 12:04, 14 August 2008 (UTC)

...and you can get lens-cleaning disks to sort that out. However, I have had several CD players that seemed to have worn out and refuse to read disks consistently no matter how much I clean them.--Shantavira|^{feed me} 12:14, 14 August 2008 (UTC)

Ive tried the lens cleaner a number of times: some times it improves matters; sometimes not. If not I then resort to a sharp bang on the case. This more often than not sorts it. What could be the problem as Im getting tired of having to bang it all the time —Preceding unsigned comment added by 79.76.203.9 (talk) 15:11, 14 August 2008 (UTC)

My guess would be some problem with the lens carriage (the part that moves the lens back and forward along the CD) - it could potentially just be sticky or have some bits of debris on it that partially jam it, which would be temporarily solved by banging the case. Giving the player a good squirt of compressed air may well clear some of it out, but it's quite possible that the motor itself has some dust in it, which may be harder to fix. ~ mazca ^{t | c} 17:00, 14 August 2008 (UTC)

My impression, at least in the older players I'm familiar with, is that the analog focus and tracking servos get "out of whack". In these older designs, they're adjustable via trim-pots but you need the manufacturer's service documentation to know which of the half-dozen or so pots to turn and under what operating conditions. But yes, ordinary mechanical failures (like a need for cleaning and lubrication on the sled drive) also take their toll over the years.

Atlant (talk) 18:48, 14 August 2008 (UTC)

Yeah, i'm pretty sure that was the problem with my car's old CD player - it would get very confused in some situations and fail to focus on the correct parts of the CD, just generally spending thirty seconds failing before giving a CD read error. But it was impossible to fix in my case, I ended up "repairing it with a new one". CD players are pretty precision-dependent instruments, and time does take its toll as parts warp and decay. ~ mazca ^{t | c} 20:36, 14 August 2008 (UTC)

Railway locomtive Steam Cycle...

I will have a look around, but does anyone here know of anything specfic thats been written on the software simulation of railway steam locomotives? I'm particualrly interested in what if any mathematical models they use. Sfan00 IMG (talk) 11:22, 14 August 2008 (UTC)

Since steam locomotives became obsolete by the time computer simulations were possible, I wouldn't expect many resources to be put into such models. However, I suppose it's always possible that someone did such a simulation to study trains of the past. StuRat (talk) 17:14, 14 August 2008 (UTC)

Obsolete? The Meiningen Steam Locomotive Works might take exception to that. Anyway, obsolete or not, if anybody knows anything about simulating locos, these guys probably will. --Heron (talk) 09:52, 16 August 2008 (UTC)

Well, in that they work on steam locomotives for museums, that goes right along with being obsolete, yes. StuRat (talk) 03:08, 22 August 2008 (UTC)

Hmmm, Thanks Sfan00 IMG (talk) 14:10, 16 August 2008 (UTC)

See Carnot cycle. Not a simulation, per se, but it is a math model. Saintrain (talk) 16:08, 15 August 2008 (UTC)

I know there are more generalised simulations of 'steam cycle' behaviour around, because they are still used for designing turbine equipment for power generation. Not Locomotives I know, but it might provide some leads. Sfan00 IMG (talk) 14:10, 16 August 2008 (UTC)

UFO sightings

Why world's most UFO sightings are reported from the United States? A quick review of the article List of UFO sightings prove Americans are more likely to see UFOs than people of any other nation. Is there any sociological analysis behind this? Otolemur crassicaudatus (talk) 14:10, 14 August 2008 (UTC)

It doesn't prove anything, unless you are certain that the methodology used to construct the list is free of systemic biases. -- Coneslayer (talk) 14:11, 14 August 2008 (UTC)

However most UFO sightings are reported in the United States. This google search result is interesting. Otolemur crassicaudatus (talk) 14:16, 14 August 2008 (UTC)

There are all sorts of possible reasons, many of which depend on your opinion of the ontological state of UFOs (that is, your opinion of what they may be):

