Wikipedia:Reference desk/Archives/Science/2008 March 20

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.

March 20

Capacitor-Batteries

Can capacitors be controlled to discharge at a near constant voltage using some electrical circuitry apparatus? In other words, I'm asking if a capacitor can be used as a battery, and how a circuit could be/is constructed to do this.

Thanks, 74.173.90.59 (talk) 00:28, 20 March 2008 (UTC) Sam

I think a capacitor that works like a battery is a battery. See battery capacity and discharging -- MacAddct  1984 ^{(talk • contribs)} 02:01, 20 March 2008 (UTC)

Ultracapacitors with carbon nanotubes covering plate surfaces to increase surface area may in the future replace common dry-cell batteries. Mac Davis (talk) 03:16, 20 March 2008 (UTC)

I think you two are totally missing the point of this question. The voltage of a battery falls off slowly as the remaining charge decreases, until the end when it falls off sharply; e.g. it could have half a charge but still 90% of the original voltage. The voltage of a capacitor, on the other hand, always falls off linearly with the remaining charge, because the energy is stored in the form of an electric field produced by separated charges. Sam here is asking whether it's possible to modify the charge-voltage relationship of a capacitor to seem more like that of a battery by using an electric circuit. I don't know the answer; I just wanted to clarify what the question is. —Keenan Pepper 05:02, 20 March 2008 (UTC)

You can use a DC-DC converter. It can take a non fixed voltage and convert it to a fixed output voltage. There will be a lower limit where the input will fail to be enough to drive the circuit. These can also be found in cameras and LED torches to extend the life of the battery. Graeme Bartlett (talk) 05:10, 20 March 2008 (UTC)

Tuberculosis

Just got back from the doctor who told me something that has left me in a bit of shock. In America where there are ample medications to treat TB vaccines are not given as in some parts of Europe, Asia and Africa where the reverse is the case. The reason is that once a person is immunized against TB they develop antibodies, the problem being that the test for TB is based on whether or not a person has the antibodies. In America people are not immunized so they can serve as indicators of an epidemic. Does this mean that in America as far as the government or health care system is concerned the it is better that the population serve as human Guinea pigs than to be immunized against a disease despite the fact that almost no one in America would be immunized against TB in the event of a major catastrophe like nuclear war, followed by bombardment with biological weapons? —Preceding unsigned comment added by 71.100.10.177 (talk • contribs)

According to the CDC, the "BCG vaccination is not generally recommended in the United States because of the low risk of infection with M. tuberculosis, the variable effectiveness of the BCG vaccine against pulmonary TB, and the vaccine’s interference with the ability to determine tuberculin reactivity."[1] table of contents. -- MacAddct  1984 ^{(talk • contribs)} 03:18, 20 March 2008 (UTC)

That would certainly confirm my doctor's comments and my fear regarding a situation where in the absence of vaccination contraction of the disease by a major portion of the US population would be too late to prevent. —Preceding unsigned comment added by 71.100.10.177 (talk • contribs)

I think basically the CDC has decided (and weighed the +/-) that the vaccine in adults is nowhere near effective enough and would just rather have doctors know if someone is infected or not. Otherwise we'd all have to get chest X-rays every year. Also there were only 646 reported deaths from TB in the US in 2005 out of 14,200+ people infected. I think a smallpox outbreak is something more more disconcerting. [2] -- MacAddct  1984 ^{(talk • contribs)} 05:13, 20 March 2008 (UTC)

Just to correct a statement by the original questioner, it's not a question of antibodies. Someone who has received BCG vaccine, or been exposed to tuberculosis, develops cellular immunity, which (rather than humoral immunity) is the mechanism responsible for positive PPD reactions. As to the other point, TB doesn't spread quickly enough to be an effective biological weapon. - Nunh-huh 06:37, 20 March 2008 (UTC)

Thanks for the clarification. However, in a situation unlike in Japan where there were intact resources surrounding the two bombings that would not be the case following a nuclear exchange today after which one might expect a combination of biological weapons meant both to kill and to exhaust resources. In other words TB might not be an effective killer but it might be one heck of a resources user along with a mired of similar lack luster bio weapons. America has been caught with its pants down before.

