Wikipedia:Reference desk/Archives/Science/2007 January 5

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[edit] January 5

[edit] Buzz

Why do transformers buzz?69.29.38.13 00:05, 5 January 2007 (UTC)

Per our article about them, the changing magnetic field causes vibration and magnetostriction (shape changes due to the magnetic field in the transformer core,) both of which can give rise to the hum or buzz that you can often hear coming from transformers. -- AJR | Talk 00:37, 5 January 2007 (UTC)
this website has an excellent and detailed description. Vespine 00:53, 5 January 2007 (UTC)
HMMMMMMMM! Magnetostriction could bee the answer! (couldnt resist this one sorrry)--Light current 01:07, 5 January 2007 (UTC)
Further to your earlier answers, there are several sources of noise in a transformer that cause it to hum:
  • Magnetostriction: when magnetic flux flows through a material (such as the steel that makes up a transformer core), there can be a physical change in the size of the material, usually a shrinkage. With each cycle of the alternating supply, the material will change in size twice, so at 100Hz in some part of the world, and 120Hz in others, such as North America.
  • Eddy currents are caused by changing magnetic fields intersecting conductors. Most transformer cores are made from sheets of steel, and the eddy currents cause these sheets (called laminations) and any other metal parts of the transformer, such as its windings, to flex to and from each other.
  • Very large transformers, such as are found at power stations or high-voltage electrical substations can have electrically-powered pumps and fans to cool them. These can make a noise as they operate, and is usually the greatest source of noise when you stand close to a substation.
In other words, the source of the hum is for the most part vibration. I hope this answers your question; but if not, perhaps you can ask for a clarification. Regards, — BillC talk 20:31, 5 January 2007 (UTC)

[edit] Talk:Lemming "so do they jump off cliffs?"

This was posted by an anon:

Yes or no (and by the way this is the number 1 reason anyone would visit this page.) there are 3-4 references to this "reputation" they have, from popular literature -- so is it a well-founded reputation???

A good question, and I hope it can be answered clearly and with lots of nifty twists and turns, which can then be supplemented into the article. - RoyBoy 800 00:41, 5 January 2007 (UTC)

They do, if Walt Disney orders them to be herded off for the sake of a nature documentary [1]. The video game variety, by contrast, must be herded to keep them from falling off.--Joel 01:35, 5 January 2007 (UTC)
The lemming article says quite clearly (in the Population fluctuations section) that lemming suicide behaviour is a myth, propogated by the Disney film White Wilderness. It also gives a link to this page which debunks the myth. I've put this same answer on Talk:Lemming. Gandalf61 11:24, 5 January 2007 (UTC)

The myth was populated by the film, but I believe the film got it from an earlier book. 80.169.64.22 16:36, 5 January 2007 (UTC)

Lemmings just GO GO GO driven by hunger and other consequences of population stress (basically, they go bonkers). Being social animals, they have a sort of pack mentality during migration. Formerly, they occasionally drowned en masse in rivers, lakes, etc. SOften enough, some could use the corpses to float to the other side, that's why the behavior has been maintained. Due to habitat restrictions (lemmings do't usually occur in terrain adjacent to rocky shores, they need juicy medaows etc to thrive) only in the rarest instances they'd fall over cliffs, and they wouldn't jump but be pushed rather, by the scores of lemmings coming after them.
Nowadays, they get more often squished en masse by road or rail traffic. Dysmorodrepanis 03:00, 6 January 2007 (UTC)
I don't think any of you have even seen the documentary, because the narrator quite clearly used the words "suicide" being a "myth" when talking about their tendency to jump off cliffs. It goes on to explain the reason lemmings are famous for jumping off cliffs: not suicide, but both migration and perhaps a clever trick to keep the area inhabitable by allowing the food to regrow, thus allowing the lemmings to avoid extinction. Far from propogating the myth, the documentary debunks it. --RockMaster 00:26, 9 January 2007 (UTC)

[edit] Twice the focal length? You must be crazy...!

