Wikipedia:Reference desk/Archives/Science/2008 February 7

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February 7

Tornados in the American South - why?

Tornados in the American South - why? Why do severe tornados occur in the Southern USA in about 5 states (Arkansas, Tennessee et al) and seemingly nowhere else in the world? Here in Australia, at about the same latitude in the Southern Hemisphere, and around the same land size, we get what we call “willy willies”, and they could hardly lift a paper bag off the ground (Think of the breeze blowing the plastic bag around in “American Beauty” and you’ll get the picture.) Is God punishing the South for some reason? Myles325a (talk) 00:15, 7 February 2008 (UTC)

See Tornado#Climatology and Tornado climatology. --Anonymous, 00:30 UTC, February 7, 2008.

Also see Dust devil to understand why willy willies are so different from tornadoes. --Allen (talk) 07:33, 7 February 2008 (UTC)

They're attracted to trailer parks? Meanwhile, also see our Tornado Alley article.

Atlant (talk) 18:42, 7 February 2008 (UTC)

I know you were just joking, Atlant, but we do have an article on tornado myths. This edit is more comprehensive, but unfortunately contained mostly copywrited information. Many of these myths were dispelled during the Super Outbreak of 1974. -- MacAddct  1984 ^{(talk • contribs)} 19:25, 7 February 2008 (UTC)

If I remember rightly, Britain has more tornadoes per square kilometre per year than Tornado Alley, it's just that they're a lot bigger in the states. -mattbuck (Talk) 02:39, 10 February 2008 (UTC)

Batteries heating up

I bought a cheap portable fan yesterday which operates on two AA size batteries. I noticed after operating it for about 10 minutes that the batteries became red hot.They worked okay in a torch that I used them in previously, so I don't think they're defective. I have only seen this once before, in a TV remote control, in which the batteries heated up and died. The next set worked fine and no problems since. As to the fan, I let the batteries cool and they worked but the problem repeated. Is the fan likely to be the problem or the batteries? LuckyThracian (Talk) 01:06, 7 February 2008 (UTC)

Neither. Anytime you draw large current (usually when powering motors) the batteries will heat up, a lot. It might be possible that the fan is drawing too much current because parts of its motor winding is shorting out, but this is unlikly.--Dacium (talk) 01:41, 7 February 2008 (UTC)

Thanks, Dacium. I don't have much savvy on matters electrical so I don't understand how a motor can control what current is drawn. LuckyThracian (Talk) 01:51, 7 February 2008 (UTC)

Any time you ask an electrical device for physical movement, you're going to use relatively big power. AA batteries can light a few LEDs or beep a peizo element till the cows come home, but a motor is a whole nother animal. In fact, if you stop the motor with your hand, the battery sees basically a length of copper wire to ground and will heat up pronto. Dacium is right that you can expect the batteries to get pretty warm (read "hot"), but I would say that "red hot" is too hot (I know that that was an exaggeration, but, still). Rarely is a non-rechargeable battery bad in such a way that it gets hot, although your anecdote about the remote control seems to be that. It's more likely to my mind that the fan is poorly designed or defective. --Milkbreath (talk) 02:17, 7 February 2008 (UTC)

Well, they became literally too hot to hold (if not red)! Okay, so the fan is likely the problem. What I can't understand is why the motor's defect affects the batteries. If it ran slowly or not at all it would make sense to me, but to run at the right speed and heat the batteries? Is it that the inductive coils in the motor determine the rate of rotation of the shaft by their length and force the batteries to discharge at an excessive rate? LuckyThracian (Talk) 02:40, 7 February 2008 (UTC)

What sort of motor is it and how big is the fan? Does it say what power rating it is? Most likly any batteries you put in it are going to get hot (do you have a thermometer?). Batteries are usually used for low current, but a fan motor is high current. If the motor is defective, it will draw excess current. A motor is basically a coil of wire. If the motor did not spin, a huge amount of current would just flow through the wire (this is why you never stop the fan spinning with your hand and hold it there, as the motor will burn out as large amount of current is flowing). As the fan spins up a back emf forms and reduces the current draw. If the motor is defective (in that some of the windings are shorted out) then the back emf will be reduced and the resistance of the motor will be reduced and excess current will be drawn, which heats the battery. Personally I don't think you have anything to worry about. I would imagine the fan needs quite a lot of current and batteries getting hot is normal.--Dacium (talk) 02:54, 7 February 2008 (UTC)