1. Americans expect to see them (due to conditioning by the media, etc.) and thus do see them more often
2. There are better/more organizations to report them to in the USA
3. When an American sees something they don't understand, they report it as a UFO; in other countries, they might just let it go or expect it is something mundane
4. There are more potential UFO phenomena in the US (more testing of military vehicles near semi-populated areas, more weather balloons, more incursions by foreign vehicles, or more aliens—feel free to take your pick)
5. In the USA, there is an existing "market" for reporting on sightings, and no political repercussions for doing so

And so forth. In my uninformed opinion it's probably a mix of the above (though I don't really suspect aliens). If an American sees a funny light on the horizon that zooms off in a funny way, he or she is probably more likely to say, "omg, a UFO". --98.217.8.46 (talk) 14:23, 14 August 2008 (UTC)

Not to mention, you have been reading the English Wikipedia, which is disproportionately contributed to by American editors due to language barriers. If you read that same article in Finnish you will see a different breakdown of sighting locations. Nimur (talk) 15:19, 14 August 2008 (UTC)

Kang and Kodos think americans about ready and would like to serve them. Mwah ha ha ha ha ha ha hah. Foolish earthlings use of military power also more interesting than watching ants. Dmcq (talk) 15:27, 14 August 2008 (UTC)

That idea is much older: The reason that we never meet our 'owners' is that they have no interest in talking with their 'livestock' (The Book of the Damned, given just to show that UFOs were hot in 1919 already. Appears to me a continuation of the parapsychology/occultism hype around 1900). --Ayacop (talk) 17:12, 14 August 2008 (UTC)

From list of UFO sightings, I counted 40 UFO reports within this decade, 13 of which were reported by Americans. Considering that the U.S. is the most populous and second largest developed country, 1/3 does not seem impressive to me. --Bowlhover (talk) 06:07, 15 August 2008 (UTC)

Er.."the U.S. is the most populous...country,"?!? I don't think so!

1. People's Republic of China: 1,325,544,000
2. India: 1,136,750,000
3. United States: 304,875,000

If you believed these sightings were "real" then there are any number of reasons why the US might be disproportionately represented. Suppose you buy the idea that the aliens learn our language from listening to TV and Radio shows they catch from way out in deep space? By far the largest proportion of those would have come from the USA - so naturally, that's where they'd go first when looking for "probing" victims. Perhaps you buy that their landings are randomly distributed across the earth but pretty rare - then the largest and most densely populated countries would have the highest probability of sightings. But if the aliens are deliberately avoiding human contact - or deliberately seeking it out - then different statistical biasses could be expected.

If you don't believe these sightings are "real" - then you have a whole pile of other statistical biasses. In countries where there is no dense media reporting, the depths of Africa perhaps, if you go nuts and claim you saw a UFO then who can you tell? Other people in your village - sure - but your report is never going to make it into the press and thus into a Wikipedia article. If you are in a country where the press are strongly muzzled - then perhaps UFO reports are squashed early on.

There are just WAY too many variables here to be able to make any kind of rational conclusions from a simple numerical count of "sightings" per country. SteveBaker (talk) 07:22, 16 August 2008 (UTC)

SteveBaker (talk) 07:22, 16 August 2008 (UTC)

"the U.S. is the most populous and second largest developed country" A farmer in China who sees a strange object in the sky isn't likely to report it to the news if he hasn't read about aliens or UFOs before. --Bowlhover (talk) 07:40, 16 August 2008 (UTC)

The answer is obvious. Americans are far more interesting. --mboverload@ 00:40, 16 August 2008 (UTC)

Skin effect in transmission lines

By what circuit techniques can the deleterious effects on pulses of skin effect be compensated in a delay line (transmission line). Not homework I'm just curious how its done in oscilloscopes and if there are other more advanced techniques.--79.76.203.9 (talk) 15:20, 14 August 2008 (UTC)

The most common physical technique is to silver-plate the conductor so the current, forced into the "skin" of the conductor, is flowing in a material with the highest possible conductivity. Otherwise, circuit techniques might include an equalizer or an all-pass filter.

