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December 21[edit]

Name this bug[edit]

Okay, like a silverfish, but 90% grey and not opaque in any way. No longer than a quarter of an inch, likely about 4 mm. It lives in the Greater Toronto Area, and invades bathrooms of commercial buildings. Any ideas? -- Zanimum 00:09, 21 December 2006 (UTC)[reply]

Would it be possible to get us a picture of it? Also, you might be interested in this website called What's That Bug? — Kieff 00:15, 21 December 2006 (UTC)[reply]

Sorry, I kill them as soon as I see them. Seeing that website, they actually look not at all like a silver fish. They're essentially a tear drop shape, and there's no visible antenna or legs. The colour I'd say is more of a Prussian blue meets Old Lavender. -- Zanimum 00:21, 21 December 2006 (UTC)[reply]

That bug site is great! The profiled sow bug was also from Toronto and had also invaded. [1] --Zeizmic 01:52, 21 December 2006 (UTC)[reply]

Pantheon building maintenance ?[edit]

Question reposted from Humanities:

How can a building with a large hole in the top be practical ? I'm assuming it's completely open to the outside air, although, with modern technology, it could be closed off with a window. There are several issues I would think having a large hole in the dome would cause, how are each of these addressed ?

Water damage from rain. StuRat 00:59, 21 December 2006 (UTC)[reply]

Sun damage from UV light. StuRat 00:59, 21 December 2006 (UTC)[reply]

Birds and flying insects living in the building. StuRat 00:59, 21 December 2006 (UTC)[reply]

Temperature control. StuRat 00:59, 21 December 2006 (UTC)[reply]

The interior is concrete (the upper part) and marble, and will suffer very little from rain and sunlight. Rome is not a very rainy city anyway, but rain will only reach the floor, and there are little holes all around the middle to drain any rain water. Rome has a mild climate and is also rarely very cold (the average low in the coldest month is 5 Celsius), and when it is hot the oculus actually helps.

I see. How do they handle birds and flying insects ? StuRat 19:50, 21 December 2006 (UTC)[reply]

5 C average isn't cold? Are you insane? When it reaches 10 degrees C here I think it's cold... :-P Nil Einne 11:37, 22 December 2006 (UTC)[reply]

But you're not a building, I presume. For building maintenance, rain followed by freezing or snow + thawing + freezing is bad, as the water gets into tiny cracks and expands when it freezes, making the cracks deeper every cycle. During the day it is (on the average) much warmer in the coldest months. --Lambiam ^Talk 17:31, 23 December 2006 (UTC)[reply]

Insects would get in anyway through the "front door". I see birds (pigeons, starlings) all the time in large public buildings, like train stations and shopping malls; here, if they get in, at least they'd know how to get out again. --Lambiam ^Talk 21:19, 21 December 2006 (UTC)[reply]

While some can get in anyway, it seems like the problem would be much worse with a giant hole in the roof. StuRat 16:08, 23 December 2006 (UTC)[reply]

From above the opening must look dark, and most bird species appear to have an inhibition against diving into a dark hole. --Lambiam ^Talk 17:31, 23 December 2006 (UTC)[reply]

Melons[edit]

How are seedless watermelons grown? Crisco 1492 01:38, 21 December 2006 (UTC)[reply]

Have you read the article seedless fruit? Anchoress 01:45, 21 December 2006 (UTC)[reply]

That would do it. I just searched specifically for melons. Crisco 1492 07:48, 21 December 2006 (UTC)[reply]

tension in a rope[edit]

If the two ends of a rope in equilibrium are pulled with forces of equal magnitude and opposite direction, why is the total tension in the rope not zero? -Sruthi

A homework question? Remember that Tension involves internal forces. --Wjbeaty 03:28, 21 December 2006 (UTC)[reply]

My high school physics teacher said that in a movie where barbarians rip a man in half by tieing him between 4 horses, they could do just as well with 2 horses and a fixed object. Maybe that will be helpful. Edison 05:59, 21 December 2006 (UTC)[reply]

Sruthi, imagine yourself in the position of Edison's man where you are being pulled in two directions with forces of equal magnitude. Though you are in equilibrium, would you not feel the tension and pain? :-). The tension in the rope would be zero if it is being pulled in just one direction by just one force, but then it wouldn't be in equilibrium. It would move in the direction of the force. -- WikiCheng | Talk 06:35, 21 December 2006 (UTC)[reply]

I think, though, that the objective of quartering someone is to rip them into four parts, not three. So two horses and two fixed objects is insufficient, although three horses and one fixed object would be (sufficient). But Edison, did you have had the same sadistic physics teacher that I had? Mine used the example of dangling one of the long-haired young women in the class from our second-story classroom window to illustrate tension -- sheesh! Atlant 12:35, 21 December 2006 (UTC)[reply]

The net force on the rope (or on any part of the rope) is zero, because the rope is in equilibrium. But the tension is not the same as the net force. The tension is the internal force in the rope (in either direction) that must exist in order to conform to Newton's third law - in other words, if you pull on a rope with force T, it pulls back on you with force T. Gandalf61 08:52, 21 December 2006 (UTC)[reply]

How can you tell if a gemstone is real?[edit]

Much help appreciated. 68.67.112.91 03:16, 21 December 2006 (UTC)[reply]

Well, Id think about the only way to be sure is to have it appraised by a jewller, or similar. Vespine 03:24, 21 December 2006 (UTC)[reply]

Well, what are things that the jeweler does that a layman can do? 68.67.112.91 03:47, 21 December 2006 (UTC)[reply]

According to Gullible.info: "Pearls will dissolve in vinegar, opals in chlorine bleach, and cubic zirconia in hydrogen peroxide." I guess you could try that, for starters. bibliomaniac 1 5 03:58, 21 December 2006 (UTC)[reply]

Apart from pearls I don't think the above is right? Could the web sites name 'gullible' have anything to do with it?87.102.7.27 11:00, 21 December 2006 (UTC)[reply]

It all depends on what type of gem it is (or might be). Each gem is different (obviously) so different tests are usually needed to distinguish each type from likely imitations of that it. DMacks 04:01, 21 December 2006 (UTC)[reply]

If you have a good eye, you can generally just tell if something is valuable. It's no coincidence that precious gemstones refract light in pleasing ways. ←Vranak→