The fan's only a tiddler with plastic casing and "blade". It's made in the shape of a standard desk-top fan but it's only got an 8cm diameter fan-blade and stands about 18cm in height. I was just intrigued by the heating effect - I only paid $3.00 for it and the motor is just the sort you'd find in a lot of small appliances, about 4cm long. Thanks for your time..I'm going to read up on EMF etc. This seems to be one of the hardest things in applied science to get one's head round. LuckyThracian (Talk) 03:21, 7 February 2008 (UTC)

You took some good steps trying to figure this out, by testing the batteries in a tourch and by noting that the fan only cost $3. Some other questions to ask are, did the fan ever work well, or is it only with these batteries that it draws too much power? Normally, I'd suggest trying different batteries, but considering how cheap the fan is, and that it is likely to be no good, and how expensive batteries are these days, I'm not sure I'd bother trying new batteries. --Gerry Ashton (talk) 07:57, 7 February 2008 (UTC)

Here's how to think about the current consumed by an ordinary DC motor: The motor, when spinning, also acts like a generator, generating a voltage that opposes that flow of electric current that the batteries would normally cause. (This is called a "counter EMF".) But the generator isn't 100% efficient so if the motor is running on (say) 3 volts and turning n RPMs, the generator only manages to generate (say) 2.5 volts. That leaves 0.5 volts to flow through the resistance of the motor's copper windings.

What this means is that a free-spinning motor draws relatively little current because its virtual generator manages to generate a counter EMF that pretty-closely approaches the voltage applied to the motor. But as you add load to the motor and the motor turns more slowly, the output voltage of the virtual generator also drops, and there's more voltage left to appear across the resistance of the motor. Stop the motor dead, and the virtual generator output stops entirely; the motor now draws current equal to V/R and the batteries die quickly.

Your fan is probably putting a pretty heavy load on the motor and so the motor is turning a lot more slowly than it would turn if it were not loaded at all.

Atlant (talk) 18:38, 7 February 2008 (UTC)

As mentioned earlier, there might be a short in the motor, or the motor and fan might have excess friction. Does it spin freely without the power on, or does it take some effort to turn it? Small batteries have operated a portable cassette recorder for quite a long time, and it has a motor. I personally would measure the current draw and compare it to the amp-hour rating of the batteries. If they are getting too hot to touch, they will probably be discharged in minutes rather than hours. Edison (talk) 01:12, 8 February 2008 (UTC)

I was off-wiki for a couple of days and just read the further comments. Thankyou all for your assistance. LuckyThracian (Talk) 04:25, 11 February 2008 (UTC)

Olympus Mons climb

Based on an earlier discussion here, what could an interplanetary mountain climber preparing to climb Olympus Mons expect? Off the top of my head I'm thinking lava-flow, dust-storms, drastic changes in temperature from night to day... The air is less breathable there than on Everest, I'm assuming? What kind of gear would such a climb require? Does such gear exist? Wrad (talk) 01:09, 7 February 2008 (UTC)

The atmosphere of Mars is predominantly carbon dioxide; at mean surface level the pressure is a little under 1% of the atmospheric pressure on Earth. At the peak of Olympus Mons – 27 km above Mars' mean surface level – the pressure is even lower: one-tenth to one-twentieth the pressure at the base of the mountain. In other words, the air isn't breathable at the base or the peak of the mountain; it's pretty much a dirty vacuum. Even if the pressure were high enough, the carbon dioxide would kill you. To breathe anywhere on Mars you're looking at a spacesuit.

The 'climb', such as it is, would at least be easy. It's a long hike from base to summit (close to 300 km) but it's not a difficult trek. The slope (typically less than five degrees) is extraordinarily shallow. (For comparison, parts of the climb up Everest are at 50 degrees or worse.) As an added bonus, you get to do the whole climb under gravity that's only a third as strong as Earth's. Now, you might have some tough climbing if you decided to explore the crater/caldera complex at the heart of Olympus Mons. There you've got craters and canyons with sheer drops of thousands of meters. TenOfAllTrades(talk) 01:32, 7 February 2008 (UTC)

because of the atmosphere (or lack of one) duststorms probably won't be a factor. Nor will the weather, as there is little to transfer heat away from you. If you have a suit that survives on the surface it shouldn't be a problem on the summit.--Dacium (talk) 01:44, 7 February 2008 (UTC)

Five degrees isn't as shallow as it sounds -- about a nine percent grade, which is a bit of a slog if you're biking it (on Earth) for more than a mile or two. But I agree it's shallow for straight up the mountain. --Trovatore (talk) 01:46, 7 February 2008 (UTC)