Atlant (talk) 18:40, 14 August 2008 (UTC)

OK As i was interested in circuit techniques (because I cant fix the transmission line itself), the equalizer seems preferable. What sort of equalizer would I need? —Preceding unsigned comment added by 79.76.203.9 (talk) 23:19, 14 August 2008 (UTC)

Use SPICE or something to model your transmission line (or equivalent) and vary the distributed series resistance so you can see the effect of rising skin resistance. Then design an equalizer that nulls that out. You may also find the group delay varying and that's why I suggested an appropriate all-pass filter that is specifically designed to cancel the group delay variance.

Atlant (talk) 23:28, 14 August 2008 (UTC)

OK do you have any details on how I should proceed to model the transmission line skin effect in Spice and then design a compensating network--79.76.203.9 (talk) 00:26, 15 August 2008 (UTC)

Electric Current

electric current is a flow of whatkind of charges? positive charges or negative charges.VaibhavIIT (talk) 15:42, 14 August 2008 (UTC)

See electric current. The answer, by the way, is negative (electrons are negatively charged). -- Aeluwas (talk) 15:46, 14 August 2008 (UTC)

But, notationally, the current direction is treated as if carriers were positive charges: See Electric current#Conventional current. -- Coneslayer (talk) 15:51, 14 August 2008 (UTC)

Well, normally (in metal wires and such). In P-type semiconductors the charge carriers are mostly positively charged holes, while in ionic solutions both positively and negatively charged ions could flow simultaneously. See charge carrier for more. Algebraist 15:58, 14 August 2008 (UTC)

In the electronics industry, the concept used by design engineers is that electricity flows (outside the generating source) from positive to negative. Electronics technicians use the concept that electricity flows the opposite way - from negative to positive. Occasionally, engineers and technicians will argue about this. The technician can always stump the engineer by asking him to explain the operation of a CRT (cathode ray tube). I think engineers have stayed with what was a very early concept, before any knowledge of "holes" and all that. To switch concepts would cause confusion when referring to text and formulas - the engineer would not know which concept was used. Magnetic polarity in motors and electrical equipment would also be involved. Technicians use the concept of electron flow, which is the true way to regard it. The concept of holes moving is ridiculous. A hole is nothing - how can nothing move? However, the concept of moving holes makes certain phenomena easier for the mind to handle. (Positive ions do not enter into most practical electronic phenomena. It is true that the atoms of a metallic conductor are constantly ionized and de-ionized as flowing electrons enter and leave orbit in the atoms; but the atoms do not move, and the ionization of metal atoms can be ignored for practical purposes in electronics and electrical work.) Andme2 (talk) 02:50, 15 August 2008 (UTC)

In my experience (and I have been involved in electronic chip design and such) most electronics engineers think in terms of potential differences and they aren't really concerned about the actual flow of charges. It's perhaps different for the materials physicists who are worrying about silicon doping levels and the design of individual transistors. But for most working electronics 0engineers, it's a matter of how long does it take this signal to get from A to B (considering resistance, capacitance, inductance, distance, cross-talk, etc) - it doesn't matter much whether it's a signal where the voltage change is from positive to negative - or negative to positive...so the direction in which electrons or holes actually move is largely beneath consideration at that level of abstraction. The direction in which information flows is what matters. SteveBaker (talk) 07:08, 16 August 2008 (UTC)

Hot water cold water

I seen video on Utube showing hot water freezes faster than cold water in the ice box. Anyone know why that might be?--79.76.203.9 (talk) 15:59, 14 August 2008 (UTC)

Mpemba effect? DMacks (talk) 16:07, 14 August 2008 (UTC)

I have read that water that has been boiled, then allowed to cool, will freeze faster than water from the same source that was not boiled. The reason is that boiling removes dissolved gases from the water; that makes it easier to freeze. Even distilled water may include dissolved gases. Also, water from different sources may contain different impurities and different amounts of dissolved gases, making a difference in the freezing temperature. (Sea water freezes at a lower temperature than fresh water.) Also, ions in water might make a difference. I have heard of "deionized" water. Andme2 (talk) 19:31, 14 August 2008 (UTC)