What can I say. The experts differ with your assessment. In a situation where healthcare resources are strained or absent, problems arising from sanitation, such as cholera and dysentery, and other diseases, they feel, are more likely to cause immediate problems than TB. The downside of BCG vaccine (ineffectiveness, and rendering tuberculin tests far less useful) are felt to outweigh the advantages (some level of immunity in part of the population). - Nunh-huh 08:37, 20 March 2008 (UTC)

United States civil defense has always been very weak. It tends to remind the voters how nasty these sorts of weapons are. --Sean 14:12, 20 March 2008 (UTC)

Worrying about whether TB would be a problem in the wake of a nuclear exchange seems, to me, to be akin to worrying about how the problem of racial prejudice would be affected after an impact event. Yes, we can dream up all sorts of mad scientist situations but if there isn't a good reason to believe them plausible (or the most effective way for anyone to get what they want) it's not worth the resources to worry about them, much less act upon those resources. There are far more effective agents for use as biological weapons than TB; if we are talking about an enemy that can start nuclear war (and would?), I'm sure they could do far worse. --Captain Ref Desk (talk) 16:57, 20 March 2008 (UTC)

LOL... You remind me of one of the reference desk librarians at our local library who likes to give her opinion whenever we consult her to find reference material and studies, such as a list of biological weapons following a nuclear attack in the order of significance of their expected impact. Alas, whenever we want reference material and studies we have learned to ask a different librarian.

Foxes in the UK

Isn't it strange that you can find foxes in Glasgow and London but not in Newcastle?Mr.K. (talk) 03:26, 20 March 2008 (UTC)

It would be strange. Do you have any references to this? -- Q Chris (talk) 12:20, 20 March 2008 (UTC)

Not more than personal opinion. Mr.K. (talk) 17:38, 20 March 2008 (UTC)

Mange perhaps? Some years ago I remember the foxes in Bristol almost died out from mange. I think outbreaks of mange remain localised.--80.176.225.249 (talk) 18:51, 20 March 2008 (UTC)

Maybe the upper class prefer to hunt in Newcastle? Nil Einne (talk) 12:06, 21 March 2008 (UTC)

sound transducer

how can i make a sound transducer by myself in home?actually i want to make a switch which can be turned on by the sound of clapping .

have a look at [3] or other links from a search for "clapper switch circuit", with which i found the given url. sorry no-one answered your simple request earlier. 153.1.253.5 (talk) 13:40, 20 March 2008 (UTC)

Quantum Mechanics: Orthogonality of Dirac Delta Function

The functions $u(x)$ and $v(x)$ are said to be orthogonal on interval $(\alpha _{1},\beta _{1})$ if their inner product is zero

$\int _{\alpha _{1}}^{\beta _{1}}u(x)v(x)\,dx=0$ (1)

For complex-valued functions or kets $f(x)=|f\rangle$ and $g(x)=|g\rangle$, they are said to be orthogonal on interval $(\alpha _{2},\beta _{2})$ when

$\langle f|g\rangle =\int _{\alpha _{2}}^{\beta _{2}}f^{*}(x)g(x)\,dx=0$ (2)

To continue my last discusstion, Quantum Mechanics: Entangled Wave Function, my question is how to prove the orthogonality of the Dirac delta function $\delta (x)$ mathematically? Or some related resource? Thanks! - Justin545 (talk) 08:53, 20 March 2008 (UTC)

This is really a maths question, but the way to prove it is to look at the definition. The dirac delta function is zero at all the places except for the argument. If you multiply this zero value by another dirac delta you, will get a zero, unless the arguments are the same, when it will be greater than zero. $\delta (t-x)\delta (t-y)=0$ if x not equal y. Graeme Bartlett. Another way to look at the dirac delta is that it is a sampling function, when you integrate its product with another function, it samples the other function at the argument to the dirac delta. (talk) 11:14, 20 March 2008 (UTC)

VHF/FM Radio Frequencies

The current commercial FM waveband is 88-108 MHz. However, I am trying to discover exactly when this spread came about.

I have a number of radio receivers, the dials on each differing significantly. One, an American set, covers 90-108Mhz. Two others, both British, cover 88-101 and 88-104 Mhz respectively. I have, however, been unable to ascertain when the full spectrum (88-108) came into use. Can anybody help?

Samilong (talk) 10:29, 20 March 2008 (UTC)samilong

Take a look at FM band, it does not answer your question, but shows even more alternatives. Graeme Bartlett (talk) 11:49, 20 March 2008 (UTC)

FM broadcasting in the USA looks like it covers more of the specifics, though it doesn't date the extension of the range from 106 MHz to 108 MHz. — Lomn 12:53, 20 March 2008 (UTC)

spin and (hyper)complex numbers -- why is the quanta of angular momentum 1/2

What is the connection between the intrinsic spin of quote-unquote elementary particles and the imaginary operator(s) defining complex numbers (and their further generalisations)? Intuitively, it seems they ought to be reasonably closely related, but this does not appear to be reflected in the "popular" literature. (One supposes that this intuitive sense could be ellaborated upon, if really necessary, but does not feel that it ought, as the connection is expected to be known, if not superficially evident.) 153.1.253.5 (talk) 13:53, 20 March 2008 (UTC)

I don't think there is any intrinsic connection.