One test, for diamonds, is that they will scratch glass. There are some other things hard enough to scratch glass, too, but if your "diamond" won't, then it's not real. StuRat 07:59, 21 December 2006 (UTC)[reply]

Of course it also depends on what you mean by real. A number of gemstones can now be produced artificially with such a quality that they are difficult if not impossible to distinguish from 'natural' gemstones. Indeed, I recall a programm quite a while back where the people claimed they could produce rubies just like 'natural' rubies. The only reason you could tell them apart was because these people chose to put some sort of UV reactive dye I believe. Similarly there was an article in New Scientist I think sometime the last year or two where it was mentioned that the only reason someone was able to tell the gemstone was artificial was because it was too perfect. I think it was diamond which agrees with this link I found [2]. Point being, if by real you mean natural, it's getting increasingly difficult to tell the difference Nil Einne 13:51, 21 December 2006 (UTC)[reply]

Yes, diamonds, for example, can be made out of coal. (In fact, they are nothing else than a form of carbon) With high enough pressure and temperature, they can be created artificially. Maybe that's the way how the natural ones were formed a few million years ago. --V. Szabolcs 16:34, 21 December 2006 (UTC)[reply]

Did somebody say Occam? ←Vranak→

Different diffraction implies different material. Mr.K. 21:44, 21 December 2006 (UTC)[reply]

Somepeople can tell what a gemstone is just by looking at it, maybe with the aid of a loupe (magnifying glass), clearly a jeweller will not use a destructive test to check a gemstone. The tests I am aware of rely on refraction - measure the refractive index, and spectrum - measure the color spectrum of the gem. Other forms of spectroscopy can also be used - such as infra-red spectroscopy. The results no doubt are compared with known values for specific types of gem.. A gem dealer or jeweler will probably have the equipment to do this in the 'back room'.87.102.11.80 13:41, 22 December 2006 (UTC)[reply]

As User:Vranak says "If you have a good eye, you can generally just tell if something is valuable." - there's no real need for the above tests I mentioned.. though they can be useful in grading a gem for quality or certificating them.87.102.11.80 13:44, 22 December 2006 (UTC)[reply]

Craft[edit]

If I built a craft that had a balloon that gave the craft lift(but not all of it to reduce ballon size) and I had engines capable of producing 300 pounds of thrust and the craft wieghed around 300 pounds what is the smallest the balloon can be?72.147.86.89 05:04, 21 December 2006 (UTC)[reply]

If the weight, including engine and payload, is W, you need a total upwards thrust of more than W for lift-off. If the upwards thrust of the engines equals T, and T is less than W, you need an additional amount exceeding W−T to be supplied by the balloon. The data you give (W is around 300 lbf, T = 300 lbf) is not sufficient to determine if you need any additional lift at all, and if so how much. Using the metric unit of kgf (where 300 lbf = 136 kgf), it is the case that for a helium balloon you get about 1 kgf of lift for every cubic metre. So if, for example, W = 150 kgf, you need a balloon of 14 m³ (or actually a bit larger because that is just on the border, and there is also the (small) weight of the balloon itself). That is about the volume of a sphere whose diameter is 3 metre. --Lambiam ^Talk 09:40, 21 December 2006 (UTC)[reply]

I Like that answer thank you.

Query if the lift needed is 150kg and I get 1kg lift per cubic meter - don't I need 150m³ balloon?87.102.11.80 13:36, 22 December 2006 (UTC)[reply]

No, because that 14 meter cubed is only giving me part of the lift while my 300 pound thrust engines are giving me the rest. I have mislabled lift it is actually buyancy that the ballon is giving be.208.61.154.198 21:21, 22 December 2006 (UTC)[reply]

The darkest hour is just before dawn.[edit]

Is there any truth in this statement. It would seem more likely that the darkest hour would be at the midpoint between sunset and sunrise. Is there a scientific explanation that would make this clear.

davidwinkelaar

I believe that night, between maybe an hour after sunset and maybe an hour before dawn, is of equal darkness, except that the position of the Moon may also have an effect. The saying means "things seem the worst right before they get better". StuRat 07:51, 21 December 2006 (UTC)[reply]

Yeah. It's a statement of psychology, I'd say. It isn't really any darker just before dawn, but it's been dark for a long time, and you're getting tired of it. --Anonymous, December 21, 08:26 (UTC)

I largely agree with StuRat except to point out it's more complicated then just the moon. Obviously stars can have an effect. Most stars are obviously in a fairly constant position in one day but planet's especially, like Venus do vary. Obviously this pales in comparison to the moon but if the moon isn't visible it could arguable have enough effect. More importantly though, cloud cover can obviously very significantly affect how dark it is. Obviously this depending on the weather, not the time of day but it's an important factor to consider. And the level of ambient light will probably vary depending on the time of night if you're in a urbanised area. Even if you can't necessarily see many lights, anyone interested in star gazing will probably know how much light 'pollution' there is in cities (obviously atmospheric pollution is another factor) . Of course, a lot of this is fairly constant but in residential areas especially, there will be a difference depending on the time of night. So in conclusion, when it's the darkest depends on a lot of factors so it's difficult to actual say. The statement is simple a metaphor Nil Einne 10:08, 21 December 2006 (UTC)[reply]

You are right, the statement is wrong - the darkest hour is indeed halfway between sunset and sunrise (excluding the moon). The coldest hours are just before dawn however. I agree with the above posters that the statement only makes sense as a metaphor. If it was used as a scientific statement it is just wrong.87.102.22.58 11:18, 21 December 2006 (UTC)[reply]

I've spent many days outside at dawn this year, and while it is most certainly coldest right before dawn, I haven't noticed any trend in the photon department. Vranak 16:44, 21 December 2006 (UTC)[reply]

I think the darkest hour is the hour before dawn (or rather, the hour before astronomical twilight). By that time, most people would have gone to sleep and turned off their lights, reducing light pollution. You might think light pollution doesn't have a large effect on sky brightness, but in large cities it has a much larger effect than even the Moon.

By the way, the Sun doesn't affect the sky anymore after astronomical twilight. It doesn't matter if it's 19 degrees below the horizon or 53 degrees below; it's set too far to matter. --Bowlhover 17:06, 21 December 2006 (UTC)[reply]

Are you sure about this? Here in the northern hemisphere, I notice that the sky still retains a slight azure tint throughout cloudless summer nights -- but during the winter, not so much. →Vranak←

In my area (Toronto), the sky is always bright blue, even in the dead of night. I think this is due to light pollution, since the sky was completely dark when I visited the Dominican Republic (which doesn't have as many city lights).