If you place a point of light at twice the focal length of a lens will it converge, and if so, at what point on the other side of the lens? It's not a homework question and I can't find answers at the lens or focal length articles. --Username132 (talk) 01:11, 5 January 2007 (UTC)

Use this formu1a 1/u + 1/v = 1/f —The preceding unsigned comment was added by 88.109.93.42 (talk) 01:24, 5 January 2007 (UTC).

You can use the formula or you can use a graphical method. In the diagram on the right any ray of light that passes through f emerges horizontally from the lense. Conversly any horizontal ray must pass through f. Drawing in the rays means the image must abe at 2f, the other side of the lense and upside down. Theresa Knott | Taste the Korn 01:45, 5 January 2007 (UTC)

[edit] Ion Implantation

I've been looking for an ion implantation vendor that can do precious metals at ~7 MeV, or at least within an order of magnitude. Of the three vendors I found on Google, two only do ~400 keV, while one does 1 Mev. The article says 5 MeV is common...am I looking in the wrong place? Can anyone recommend a good vendor? Thanks in advance. --Joel 01:50, 5 January 2007 (UTC)

Using these search terms <"Ion implantation" services MeV> there seem to be many google hits. --JWSchmidt 03:14, 6 January 2007 (UTC)

[edit] Crop circles

What exactly are "crop circles" 59.95.4.83 10:22, 5 January 2007 (UTC)

Unsurprisingly, they are circles on crop fields. See the article: crop circle — Kieff 10:30, 5 January 2007 (UTC)
If you mean how they're made, they're made by pranksters with ingenious ideas but simple tools. --Bowlhover 17:52, 5 January 2007 (UTC)
At least the elaborate ones you see nowadays. The circle-shaped ones (as opposed to the disk-shaped ones) that sometimes occur (and perhaps inspired the phenomenon) might also be the product of roe deer in heat, whereas rough disk shapes of flattened crops may be produced by dust devils on occasion. Dysmorodrepanis 03:04, 6 January 2007 (UTC)

[edit] soil microbiology

what are the required conditions for growing (pH,TEMPERATURE,CULTUREMEDIA)sulphur bacteria and denitrifying bacteria —The preceding unsigned comment was added by 59.163.146.11 (talk) 12:33, 5 January 2007 (UTC).

Search here for <culture conditions soil bacteria> --JWSchmidt 03:01, 6 January 2007 (UTC)

[edit] resistance of wires

i'm doing a report on the resistivity of wires at different temperatures, and from one of my graphs i have the resistivity due to impurities and lattice defects as a negative number, i have a feeling this is just my experiment going wrong as the coefficient of resistivity is also outside the error margins, i'm using a "pure" wire of copper, so is there anyway that impurities or defects would increase the conductivity of the wire, even in small enough quantity's to not normally be recorded? as i was thinking anything added to the lattice will simply distort the repeating pattern making the electrons "hit" the ions more often, thereby creating a positive resistivity due to impurities.--86.29.62.113 13:32, 5 January 2007 (UTC)

You're right. Theoretically, the most conductive Cu is a single, pure crystal with no dislocations or vacancies in it. It's possible that your wire is slightly thicker or shorter than you thought, giving it a lower resistance than a theoretical pure wire of the nominal size. Otherwise, you might want to check your calculations. --128.115.27.10 01:23, 6 January 2007 (UTC)

[edit] Ortho-effect & ortho-para positioning(Organic Chemistry)

Sir,

I'd like to know about ortho-effect (definition & short detail) and when the ortho/para product will be the main product(i.e. temperature or steric effect).