One reason why spins are often defined as 1/2 or -1/2 is that the spin difference between the two states is one . 1. A nice simple number and ideal for quantisation. Quantisation means allowing a property to have only integer multiples of a value. Eg if energy can be 10,20,30,etc (and not 22.3, 35, or 44) Joules then that energy is said to be quantised. Quantisation can also refer to methods that treat non-quantised properties as being quantised for the purposes of simplicity (eg when contructing a model or theory)

Spin is assumed to be intrinsically quatised; why that is so is another question.83.100.183.180 (talk) 18:17, 20 March 2008 (UTC)

The reason imaginary operators/complex numbers were not used is simply that they were not needed to describe the situation. Negative number possess sufficient weirdness to do the job. Have a look at Quaternion for a 4d consistent system, (IMHO) Its only a matter of time before this is used to describe a physical processes. You might find this interesting from a quantum point of view Spin-statistics theorem. GameKeeper (talk) 21:34, 20 March 2008 (UTC)

Quaternions are used to represent rotations in 3D and 4D space - see Quaternions and spatial rotation. Indeed, part of William Rowan Hamilton's reasoning that lead to his discovery of quaternions was a search for a higher dimensional analogue of the way on which 2D rotations can be represented as multiplication by complex numbers with unit magnitude. Gandalf61 (talk) 10:46, 21 March 2008 (UTC)

Tapdancing gulls

We haven't had a gull themed question in a while so here goes. Some local species of gull tap their feet onto the ground. My guess is it's either some strange courtship ritual or perhaps a method to draw insects out of the ground (I only see them do this on grassy fields). What do you guys think? PvT (talk) 14:11, 20 March 2008 (UTC)

It makes worms come up. This answer says the sound annoys the worms, although I've heard other explanations that have it that the worms think the sound is rainfall and come up for a drinky. -- Finlay McWalter | Talk 14:18, 20 March 2008 (UTC)

I thought worms got all the moisture that they need from the ground and the only reason they come above ground when it rains is so that they don't drown. Dismas|^(talk) 15:06, 20 March 2008 (UTC)

Does it look a bit like they're doing an Irish jig? if so, yes - they're trying to get the worms to come to the surface. There's a good video of several different gulls 'rain dancing' here. I've often wondered how it is that the gulls know exactly where they need to 'dance' in order to bring up a worm. There definitely seems to be an element of skill to it - they'll wander back and forth with their heads cocked, trying to find just the right spot. Are gulls' ears sensitive enough to hear the worms moving beneath the surface? Maybe they're just looking for fresh worm casts (I personally wouldn't put it beyond a gull's intelligence to know what a worm cast is...)? --Kurt Shaped Box (talk) 18:46, 20 March 2008 (UTC)

I'd expect that any decent soil (as opposed to sand, clay, or rock) would contain worms, since they are common enough that I don't see a need to search for individual worms. I now have the urge to sneak up on a sleeping gull and glue on a pair of tap shoes, to make the dance even more entertaining. StuRat (talk) 19:12, 20 March 2008 (UTC)

If you think watching gulls do it is entertaining, try watching flamingos. Daniel (‽) 20:33, 20 March 2008 (UTC)

It's even funnier to see a young gull (it's always the recently-fledged ones I've seen doing it) determinedly dancing for worms on concrete or tarmac. I even saw one doing it on a flat rooftop once. Not *quite* as funny as seeing a woodpecker attempting to drill a steel lamppost - but still... ;) --Kurt Shaped Box (talk) 21:15, 20 March 2008 (UTC)

Thanks for the answer guys. I'm wondering though. How do gulls learn this behaviour? Is it instinctive? Is the dance itself instinctive? Obviously they don't know by themselves that their trick won't work on solid surfaces, there's some trial and error involved. This would be an interesting research project. Sure beats finding out how far penguins can poop...PvT (talk) 11:09, 21 March 2008 (UTC)

I doubt if it's 100% instinctive. At best it's probably like kittens, which know to scratch the ground after they urinate or defecate, but need to be shown by mom to put that action to good use to bury what they've produced (so as to hide their scent). StuRat (talk) 21:19, 22 March 2008 (UTC)

Ever see a gull soaking a piece of bread, a biscuit, a crisp or similar in water (swishing it back and forth) in order to soften it up before swallowing? That's never stuck me as a behaviour that has any business being hard-wired. --Kurt Shaped Box (talk) 13:14, 23 March 2008 (UTC)

What the gulls are doing is something like (or actually is) worm charming and it's likely passed on down the generations carefully from gull to gull so that this skill won't die out. Julia Rossi (talk) 11:52, 21 March 2008 (UTC)