Although the Moon does affect sky brightness, the stars and planets have a negligible effect. Even the Milky Way, the zodiacal light, and the [[gegenschein] contribute more to sky glow. --Bowlhover 04:44, 22 December 2006 (UTC)[reply]

I guess the key question is in the absence of the moon (as I mentioned), light polution (i.e. in a very rural area), the zodiacal light (which AFAIK doesn't occur in equtorial regions so this isn't that uncommon) whether the stars and planets will still have a neglible effect Nil Einne 05:16, 22 December 2006 (UTC)[reply]

I am aware of the metaphorical meaning of the statement. Also, the moon, stars, city lights, cloud cover, etc. are all variable and would all have an impact on a given day. Living in the northern hemisphere, latitude 54, I am also aware that the sky in the summer can glow long after sunset, and begin to glow long before sunrise, even on a cloudless day, but if we could eleminate the effects of all these and other variables, I was wondering if there could be some scientific reason I am not aware of for the literal meaning of the statement. [davidwinkelaar]

The saying is perfectly true for humans, and refers to what we see, not to the optical instrument measurement of light levels. It describes our experience, not the reality of that which is outside of ourselves. Given constant low light levels through the night, humans see less just before dawn. Reason: Due to a diurnal cyclic changes, the threshold for discrimination is at its highest just before dawn, meaning it takes more light to trigger the optic system. Therefore it seems darkest. See this article:Visual resolution in humans fluctuates over the 24 h period. Remember high threshold = low sensitivity. Possible problem: Given the very high levels of light contamination in Northern hemisphere countries (see The World Atlas of the Artificial Night Sky Brightness, especially the detailed maps), it may be that a present day citizen of those countries would never in his life discover this basic fact which has been known to humans for millenia. --Seejyb 09:45, 22 December 2006 (UTC)[reply]

Yes - that would be great - if it were true - but it starts to get brighter hours before dawn. Which people can see, with their eyes..87.102.11.80 12:33, 22 December 2006 (UTC)[reply]

You seem to be aware of the phenomena associated with twilight, but you have not experienced the phenomenon to which davidwinkelaar is referring. The fact that he asked means that it was strange to him as well, and that what the saying was claiming is inconsistent with his observations, which you are simply now repeating. In this case, the saying is valid, the observations invalid, for reasons unknown to the observer such as yourself. Your observations are limited to your experience, as determined by your environment and the number of times that you have been outside in the cloudless, moonless, dark at the appropriate hours, removed from all light pollution. I suspect you may hail from a country where light pollution over virtually the whole land area is in the gray areas of the map referred to above. In unpolluted (by light) night such as humans all knew until 150 years ago, the observation is valid in the right circumstances. Before dawn refers to before astronomical twilight, as mentioned by Bowlhover. The brightening before sunrise that you refer to is that twilight, as described in the twilight article. A point which I did not make, is that the days is coldest just after dawn, and not, as claimed in one of the replies above, just before dawn; but that is another issue. --Seejyb 02:31, 24 December 2006 (UTC)[reply]

Plus 'visual resolution' means the accuracy with with you can see - it doesn't mean 'light sensistivity'.87.102.11.80 12:36, 22 December 2006 (UTC)[reply]

Yes, the title of the paper does seem not to answer the question, but reading it would correct your view. If after reading it, you do not understand that in the human mind a drop in resolution (in darkness) means an increase in the sensation of darkness, feel free to ask again. It translates to "I don't see so well a while before dawn, it seem to be darker."--Seejyb 02:31, 24 December 2006 (UTC)[reply]

Water and Sodium Electric Power[edit]

I'll appreciate to receive information about water and sodium can make electric power. It's related to fuel cells. Thank you

Don't know much about this but fuel cell has a list of which Direct borohydride fuel cell and Molten-carbonate fuel cell may be of interest. I don't think anyone has ever suggested elemental sodium be used for a fuel cell. I don't think this would be a good idea somehow. If you're interested in what happens when you mix sodium and water, take a look at this link [3] which has a bunch of videos such as [4] (although as far as I know the skipping behaviour has been observed and was known about long before this experiment) Nil Einne 10:45, 21 December 2006 (UTC)[reply]

It's simple - simply connect a sodium rod to a wire, connect another wire to another elctrode (copper or iron would do), connect the two wires to a load eg lightbulb/ dc motor etc and then put both the wires in water - it's an electrochemical cell - however from the above posts you can see that this is not a safe thing to do.

The electrode reactions would be...

Na >> Na⁺ + e-     2e- + 2H₂0 >> H_{2 (gas)} +2OH^-

Sodium is a very reactive metal - the "sodium rod" would not make a particularly good electrode. The half cell reactions would proceed quite fast. In a fuel cell, you typically want slow, sustained reactions. --HappyCamper 12:20, 21 December 2006 (UTC)[reply]

Yeah like the videos I linked to above so. Not something you would want for a fuel cell IMHO :-P Nil Einne 11:47, 22 December 2006 (UTC)[reply]

I found Molten salt battery and a molten pure sodium battery. [5] No such fuel cells though. --Zeizmic 14:13, 21 December 2006 (UTC)[reply]

My guess is that Nil Einne's answer is what the questioner was asking about, and possibly for a public relations reason. In the DaimlerChrysler article about their minibus, they use the phrase "...the minivan, which runs on salt and water,..." This may well become the popular concept of what a sodium borohydride fuel cell is. Calling the thing "Natrium" makes this even more likely. Is this then a PR job: homely, simple, "obviously safe", making it more man in the street friendly? --Seejyb 20:59, 21 December 2006 (UTC)[reply]

Actually I don't really know whether calling the thing natrium or sodium would be better. What happens when videos of sodium explosions start propping up and people don't understand the difference? I do think saying the thing runs on salt is a bit misleading, since sodium borohydride isn't what people think of when you talk about salt and I wouldn't want someone using it instead of normal table salt... But I guess that doesn't stop companies using it :-P Nil Einne 11:43, 22 December 2006 (UTC)[reply]

Papular Mucinosis[edit]

Papular Mucinosis

Any extra information on this condition would be greatly appreciated.

