Wikipedia:Reference desk/Archives/Science/2007 September 12

From Wikipedia, the free encyclopedia

< Wikipedia:Reference desk | Archives | Science

Science desk
< September 11	<< Aug \| September \| Oct >>	September 13 >

Welcome to the Wikipedia Science Reference Desk Archives
The page you are currently viewing is an archive page. While you can leave answers for any questions shown below, please ask new questions on one of the current reference desk pages.

1 September 12

[edit] September 12

[edit] White bread - is it bad for the gulls?

I've seen this subject raised in various places online by those with an interest in feeding the larids. Is white bread *really* an unhealthy food to give to the gulls? The 'common wisdom' seems to be that one must only feed them wholemeal bread, as white bread is nothing but 'empty bulk' and 'bad for the babies'. Anyone actually know the facts? I very rarely feed them the above for this very reason - but I'm beginning to wonder if there's actually anything to it. --Kurt Shaped Box 00:50, 12 September 2007 (UTC)

Kurt, if anyone else had asked this question, we would have been waiting for you to answer it! DuncanHill 00:53, 12 September 2007 (UTC)

In Australia, white bread is bad for kangaroos and wallabies as it causes their teeth to rot. Your gulls don't have teeth do they? I expect that they may suffer a vitamin and protein defficiency if you fed them too much white bread. White bread with sugar on it has been called white death. Graeme Bartlett 01:12, 12 September 2007 (UTC)

Hmmm. I wonder if sugar causes 'beak rot' in birds? Never considered that before. I know that I used to envy my pet birds for having no teeth when I was a kid - my mouth looked like (and still does look like) Shane MacGowan's due to a genetic thing and causes me much pain and hassle. :) I've noticed that the gulls will only really eat bread if there is nothing else to be had. Bread is considered the standard 'bird food' but I've noticed that they prefer meat, boiled eggs and cheese. Guess that you can't teach an old carnivore new tricks. :) --Kurt Shaped Box 01:30, 12 September 2007 (UTC)

I think it's exaggeration to call white bread "empty bulk." I'm comparing a loaf of white vs. wheat right now, and I see exactly 186 calories per four ounce serving in both nutritional fact labels. Surely, the methodology may be called into question; whether the numbers are exact; how much actual caloric content can be absorbed by a human dietary tract (let alone a bird!) Nonetheless, I think that's a largely false claim that the wheat bread is more nutritious. I see equivalent carbohydrate, sodium, and even protein, in both of these loaf labels. Nimur 01:34, 12 September 2007 (UTC)

While I'm not disputing that there may be too much hysteria about the 'badness' of white bread, bear in mind that one of the disadvantages of white breads or more generally refined cereals is that they tend to have higher Glycemic index. Although in the case of breads according to the GI articles some brown breads can be just as bad due to the addition of enzymes. White breads or refined cereals also tend to have less Dietary fiber and lower vitamin and mineral levels. (Not talking about sodium or potassium here which most people probably don't have trouble getting) The low vitamin and mineral levels are the predominant reason AFAIK why things tend to be called 'empty bulk'. Nil Einne 20:09, 12 September 2007 (UTC)

And deciding what you should do depends on what you are trying to achieve. Do you want segulls to pester you for food, in which case feed them (anything). But if no one ever supplied them with food, they would leave humans alone. Are you feeding a pet that you want to keep healthy, or just some random birds that will appreaciate anything better fresher than the normal tip food that they consume? Graeme Bartlett 01:47, 12 September 2007 (UTC)

This is probably irrelevant, but I've heard that feeding Paracetamol or Panadol to gulls causes them to die, but this may be a myth. But I think that feeding animals food which is not their normal food causes them to be too reliant on this food, and thus lose the ability, or want to look for their own food. A recent study which I remember ready did a survey on innercity seagulls and seagulls near the coast with no humans around. The inner city ones had a higher death rate and were fatter; with less muscle. The coastal ones were leaner and more muscular and capable of living longer. So.. that's why we are often told not to feed wild animals. (could be a part reason). Phgao 04:02, 12 September 2007 (UTC)

When i worked on a gull colony 23 miles away from San Francisco the gulls there would feed their chicks any amount of human rubbish from the city. Gulls are natural oportunists, but I doubt Giants game hotdogs are the best food for growing chicks! Sabine's Sunbird talk 04:44, 12 September 2007 (UTC)

Eating one panadol pill for a seagull, is probably like a human eating 50 or 60 pills. It's bound to have a bad effect! My science teahcer used to feed sodium metal to gulls, that was unhealthy. I think they will eat anything white. But they won't touch fruit and vegetables! Graeme Bartlett 04:47, 12 September 2007 (UTC)