For bicycling, perhaps five degrees isn't shallow, but for hiking, it's easy enough. I don't consider a slope "steep" until it reaches about ten degrees. --Carnildo (talk) 21:11, 7 February 2008 (UTC)

Actually, "Mars ... has the largest dust storms in the Solar System". So large, in fact, that they can cover the entire planet, although I believe at least sometimes the summit of Olympus sticks out the top. Confusing Manifestation(Say hi!) 03:36, 7 February 2008 (UTC)

Perhaps a pressure suit of the grade used in aircraft would suffice for the Martian atmosphere. Retarius | Talk 03:46, 7 February 2008 (UTC)

I would be most worried about the radiation - at it's lowest level, it's not too bad, but if a solar flare hits (small ones occur every few days on Mars, and bad ones every few weeks), or even worse, a solar proton event, you can get cooked pretty good, although only the latter would be likely to kill you on the spot - the rest would probably just shorten your expected lifespan by a lot. And there's unfortunately not a lot that you can do, unless there's a nice deep cave handy (unlikely). --Bmk (talk) 06:58, 7 February 2008 (UTC)

"Largest dust storms" is quite misleading. A Martian dust storm may cover a huge fraction of the planet, but the thin atmosphere means that the dust is equally thin. It would be more accurate to compare it to an overcast day or a light fog than to an Earth duststorm. --Carnildo (talk) 21:13, 7 February 2008 (UTC)

Lava flows are not a risk. Olympus Mons has been extinct for a long, long time. Algebraist 10:20, 7 February 2008 (UTC)

(The same goes for the rest of Mars; Mars is thought to be entirely inactive, geologically. --Bmk (talk) 18:01, 7 February 2008 (UTC)

Mars is actually quite similar to the Moon (Earth's Moon, that is) in many respects. Thus, the gear used for the Moon landings would certainly work on Mars. Solar radiation is definitely a threat; however, there are warning systems for solar flares / CMEs so taking cover seems possible. One would certainly need a pressurized transport vehicle of some sort: to hold supplies, to house the communication equippment, and, most important, to change the spacesuit (or at least the diapers) every few hours. I do not know if such a vehicle would provide a sufficient protection from the Solar radiation, but it probably would provide some protection at least. (Note: ISS is not a good example, its orbit lies within the Earth's magnetoshere so the conditions there are quite different). Note also that Moon EVAs took less than 8 hours each, on every mission so far (Apollo 11,12,14-17); I don't think one can (or should) design a suit that would last much longer, for it probably will be very, very cumbersome and uncomfortable. Cheers, --Dr Dima (talk) 14:09, 7 February 2008 (UTC)

Hmm, with such low gravity, would the fall into one of those canyons of a 1000 meters kill you? How far could you safely fall? Snorgle (talk) 16:39, 8 February 2008 (UTC)

Martian gravity is about a third of Earth's, so a rough estimate is that you can survive a fall three times higher. --Carnildo (talk) 21:21, 8 February 2008 (UTC)

A 1000 meter fall on Mars would result in you hitting the ground at a higher velocity than a similar fall on Earth. Earth atmosphere prevents you from accelerating indefinitely. The terminal velocity for a human on Earth is 50-60 m/s, so most of the 1000 m fall on Earth you move approximately at that constant velocity, until you hit the ground. Mars atmosphere OTOH is far less dense, so the terminal velocity on Mars is higher. Neglecting Mars atmosphere effect altogether, your vertical velocity after 1000 m fall would be about 80 m/s. --Dr Dima (talk) 07:22, 9 February 2008 (UTC)

Device to locate devices

Is there a device that can scan and locate electrical devices (cell phones, tape recorders, PCs - whatever) running near it?217.168.0.55 (talk) 04:44, 7 February 2008 (UTC)

You could try looking around in Technical Surveillance Counter-Measures; many of the devices designed to sweep for listening devices could be applied to many electrical devices. I think Nonlinear junction detector is a particularly cool one (and probably the best answer to your question that I've found). --Bmk (talk) 06:39, 7 February 2008 (UTC)

Actually, after reading more, it seems that it is kind of impractical, and difficult to use. --Bmk (talk) 06:41, 7 February 2008 (UTC)

I personally am looking forward to the day when every object I own is tagged with an RFID tag and I can just walk around the house with a scanner to find my glasses. --Sean 15:02, 7 February 2008 (UTC)

But can you find the scanner without your glasses ? Gandalf61 (talk) 15:19, 7 February 2008 (UTC)

Most of us just depend on our spouses (etc.): "Honey, where'd I leave my glasses?" "Top of your head, dear!"