Wikipedia does not yet have usable information. You can look for "Lichen Myxedematosus" on the web. The sites rarediseases.about.com and www.emedicine.com are places to start. The former is much easier to understand than the latter (for which you may have to look up every third word:-)). --Seejyb 22:34, 21 December 2006 (UTC)[reply]

Vacuum balloon[edit]

Is there a material which is strong enough that a hollow sphere (or other form) of it can be evacuated without collapsing, yet light enough that with a vacuum inside it, the average density of the structure becomes less than that of air, and therefore it floats? La ï ka 14:58, 21 December 2006 (UTC)[reply]

Great question. I saw a poster in FSU's physics building that discussed assembling a lot of long, thin balloons into a cylinder, covering the ends, then evacuating the center of the cylinder. They had all these fancy equilibrium equations that proved it could work, and I think someone had already built one. Unfortunately I can't find any mention of it with Google, because I can't think of any good keywords. —Keenan Pepper 15:36, 21 December 2006 (UTC)[reply]

http://www.freepatentsonline.com/4534525.html seems to involve something like it. —Keenan Pepper 15:39, 21 December 2006 (UTC)[reply]

Well if the thickness is t (the density of the substance d) and radius is r the mass of the sphere is 4πr²td, and the volume 4/3 πr³. The density is of air 1.2 kg/m3, so the thickness must be given by

4πr²dt = 4/3 πr³ x 1.2

dt = 1.2r/3

t = 1.2r/(3d) (or less) (in SI units d is in units of kg/m3)

Now all you need is the thickness of a given solid capable of not breaking under a pressure of ~10⁵ Pa (Nm^-2)... Maybe someone else can help here.87.102.22.58 16:16, 21 December 2006 (UTC)[reply]

Also if S is the force a planar sheet of a material can support without breaking there is the relationsip (note S is the maximum tensile or compressive load)

S = Pr where P is the pressure, and r is the radius of curvature of the sphere.

So if S is proportional to t the thickness we get: (S=kt)

S/k =t and t <= 1.2r/(3d)

Pr/k <= 1.2r/(3d)

3Pd/k <= 1.2

3Pd/1.2 <= k

So we need a material that has k (the ratio of breaking force to a given thickness "Newtons per meter") that is greater than 3Pd/1.2 = 250,000d where d is the density (in kgcm^-3).

Can anyone supply this figure for different substances?87.102.22.58 16:54, 21 December 2006 (UTC)[reply]

I think we can safely assume that a sphere is the optimal shape since it would enclose the most vacuum (can you enclose nothingness?) for the least surface area (so weight of surface material). Plus, its uniform shape would best withstand the compressive forces of the outside atmosphere.

Still, my engineering gut feel suggests there's no material yet that's strong enough, although I'd bet that a carbon composite material comes close, and I'd love to be proven wrong just for the fun of seeing a practical device.

Atlant 16:57, 21 December 2006 (UTC)[reply]

I think I made a mistake in the above relating force required to support a spherical arch - I've corrected that now.

Given that k is the ration of the maximum tensile strength to thickness k will be the yield strength eg for glass ~4000MPa, steel ~400MPa,

So 3.10⁵6000(density steel)1.2=1,500,000,000Pa (1,500MPa>400MPa not strong enough)

and 3.10⁵4000(density steel)1.2=1000,000,000Pa (1000MPa<4000MPa strong enough)

So it looks like (in my simplification) that a glass sphere would be strong enough - but not a steel one. (I can't guarantee I haven't made a mistake here - the equations I have used don't take into account the change from tensile load to compressive load as one goes from outside the sphere to inside..double correction - the load is all compressive unless the sphere deforms..)

But it looks to me a the strength to weight ratio of a glass (and by extension a ceramic such as alumina) is great enough to hold back the pressures inside a sphere containing a vacuum of sufficient size to make the overall thing big enough to float.87.102.22.58 17:33, 21 December 2006 (UTC)[reply]

However actually making such a thing is another thing...87.102.22.58 17:34, 21 December 2006 (UTC)[reply]

There seems to be a lot of discussion on this, found with Google. From my engineering perspective, the structural forces are huge, and you have very little lift force. You can see from the typical construction of a bell jar (sad article), that glass has to be very thick, and a steel vacuum chamber is also very heavy. --Zeizmic 17:53, 21 December 2006 (UTC)[reply]

yes bell jars are thick (1cm to 1inch) but much thinner glass vessels will support vacuums - provided there are no hairline cracks! - a big factor in the thickness of bell jars is pure safety.87.102.22.58 18:03, 21 December 2006 (UTC)[reply]

I agree that bell jars are 'overbuilt'. I pull hard vacuum on regular lab vials all the time and I've never had one break. ike9898 18:22, 21 December 2006 (UTC)[reply]

Agree about the forces - using the method above the thickness seems to be 0.1/(yield strength in MPa) times radius - so for a 1m glass balloon the thickness will be 0.1/4000 m = 0.025mm - quite thin! (but not impossible!!)87.102.22.58 18:00, 21 December 2006 (UTC)[reply]

The implosion of an evacuated glass balloon would be a wonderful thing to see, but I would not want to be riding in its basket! Still, this is a wonderful example of Ref Desk at work: to run the calcs and compare the required material properties with the existing. Thre would have to be a safety factor, so that the strength would need to be some multiple of the calculated minimum. Edison 19:19, 21 December 2006 (UTC)[reply]

As an aside I can't help thinking of blowing a bubble of 'bubble gum' - made of a polymer that cross links when exposed to uv light - when the bubble is big enough - turn on the uv - to cross link and harden - then evacuate...83.100.255.234 19:23, 21 December 2006 (UTC)[reply]

But that ex-post-facto evacuation will be problematic; one of the many reason's you'll need Edison's safety margin is that the omphalos from which you evacuate the sphere will be a point where implosion stress accumulates.

Atlant 14:02, 22 December 2006 (UTC)[reply]

The join between the sphere and the inlet could easily be specifically strengthened - it would only contribute a miniscule amount to the total mass if the balloon is 1cubic meter, and the 'evacuation pipe' is say 0.5cm.. (Thanks for saying omphalos by the way)87.102.11.80 15:10, 22 December 2006 (UTC)[reply]

(You're welcome. How often does one get a chance to use that word, ehh ;-) ? Atlant 19:28, 22 December 2006 (UTC))[reply]

Rather than discuss if it's possible, let me assume that it is, and instead discuss if it's desirable. That is, would such a device have a superior lift per volume than a similar-volume balloon, filled with hydrogen or helium, floating in air.