Why did he (your teacher) do that? Sounds like an arsehole to me. Gulls will eat boiled potatoes and sliced apple, FYI. --Kurt Shaped Box 19:49, 12 September 2007 (UTC)

Hi Graeme. I basically like to feed my local flock of gulls a decent meal three or four times a week. There are about 25 birds, mostly lesser black-backed gulls that hang out in my street and build their nests on the rooftops in the surrounding area. I first got 'into gulls' in a big way (though I always thought they were interesting) after I hand-raised an orphaned gull chick (she turned out to be a great black backed gull - an absolute tank of a bird) and started feeding her every day after she fledged. The other gulls started coming for food at the same time, I started to recognise individuals and it all went on from there, really. Quite a few of my neighbours feed them too. --Kurt Shaped Box 19:59, 12 September 2007 (UTC)

If we can accept that refined white flour is bad for people, certainly it would be bad for all mammals. Metabolism and malnutrition are the same whether you're a chipmunk or a beluga. Vranak 17:30, 12 September 2007 (UTC)

But a seagull isn't a mammal. SteveBaker 18:42, 12 September 2007 (UTC)

I would assume that feeding them anything other than their natural diet of fish or whatever would be bad for them. It doesn't matter what's in the bread so much as it's filling the bird's stomach with something that isn't full of the things that fish are full of. SteveBaker 18:45, 12 September 2007 (UTC)

I don't actually give the gulls that much bread of any kind myself - just the odd few slices here and there to save it going to waste. I generally give them cooked beef/chicken/pork/sheep (my local butcher does me a deal on offcuts and offal - it all goes in a big pot and gets cooked up), boiled potato and boiled eggs. They seem to be doing well on it - loads of large, strong-looking GBB/LBB fledglings with good plumage this year. --Kurt Shaped Box 19:49, 12 September 2007 (UTC)

Why feed them at all? Will they starve to death without your intervention? --24.249.108.133 19:41, 13 September 2007 (UTC)

[edit] a wild and crazy question

I know this is a wild and crazy question and one that will require a computer but here goes. How much of the central valley would fill with water and how long would it take to fill if you replaced the Golden Gate Bridge with the tallest possible dam? Clem 05:39, 12 September 2007 (UTC)

Do you mean a dam joining the highest point in San Francisco with an equal height in Marin, or what? (Hm, how high is the saddle point of the San Bruno gap?) —Tamfang 07:10, 12 September 2007 (UTC)

...with and without all coastal land mass gaps filled in to the same height as the dam. Clem 07:49, 12 September 2007 (UTC)

I'm not familiar with the water source. Is it the sacramento river? The would keep rising up through the source whihc would probably be a larer volume than you are contemplating. --DHeyward 07:24, 12 September 2007 (UTC)

The Central Valley is immensely big. You'd need to estimate its volume though to start with a question like this. My quick Google Earth measurements make it feel like it can be approximated as an prolate spheroid with dimensions 400 mi * 64 mi * 1000 ft or so. You do the math. --72.83.170.138 12:19, 12 September 2007 (UTC)

Current weather conditions give a non-tidal flow rate of about 1000 cubic feet per second[1] (at the Folsom Dam at least - it's the only figure I could find, and I'm assuming no rain is going to fall, which would speed things up, nor water evaporating, which would slow things down (hopefully cancelling each other out!)). Using 72.83...'s estimate slightly tweaked (taking half of a 400 mi * 64 mi * 2000 ft ellipsoid instead), I get a volume of 3 × 10¹⁵ cubic feet (20000 cubic miles). This would take 95,000 years to fill if dammed.

The Golden Gate Bridge however is only 27 metres (90 feet) high. At this level, I can assume the valley is an elliptical prism 400 mi * 64 mi * 90 ft, which would take only (!) 6,400 years to fill before water begins to lap over the edges. I don't know about what would happen without all the other land being filled in - I'd imagine that it would fairly quickly find another route to the sea over the San Francisco peninsula. Laïka 15:11, 12 September 2007 (UTC)

And when you're talking about such geologic size scales, you can't consider things like ground to be water-tight! See groundwater for an introduction. Just because you "seal off" the above-ground flow pathways does not mean the water will stay inside the basin! Incidentally, we have an entire article on groundwater modeling, including links to software packages such as FEFLOW and Visual MODFLOW. These look to be commercial software simulators. Nimur 15:42, 12 September 2007 (UTC)

If the present bridge were the top of a dam, where else would it spill over? I don't know of any pass that low. —Tamfang 05:00, 16 September 2007 (UTC)

I hope you're not planning to build a dam there. I happen to live in the central valley and would be very upset. — Daniel 03:04, 13 September 2007 (UTC)