Atlant (talk) 18:27, 7 February 2008 (UTC)

properties of electromagnets?

can a bar magnet be turned into an electromagnet? would an electromagnet discharge its electricity to any conductive material that was grounded? and if so would there be a way to insulate the electromagnet so as to keep the electric charge and still have increased attractive forces?

Automatedlearner (talk) 06:02, 7 February 2008 (UTC)

You may want to read a bit about what an Electromagnet is. The magnetic field of a bar magnet (or ferromagnet) is basically produced by the magnetic fields of unpaired electrons in the bar; it's actually quite complicated, and it involves phenomena on many different length scales. The magnetic field of an electromagnet is generated by electriccurrents (not charge; an electromagnet need have no net charge to produce a magnetic field). As for whether a bar magnet could be turned into an electromagnet, the answer is...sort of, but probably not in the way you're imagining. Any conductive substance carrying an electric current produces a magnetic field, and is therefore an electromagnet, so you could use a ferromagnetic material to form an electromagnet. May I ask why you want to know? You seem to have something specific in mind. --Bmk (talk) 06:48, 7 February 2008 (UTC)

A bar or horseshoe shaped piece of iron or steel becomes an electromagnet when one or more turns of insulated wire are wrapped around it and electrical current flows through the wire. A strong electromagnet typically has a great many turns, perhaps in several layers. If a bar magnet were used to wrap the wire around, then the effect of the electromagnetism might add to or subtract from the magnetism originally present. A bar magnet would typically be steel or a "hard" ferrous substance which retains a portion of the applied magnetic field when the current is turned off. The electric current is a property under normal conditions of the wire wrapped around the bar of iron, not of the iron itself, so unless there was a short of the wire to the bar, it would make no difference whether the bar was grounded. An electromagnet can be made by wrapping insulated wire wrapped around an iron bar and connected to a battery (with suitable construction to avoid overheating or electric shocks) and with no ground connection whatsoever. An "electric charge" sounds like you have static electricity in mind, so please note that it is a current of electricity through the windings, not an electric charge, that creates the electromagnetism. Edison (talk) 01:07, 8 February 2008 (UTC)

How to select marker bits for data communication

Hi friends!, I have posted it under Science and computing section too. Since this type of question isn't common in this discussion room, I'm unaware of where to ask.ok I'm doing a master project in SDR using vlsi-fpga. Well, now the main issue is that I use some predefined values infront of the data stream. Data from codec is 32-bits(Lch&Rch). I place few bits as marker in front of this audio data to denote the reciver the starting point of my data since data has to be exactly placed in lch and rch of the codec. a single bit reversal or mis-match causes serious noise.Now what happens is that data bits sometimes happen to be the marker value and mimic the marker. So the reciver when tuned on, considers that data bits as marker and starts processing from that point.so next to this action, all info becomes useless. No matter it helps if i increase the marker bits!. Simply it adds overhead only!...Is there anyway tat using a special marker can help in receiving the data without these problems?...Any other new ideas if you think, please suggest me...Thanks —Preceding unsigned comment added by Balan rajan (talk • contribs) 06:53, 7 February 2008 (UTC)

Is your SDR Software defined radio? You will have to have some extra in your data stream to detect if you have got into an error state or not. Perhaps a cyclic redundancy check or parity. If you slip a bit or gain a bit you will have lost sync as well with lots of noise as a result. In some systems there is an extra out of band method to indicate the start of words, or there could be a superfame that contains data in a particular order. Then you have to align your superframe (eg in E1) There will also have to be a way to see if you have slipped a bit in that too. Graeme Bartlett (talk) 09:11, 7 February 2008 (UTC)

YEs, I am doing a project on SDR only. You are correct. there are very possibilities that the bits might slip or gain when sampled in the receiver unit. But how does a CRC check may gonna save this?. My intension is not the error correction or detection. Because I consider that a perfect synchronization will not generate any errors upon receiving except the medium which may generate noise. But however I will use digital modulation. So I guess I can avoid those noises easily. I couldbe wrong, but for now, I believe that I need a way to sync my pulses at the start. Is there any idea of using marker bits?...Any relevant article or information you suggest?....Thanks —Preceding unsigned comment added by Balan rajan (talk • contribs) 13:38, 7 February 2008 (UTC)

You could insert some simple logic in the data path coming from your codec to block the marker value. For example, if your marker value is #FFFFFFFF, then the logic could change all instances of #FFFFFFFF coming from the codec to #FFFEFFFE. This is unlikely to have an audible effect on the data, assuming that #FFFFFFFF corresponds to maximum amplitude. Then, when your receiver sees #FFFFFFFF, it knows that it cannot have originated from the codec so it must be a marker.