Air = 1.293  g/L
He  = 0.1785 g/L
H   = 0.089  g/L

Helium has a density of 14% of air and hydrogen has a density of 7% of air. So, the best improvement you could hope to make on lift is 14% (16%, technically) relative to a helium balloon and 7% (8%, technically) relative to a hydrogen balloon. I don't see any possibility of creating a pressure structure adequate to withstand normal air pressure for less than a 16% weight gain. So, it's a completely impractical way to proceed. StuRat 19:34, 21 December 2006 (UTC)[reply]

Is everybody just ignoring what I said? You stick a bunch of inflated balloons together, then evacuate the hollow space between the balloons. It's the only good way to do it. StuRat, you're ignoring that helium costs money. —Keenan Pepper 02:32, 22 December 2006 (UTC)[reply]

But how would you evacuate the air without making the balloons' skins stick to each other? About the cost of helium: it's pretty cheap, but is it cheaper than making a container that can withstand vacuum? Considering what the other users said about using a thin layer of glass, I really don't know. --Bowlhover 05:04, 22 December 2006 (UTC)[reply]

The poster just asked if it were possible - not for a cost analysis.. (By the way no way will "You stick a bunch of inflated balloons together, then evacuate the hollow space between the balloons" ever work - unless I have greatly misunderstood.87.102.11.80 12:42, 22 December 2006 (UTC)[reply]

Bowlhover, the balloons are wedged tightly together, and the vacuum is helping to keep them together. 87.102.11.80, what do you think will happen? —Keenan Pepper 22:41, 22 December 2006 (UTC)[reply]

I don't understand. If the balloons are together, that means there's no vacuum. --Bowlhover 02:06, 23 December 2006 (UTC)[reply]

Come on, think a little harder. Say there are ten long, thin balloons arranged in a cylinder. Balloon number one is touching balloon number two, but balloon number one is not touching balloon number three. There's a big hollow space in the middle, which can be sealed off with two end caps, and it's not going to implode if you suck the air out of it. The system is in mechanical equilibrium, and the overall density is less than that of air. —Keenan Pepper 04:44, 23 December 2006 (UTC)[reply]

If you try to evacuate such a space the balloons would just expand inwards to fill it. Think about a balloon in a bell jar - when the pressure is decreased the balloon expands.87.102.4.34 10:35, 23 December 2006 (UTC)[reply]

They expand inwards a little, but not to the center (making them wedge-shaped, with sharp corners?) because they're not infinitely flexible. This has actually been built, so stop trying to find reasons why it's impossible. —Keenan Pepper 23:06, 23 December 2006 (UTC)[reply]

This document http://www.ualberta.ca/~edtechpd/tutorials/probes/activities/LessonIdeas-ExtraProbes.doc from the answer to another question shows that ballons burst at 1.1 to 1.2 atm (see page 4 Graph of pressure as balloon is inflated)- thats when the pressure differential is 0.1 to 0.2atm - in your example the pressure differential would be at least 1atm (since the balloons must inflated to at least 1atm to inflate at all) - in this case the ballons will have surely burst already - unless they were very special balloons. Explain that then (or give a link to this balloon assemblage).87.102.4.126 11:16, 24 December 2006 (UTC)[reply]

Sorry, I can't read that because it's in Microsoft Word format. Does it say all balloons are equally strong and break at the same pressure, or does it say balloons of some particular kind break at that pressure? I said I got the idea from a poster; I've been trying to find something about it on the Web, but no luck so far. Feel free to continue this discussion on my talk page. —Keenan Pepper 01:34, 25 December 2006 (UTC)[reply]

(New ident) It does make a kind of sense to me; the balloon would collapse, but as it did so, the pressure inside the balloons would increase and begin to force the balloons back out. Incidently, some balloons must be able to withstand near-vacuums; a weather balloon can reach 40 km, where the pressure is 0.00286 atm. La ï ka 22:39, 25 December 2006 (UTC)[reply]

Right, it's in mechanical equilibrium, because the pressure inside the compartments gives the structure rigidity and prevents the hollow core from collapsing. Weather balloons, though, expand as they rise, so the pressure difference between the inside and outside (the important factor) is much less than one atmosphere, so that's a bad example. —Keenan Pepper 00:08, 26 December 2006 (UTC)[reply]

Evolutionary Biology perspectives on Fear of Speaking in Public[edit]

In the past years I've been amazed at how so many of the strange social behaviors and feelings of humans and other animals have been explained so well by evolutionary biology.

My question is -- has anyone heard of an evolutionary explanation for the very prevalent phobia of public speaking (glossophobia)? Wikipedia says that 75% of people have some sort of fear of public speaking -- yet, my intuition tells me that being able to comfortably speak in front of a group would greatly improve one's chance of gaining a mate. Why the disconnect? Why are so many brains afraid of something so socially beneficial?

Thanks in advance for any answers, theories, ideas! -Quasipalm 19:38, 21 December 2006 (UTC)[reply]

I have studied evolutionary strategies extensively, as well as the human condition. Although it may be human vanity that leads us to draw a line between ourselves and 'the animals', this distinction is not altogether a bad one to hold.

Human society is much more complex and convoluted than animal colonies, so it's not always a good idea to apply methodologies used to study the latter to the former.

In layman's terms: us humans are a relatively messed-up bunch. Fear of speaking in front of others is more a reflection of that, than features inherited through evolution, I think.

Caveat: When I say 'us humans', I can and only should speak for the society I'm most familiar with: Anglo-North American. ←Vranak→

Speaking in front of a group puts one in the focal point of atention. This is not a spot a lot of people want to be in. You are screaming hey, look at me I'm important listen! In a sense you are placing yourself above the others since they are supposed to listen to you. now this isn't a big problem with small groups of people or people you know. It gets iffy when you need to speak to a group of strangers. Imagine being a prehistoric human meeting a strange tribe. Would you wnat to draw atention to yourself? Heck no, you want to get the heck outa there.