Only if my mom increases my allowance, like she promised, and stops making me buy my own lunch at school. Clem 04:35, 13 September 2007 (UTC)

Well I'm glad we're not being unreasonable here. —Tamfang 04:55, 16 September 2007 (UTC)

[edit] Embarrassingly simple work question

Heyo there. I have a rather simple (and therefore embarrassing) question regarding mechanical work. According to the article work is defined as the scalar product of displacement and force. I had some difficulty understanding that article as it also mentions distance, but my textbooks support the idea of force x displacement. If I were to pick up an object such as a pen and raise it by a distance of x metres vertically and then drop it to the floor, would I be doing any work on the pen? I would argue that since displacement is 0 then work must be 0, but I could also see it as being the sum of the work required to raise the object and the work done while it drops. Which of these interpretations is correct? Many thanks Vvitor 07:55, 12 September 2007 (UTC)

While raising the pen, you are doing work on the pen. Simultaneously, gravity is doing negative work on the pen. For the time period in which the pen is moving up with constant velocity, the force of gravity equals your force raising the pen, and so in this period of time no net work is done on the pen. And so during this time, while the pen is gaining gravitational potential energy thanks to your own work, it gains no kinetic energy. Remember, the net mechanical work done on an object over a period of time equals its change in kinetic energy. When you first begin raising it, you must exceed the force of gravity to accelerate the pen upwards, and in doing so net work is done on the pen (confirmed further by the fact that it has gained kinetic energy). When you stop raising it, your force is less than that of gravity, to allow it to slow to come to a stop (confirmed as the pen loses kinetic energy). Since the pen began at rest, and stopped at rest, it gains no kinetic energy in the whole process of raising it, and so no work has been done on it. When you release the pen, gravity is the only force acting (ignoring air resistance), thereby allowing it to gain kinetic energy, and so work is being done on it as it is falling, as potential energy is converted into kinetic (gravitational potential energy is really nothing more than an expression for the work that gravity is capable of doing). So, in short, in raising the pen you do work on it, but all the work is cancelled out by gravity and turned into potential energy. And net displacement is not inherent to the equation for work, only the differential displacement. Net displacement is merely sufficient for calculating work done by conservative force fields, like gravitational fields and electric fields of stationary charges. Someguy1221 08:25, 12 September 2007 (UTC)

This is completely wrong, because you are forgetting about potential energy! The work done to lift something is w = mgh (assuming no net change in kinetic energy). --131.152.105.20 14:45, 14 September 2007 (UTC)

Ah, looking at the article, I see where the confusion may have arisen. The actual definition of mechanical work is the integral equation. The two algebraic equations above it are less-than-accurate expressions that only suffice for some simple examples. Someguy1221 08:29, 12 September 2007 (UTC)

That is rather...surprising, I must say. I was taught that a change in gravitational energy equals work, but your reasoning sounds clear enough. Thank you so much for your explanation. The reason I am asking this question is because I received something similar on a test, and I wasn't sure I agreed with the way it was marked. In the test question, a worker wishes to carry 850 tiles to a roof and he can carry 10 at a time. To lift them, he stands upon a barrel and lifts the load the rest of the way, steps down, repeats 85 times. Apparently the correct answer involved calculating the work done in one load, involving the worker's mass, the mass of 10 tiles and the formula Ep=mgh, and multiplying it by 85. I'm going to have to consider your explanation and read up a great deal more on what I initially thought was a simple concept. Thank you once again! Vvitor 08:38, 12 September 2007 (UTC)

You're very welcome, but let me just reiterate...A point that should be made very clear in any homework or test question is the destiction between work done by and work done on. Work is certainly done by the worker, but no net work is done on the tiles, overall. Someguy1221 08:42, 12 September 2007 (UTC)

I tend to think of work as the amount of energy you expend transporting a test particle in a conservative field, i.e. I think of $W = \Delta V = \int_{\mathbf{x}_i}^{\mathbf{x}_f} -\mathbf{F}(\mathbf{x}) \cdot d\mathbf{x}$ as the integral form of $\mathbf{F} = -\nabla V$ . This was clearly the intended interpretation in Vvitor's test question. In this interpretation kinetic energy isn't involved at all, and the work involved in transporting the tiles isn't zero. One can argue about whether the work is done on the tiles or on the field, but in any case there's a net energy transfer which is later reversed when the tiles are transported back to ground level, and W measures the amount of that transfer. -- BenRG 11:47, 12 September 2007 (UTC)

My apologies for the poor wording of the question. The worker (no pun intended) steps up onto a barrel, moves 10 tiles onto the roof of the house, steps down and repeats. At the end of the 85 loads, the 850 tiles are on the roof and the tiler is on the ground. The exact wording of the question, such as "work done by...on..." is also unclear because of my poor memory. I recall the question asking "how much work does he do". It's ambiguous, I know, but I've sadly come to expect this kind of ambiguity on physics tests. Thank you for your insight Vvitor 14:05, 12 September 2007 (UTC)