If you are not allowed to tamper with your data at all then the solution will be more complex, but it can still be done. You would need to use an escape sequence rather than a single marker value. --Heron (talk) 08:35, 8 February 2008 (UTC)

Thanks!...One bit slip in data is under tolerance... and it's not a CAA project :-) —Preceding unsigned comment added by 122.164.50.218 (talk) 05:50, 9 February 2008 (UTC)

biology

brieftly explain how concentration and temperature can influence the results of a clock reaction. —Preceding unsigned comment added by Mabetoa (talk • contribs) 10:43, 7 February 2008 (UTC)

There are various kinds of clock reaction. See chemical clock.--Shantavira|^{feed me} 13:56, 7 February 2008 (UTC)

Minimum amount of useful sleep?

First off, I just want to make clear that I'm not seeking medical advice. So let's say I set my alarm clock for 7am. However, I wake up at 6:30 because the phone rings, or there's a loud noise outside or whatever. It is worth going back to sleep for such a short period of time, or am I better off just getting up? What if I wake up 15 minutes before the alarm goes off? What's the minimum amount of time of useful additional sleep can a person get? --24.249.108.133 (talk) 16:20, 7 February 2008 (UTC)

Short periods of sleep can be very beneficial. See our article Power nap.--Eriastrum (talk) 16:36, 7 February 2008 (UTC)

I can tell you anecdotally that when such a thing happens to me, I often dream in that short amount of sleep between 6:30 and 7:00. This suggests that I am having REM sleep, which some theories consider a part of memory consolidation. Thus I would hypothesize that this small amount of sleep can be beneficial. I would also venture to say that the usefulness of the short sleep period may relate to the degree of wakefulness achieved during the interruption. I have seen bed partners wake up for very short amounts of time, appearing responsive, then fall immediately back to sleep, apparently deep, and not remember it in the morning. I have also been awakened and found myself unable to fall back asleep. Where you were in the sleep cycle when you wake up may also have a bearing on the answer to your question. I think the shortest answer to your question is probably, "it depends..." Tuckerekcut (talk) 16:39, 7 February 2008 (UTC)

I wouldn't say some theories consider REM sleep a part of memory consolidation, there is enough evidence from animal and human studies to state this as fact. --Rwst (talk) 18:02, 7 February 2008 (UTC)

I would disagree, to this day sleep physiologists disagree as to the purpose of dreams, non REM sleep and REM sleep. Scientifically, a fact is merely an undeniable observation that needs to be explained. Wisdom89 (talk) 18:37, 7 February 2008 (UTC)

Even minimal amounts of sleep like microsleeps can be physiologically significant. The question in your case is if you are able to fall asleep again in such a short period of time. If you need to go on without too much sleep, I can recommend the caffeine nap cited in powernap, specially if you suffer from jet leg or need to regulate abruptly your sleep pattern. Mr.K. (talk) 19:54, 7 February 2008 (UTC)

Regarding the statement above that dreams can occur, indicating REM sleep, during end-of-the night brief sleeps, there was a very funny radio play on National Public Radio in which someone was hitting the "snooze" button repestedly to get additional napping and having vivid dreams, the end of each incorporating whatever the commentator or newsman was saying when the radio came back on (dreams can swerve abruptly to incorporate environmemtal stimuli in a vain effort to maintain the sleep state). Edison (talk) 00:58, 8 February 2008 (UTC)

Disowning our distant relatives.

I realise that by definition this isn't really science, but I was hoping some biologists would have an answer. Is there a dominant creationist position on the other members of the homo genus? Is there an explanation commonly given for the remains? AlmostReadytoFly (talk) 18:18, 7 February 2008 (UTC)

My guess would be that most creationists would refuse to accept one of the following 1.)That they are our extinct ancestors (just as much as they do not accept apes, monkeys and humans evolved along parallel lines) 2.)They aren't as old as the carbon dating techniques used tell us. Wisdom89 (talk) 18:34, 7 February 2008 (UTC)

Those that reject the fact of evolution may often use the vague and undefined term "kinds". Something I've just thought of, even usage of the biological classification system is acceptance of evolution through biological and genetic relationship to other species. -- MacAddct  1984 ^{(talk • contribs)} 18:55, 7 February 2008 (UTC)

