Wikipedia:Reference desk/Archives/Science/2008 December 25

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December 25

Swimming in Purified Water

With so much controversy over the science of swimming during these most recent Olympic games (LZR suits, lane standards, turbulence-reducing walls etc.), I am curious: is the density of the water solution the athletes swim in an issue? Would the difference of buoyancy in a 100% pure water pool without chlorine and other additives be noticable? Sappysap (talk) 04:10, 25 December 2008 (UTC)

While I can't definitely answer the question there doesn't seem to be any regulations on this [1] or Olympic-size swimming pool. Obviously pure water is not what you want, that will just make you slower if anything. A salt water pool may make you faster. This ref mentions they used to contain salt water [2] but no mention if why they changed Nil Einne (talk) 11:13, 26 December 2008 (UTC)

It's worth noting that researchers found that swimming in water or viscous syrup made little difference to times[3]. --Maltelauridsbrigge (talk) 18:43, 26 December 2008 (UTC)

Would a human float/swim more easily in Heavy water, neglecting likely ill effects of drinking the stuff? It appears to be denser, promoting flotation, but more viscous, possibly impairing swimming. Edison (talk) 05:12, 27 December 2008 (UTC)

Yes, that's right. Axl ¤ [Talk] 07:29, 30 December 2008 (UTC)

Heavy water is toxic, but you need to ingest high purity heavy water nearly exclusively over several days for you to be poisoned. You could go swimming in a pool of it everyday as long as you didn't drink the pool water and drank regular water like normal. 152.16.15.23 (talk) 05:24, 31 December 2008 (UTC)

Testing for lead

Lead testing kits for sale in stores cost 8 to 10 dollars for enough reagent to do 4 swab tests, such as the First Alert LT1 test kit. It has a tiny bottle of a clear reagent which changes color when swabbed on something containing lead. Where can such a reagent be purchased in bulk? Edison (talk) 06:42, 25 December 2008 (UTC)

In particular, I am interested in testing pieces of fabric. It could be, preferably, dunked in a test tube to leach out any lead, or swabbed with something. I note that Wikipedia has an article on Lead and one on Lead poisoning but none on Lead test. There is great interest in the presence of trace amounts of lead in paint, and with California Proposition 65 (1986) in lead present in, for instance, children's jewelry and even brass keys. There is California's Assembly Bill 2901 of 2008, dealing specifically with lead in children's jewelry. Then there is the Consumer Product Safety Improvement Act which kicks in in the U.S. early next year, and which is expected to have devastating effects on small businesses such as clothing manufacturers due to the apparent requirement for third part lab testing of every minor product variation, and tests required for items already in stores or en route to stores, with the number of tests required far beyond the capabilities of all the qualified testing labs. Edison (talk) 16:27, 26 December 2008 (UTC)

There is some concern about the reliability of the test kits on the market. CPSC Staff Study: Home Lead Test Kits Unreliable says "Of 104 total test results, more than half (56) were false negatives, and two were false positives." As for reagents, that page links to the results of the study, where we find "The two types of test kits that are currently available are based on chemical reactions of either the rhodizonate ion, which produces a pink or red color in the presence of lead, or the sulfide ion, which produces a gray, brown or black color in the presence of lead." 152.16.59.190 (talk) 23:13, 26 December 2008 (UTC)

If trace amounts of lead in consumer products are the great threat they are made out to be by all the recent legislation, why are there no accurate tests available for common use? Edison (talk) 05:07, 27 December 2008 (UTC)

Technology is a wonderful thing - but sometimes what is desirable is just hard. It may just be really hard to make an easy-to-use kit that could test objects non-destructively using non-toxic chemicals - that's cheap, sufficiently sensitive, not TOO sensitive and easy to interpret, etc. Given the relatively small number you'd probably sell...you could easily imagine why they don't exist. SteveBaker (talk) 16:18, 27 December 2008 (UTC)

electric fuse

i read that thick wires shouldn't be used to make elctric fuses because their resistance is low, so the heat produced will be low. but its low resistance means more current will flow & heat is directly proportional to current flowing. so, the heat produced will be more. so, why shouldn't thick wire be used? —Preceding unsigned comment added by 122.50.128.178 (talk) 06:52, 25 December 2008 (UTC)