I'll be highly obliged if you kindly answer my question.To clear any ambiguity in my question please contact- (email removed)

Thanking you, ours faithfully, Tirtha —The preceding unsigned comment was added by Tirtharahaman (talk • contribs) 16:20, 5 January 2007 (UTC).

ortho/para directors are primarily encountered in electrophilic aromatic substitution reactions, and there's some information about it on our electrophilic aromatic substitution page. DMacks 17:09, 5 January 2007 (UTC)

Sir,

   I wanted know about the ortho effect.Is the article deals with it?!!!Also I want to know the effect of temperature & others in specific ortho-para positioning?To clear ambiguity- tirtharahaman@gmail.com

[edit] Hippocratic constraints

What are the hippocratic constraints(if any) regarding doctors emailing their patients?

Thanks, HighAnxiety32 16:34, 5 January 2007 (UTC) Jesse

Hippocratic does constraint nobody anymore, because doctors dont do this old ritual anymore (I think for centuries). It's normal laws know!--Stone 17:01, 5 January 2007 (UTC)
And for the rest Hippocratic Oath. --Stone 17:05, 5 January 2007 (UTC)

The question is whether any pledges in the Hippocratic oath constrain email. The answer is yes, the promise of confidentiality, since email is often not secure. While this is only a potential violation of the Hippocratic oath, the use of unencrypted email is a violation of HIPPA-cratic law in the US. alteripse 15:08, 6 January 2007 (UTC)

[edit] Biasing a BJT Common Emitter amplifier

In a Common Emitter amplifier (based on a Bipolar Junction Transistor), if I choose to use the constant VBE biasing (using a voltage divider at the base), how should I establish a constant collector current? Should I use a bypass capacitor between emitter and ground or can I use an appropriate voltage divider between +V and ground with the central node connected to the collector in order to establish a constant voltage and, therefore, a constant IC? —The preceding unsigned comment was added by 84.90.42.248 (talk) 16:58, 5 January 2007 (UTC).

I'm not certain I understand the constraints implied in your question. If you hold the base at a constant voltage (relative to the circuit's overall VCC), then the emitter must be about one diode drop (so, usually, about 0.6v) below the base voltage. IE is then determined by the sum of the external and internal emitter resistances and IC will be IE-IB (which, for most "large-ish" values of β (Beta) can be assumed to be the same as IE).
If the external emitter resistance is small or zero, then you really can't use a constant base voltage as a bias as it makes things way too sensitive to variations in RE and β. In this case, the base voltage divider is usually connected to the transistor's collector rather than VCC, providing negative feedback that stabilizes IE and IC.
Meanwhile, the purpose of the bypass capacitor around any external emitter resistance is to make that impedance low for AC signals while keeping the effects of the resistance for DC signals. This has the effect of maintaining a stable (and low-gain) DC operating point while allowing more gain for AC signals. Remember, the gain of a common emitter amplifier is (roughly) the ratio of the collector resistance over the total emitter resistance (counting the internal RE of the transistor plus any external resistance/impedance that you add); the bypass capacitor brings the total emitter resistance down to the typical few ohms or tens of ohms RE inherent in the transistor itself.
Atlant 18:25, 5 January 2007 (UTC)

[edit] Bio Reserve

need to know everything about bio reserve---- —The preceding unsigned comment was added by 203.101.13.198 (talk) 17:08, 5 January 2007 (UTC).

Do you mean a nature reserve? --Bowlhover 17:49, 5 January 2007 (UTC)
Biosphere reserve? Rmhermen 22:02, 5 January 2007 (UTC)

[edit] 48 to 46

i find it strange that no wiki article covers this aspect. My question is that although genes evolve slowly with time, but the change in number of chromosomes in hominids could only have occured at a certain point. Does anybody has an idea when this change occured.nids(♂) 18:37, 5 January 2007 (UTC)