It turns out to be a really complicated thing. Stuff like the conductivity of hot water being relatively poor - so the edges and top of the liquid that are exposed to the cold freeze quickly because relatively little heat is being transported from the interior of the liquid. With cold water, warmth from the interior of the liquid is conducted to the outside more easily - so the whole volume pretty much has to freeze at once. At first sight, it looks like the hot water froze first - when in fact, the interior of the ice block is still liquid. Then there is lots of complexity due to whether the hot water has been boiled (driving out dissolved gasses) and the cold water hasn't. There are a TON of complicating factors. It's still cool though! SteveBaker (talk) 03:23, 15 August 2008 (UTC)

Since the water is hot it evaporates faster. Thus there is less water to freeze, thus it freezes faster. I don't know if that's true but it makes sense to me. --mboverload@ 00:36, 16 August 2008 (UTC)

I don't buy that. The Mpemba effect works best when the two containers start out at 35 degC and 5 degC. The amount of evaporation from a 35 degC liquid over the half hour it might take to freeze is pretty negligable - yet the total amount of heat that has to be removed from it is seven times higher than from the same amount of liquid at 5 degC. Nah - that may be a tiny contributory factor - but it's not enough. SteveBaker (talk) 07:01, 16 August 2008 (UTC)

Electromagnet

If two 7 oz with 100 wraps of wire electromagnets were repeling each other, what would be the force in newtons? Hovercraft Experts (talk) 16:05, 14 August 2008 (UTC)

Practically zero...they are too far away to affect each other and there's no current flowing through the wire. Or did you forget to include some information while copying what looks like a homework problem? Our article about Electromagnets has lots of equations to help you. DMacks (talk) 16:10, 14 August 2008 (UTC)

You need to specify the core material,the current, the geometry, and the distance between magnets for there to be any meaningful answer to the question. Edison (talk) 03:13, 15 August 2008 (UTC)

The End Of The World as we know it!

Inspired from a previous question, what is theorized about charged micro black holes? If they were able to chew up planets or stars we would have seen suspiciously small black holes in space, too small to have been collapsed under there own gravity. But what if we could produce micro black holes with a half life of thousand years, would it be possible to create totally new materials? The article on exotic atoms doesn't mention anything like that --- well, myons have a half life of 2 microseconds, not enough to exploit the chemical properties of myonic atoms. But I miss any note that myons and other fermions don't share orbits with electrons. 93.132.156.107 (talk) 20:17, 14 August 2008 (UTC)

Even if we did have stable micro-black holes that somehow created them, nearly all exotic particles (ie other than proton/neutron/electron and some neutrinos) are just inherently unstable with respect to those, so even if (somehow) a micro-black hole allowed us to produce muon-orbited atoms, say, it wouldn't make them any less massively unstable. ~ mazca ^{t | c} 21:15, 14 August 2008 (UTC)

You misunderstand. His question is whether a charged micro-blackhole could itself substitute for an electron or proton, thus giving rise to "atoms" with different chemical properties. Dragons flight (talk) 21:51, 14 August 2008 (UTC)

Ah, thanks for clarifying. In that case I'd still suspect it would be unlikely due to instability - assuming the theory of Hawking radiation is correct, a micro-black hole small enough to substitute for a subatomic particle would have a lifespan measured in tiny fractions of a second. ~ mazca ^{t | c} 22:17, 14 August 2008 (UTC)

That's a false conclusion. If you compare the lifetime estimate from Hawking radiation to Schwarzchild radius there is a substanitial population of hypothetical black holes with masses from 10⁷ to 10¹⁶ kg where the event horizon is smaller than 1 angstrom (a typical atomic size) but the lifetime is from days (at 10⁷ kg) to longer than the age of the universe (at > 10¹¹ kg). Setting aside the intractable problem of how you create or find a 10¹⁰ kg black hole in the first place, they would appear to be the right size and relatively stable. Dragons flight (talk) 23:30, 14 August 2008 (UTC)