The fuse should melt and interrupt the current long before the actual wires heat up and start a fire. That is why large low resistance wires generally make poor fuses. The fuse wire is also likely to be a material which melts at a lower temperature than the actual wires carrying the current from the fuse box to the load. Since the fuse wires are themselves extremely short, like a fraction of an inch in some cases, the higher resistance per foot does not cause much of a voltage drop during normal operation. More current flowing through a larger wire would not necessarily produce more heat, since the resistance of the larger wire would be lower, and the heat is inversely proportional to the resistance. Edison (talk) 07:02, 25 December 2008 (UTC)

As noted below, I should have said "square of the resistance." Edison (talk) 21:33, 26 December 2008 (UTC)

A fuse of thick wire, as compared to one of thinner wire, will make very little difference in current flow. Current depends on total circuit resistance, of which a fuse (thick or thin) is only a very small part. It must be a small part otherwise electrical power would be wasted in heating the fuse (assuming current is not enough to melt the fuse). – GlowWorm

I don't know, but just thinking about it, I suppose that when the fuse blows by the melting of the wire, you want as little molten metal around as possible. Also, a thick wire would sag and maintain current flow longer before melting than a thin wire, which would come apart quicker mechanically or simply vaporize. As for the blowing, that's caused by power dissipation, not current per se, so the voltage enters into it. Look at Ohm's Law. P=I²R --Milkbreath (talk) 17:28, 26 December 2008 (UTC)

"Smart fuses " have been around many years. They have electronic circuits which measure the current, and if it is above the current or above the curve selected, an explosive charge blows the fuse element into dry sand which surrounds the current element. The current carrying element can be silver for low heating. It is all or none with no partial melting leading to changes in the current interrupting characteristics. Edison (talk) 05:05, 27 December 2008 (UTC)

Also see Miniature circuit breaker for more advanced solutions.--GreenSpigot (talk) 05:09, 27 December 2008 (UTC)

Lead test kit

How do home lead tests work? (I'm assuming they at least give some idea if lead is present even if they don't tell you how much). What compound do they use? Nadando (talk) 07:23, 25 December 2008 (UTC)

Oh, I see Edison asked pretty much the same question above me. Disregard this. Nadando (talk) 07:49, 25 December 2008 (UTC)

They don't. Even worse, they fail by giving false negatives. There was a recent Reference Desk discussion about them, citing this study from the CPSC. --Sean 23:58, 25 December 2008 (UTC)

determination of muscle mass

Hello. Since blood creatinine depends on GFR and muscle mass, is it possible, if one has the GFR (from cystatine C eg) and the creatinine to compute muscle mass? If not: How can overall muscle mass be determined most accurately? --Ayacop (talk) 08:39, 25 December 2008 (UTC)

This reference looks helpful. Of note "While it is generally accepted, based on a review of the literature, that urinary creatinine output on

a constant diet composition is proportional to muscle mass, it is also well known that even under these conditions the day to day variations in creatinine excretion can be of the order of 4 to 8 per cent which cannot be explained by small variations in diet or physical activity." I think that Magnetic resonance imaging is the most accurate (although expensive and labour-intensive). Quantitative computed tomography and dual energy X-ray absorptiometry are also highly accurate. Axl ¤ [Talk] 00:22, 26 December 2008 (UTC)

Indeed, many thanks. Always a pity that it's not that simple and needs further work. --Ayacop (talk) 08:08, 26 December 2008 (UTC)

Chemical reaction (was "test for fullerenes")

Decades ago I dissolved naphtalene in formaldehyde and added concentrated sulfuric acid. The solution turned black instantly. Now when I remember this I wonder if I was creating nanotubes, fullerenes or something like that. The reaction doesn't work when one of the components is absent. The reaction was reproduced using chemicals from different source, so it's unlikely that they were contaminated by something that went into the reaction. Can anyone give a clue what I produced? 93.132.134.138 (talk) 09:12, 25 December 2008 (UTC)

Toxic waste! The black will be short chain carbon, or soot (carbon plus other chemicals) but probably not fullerenes. Napthalene is 2 conjoined 6-carbon rings, which will break down into something related to coal tar. Thus says my hubby who did his degree in chemistry: I merely relay the information.