[edit] microorganisms

what are the benefits of being small to a bacteria such as E.coli in comparism to a eukaryote such as paramecium.discuss —Preceding unsigned comment added by 82.206.134.148 (talk) 07:58, 12 September 2007 (UTC)

A friendly reminder that, at the top of this page, it says: Do your own homework. The reference desk will not give you answers for your homework, although we will try to help you out if there is a specific part of your homework you do not understand. Make an effort to show that you have tried solving it first. Rockpocket 08:10, 12 September 2007 (UTC)

There's a template for that do your own homework thing. It's {{dyoh}}. — Daniel 03:00, 13 September 2007 (UTC)

[edit] Measure of temperatures

The records and the measures of temperatures are in umbra or under sun? --Vess 10:05, 12 September 2007 (UTC)

If you are talking about the weather, air temperatures are normally measured in the shade.--Shantavira|^{feed me} 10:44, 12 September 2007 (UTC)

The temperature of an object out there in the world depends on a balance between the energy it absorbs by conduction through the air, the input of energy from the sun and the sky - versus the energy it loses due to infrared radiation and conduction out into the air. Hence, if the sun is beating down on your thermometer, the solar input can push it's temperature up much higher than the surrounding air - and the temperature it'll show won't be the air temperature - it'll be it's own temperature. Putting it in the shade helps that somewhat by shutting off the solar input contribution - and whilst it's still absorbing a little extra energy from the sky, the temperature it reads will reflect much more accurately the energy from the surrounding air. Hence - only readings made in the shade are meaningful measures of air temperature with most kinds of thermometer. SteveBaker 13:17, 12 September 2007 (UTC)

A Stevenson screen is used to provide both shade and protection from wind chill. DuncanHill 13:27, 12 September 2007 (UTC)

Actually a Stevenson screen is specifically designed NOT to provide any wind chill protection (not that the term 'wind chill' has any good definition when not relating to clothed humans). Wind chill comes about in humans because when there is no wind, our bodies warm up a boundary layer of warm air. Our experience of (say) 10degreeC is not of air at 10degreeC touching our skin it's something warmer than that because of that boundary effect. When the wind blows, it moves that boundary layer away and we feel what 10 degreesC actually feels like - which is to say colder than it feels in still air. So "wind chill" is a downward adjustment of the temperature to a level that represents what temperature still air would have to be at in order to produce the same sensation. It's beloved by TV meteorologists - but completely bogus as a scientific measurement. So: Far from protecting any kind of boundary layer around the thermometer, you most definitely want it to be exposed to the wind in order to get an accurate measurements of the true air temperature! That's why the Stevenson screen has slats all round it to let the wind blow through the box. SteveBaker 18:40, 12 September 2007 (UTC)

[edit] GPS TIME SYNCH

HI SIR, i searched in ur wink guide about the gps but i dint find the relevant info i need...so plz do the needful.... i wan the PRO'S AND CONS how the gps has its hand on relay time synchronisation —Preceding unsigned comment added by 203.145.162.226 (talk) 10:53, 12 September 2007 (UTC)

I'm not sure I understand your question - but GPS relies heavily on time, each of the satellites of the GPS system broadcasts the time to high precision. GPS units on the ground read the time signals sent by each satellite all at the same instant. Because the satellites are all at different distances, their time signals are delayed by different amounts due to the speed of light. Knowing where the satellites are in space and the time they sent their signals allows the GPS unit on the ground to figure out where it is. Clearly, it's essential that the clocks on all of the satellites are accurately synchronised and the guys at the ground station that run the GPS system make sure that this is very precisely the case. Your GPS unit therefore has a very good and extremely accurate knowledge of the time just as a part of what it does. As for the Pro's and Con's:

CONS:
- A GPS unit is a lot more expensive and a lot bulkier than a wrist watch.
- GPS doesn't work underground or inside some buildings (although the GPS unit may continue to give you the time, it won't be being synchronised with the satellites anymore).
- The GPS system is under the control of the US military - I guess that in some applications you might be concerned about whether they might turn it off in times of war - but since an increasing number of critical systems use GPS, this is becoming less and less likely to happen as time goes on.
- Another problem with GPS is that if the ground unit has been switched off for an extended period of time, or if it's been turned off, then moved a hundred miles and turned back on again - then it can take it quite a while to lock on to enough satellites to get a decent result. Normally, it uses its internal clock to estimate the positions of the satellites, so things go fairly quickly.
PROS:
- It is exceedingly accurate - to within a few nanoseconds in fact!
- The time-zone can be generated automatically because the GPS unit knows where it is in the world.