Elaboration: Also, creationists assert that every living animal existed at the same time. So they may not deny that Neanderthals existed, but they will deny a common ancestor with homo sapiens (or any species, for that matter). -- MacAddct  1984 ^{(talk • contribs)} 19:02, 7 February 2008 (UTC)

Taxonomy (biological classification) existed long before any theory of evolution (Linneaus was a creationist, of a sort). Evolution means you believe the taxonomic tree is fluid and related; creationism implies that you do not, that you believed in "fixed kinds". That the kinds should look similar, for a creationist, simply implies they were fashioned by the same hand. Just pointing that out. That a "fixed" or creationist view would have held on for so long even among imminent biologists (who were in many cases not wed at all to Biblical literalism) makes more sense when you think about how biologists interacted with the idea of species—not as some sort of flexible collection of all life, but as a real marker of difference between very different types that, as far as they could tell, always "bred true" to their type no matter what. It's wrong, but it's not nonsensical. --98.217.18.109 (talk) 00:25, 8 February 2008 (UTC)

Wisdom89, I must correct you. Humans ARE Apes. See, Great Apes. 64.236.121.129 (talk) 19:23, 7 February 2008 (UTC)

Good catch, that was actually just semantics on my part, but thanks for the correction ; ). My point was that creationists deny common ancestry, or the ancestral gene pool, and therefore disavow organisms sharing extinct ancestors - in this case it would be an extinct ape. Yes? Wisdom89 (talk) 19:28, 7 February 2008 (UTC)

Actually, when they get close enough looking to humans, most creationists argue that they are humans. Neanderthals simply had some sort of bone disease and scientists are cherry-picking the data to make it seem as if there were a group of them living together for longer than the real age of the earth. — Ƶ§œš¹ _{[aɪm ˈfɻɛ̃ⁿdˡi]} 20:52, 7 February 2008 (UTC)

Oh, that's right, Lucy just had a really bad case of rickets -- MacAddct  1984 ^{(talk • contribs)} 21:03, 7 February 2008 (UTC)

talk.origins has a nice table on the different classifications of different finds by different creationists. --Stephan Schulz (talk) 21:14, 7 February 2008 (UTC)

My impression is that you can't paint the beliefs of all evolution-deniers with the same brush. Being ideologically obliged to refute easily verifiable facts (e.g., that some stars are more than 6000 light-years away) has unsurprisingly led them to a variety of creative interpretations of the facts on/in the ground. --Sean 21:21, 7 February 2008 (UTC)

Don't you know? God made the stars with the light sticking out! --Carnildo (talk) 21:29, 8 February 2008 (UTC)

Calculating the speed at which pressure travels

If I open a long pressurised tube on one end, how long does it take for the pressure to change on the other side of the tube? I realise the answer will be a function of the length, diameter, initial pressure, viscosity, tube roughness and such, but I'm having trouble finding an equation to use.

For example, if a 100 metre long tube is filled with pressurised air to a pressure of 5 bar, and one end is opened, how long does it take for a noticable change to occur on the other side (apart from the pressure wave of course, which will travel at the standard speed of sound).

Thanks in advance. -- Ec5618 19:40, 7 February 2008 (UTC)

I think you're looking at it the wrong way. It's not the pressure that's traveling, it's the substance itself. I would suggest reading into fluid dynamics -- MacAddct  1984 ^{(talk • contribs)} 20:07, 7 February 2008 (UTC)

The speed of sound is the rate at which pressure changes propogate through the medium. Dragons flight (talk) 20:21, 7 February 2008 (UTC)

• You seem to have a pretty good idea of all the variables involved already; I would add that your particular definition of "noticeable" change is also going to affect the answer. You have correctly noted that the pressure wave travels at the speed of sound. We can be reasonably confident that nothing will happen faster than the sound speed, so one possible answer is that with a sufficiently good sensor, you'll see some effects starting at one sound-propagation time after the other end opens.

We can also put a lower bound on how fast the tube will reach atmospheric pressure. Unless you have some fairly unusual circumstances, the air exiting the tube at the open end won't travel faster than the sound speed. When the fluid velocity reaches the sound speed, you'll have a shock wave at the exit and you'll get "choked flow". —Preceding unsigned comment added by JohnAspinall (talk • contribs) 20:44, 7 February 2008 (UTC)

If you want to go farther, you might be able to find that someone has already done the experiments or written the computer models that could give you a more comprehensive answer (say, pressure as a function of time). This kind of question is fairly common in modelling the safety of things cooled by forced heat transfer. For example, a reactor safety engineer might ask what happens when a pipe carrying the high pressure and temperature water cooling the reactor, breaks? How long before heat transfer to the pipe is lost? Try searching for "reactor safety" and "blowdown experiment".