Not to mention the fear that you might fail. You might get ridiculed or worse. Otherwise I can't really think of anything else right now. 62.194.89.68 20:38, 21 December 2006 (UTC)[reply]

"Blending in" is an excellent survival strategy. Until recently, an invading army would often execute all opposing "public figures", but leave the bulk of the population alive. So, speaking in public and thus becoming a public figure, was not the best survival strategy. Also, speaking in public when you said something that people disagreed with would often get you killed, as well. Keeping your opinions to yourself can help to keep your head on your shoulders: "Swear allegiance to the flag, whatever flag they offer, never hint at what you really feel." - Silent Running - Mike and the Mechanics. StuRat 20:56, 21 December 2006 (UTC)[reply]

Whatever the theory, it would have to be based on genetics, with a reasonable correlation between a fear of public speaking and an increased rate of reproduction. That seems rather unlikely to me. Less tenuous may be that an appropriate fear of failure may lead to an increase in survival and reproductive efficiency. Could it be that 75% of persons get appropriate and inappropriate fear "mixed up", as Vranak suggested, and that this inappropriate fear is determined to a large extent by upbringing, education and environment, not by your evolutionary biology? Then again it may be that inappropriate fear is indeed a characteristic that improves survival and reproductive rate, so that fear of public speaking is but a manifestation of this irrationality that may have evolutionary advantages. --Seejyb 22:23, 21 December 2006 (UTC)[reply]

Just a thought, fear of public speaking = fear of rejection, by peers and possible mates, this whould certianly have serious consequences on one's reporductive fitness. --Cody.Pope 22:35, 21 December 2006 (UTC)[reply]

This seems like a rather complex phenomena, some mixture of generic fear and things of that nature. I too am skeptical that there could be a good evolutionary explanation that wasn't just a just-so story. --24.147.86.187 23:35, 21 December 2006 (UTC)[reply]

I guess I would rephrase my question as performance anxiety rather than just fear of public speaking. I've read up on it and the fear many (most) people experience when speaking in public is often cited as a fight-or-flight response. Including the release of adrenalin, increased heart and lung action, etc. That's what got me thinking of the causes of a response and evolution. It seemed strange to me that standing in front of a crowd and speaking would illicit such a response. Overall these are very thought provoking responses everyone! -Quasipalm 00:26, 22 December 2006 (UTC)[reply]

Well the "fight or flight" response is a general response to stressful situations and for the reasons described above, it is a stressful situation. Perhaps the name is confusing you, althought it's called that, don't assume your response must be to literally fight or flee, especially given the complexities of human social interactions. Of course you are fighting in a way. Speaking in public can be taught of as a form of "fighting". The alternative is "flight" i.e. don't take part. At a guess, I would say quite a number of people, especially teens exhibit the same response in response to infatuations but again, it's not fighting literally but pursuing the infactuation is a form of fighting (e.g. calling the person, asking the person out on a date etc) Nil Einne 05:11, 22 December 2006 (UTC)[reply]

You talk of fear and anxiety but let’s not forget quite a few people actually get a big kick out of it (is it adrenalin?) some I’m sure to the point of a form of addiction. Keria 11:17, 22 December 2006 (UTC)[reply]

"Stable" Hydrogen?[edit]

This is purely a construct; I need a bit of "pseudo science" for a book plot:

Is there a plausible chemical method to render hydrogen less reactive? I'm looking for a cheaper alternative to helium to use as a lifting gas for giant blimps, "cities in the sky" etc., but without the dangerous flammability of hydrogen. Yes, I know it isn't actually possible, what I want is a plausible lie. The plot may connect the substance to a search for a safer way to store and transport hydrogen for the coming "hydrogen economy," so spending a few megabucks in research would be plausible.

It seems to me that, as carbon can form stable forms via co-bonding with other carbon atoms, so, perhaps, "stable hydrogen" might be formed by similar hydrogen-hydrogen bonds? The result would be a diatomic molecule, functionally similar to monatomic helium, but without the messy nuclear fusion reaction needed to create real helium.

Any ideas?

Well, it's completely nonflammable in the absence of oxygen, so why not use that ? You could also have it mixed with some gas, like Halon, which bonds with any free oxygen it finds. StuRat 19:53, 21 December 2006 (UTC)[reply]

A couple of minor nitpicks. The Halons don't bond with free oxygen; rather, they quench radicals that are generated as part of the burning process (species like H· and ·OH) which normally perpetuate the combustion reaction. Bear in mind that adding Halon in sufficient quantities to prevent combustion (typically about 5% by volume) will totally blow the buoyancy benefit you got from using hydrogen.

That said, might you be able to get away with a double-envelope design? Make the bulk of the envelope hydrogen, surround it with a layer of helium (or even fire suppressant of some sort) and add sensors to the outer envelope to detect hydrogen. Again, it's a question of whether or not the extra envelope weight blows the buoyancy benefit of hydrogen. (Of course, if I'm holding up a city then I'd want the balloon to have double walls and lots of compartments anyway....)

Or, for floating cities, get rid of the lifting gas entirely. Suspend the whole thing on carbon nanotube cables (see skyhook (structure) for various related principles).

It all depends on how far in the future you want to set this, and which sorts of techniques might make useful plot devices. :D TenOfAllTrades(talk) 20:14, 21 December 2006 (UTC)[reply]

Thanks for ideas so far, but we're moving too far afield. I'm not looking for plot devices, just a stable form of hydrogen. Any chemists out there?

I do not see stable hydrogen being easy to make in plausible way, but you do have the claims of cold fusion to work with, i.e. it produces helium (and energy) from hydrogen at low temperatures in the appropriate circumstances. You have a reasonable literature on the possibility of such processes already, and you are not bound by the need for real life observations to develop it further. Such a process may be modified specifically for helium production, with energy as merely a beneficial byproduct. --Seejyb 21:56, 21 December 2006 (UTC)[reply]

"The University of Queensland, Australia, has just spun off a company to manufacture a new storage medium for hydrogen, using magnesium allows (sic) with a sponge-like nanostructure"[6] So perhaps some nano-engineered metal-hydride sponge structure that allows an unprecedented packing efficiency. Use for example palladium [7], which might be doped with rare earth metals. --Lambiam ^Talk 23:09, 21 December 2006 (UTC)[reply]

Lets call it Aerogel but from metal. Acting like the copper net used for miners lamps to make it impossible that the fire goes through the net, because the heat is absorbed so fast that the flame deminishes. This small metal bubles are filled with hydrogen and its like building a wooden raft you simply take a few m3 of this airfloating material and you stand on it. Take several km3 and build your city in the sky!--Stone 23:17, 21 December 2006 (UTC)[reply]