I hope this answers your question - if not, please rephrase and clarify what you need to know. SteveBaker 13:11, 12 September 2007 (UTC)

One thing to keep in mind, if you actually need the time accuracy you can get from GPS, is that it isn't exactly the same as civil time. I think it's a UTC0 versus UTC1 difference or something like that. It appears to be roughly a half-second difference from my observation, comparing a clock synched with WWV to a GPS handheld. Admittedly it's also possible that the response time of the LCD display is different between the two instruments. --Trovatore 02:15, 13 September 2007 (UTC)

[edit] About Sinter & Pellet (copied from Village Pump (technical))

Dear Sir,

I would like to ask a question that What is the difference between Sintering & Pelletizing (Of Iron Ore Fines)?

Could you help me in this? —Preceding unsigned comment added by 122.168.70.129 (talk) 04:01, 12 September 2007 (UTC)

I am copying your question to the Science reference desk which is a better place for this type of question. DuncanHill 12:45, 12 September 2007 (UTC)

Sintering which see, is a general term for joining things (powder usually) using heat, pelletisation almost certainly refers to making pellets, which could be by sintering though other methods are possible (such as removing liquid from a solution/suspension of the fines maybe)87.102.16.32 14:35, 12 September 2007 (UTC)

note, there is an article on Pelletizing though I'm not convinced that it's written in english - take a look anyway - and improve it if you can...87.102.16.32 14:38, 12 September 2007 (UTC)

Typically iron dust is formed into balls (maybe as a wet sludge) then heated(sintered) to dry out and fix in shape. This 'sintering' wouldn't be/need to be as extreme as the sintering used in forming ceramics/cermets etc...87.102.16.32 14:50, 12 September 2007 (UTC)

Also pelletization of iron ore can use a binder see http://www.cs.akzonobel.com/MarketSegments/Mining/Pelletization.htm (which may not need sintering - just drying)

also see this diagram http://www.kudremukhore.co.in/pelpro1.htm which should give a rough idea of what happens - note the separate stages of mixing, 'balling','rolling' and heating last - 87.102.16.32 14:57, 12 September 2007 (UTC)

There are some good pages about both these subjects if you want to know more - just to a web search for either of the two terms.87.102.16.32 15:00, 12 September 2007 (UTC)

[edit] Stupid question about light reflection.

Pardon my ignorance. White objects are only white because they are reflecting all of the wavelengths of light that make up white light. But if they are reflecting all of the visible spectrum why aren't they truly reflective like a mirror? Jooler 13:01, 12 September 2007 (UTC)

Because their surface is diffuse. A mirror is specular. Capuchin 13:12, 12 September 2007 (UTC)

Because dull surfaces are microscopically bumpy - each tiny part of the surface reflects light off in a different direction - so the light is scattered more or less evenly in all directions. Very shiney surfaces (such as mirrors) are polished flat so that all of the light is reflected in one direction. Some surfaces are bumpy but have a larger fraction of the surface pointed in more or less the same direction - these are the majority of objects that show shiney highlights - but aren't as flat as a mirror. When you wax your car, you are using the soft wax to fill in the microscopic cracks and dimples in the paint - and the result is a shinier car. SteveBaker 13:25, 12 September 2007 (UTC)

Hm... But you'll never polish a white car to the point where it reflects like a mirror, and if you take tin-foil and keep on crunching it up and flattening it out, it will never look white. Jooler 18:39, 12 September 2007 (UTC)

The car has a layer of the bumpy white paint, which is then covered by a smooth layer of varnish - this is howe they can be both white and shiny at the same time. For the tinfoil, you'd need to get the surface really bumpy. While you would not be able to get it this rough by crumpling it, if you were to rub it quickly with sandpaper, it would definitely appear much whiter than it did before. Laïka 19:04, 12 September 2007 (UTC)

What a nice experiment! I just tried it with a ScrotchBrite pad and it turned the foil from reflective to dull white. I'll have to remember this one! - hydnjo talk 02:16, 13 September 2007 (UTC)

Hm.. I just tried it with some foil and some fine grained sandpaper, but the foil just looked scratched and torn rather than white. Jooler 21:52, 13 September 2007 (UTC)

The point here is that we're talking MICROSCOPICALLY bumpy. Crumpling foil up still leaves flat bits maybe a millimeter across. I'm talking about bumps that are maybe 1000 times smaller than that. Well maintained, polished car paint DOES act like a mirror - you can easily see your reflection in it. Take a look at my 1963 Mini Image:1963 MkI Mini.jpg - look to the right of the nearest headlamp and you can see my wife's silver Mazda reflected in the green paint. That doesn't mean that the paint is reflecting all colours equally though - it may be absorbing some of it (which is why that car is green). Car paint cleanly reflects some percentage of the light (from the surface of the clear-coat) - and scatters the remainder (that penetrates through to the pigmented paint beneath). SteveBaker 02:33, 13 September 2007 (UTC)