If you are really ambitious (and have the programming chops), you could write a simple computer model yourself. If the pipe is long, compared to its diameter, a one spatial dimension model is probably pretty good. You will end up with coupled first order hyperbolic PDEs in three state variables such as density, velocity and enthalpy for example. The classic text for all this stuff is "BirdsFoot". JohnAspinall (talk) 20:26, 7 February 2008 (UTC)

Tandem genes

I saw a special about how they are now specially breeding dogs to meet specific needs. One thing they talked about is tandem genes and how these repeating ACTG's are constantly repeating. They mentioned how it is these genes are repsonsible for physica traits at least they say when they change them there is a change in the physical appearance (i.e. the snout will be turned up or down, same thing with the tail. My question is how do they know which part of the repeating code to change and what method do they implore to cause the changes to take effect? Also a quick side note, supposedly there is a small change or difference in the repeating act's that is changed I am not sure if the location on the chromosome and which chromosome strand the tandem genes are located on make a difference or have an effect with the physical appearance. —Preceding unsigned comment added by 67.127.175.202 (talk) 21:36, 7 February 2008 (UTC)

It's unclear if you mean tandem repeats in a gene or tandem genes. As far as breeding is concerned they would use the phenotype so they would not need to know the specific DNA sequences that are being selected (as breeders have always done). Or are you impying these breeds are being genetically engineered? —Preceding unsigned comment added by David D. (talk • contribs) 22:42, 7 February 2008 (UTC)

I just did a quick google and you do mean tandem repeats. Here is a nice discussion on the topic. David D. (Talk) 22:47, 7 February 2008 (UTC)

I believe the breeds were being genetically engineered. How would that change the situation of the approach taken to change the repeats, which part of the repeats would be changed? Would it be a possibility that somewhere in the hundreds of repeats a piece of the code was not a repeat or the same? —Preceding unsigned comment added by 67.127.173.111 (talk) 19:00, 9 February 2008 (UTC)

Sounds like they know the effect that different numbers of repeats have on various breeds. Genetically engineering the desired number of a repeats in a specific breed it will speed up the breeding process dramatically rather than having to wait for a natural variant in the repeat number. I don't know enough about the specific function of the tandem repeats to know if having a insert that is not the same as the repeats would have a dramatic effect or not. David D. (Talk) 22:38, 11 February 2008 (UTC)

is eating potato skins unhealthy?

Is it unhealthy to eat the skin of, say, a baked potato?

Thank you! —Preceding unsigned comment added by 212.51.122.26 (talk) 21:44, 7 February 2008 (UTC)

Assuming the potato has been washed prior to baking, no. --LarryMac | Talk 21:47, 7 February 2008 (UTC)

Actually, green potato skins have glycoalkaloids, a toxin. See: Potato#Toxic_compounds_in_potatoes David D. (Talk) 21:49, 7 February 2008 (UTC)

wow, those two seem like pretty opposite opinions! So, which is it? Can we have a reference for the former? The fact that thiere is a toxin in it doesn't mean anything, there's also gold in my dog's feces. The question is 'how much'. So is eating a potato unhealthy? —Preceding unsigned comment added by 212.51.122.26 (talk) 21:58, 7 February 2008 (UTC)

Note: green potato skin does not equate to baked potato skin most of the time. As to how much, who can say, every potato has a different concentration. David D. (Talk) 22:15, 7 February 2008 (UTC)

In the section that David D. linked, it specifically says "The National Toxicology Program suggests that the average American consumes at most 12.5 mg/day of solanine from potatoes (note that the toxic dose is actually several times this, depending on body weight). Dr. Douglas L. Holt, the State Extension Specialist for Food Safety at the University of Missouri - Columbia, notes that no reported cases of potato-source solanine poisoning have occurred in the U.S. in the last 50 years and most cases involved eating green potatoes or drinking potato-leaf tea." (emphasis added). It also states that the glycoalkaloids are broken down by high heat. Baking involves high heat. And it states the differences between the amounts of those compounds in wild and farmed potatoes, so your question of "how much" has been addressed.