--TammyMoet (talk) 09:59, 25 December 2008 (UTC)

I don't understand that properly. Naphtalene has 10 C and if it 'breaks down' there would be even less. Are there C-chains that short with a black color? Neither the article soot nor coal tar gives a concise chemical description. For the toxicity, I guess it's something in between of the original formaldehyde and the soot from christmas candles. (I never planed to eat, inhale, drink or smoke it, not even to use it as an ointment against dandruff). 93.132.134.138 (talk) 10:46, 25 December 2008 (UTC)

You are making an uncharacterizable mess. The sulfuric acid will act to dehydrogenate the naphthalene, which already has very few hydrogens to begin with (it only has 8 per 10 carbon molecule). What gets left behind is essentially lots of carbon molecules, but nothing as interesting as graphite or fullerenes; just essentially unorganized carbon molecules of indeterminate shape and size. The molecules may cross-link to make larger molecules, or be relatively short, but its not going to be any consistant or predominant form of Carbon, just a random mess. --Jayron32.talk.contribs 01:29, 27 December 2008 (UTC)

I'm afraid you are right but, I still don't understand the mechanism or have heard anything that would exclude (or diminish) the possibility that there was more in it. 95.112.152.70 (talk) 09:46, 27 December 2008 (UTC)

The fact that the reaction is not producing a solid hunk of carbon (like the sulphuric acid with solid sucrose reaction) but stays as a solution means the dehydrogenation is not complete, so you are producing lots of small unstable carbon based molecules (radicals? but highly unstable nevertheless) which then polymerize together into large messy tangles. The black color like you are describing means that you have rather large complex molecules, but that large means a random mess rather than anything as orderly as a fullerine.

The large positive enthalpy of formation value for complex molecules like carbon nanotubes and buckyballs as well as the lack of any sort of selective conditions promoting their formation will limit the formation of buckyballs and fullerines. Effectively, complex orderly molecules like fullerines and nanotubes will not spontaneously form unless energy is added to the system. That energy can come in the form of heat, which is being produced by the production of other more random/energetically favorable molecules around them which have a negative enthalpy of formation (meaning they give off energy when they are made). It will take a lot of other molecules being produced (I am guessing at least the same number of carbons or more) to produce the energy needed for just one fullerine, which will sap the pool of carbon available to make them. The second problem is that the reaction is not effectively selecting for the production of these complex carbon molecules versus other things. Many other complex molecules which have positive enthalpies of formation will be competing for that same heat energy. There are myriads of other compounds with positive but lower enthalpies of formation, which means they will form much more often. Things with negative enthalpies of formation will form with more probability than either of the previous. Basically statistics is working against you to severely limit these things from forming. Taking a wild stab without exact knowledge of the statistics and assuming a 1mL reaction quantity I am going to guess you have close to zero of these molecules forming. 152.16.15.23 (talk) 05:13, 31 December 2008 (UTC)

Band Planet

Hi. I was leafing through an old high school physics textbook, and stumbled across an interesting problem:

A science fiction tale describes an artificial "planet" in the form of a band completely encircling a sun. The inhabitants live on the inside surface (where it is always noon). Imagine that the sun is exactly like our own, that the distance to the band is the same as the Earth-Sun distance (to make the climate temperate). and that the ring rotates quickly enough to produce an apparent gravity of g as on Earth. What will be the period of revolution, this planet's year, in Earth days?

The back of the book said the answer was 9 days. But I still want to know:

1) Is this a real science fiction tale the book is describing?

2) Could humans survive on the outer surface of the band?