Regarding the diatomic hydrogen being stable, hydrogen already exists as H₂, which is presumably what hydrogen balloons used, so that wouldn't be the answer. 80.169.64.22 11:45, 22 December 2006 (UTC)[reply]

H₂ is stable - but burns/explodes with air - eg Hindenberg disaster.87.102.11.80 13:29, 22 December 2006 (UTC)[reply]

I would suggest if you going to use psuedo science why not go the whole hog and totally make up a new element.87.102.11.80 13:29, 22 December 2006 (UTC)[reply]

Two suggestions 1) A Device that disables the ability of the hydrogen to form chemical bonds. 2) A Device that fills the blimp with free electrons. The electrons natural repulsion would fill the blimp, and the electrons have even less mass than Hydrogen. The blimp material would have to be a non conductor, and/or the device would have to be able to repentish the electrons as fast as they bleed away. Of course, the fire hazard would be replaced by danger of electrocution. Jokem 14:53, 22 December 2006 (UTC)[reply]

I don't know about anyone else, but I personally hate science fiction writers who give "scientific" explanations which are demonstably wrong (with current scientific knowledge). It's sort of an uncanny valley type of effect. I have no issue with science fiction devices and explanations which are theoretically possible but not yet feasible (e.g. space elevator) and even those which the theoretical possiblity is still debatable (wormholes, time travel). On the other side, I can completely except and enjoy science fantasy and even fantasy writing where the author deliberately ignores physical reality and just waves his hands about explanations. What really annoys me is those authors who want to have it both ways -- make up devices that we know *can't* exist, and then make up some pseudoscientific explanation, flaunting the fact that they've read A brief history of time (but probably not understood much of it). Unfortunately, as I understand it, simultaneously non-flammable and buoyant hydrogen is something current science "knows" can't exist. Sure there are some theories like Blacklight Power's hydrinos, but those are pretty universally held to be quackery. My suggestion to you is to go the Science Fantasy route -- wave your hands. E.g.:

Naive Farm-Planet Raised Protagonist: "They're filled with hydrogen? Isn't that dangerous?"

Knowledgeable City Dweller, Who Takes the Protagonist Under His Wing: "Not at all. They call it 'The Soltero Process.' You have to be a Fifth-Level Quantum Physicist to understand the math, but it renders hydrogen completely safe."

You can thus "explain" why hydrogen doesn't react, but not have the explanation be grating to someone who is more knowledgeable on the topic than you are. -- 16:23, 22 December 2006 (UTC)

Of course, in a science fiction setting, Helium should be cheap. It is the second most common element in the universe ... WilyD 17:44, 22 December 2006 (UTC)[reply]

You could say that because it's the future, there are materials strong enough to make a vacuum balloon (see two posts above). This would be both incredibly safe (you can't set fire to a vacuum!) and fairly cheap (assuming it's the future, some sort of carbon fibre, which could be made from just about anything, would be a lot cheaper than hard to get helium). La ï ka 18:47, 22 December 2006 (UTC)[reply]

EE - Why do fast battery chargers have larger power bricks?[edit]

I was looking into rechargable NiMH AA and AAA batteries. I noticed that the faster the charger, the larger the "power brick" needed for the unit. "Slow" chargers (4-8 hours) don't even use power bricks. But 60 min charger use a medium sized brick and 15 min chargers use huge bricks that are larger than the actual charger. Why is this? --24.249.108.133 22:56, 21 December 2006 (UTC)[reply]

An interesting observation. Most likely because it takes more watts and more amps to charge the batteries to the same volt-amp rating in a shorter time. This requires bigger wire in the transformer windings to limit voltage drop and heating of the conductor, as well as a larger cross section in the iron laminations of the transformer core to prevent saturation of the metal. Edison 00:29, 22 December 2006 (UTC)[reply]

I also noticed this when I worked in a power tool shop. Cordless tools follow a similar pattern, the crappy chargers are small no frills type things while the super duper fast chargers are quite large and fancy. I don't know the details but the fancy super fast chargers actually have quite a lot of circuitry in them, I always guessed to monitor temperature and voltage and vary the input for the optimum effect. Vespine 03:08, 22 December 2006 (UTC)[reply]

The problem is likely to be that as charging time decreases, current increases in inverse proportion, e.g. from a 60min to a 20min charger, the current has to triple. But, according to Joule's law, heat production increases exponentially with current, i.e. by the square of the current. So in our example we would have nine times the heat production, and have to build the transformer correspondingly more robust. For the slowest and quickest of the chargers you described, the difference in heat production is (24x24) 576 fold .--Seejyb 11:17, 22 December 2006 (UTC)[reply]

Listing of AC power cord plugs?[edit]

While I found Electrical_plug very useful, I'm also looking for a list of the female ends of AC power cords. There seem to be two popular styles in the USA. There is the triangular three hole used to power computers, monitors, Xbox360 and other high-draw equipment. And there is the two hole type used to power VCRs, DVD players, and other consumer electronics (some are rounded on the left and the right while others are squared off on one side). What are their official names? --24.249.108.133 23:11, 21 December 2006 (UTC)[reply]

Aren't these just the "Type A" and "Type B" North American ones described on that Electrical plug page? Those descriptions refer to the NEMA connector page, where you can learn about the different specific designations of them. DMacks 23:44, 21 December 2006 (UTC)[reply]

I know what you mean - see IEC connector - was I right?87.102.22.65 02:26, 22 December 2006 (UTC)[reply]

You are surely correct; there are several styles of "IEC Inlet Connectors" depending on the amount of current and the need (or not) for a safety ground/earth connection. These are worldwide standards and make it easier for the manufacturer to sell their equipment anywhere in the world, simply by exchanging the power cords (and assuming, of course, the ability to work with both "120" and "240" volt power).