That other famous gull hunting skill, namely cracking open bivalves and crustaceans by flying up and dropping them onto a hard surface is (IIRC - no source to hand, sorry) learned by young gulls from observing their parents/older members of the flock then perfected through trial and error. I'd imagine that it's a similar thing with the worm-charming. Gulls have a long childhood/adolescence - going from meek, timid, dopey and needy to the precise opposite of all those things in four years or so. They learn and grow as individuals. --Kurt Shaped Box (talk) 20:38, 21 March 2008 (UTC)

Hallucinations making everything appear very big or very small

I'm vagually sure I remember reading about this phenomenon (see subject line) a while ago, but I can't find any references to it now. Is this a recognized type of hallucination? Is there a word for it? How common is it? --86.135.178.19 (talk) 20:40, 20 March 2008 (UTC)

Micropsia or macropsia, depending. - Nunh-huh 01:35, 21 March 2008 (UTC)

Radioactive decay

How half-life work on individual atoms? If I have 8 atoms of substance X, which has a half life of 5s, then after 5s, I obviously have 4 atoms of X, plus whatever it decayed into and some radiation, right? So what if I have 1 atom of X? Is there a 50% chance that it will decay after 5s? If so, is that true for each atom in my 8 atom sample, i.e. that after 5s, there is a remote possibility that all of the atoms will have decayed, or that none of them will have? I'm very confused. 72.155.207.33 (talk) 22:31, 20 March 2008 (UTC)

Half-life, especially as it relates to radioactive decay describes a statistical probability for a "large" sample. The two articles I linked have more information about what "really " happens, especially for small collections of decaying particles. DMacks (talk) 22:54, 20 March 2008 (UTC)

To answer the specific example, your last statement is correct. With x atoms after one half-life, the most likely number of atoms remaining is x/2, though any number from 0 to x is possible. Additionally, in virtually all cases, x/2 is not probable -- that is, the odds are usually better than 50/50 that there are not x/2 atoms remaining. The various probabilities can be expressed via the combination function. — Lomn 23:51, 20 March 2008 (UTC)

So what determines what amount decays? 72.155.207.33 (talk) 04:52, 21 March 2008 (UTC)

Pure chance, as far as we understand it. Every atom has a 50% chance of decaying within one half life. That means that the probability for k out of n atoms decaying within one half life is ${\tbinom {n}{k}}\cdot 0.5^{n}$. If you are unfamiliar with this mathematical notation, see Binomial coefficient. The expectancy is n/2, the standard deviation is $0.5\cdot {\sqrt {n}}$. With 8 atoms, the standard deviation is thus about 1.4. The individual probabilities are:

0 atoms will decay: 1/256

1: 8/256

2: 28/256

3: 56/256

4: 70/256

5: 56/256

6: 28/256

7: 8/256

8: 1/256

Icek (talk) 06:45, 21 March 2008 (UTC)

Note also that for radioactive decay, you are in the "spooky" "God ... play[s] dice with the universe" world of quantum mechanics, where you pretty much have to leave your intuition at the door. (In actuality, with one radioactive atom you can have the situation where it is both decayed and not decayed simultaneously. Just ask Schrödinger's cat.) -- 128.104.112.85 (talk) 00:49, 22 March 2008 (UTC)

Wow, that's weird! Thank's for the link. Is this actual science, or is it an If a tree falls in a forest and no one is there to hear it sort of philosophical thing? 72.155.207.33 (talk) 01:01, 22 March 2008 (UTC)

That quantum mechanics is generally not as simple as "if no one hears the tree falling" can be seen by Bell's theorem for entangled particles. Icek (talk) 07:08, 22 March 2008 (UTC)

scientific name

what is the scientific name for fucking the shit out of someone excuse my french (yes i mean sexually)

Coitus (wiktionary) is a commonly-accepted term. — Lomn 23:52, 20 March 2008 (UTC)

Anal sex might fit the questioner's term more closely. Edison (talk) 23:55, 20 March 2008 (UTC)

There is no scientific term for unusually hard sex, if that's what you're asking. Mac Davis (talk) 00:10, 21 March 2008 (UTC)

But hey, let's make one up! How about Coitus maximus? (Someone who knows Latin could probably do better)--Captain Ref Desk (talk) 01:07, 21 March 2008 (UTC)

How about Spanish instead ? "cópula hasta muerte". StuRat (talk) 02:05, 21 March 2008 (UTC)

"Coger hasta que no haya mierde en el cuerpo" (only in latin america)

Where do I nominate questions for "best of the ref desks"? HYENASTE 16:31, 24 March 2008 (UTC)