Okay. But I can put a nice shine on a black car and see my reflection in it nearly as well as a shiny white car, and yet supposedly the black car is absorbing light of all wavelengths and the white car none. Jooler 21:41, 13 September 2007 (UTC)

As SteveBaker says, it's the clear-coat that makes the car shiny - it is this that reflects your image. What is under the coat doesn't matter, as either way, the paint is not specular reflective. Laïka 11:54, 14 September 2007 (UTC)

Yep. In detail, it's something like this: 50% of the light striking the car is reflected (as in 'angle of incidence equals angle of reflection') back as a 'mirror-like' reflection by the clear-coat. The remainder of the light hits the underlying pigment in the paint. Depending on the colour of the pigment, some wavelengths of the light are absorbed and the remainder scattered off in all directions by the irregular surface of the pigment. If you have a red car - then it absorbs all of the frequencies that are not red and scatters red light in all directions. This scattered light is mixed up with the 50% of light that was reflected by the clearcoat. In fact, (to correct Laïka) normal 'gloss' paint does reflect. What happens is that as the paint dries, the pigment settles towards the underlying surface leaving the oily varnishes to float to the outside - so again, there is a thin layer that works kinda like clear coat on a car...but not as well. Also, when you think about the irregular surface of the pigment, it only scatters light in all directions if the surface it utterly random. Generally there are more microscopic parts of the surface that are parallel to the underlying sheet metal than there are parts that point off at crazy angles - so the reflected light won't be truly uniformly scattered unless the pain is 'matt' paint that's designed to do that. Gloss paint settles down with a larger proportion of the final surface being smooth. Car paint is becoming very complex stuff though - my orange MINI Cooper had paint that looked orange from some angles, yellowish from others, pinkish from others - and that's down to some seriously complicated surface chemistry. SteveBaker 14:46, 14 September 2007 (UTC)

Thank you for your answers. Jooler 19:38, 14 September 2007 (UTC)

[edit] Airspray

I have a can of airspray, which sprays out plain old air at a high pressure. The air is kept under pressure by propane, butane, and "aliphatic hydrocarbons", whatever those are. The can gets extremely cold to the touch after only minutes of continuous use. Why does this happen? JIP | Talk 14:16, 12 September 2007 (UTC)

Break out the Gas laws. (See especially Charles' law and Gay-Lussac's law.) TenOfAllTrades(talk) 14:22, 12 September 2007 (UTC)

Actually the butane and propane will be (probably) liquids in the can due to being under pressure.. releasing the valve lets these liquids boil/vapourise - which requires energy.. - shake the can - you should be able to here the liquids sploshing about.. (it is possible of course just to compress air without using the hydrocarbons.87.102.16.32 14:33, 12 September 2007 (UTC)

Those air sprays don't actually spray much (if any) air. You could theoretically pressurise air like that but the pressures would be huge and you couldn't get much into the can. Using liquids that boil at close to room temperature/pressure allows MUCH more gas to be stored without a heavy pressure vessel. But yeah - boiling those liquids takes energy and that cools the resulting gas considerably. SteveBaker 16:45, 12 September 2007 (UTC)

They sell instant flame throwers? I thought canned air is non-flammable. --antilived^{T | C | G} 09:41, 13 September 2007 (UTC)

The can I have on my desk (Jessops own brand) is marked as "Extremely Flammable" and "Contains petroleum gas (LPG)". DuncanHill 09:52, 13 September 2007 (UTC)

Yep - the same with mine. SteveBaker 11:52, 13 September 2007 (UTC)

[edit] transistor

Wikipedia:Reference_desk/Computing#transitor/logic_gate_switching_energy

I asked a question over there, but someone here might be able to answer - hence the link.87.102.16.32 14:48, 12 September 2007 (UTC)

[edit] Coprolalia - How does person learn the obscenes words?

I posted on the talk page to suggest it should be in the article, and then thought, why not look here? Sorry if this should be in language - but I'm more interested in how certain words are "plucked out" and then used by someone with Tourettes Syndrome or something else. However, it could also be a question of language, becasue you can't use words you don't know.

My question is, okay, someone with this will automatically start using taboo words, right? But, what if they have never *heard* the taboo word in question. They can't use a word they don't know.