You may want to take a look at this review of theories. - Nunh-huh 18:41, 5 January 2007 (UTC)
Or, right here, Theophilus Painter. Deltabeignet 18:51, 5 January 2007 (UTC)
There's nothing in the Theophilus Painter article that addresses the question of an actual change in the number of chromosomes. It refers to a correction of a counting error. - Nunh-huh 19:09, 5 January 2007 (UTC)
{edit conflict}::Nunh-huh, the link was really helpful, but it doesnt give any specific dates. I understand that we actually dont know the date for this change.
BTW, Deltabeignet, how does the link that you gave answers my question.nids(♂) 19:17, 5 January 2007 (UTC)
Was there a bottle neck 350 000 years ago? Are we definitely certain that the number did not go up from 46 to 48? --Seejyb 23:31, 5 January 2007 (UTC)

Have you seen this page? I cannot find anything better than Genomic Structure and Evolution of the Ancestral Chromosome Fusion Site in 2q13-2q14.1 and Paralogous Regions on Other Human Chromosomes. --JWSchmidt 02:43, 6 January 2007 (UTC)

One thing to note is that people can, and do, survive with either extra chromosomes or missing parts of chromosomes. I suspect that there is a great deal of duplication of genes between the chromosomes to allow this. Otherwise, missing a chromosome, or even part of one, would certainly be fatal. It's not too much of a jump, then, to imagine one population developing with a different number of chromosomes than the majority, and eventually replacing the majority, if the new number of chromosomes had some advantage. StuRat 02:45, 6 January 2007 (UTC)
Well Stu, people do not survive when missing a chromosome. An exception is Turner syndrome. However one may survive if two chromosomes have fused and there is actually no loss of genetic material from the genome but only a reduction in their number. nids(♂) 06:19, 6 January 2007 (UTC)
Even those with Turner syndrome aren't really missing a chromosome, because in ordinary females, only one X chromosome is expressed. --Bowlhover 07:01, 6 January 2007 (UTC)
This article compares chromosome structures in humans and modern apes. It concludes that our common ancestor probably had 48 chromsomes, and that a chromosome fusion somewhere in the human ancestors line led to modern humans having 46 chromosomes. It also presents evidence from wild and domestic animal populations that chromosome fusion can produce fertile individuals who can still interbreed with the "normal" population. Gandalf61 11:00, 6 January 2007 (UTC)
... and our article on Chromosome 2 (human) and this page comparing human and chimpanzee chromosomes support the theory that Human Chromosome 2 is a fusion of ancestral chromosomes. But I can't see any suggestion of a timeline for this event. Gandalf61 11:16, 6 January 2007 (UTC)

[edit] Nuclear Fusion

Can you tell me about nuclear fusion and how it works, or link to an article? —The preceding unsigned comment was added by 81.132.66.73 (talk) 19:20, 5 January 2007 (UTC).

see Nuclear fusion for start.nids(♂) 19:28, 5 January 2007 (UTC)
See also Binding energy and note in particular the graph towards the bottom of the article. (Almost) everything tends toward those isotopes in the center; lighter isotopes fuse (join together) towards those middleweight isotopes, releasing energy. The heavier isotopes fission (split apart) towards those middleweight isotopes, releasing energy.
Atlant 22:20, 6 January 2007 (UTC)

[edit] metal reaction

what other chemical is made if a metal reacts with gas? i already have hydrogen gas as one of my answers but i need another81.132.103.167 19:43, 5 January 2007 (UTC)----

It depends. What metal, and what gas? The product of reacting iron with oxygen are very different from the product of reacting sodium with chlorine. --Carnildo 20:31, 5 January 2007 (UTC)

Metals tend to reduce - so the product with water gas, HCl gas, HBr gas will be hydrogen plus a salt - an oxide, chlordie and bromide respectively. With other gases the reaction does not produce hydrogen eg chlorine gas reacts with a metal to produce a chloride and probably heat(energy).87.102.33.170 16:02, 6 January 2007 (UTC)

In general I think it can be said they'll form either a salt or an oxide, reacting with a gas alone will often leave little other product, because most elemental gasses will simple react to form the salt (IE Cl gas and sodium, or any halogen and alkali metal really) Wintermut3 03:14, 7 January 2007 (UTC)