3) Is there anything else about the ring, it's environment, or it's inhabitants that would differ from us and that on Earth? --Ye Olde Luke (talk) 23:15, 25 December 2008 (UTC)

I remember there was an episode of Star Trek that had something similar to this, the enterprise got sucked into it because it had such huge gravity. If I recall there were no people living in it but they didn't give a reason why it was abandoned. SN0WKITT3N 23:24, 25 December 2008 (UTC)

Episode was Relics. The Dyson sphere and Dyson spheres in fiction articles might have more info. SN0WKITT3N 23:27, 25 December 2008 (UTC)

That would be a rather unpleasant place. As it is always noon, at least unless the band is comparatively thin, it will be extremely hot on the inside. You also have the problem that, unlike in Columbus case, there *is* a border at which the sea and the atmosphere simply fall down into space, because as you remember the ring accelerates at 1g. —Preceding unsigned comment added by 84.187.72.95 (talk) 01:34, 26 December 2008 (UTC)

The problem of high temps could easily be addressed by placing the ring slightly farther from the Sun than the Earth's orbit, and the problem of losing the atmosphere into space could be addressed by making the cross section of the ring concave. StuRat (talk) 04:04, 26 December 2008 (UTC)

The book (or books, because there are several sequels) is Larry Niven's Ringworld. It's a classic - but some of the details are better than described in your book. Nivan supposes there would be gigantic plates tied together with stong cables in orbit around the sun between the ringworld and the sun. These he calls 'shadow squares' - and they are designed to provide an earth-like 24 hour day/night cycle (although 'sunrise' and 'sunset' don't happen at the horizon - they happen vertically overheads.

Sadly - this idea doesn't work in practice - aside from the practical matter of finding a material strong enough to form the substrate - the resulting ring structure would not be stable. It would rapidly become wobbly - then Very Bad Things would happen.

SteveBaker (talk) 01:36, 26 December 2008 (UTC)

Niven may not have realized that the Ringworld would be unstable when he wrote the original novel, but he made it a major plot element in the sequels, revealing that it was artificially stabilized by a series of motors along the rims. Incidentally, the Ringworld series is just a part of Niven's larger Known Space series, and was written after most of the other stories and novels in the series. Consequently, it uses a number of species and devices with little introduction, as it might be assumed that the reader would have been familiar with them from the earlier books. --Anonymous, 03:06 UTC, December 26, 2008.

Is it just me, or isn't it significant that they don't give the mass of the ring? --Shaggorama (talk) 07:52, 26 December 2008 (UTC)

Well, the Wikipedia article does give a mass. I don't know if it's mentioned in one of the novels or comes from another source. I don't see any great significance to it anyway. --Anonymous, 03:06 UTC, December 26, 2008.

Oh, and to answer question 2: the outer surface of the ring is not habitable. Not only does the centrifugal "gravity" pull upwards, but it's in vacuum. --Anon, 03:08 UTC, December 26, 2008.

Don't forget Gauss' law for gravity There's probably enough material to create a net gravitational force inward. It would depend on the structure of the "band". Nimur (talk) 16:27, 26 December 2008 (UTC)

Parades on a "band" planet would be awesome, but it could get ugly when everyone was selling Band Candy at the same time. BTW, the mass is hundreds of times greater than that of the Earth. The notion seems quite silly, unless magic is proposed as the means of building it, or it is so far in the technological future that most things they do would seem like magic to us. Edison (talk) 16:17, 26 December 2008 (UTC)

It seems to have huge advantages over a Dyson sphere, where you probably need more material than exists (outside the Sun) in our solar system, and you also have the problem that the material at the poles is unsupported. With the band, it could be constructed over millions of years, as a partial band could still work. If we used all the planets we could probably get enough material, as well. StuRat (talk) 21:14, 26 December 2008 (UTC)

As far as construction is concerned it should be noted that we don't know what material would work to support such a structure. In the Ringworld novel human's and their fellow aliens were unfamiliar with the foundation material of the ring. It would have to have truly amazing properties. 76.97.245.5 (talk) 08:21, 28 December 2008 (UTC)

If it has a net gravity, then yes. However, if it's placed at a distance and rotated at a speed so they balance each other, then there would be no net forces, but also no gravity. In such a place the design would need to be more like a greenhouse, where crops could be grown in weightlessness. StuRat (talk) 14:43, 28 December 2008 (UTC)