Original research time - 1) I have eaten more than my fair share of baked potato skins in my life, and I'm still typing nearly 50 years on. 2) Some people make an appetizer of "potato skins" which are mostly devoid of potato, but full of cheese and bacon. It's those last two that are going to kill you, not the potato skin. 3) I could stop at the supermarket on my way home and buy redskin potato salad, which is made with chunks of potatoes that have their skin still attached. It is not likely the supermarket would be selling me toxic food. --LarryMac | Talk 22:08, 7 February 2008 (UTC)

Dietitians might recommend against eating baked potato skins because of the concentration of fat. They might also recommend against eating potatoes at all, or at least limiting their ingestation, because of the starch content. But it would probably depend on the eater's dietary and metabolic circumstances. -- JackofOz (talk) 22:13, 7 February 2008 (UTC)

Fat? in potato - is this from added butter perhaps?87.102.118.73 (talk) 13:55, 8 February 2008 (UTC)

[Edit conflict]Yes, it is safe to eat potato skins. If you read the link David D provided above (in the Potato article), there have been no reports of potato poisoning in the US in the past 50 years. It seems that it is even safer if you store them in the dark (as light exposure can lead to increased glycoalkaloid concentration), and avoid green or spoiled potatoes, and always get rid of sprouts on the potatoes. It is even safer if you cook them (which, of course you would - who eats raw potatoes?). The Solanine article is a bit scary-sounding (solanine is the glycoalkaloid in potatoes), but the citation, found here, indicates that potatoes are perfectly safe to eat if you take routine precautions. If I were female and pregnant, I might avoid potatoes just to be on the safe side, since there have been some (inconclusive) indications of increased risk of birth defects. All this being said, nothing you find on this page, or anywhere else on wikipedia should be mistaken for reliable medical information. In short, I am a physics nerd having a little fun on wikipedia, not a toxicological expert. --Bmk (talk) 22:23, 7 February 2008 (UTC)

Good chance is that no one in the US eats green potato skins intentionally. David D. (Talk) 22:33, 7 February 2008 (UTC)

It is reasonable common to eat potato skins in other parts of the world. Personally I never peel potatos anymore or for that matter most vegetables. One of the reasons why it is a good idea to keep the skin on is because vitamins tend to be concentrated on the layer just under the skin so when you peel of the skin you are usually peeling of part of this layer as well. As others have stated, unless the potato is starting to green it will usually be fine. Nil Einne (talk) 06:15, 8 February 2008 (UTC)

Which is why potatoes are often peeled as the solanine is concentrated right near the skin as well. Note also that potatoes are commercially bred to minimize solanine levels. Rmhermen (talk) 23:21, 8 February 2008 (UTC)

Drugs and GI motility

So i've heard that opiates cause constipation. I was wondering why this is so because i thought opiates activated the parasympathetic nervous system, which increases G.I. motility. Also, i have been told coffee increases G.I. motility, and i know caffeine stimulates the parasympathetic nervous system which would decrease G.I. motility. Thank you very much for your time! 24.88.103.234 (talk) 22:31, 7 February 2008 (UTC)

One of the major opioid receptors, mu, located in CNS results in decreased GI motility. Wisdom89 (talk) 22:34, 7 February 2008 (UTC)

Thanks! Anyone know anything about the coffeee? 24.88.103.234 (talk) 17:12, 8 February 2008 (UTC)

It's been known for sometime that caffeine does have an effect on the GI system - The only effects that I can remember off the top of my head is stimulation of gastric acid secretion in the stomach (by increasing hydrogen Ion secretion from the stomach mucosa) and increased smooth muscle contractility. This link might be useful. [1]Wisdom89 (talk) 03:00, 9 February 2008 (UTC)

Which was the LCA of plants and animals?

So that's the question. When did the last common ancestor of plants and animals live, and what kind of form was it? Finally, if one were to list plants, fungi, protozoa, protists and monera in order of decreasing closeness to Animalia, what would it be like? Thanks. -- Leptictidium (mammal talk!) 23:13, 7 February 2008 (UTC)

I don't know the answer to the first question. But the second... choanoflagellates, which are protozoa, are thought to be more closely related to animals than are any other extant non-animals. But except for them, fungi are more closely related to animals than are any of the other groups. Monerans are more distantly related to us than are the other groups you named. As for plants and protists and protozoans, similar answers could be given, but I think the best way to understand the relationships is not to try to list these non-cladistic groups in order of relatedness, but rather to look at a phylogenetic tree and understand which groups these words refer to. --Allen (talk) 01:31, 8 February 2008 (UTC)

Actually, I think I may have made up that use of the word "non-cladistic." What I should have said is that the groups "protozoa," "protists," and "monera" are paraphyletic. --Allen (talk) 01:40, 8 February 2008 (UTC)