I'm presuming, of course, that it works this way - say I know someone with TS who has never heard ****, putting 4 stars to mean a generic curse word. He hears it, I act shocked, and so he figures it's taboo. Does the part of his mind that says not to say it then turn off, or what?

Granted, it's hard nowadays to grow up and never hear a curse wrod, but that actually brings to mind a related question. What about someone growing up in an environment where the words are *not* taboo, like some inner cities where every 2nd word is ****? Does this person then *not* say the taboo word becasue where they've grown up, they haven't learned it is taboo?

I think the first may relate to why only a minority of people with TS use coprolalia, and maybe the 2nd does a little. I've alwyas thought the stigma of someone like that always using it was dumb becasue it ignored the basic rule of language I cited above - you can't use words you've never heard. But, I'm curious as to how that happens, and also how it's effected by a culture in which the certain words aren't taboo. Would a child of bizarre parents who taught him that the word "table" or "chair" is taboo shout "table" or "chair"? 209.244.187.155 15:56, 12 September 2007 (UTC)

Yes. Edison 16:07, 12 September 2007 (UTC)

Maybe it is the society which perceives the disease, and not the individual who has the disease. Anyway, I think any introduction to Psychology will quickly make it very clear that social norms are always relative to the surroundings. Nimur 16:21, 12 September 2007 (UTC)

Or cultural anthropology, for that matter. Shakespeare in the Bush is a classic account, IMHO. --Kjoonlee 18:26, 12 September 2007 (UTC)

Reminds me of To Kill a Mockingbird: "Bad language is a stage all children go through, and it dies with time when they learn they're not attracting attention with it." --antilived^{T | C | G} 09:39, 13 September 2007 (UTC)

[edit] Seal identification

Hi, can someone tell me what species this seal belongs to, please? I suppose it's an earless seal of some sort, but I think this image would be a nice addition to the relevant species article. --Kjoonlee 17:24, 12 September 2007 (UTC)

It looks a lot like this Common Seal here: [2]. That said, they all kind of look the same to me. --Sean 18:53, 12 September 2007 (UTC)

I agree with Sean that it is a Common Seal (or Harbor Seal) Phoca vitulina.--Eriastrum 19:33, 12 September 2007 (UTC)

[edit] Regeneration of limbs

I've heard, from two seperate sources, of a young boy who was very interested in Salamanders and such creatures who could regeneration limbs. The boy then was involved in an accident which caused him to lose a leg (or an arm, not sure) As he'd witnessed the regeneration of limbs with Salamanders, and no one told him otherwise, he believed that he could also grow a new leg. No one contradicted him, and in time this is exactly what he did. One of the people who told me this say they saw photographs of this, in stages, on the web. I have, despite numerous searches of various sites, never found such a thing. Has anyone out there seen or heard of this? It sounds the stuff of sciencfiction, but one person who swears this is true is usually a sane and sound person? —Preceding unsigned comment added by 159.134.229.90 (talk) 20:08, 12 September 2007 (UTC)

This is definitely false because other then the biological impossiblity, if he had happened believe me it wouldn't be hard to find stuff about it. Definitely the mind can have a major affect on your health, see Placebo effect but it is most definitely not capable of causing the regeneration of a limb Nil Einne 20:46, 12 September 2007 (UTC)

Sorry, why is this a biological impossibility? If lizards can re-grow tails, if salamanders can re-grow limbs, and if humans can re-grow tonsils, why would it be an utter impossibility to re-grow limbs? Just because it's never been documented it doesn't mean a declaration of impossibility is appropriate. Impossible is a rather strong word. Vranak 01:38, 13 September 2007 (UTC)

It'd be more accurate probably to say there's no known way this can happen. Friday (talk) 02:43, 13 September 2007 (UTC)

Agreed. Vranak 03:03, 13 September 2007 (UTC)

We're talking humans here, not salamanders. And IMHO it's accurate based on our understanding of human biology. You have to consider what't were talking about here. Bear in mind the poster wasn't just talking about a limb growing back but a limb growing back because the person wanted it to grow back. This would imply that the person through willpower alone was able to send the specific molecular signals resulting in limb generation. Would you say it's possible for a female who wants to be a male to grow a penis because they think they can? You can say "there's no known way this can happen" if you want. I prefer to say it is impossible Nil Einne 20:11, 18 September 2007 (UTC)

I was unable to find mention of this on snopes, but as said above- if this had actually happened we'd expect it would be major news. Since it's not... Friday (talk) 20:58, 12 September 2007 (UTC)

Somewhere I read that limb regeneration in mammals is prevented by scar formation. If scarring can be stopped, far more regeneration could take place (but at a slower healing rate than with a scar). Graeme Bartlett 02:07, 13 September 2007 (UTC)