Atlant 14:10, 22 December 2006 (UTC)[reply]

Spot on! Thank yo very much! --24.249.108.133 22:15, 22 December 2006 (UTC)[reply]

black holes[edit]

black holes are theorized to form from neeutron stars right. so when a neutron star collapses onitself it slows light down more and more until bam it can escape, so would a black hole apear to be a neutron star, based on the same theory that makes it apear as tho an object goin into a black hole issuspended in space. iwould appreciate someone giving their thoughts on this because i dont know enough fysics yet but am doin a paper on them. --69.140.210.163 02:30, 22 December 2006 (UTC)[reply]

Once I figured out what you were saying it seems exactly right. Quote from the article Black hole:

Such objects for a while were called frozen stars since the collapse would be observed to rapidly slow down and become heavily redshifted near the Schwarzschild radius. The mathematics showed that an outside observer would see the surface of the star frozen in time at the instant where it crosses that radius. —Keenan Pepper 02:36, 22 December 2006 (UTC)[reply]

One correction. It doesn't slow light down, it just bends its path so it can't get out. Clarityfiend 04:28, 22 December 2006 (UTC)[reply]

In order for you to continue seeing something, photons have to keep hitting your eye. Those photons would normally be continuously emitted by the star, but that's not the case for black holes. It may be that the last light to escape the star would show you how it looked just before collapse, but that light would not remain visible for long. — BRIAN0918 • 2006-12-22 17:04Z

I recall my undergrad astronomy teacher saying that the "final image" you would see would eventually fade into blackness. That never made sense to me — I would expect it to be a sudden disapperance, as the light either could or could not cross the radius. But I am neither a physicist or an astronomer. --Fastfission 17:39, 22 December 2006 (UTC)[reply]

No, fade into blackness is correct. Basically, the "time dialation" you see for the object increases to infinity as it approaches the event horizon, and so you never see it cross the event horizon. But because time passes for the object very slowly (from your perspective) it releases less and less photons per second, and so gets dimmer and dimmer (plus increased gravitational redshifting makes it redder and redder. WilyD 17:42, 22 December 2006 (UTC)[reply]

Poseidon[edit]

Is the wave phenomenon used in the movie Poseidon real, and if so, what is its cause? I was watching the movie yesterday and the wave broke my suspension of disbelief because it seemed utterly impossible. I considered a tsunami, but the ship was too far out to see for that to have had an effect. Crisco 1492 03:00, 22 December 2006 (UTC)[reply]

It's real (although, not having seen the movie, I don't know how close the depicted wave was to real ones). See rogue waves, and its section on causes. --Anonymous, December 22, 2006, 03:22 (UTC).

If i remember well in the first movie the wave occured by a clear calm night and as such doesn't seem possible. Keria 11:08, 22 December 2006 (UTC)[reply]

Rogue waves can travel great distances, far from the storms that provoke them. Essentially, they are just the extremes of the stochastic processes that produce any ocean waves. IIRC, there was once a famous one that swept hikers off an oceanside cliff in Maine.

Atlant 14:21, 22 December 2006 (UTC)[reply]

The main believability problem I had with the wave scenario is that at sea, waves tend to have a very minor amplitude since they have a LOT of water to work with, and then when they come ashore the full strength of the surge is realized as water gets shallow and the energy becomes concentrated. I am no environmental scientist, but I really doubt a several hundred foot high wave can exist for more than a mile or two out at sea before it flattens dramatically. --66.195.232.121 16:07, 22 December 2006 (UTC)[reply]

66.195.232.121 is correct. Tsunamis and other waves in the deep ocean do not have large amplitudes at the surface; this occurs near land when the water bottom shallows. Geologyguy 16:22, 22 December 2006 (UTC)[reply]

Read the article. --Anon, Dec. 22, 18:30 (UTC).

RTFA? (Read the fucking article?) X [Mac Davis] (DESK|How's my driving?) 01:42, 23 December 2006 (UTC)[reply]

The wave occurs on a clear night, like the original. And, having read the article, I'm still confused. It seems beyond the realm of possibility for said wave to occur, although if they've been tracked scientifically, it may be true. I personally don't think too many people are aware of the Rogue wave phenomenon, and as such, most critical viewers would lost said suspension of disbelief. Crisco 1492 23:30, 22 December 2006 (UTC)[reply]

Until they see the evidence! We have a few satellites that have tracked them, I bet you can find sites in the ex links. X [Mac Davis] (DESK|How's my driving?) 01:42, 23 December 2006 (UTC)[reply]

This site has a nice pic of an approximately 25 meter (81 foot) high rogue waves breaking over the deck of an oil freighter: [8]. StuRat 15:57, 23 December 2006 (UTC)[reply]

Awesome pic. Shame it isn't in the public domain. Crisco 1492 00:51, 24 December 2006 (UTC)[reply]

Solar system and stardust[edit]

Where did it come from. If we are all made of stardust, which star?--Light current 04:57, 22 December 2006 (UTC)[reply]

I think Timeline of the Big Bang and Big Bang should provide your answers Nil Einne 05:03, 22 December 2006 (UTC)[reply]

If any star contributed to the nebula that formed the solar system, it probably contributed by adding supernova debris. Any star that went through a supernova is dead, and therefore not a star anymore. --Bowlhover 05:15, 22 December 2006 (UTC)[reply]

The sun is at least a third generation star (based on the relative abundance of heavy elements), meaning that it is star that coalesced from material that had been mixed with supernova ejecta from previous stars that had in turn already been mixed from supernova ejecta of previous stars. Dragons flight 09:57, 22 December 2006 (UTC)[reply]

Yes so if were all made fronm the original nebula, both the sun and the planets were made from the same stuff?--Light current 15:38, 22 December 2006 (UTC)[reply]

The sun and planets originally formed from the same stuff, but over time, objects have bombarded the planets, adding new stuff. — BRIAN0918 • 2006-12-22 16:50Z

I would say both were, and are, made of the same stuff. However, the original mix was very heavily weighted towards hydrogen and helium, but most of this was lost from the planets, especially the terrestrial planets, as they have insufficient gravity to hold light elements in the atmosphere. Such elements tend to be blown away by the solar wind. Thus, free hydrogen and helium are now somewhat rare on the terrestrial planets. StuRat 15:51, 23 December 2006 (UTC)[reply]

The stellar population article tells that "As of 2006, no Population III stars have been found", meaning that original nucleosynthesis of elements heavier than helium, such as carbon and oxygen that form organic molecules is still a theory : We are stardust, but of virtual stars.

The fact that we cannot observe those stars or reproduce the exact condition prevailing when they were born does not mean that science requires acts of faith. It just means that some science facts are not in the reproductible paradigm, thanks G.D. -- DLL ^{.. T} 19:18, 22 December 2006 (UTC)[reply]