Are your numbers with respect to charged or uncharged? 100 TeV is not 10⁷ kg by far. So to become heavy enough to be stable it would need too much time, wouldn't it? --Ayacop (talk) 08:17, 15 August 2008 (UTC)

Thanks for your answers. As I understand, a micro black hole that is to last in the range of seconds would need a mass > 10⁸ kg and that makes it quite unsuitable to exploit chemical properties of any material it may build. But, say, if a 100TeV MBH manages to swallow just one electron and gain its charge, would the formula for hawking radiation still be the same? I think it can't go on evaporating, keeping the charge, nor could it loose charge gradually. So I guess it would have to spit out the electron in a non-continuous process. Is it that way? 93.132.140.20 (talk) 07:15, 15 August 2008 (UTC) (Sorry, this should have gone to the end of the section 93.132.140.20 (talk) 07:35, 15 August 2008 (UTC))

But would a black hole of that size produce appropriate amounts of strong & weak nuclear forces to hold the bulk material together against it's own gravitational field? Could we get such a hole into a state where it has a positive charge equal to exactly the desired number of electrons? As a practical matter, it seems pretty intractable. SteveBaker (talk) 00:13, 15 August 2008 (UTC)

We are talking about "atoms" that have the same weight as mountains, and it is the charge that strikes you as implausible? Forget everything else, there is no material on Earth that could support that weight and keep a 10¹⁰ kg atom from falling into the core of the Earth. Dragons flight (talk) 04:14, 15 August 2008 (UTC)

Who says we have to assemble these hypothetical materials in the vicinity of a large gravity well? Indeed, as you point out, that's not likely to work too well. But suppose we were trying to make a nice big chunk of "blackholeium" out in deep space somewhere? The black holes will stay where we put them - and so long as they don't get VERY close to each other, they won't attract each other all that strongly by gravity alone. So I guess the question is: If we could charge them with just a few positive charges - set an equal number of electrons off in a cloud around them - would that make something that behaved somewhat like an atom - apart from its spectacular mass? Would there be strong & weak nuclear forces escaping the mini-black holes to enable them to form stable bulk materials? SteveBaker (talk) 06:57, 16 August 2008 (UTC)

It looks like any useful massed black hole would decay too quickly. So if you had a 1000 H mass black hole, does theory predict it will decay in a tiny fraction of a second? Or could it get stuck in a level where decay was too improbably to happen? Graeme Bartlett (talk) 05:02, 15 August 2008 (UTC)

Causality

Problem:
First, let us assume a hypothetical scenario, the earth orbits around the sun and it takes light about 8 minutes to travel from the Sun to the earth (this is not the hypothetical part), following this, let us say that by some unexplained phenomenon, the Sun were to disappear all of a sudden leaving a void in it's place and it would take 8 minutes for a species on earth to figure out that the sun is missing, if they were to depend on light for that information. We also know from the general theory that even the gravitational waves in space-time shall travel at the speed of light and hence the path of earth shall be a conic section for 8 more minutes after the Sun disappers, then it shall start moving tangentially to the conic section.
The question is why can't gravity be an instantaneous force, i.e. how would causality be effected IF gravity was an instantaneous effect? because it really doesn't have to matter if the species can see or not see the Sun, as seeing does not imply the Sun's existence, so even if the light from the sun would still be coming, the gravity would tell us that the Sun is missing, and no causality would be broken. Also, can this problem be modifed in order to suggest the breaking of causality? because in no way can I think the effect of gravity can take place before the Sun has disappeared.
Aah! The simultaneity is the issue, there would be a frame of reference in which the laws of physics act in a manner that Earth changes it's orbit before the Sun disappears. Sorry, I got the answer myself, (the laws of physics should hold in ALL frames of reference), but an interesting question nevertheless, open to discussion too. Maybe someone might have an interesting insight. Aks... (talk) 21:54, 14 August 2008 (UTC)