But if this had happened, it would be REALLY common knowledge - it certainly wouldn't be hard to find. I don't believe it. SteveBaker 02:28, 13 September 2007 (UTC)

Most of the people here on the reference desk have grown a limb or four at some ealier stage in their life, so it is known that humans can do it. It is under the circumstances of the OP that this is not possible (after amputation). And will to regrow a limb would make no difference either. Stem cells for regrowth are found in the finger nail bed. But how can you make them grow a whole arm, that is the tricky part. Graeme Bartlett 02:40, 13 September 2007 (UTC)

It sounds like a fiction children's book to me. Reminds me of a Paul Jennings one where a boy grows back his finger repeatedly. Aaadddaaammm 03:40, 13 September 2007 (UTC)

Fingertips can grow back. But I have to believe that regrowing a complete adult arm, joints and all, off of a stump, is quite a different thing than growing it from nothing in the womb. Someguy1221 07:20, 13 September 2007 (UTC)

Darn it, another reclassification under urban myth!!! Richard Avery 07:43, 13 September 2007 (UTC)

Extraordinary claims require extraordinary evidence. "A guy I know says he saw it on a web site" is not extraordinary evidence. --Sean 14:58, 13 September 2007 (UTC)

Yeah, I think that's understood. The OP is not asserting that the story is real, he's asking if anyone knows the source for the story, presumably the one that had pictures. The story is also ringing a very faint bell for me, but I don't recall ever seeing photos of the supposed re-growth. Matt Deres 16:22, 14 September 2007 (UTC)

That kind of thing would be ridiculously easy to fake. If it was real, it would be easy to find. SteveBaker 20:45, 14 September 2007 (UTC)

I think the OP was unsure if this was real which was I (and I guess others) pointed out it was obviously fake (since if it was real it wouldn't be so hard to find out about it). Specifically the OP said "It sounds the stuff of sciencfiction, but one person who swears this is true is usually a sane and sound person" Nil Einne 20:15, 18 September 2007 (UTC)

[edit] terms related to livestock management (Spanish translation)

How do you translate into Spanish the following terms: - Stockbook = ? - Stud enterprise = ? - Register stockbook = ? —190.66.253.179 21:16, 12 September 2007 (UTC)

This isn't really a question for the science desk - may I suggest you ask at the language desk instead? SteveBaker 03:40, 13 September 2007 (UTC)

[edit] Genetic Relatedness of the Human Species?

I recently read that a new study shows that human beings are less related to each other than was previously thought. I was wondering how this new data effects other scientific data regarding human genetics. For instance it had been thought that modern Homo Sapiens branched off from archaic Homo Sapiens about 225,000 years ago. Since that date is based on calculations involving genetic relatedness is that date still accurate? Similarly is it still true that Caucasoids branched off from Negroids 110,000 years ago and that Mongoloids branched off from Caucasoids about 40,000 years ago? 207.69.139.139 21:44, 12 September 2007 (UTC)

These dates would have been estimated by looking at the vaiations in a small number of genes. By considering more and more genes a more accurate estimate could be made. However just because there is a great deal of variation accross people does not mean that the branch point is different. The variations could have been in the population the whole time, and the variations could be in genes that do not matter in a negative way whether or not they differ. What it means is that whole (or a big sample) of populations of people need to be compared with each other to estimate a divergence point, rather than considering one sample individual. Graeme Bartlett 02:05, 13 September 2007 (UTC)

If I'm understanding you right - I think there is a misconception. You said "The variations could have been in the population the whole time" - but that's not possible. At the point of a genetic mutation - all of the offspring that carry the mutation are descended from a single individual (although possibly with more than one mate - but you can eliminate that issue by using mitochondrial DNA). That sharply limits the diversity of the gene line from that point onwards. So, for example, at the point where Homo Sapiens split from the rest of Hominidae, one creature and one only had the mutated gene that made them different from the other great apes. Every single one of us is descended from that one individual. That's why this genetic distance thing works. If we had the genetic map of that individual, we could count the number of differences from that in the 'junk' DNA and because this has nothing to do with evolution and those differences happen at a reasonably predictable rate, we could estimate the elapsed time since that common ancestor. However, we don't have the DNA from that individual - so we have to deduce what it's DNA was like by examining the junk DNA from a lot of modern people and analysing the patterns of mutation to figure out what our common ancestors DNA must have been like. The reason our number for that is approximate is because we keep finding more individuals with new differences in that area of their DNA and that fact pushes the date of that last common ancestor back or forwards in time. But there is no ambiguity - every single change has happened since that common branch point. SteveBaker 02:26, 13 September 2007 (UTC)

You might want to take a look at race which has a fair amount of info on the genetic basis (or lack thereof) for race Nil Einne 20:33, 18 September 2007 (UTC)