Asking "what if" with an utterly inconsistent thing like this cannot lead to any kind of a consistent answer or even meaningful speculation. We know gravity isn't an instantaneous effect precisely because everything else would end up being inconsistent if it were. It's kinda like asking "What would the world be like if all of the rules of arithmetic were absolutely normal except that 2+2=7 ?" - the answer is...what? There can't be an answer. SteveBaker (talk) 00:26, 15 August 2008 (UTC)

I believe the question which I put forth was not what if, but why? As far as I understand, the general theory predictions about the speed of gravitational effects came about due to the loss of causality if it were infinite. Hence, even before tests were done, it was suggested that Gravity was NOT instantaneous. Hence, the issue is what were the problems in causality that suggests a limited speed. Therefore, the question is not irrelevant, as there has to be an underlying inconsistency if the physicists believed it was instantaneous. To put it in your way, the question is if all the rules of arithmetic were normal, and we did not know what the value of 2+2 was, why couldn't it be 7? what would lead you to believe that it is NOT 7. Quoting SteveBaker "We know gravity isn't an instantaneous effect precisely because everything else would end up being inconsistent if it were.", what are those inconsistencies? I believe the inconsistencies came before the idea that the gravitational effects are not instantaneous, hence the inconsistencies must have been due to that instantaneous effect assumption. Its not as if the effect of gravity has always been thought of as finite, infact due to hitting road blocks with the infinite speed assumption, the physicists started questioning their fundamental assumptions, so again, what were those road blocks? Aks... (talk) 09:34, 15 August 2008 (UTC)

Well, I see you got the answer (the Earth would move before the Sun disappeared), but have you seen Faster-than-light? 81.174.226.229 (talk) 11:19, 15 August 2008 (UTC)

Uses of propane vs. uses of natural gas

Why is it that in residential uses propane has become specialized for use in portable applications (e.g. some barbeques) while natural gas is used in fixed applications (e.g. stovetops)? Seemingly, propane would be the best overall gas since its energy density is higher than that of natural gas, but obviously there must be other effects in play. Thanks --Colonel Cow (talk) 22:03, 14 August 2008 (UTC)

As far as I'm aware, propane - as a pure substance that has to be derived sequentially by cracking of oil - is substantially more expensive to produce than raw, methane-based natural gas which is extracted directly in a near-usable form from gas wells. In terms of camping-gas stoves, for instance, the predictability of a pure fuel combined with the maximum energy-to-weight ratio is worth the extra expense, but when you're piping the substance to houses or power plants, the lower cost of natural gas more than outweighs the lower energy density. ~ mazca ^{t | c} 22:20, 14 August 2008 (UTC)

Propane is heavier than air - natural gas is lighter. This is very important - you don't want your basement filling up with unburned cooking gas! Also, if there are any small leaks in the gas pipeline, natural gas doesn't pool in underground puddles ready to explode at some inopportune moment as propane would. Instead it drifts up into the air and is soon diluted. But for barbeques and such propane is easier to liquify than natural gas - so you get more of it in a cheaper cylinder - it also produces more energy per kilogram - so you don't need as much to cook your food and that makes it better for portable applications. SteveBaker (talk) 00:37, 15 August 2008 (UTC)

(ec)Is natural gas lighter than air? I have made soap bubbles with it, and they sink in air. It also pools in a cooker hob if the gas ring is left on unlit. DuncanHill (talk) 00:51, 15 August 2008 (UTC)

Yes - sure it's lighter than air - it's mostly methane which weighs about 0.7 kg/m³ versus air at 1.2 kg/m³ (our article Natural gas agrees with me - which is always a bonus!). But perhaps it's not sufficiently light to lift the weight of the (heavier than air) soap in the soap bubble? Hmmm...you get maybe 30cc's of gas in a bubble about an inch across - so with a density difference of 0.5 kg/m³ you've got about 15mg of lift...the wall of the bubble is very thin (our article says 25 nanometers) - so with a surface area of maybe 20 cm² and a thickness of only 25nm - your bubble should weigh 0.0005 mg - it should have PLENTY of lift! I don't know why it wouldn't rise. Are you SURE you have natural gas and not propane? SteveBaker (talk) 03:17, 15 August 2008 (UTC)

I don't have gas at all here. I did it years ago - I was following some experiments in an old science textbook, which described doing it with coal gas (in which case the bubbles would float, it is mainly hydrogen). As I was in the UK, and using mains gas, and it was post conversion, it must have been natural gas. DuncanHill (talk) 14:12, 15 August 2008 (UTC)

A caution about the weight of bubbles—25 nm is the approximate minimum wall thickness. Soap bubbles often start out with quite a bit more material; thickness decreases with time due to evaporation and gravitational settling of liquid to the bottom part of the bubble. Small drops of liquid may even adhere to the bubble as it is formed and dangle from its lower surface, further increasing its weight. A bubble that actually contained only five micrograms of material wouldn't leave any visible residue when burst on a sheet of paper or other surface—a result obviously contradicted by experiment. TenOfAllTrades(talk) 14:45, 15 August 2008 (UTC)

Yes - of course - the liquid slides down to the base of the bubble under gravity - so the wall thickness is only 25nm at the top. Doh! But still - bubbles with nothing but air inside float for a long time - supported by nothing more than the warmth of our breath lowering the density of the air inside (and dragged down somewhat by the increased CO2 content of our breath as well as the weight of the liquid) - the difference between the densities of air and methane is HUGE. A natural gas should rise easily. Perhaps UK's natural gas of that era was relatively unprocessed and maybe contained a higher percentage of propane and heavier 'stuff'. Sadly, my house is all-electric so I can't test the theory. Can anyone help us with an experiment? The world needs to know! (Well, OK, *I* need to know!) SteveBaker (talk) 15:51, 15 August 2008 (UTC)

Note that small bubbles are likely to move up or down due to subtle diffs in temp and air currents that may not otherwise be apparent, and this may mask any difference in density. StuRat (talk) 17:17, 15 August 2008 (UTC)

Many homes in the U.S. use propane for heating. See this map on which dark blue indicates counties where propane has 25% or more of the market share for home heating. The article from which the map comes says that in 2005, propane accounted for roughly 6% of residential heating. Another chart shows how the 7 quadrillion BTUs of energy for home heating are divided; according to this, propane accounts for nearly twice the quads of electricity, and 70% of the quads provided by heating oil. — OtherDave (talk) 00:49, 15 August 2008 (UTC)

Not just for home heating but for clothes drying, water heating, and cooking ranges. Rmhermen (talk) 15:06, 15 August 2008 (UTC)

Wikipedia discusses these heating gases under "propane", "natural gas", and "butane". These articles give lots of information but (in my opinion) are rather unorganized write-ups. They need editing. But my thanks to those who did the write-ups for taking the time to provide the info - they knew their subject but undoubtedly were not professional writers. Andme2 (talk) 01:08, 15 August 2008 (UTC)

See Liquefied petroleum gas (lp gas) as well. Rmhermen (talk) 15:06, 15 August 2008 (UTC)

More important than the "heavier/lighter" than air question is the fact that propane can be stored in liquid form at normal ambient temperature, allowing a huge amount of energy to be stores in a small tank at fairlty low pressure and without the extreme cooling needed to keep natural gas/methane a liquid. Edison (talk) 03:11, 15 August 2008 (UTC)

Also to mazca, propane is not derived by "cracking" of oil. In fact propane is derived from both natural gas and petroleum by a relatively simple fractionation process. Natural gas is fractionated to remove the CO2 component, the hydrogen sulfide contaminant in sour gas, and the natural gas liquids including propane. Removal of the NGL is important, at typical pipeline pressures the propane can liquefy and hit the pump compressor blades. See Natural gas processing. Propane is also relatively easily extracted from the top output of a petroleum refinery main fractionator, using a depropanizer. We are sadly lacking in links for this stuff, I should fix that someday! Franamax (talk) 23:11, 16 August 2008 (UTC)