Wikipedia:Reference desk/Archives/Science/2008 April 4

= April 4 =

the earth ; a superconductor ?
soil is conductor to electricity. and as we know as the volume of conductor increases it's resistance decreases. the earth is huge in size and contain lot of soil(which is conductor). so theoriticaly the electrical resistance of the earth should be near to zero. is the earth a super conductor ? —Preceding unsigned comment added by Shamiul (talk • contribs) 06:30, 4 April 2008 (UTC)


 * No. It would have to be one continuous conductor for that logic to apply.  There are many insulating materials, such as air pockets, in the soil, each of which will increase the resistance. StuRat (talk) 07:13, 4 April 2008 (UTC)
 * what's the conductance of the earth ? is the a earth a better conductor than copper wire ? —Preceding unsigned comment added by 117.18.224.173 (talk) 07:45, 4 April 2008 (UTC)
 * No, the Earth is a poor conductor. The law that "as the volume [actually cross-sectional area] of conductor increases it's resistance decreases" is only approximately correct for long, thin wires. If the length of the wire is short in comparison with its thickness, the law stops working. The Earth is practically a sphere, not a wire at all, so the law is meaningless when applied to the Earth. If I drive two big metal spikes into the ground, one here and the other a mile away, the resistance between them will be very high. If one of the spikes is halfway around the world, there will be no measurable signal at all (at least for DC; maybe for ELF waves you could see something, but it would be quite difficult). —Keenan Pepper 13:58, 4 April 2008 (UTC)
 * If the Earth wasn't a good conducter of electricity, it wouldn't work as a ground. Although the Earth isn't a wire, the size of it makes resistance largely based on the size of the electrodes, rather than the distance between them. — DanielLC 15:44, 4 April 2008 (UTC)
 * DanielLC is right. The local conditions dominate the resistance between two buried electrodes. The earth resistance can be modelled as a set of concentric shells around each electrode. The inner shells have a small surface area, so they need to have a low resistivity in order to produce a low total resistance. This is why it matters where you bury the electrode. Beyond a certain distance, called the 'sphere of influence' of the electrode, the shells have such a huge surface area that even poorly conducting soil has a low resistance when summed over the shells' area. That means that if you planted two electrodes on opposite sides of the Earth, you would get pretty much the same resistance, for a given soil type, as if you planted them a hundred yards apart.  The Earth is indeed used as a return conductor in some power transmission applications, but it's risky because you can set up unwanted voltage differences in the ground.


 * Anyway, returning to the question, it doesn't matter how big the Earth is or how big the electrodes are, the planet isn't a superconductor because it isn't made of superconducting materials. Superconductors conduct electricity in a qualitatively different way from normal conductors. You can make the resistance of a conductor arbitrarily small by enlarging the conductor, but you can't make the resistance exactly zero: only superconductors have that property. --Heron (talk) 18:54, 4 April 2008 (UTC)


 * As far as not being made of superconducting materials, I have to disagree. All we've got to do is cool the place down to a few Kelvin and apply a few MBar of pressure and there's enough in the crust that it just might work. Shame we wont be around to see that  Furmanj (talk) 10:00, 5 April 2008 (UTC)


 * Good answer, Heron. It does depend mainly on the quality of the electrodes. I take back part of my answer, which was misleading if not wrong. —Keenan Pepper 05:14, 5 April 2008 (UTC)

"The earth resistance to the current, compared with that of a long line, is next to nothing," per Chambers's Encyclopedia,(1883) p438. The resistance is basically that of the grounding electrodes, so that a large and deep electrode, or multiple long ground rods in parallel, or one consisting of the water mains of the town lowers the resistance between the ends of the circuit far lower than any practical metallic conductor connecting two distant points. The earth is not a superconductor, but with its numerous parallel conduction paths it can be a very good conductor for the earth return used with long metallic single conductor circuits. Edison (talk) 00:47, 5 April 2008 (UTC)

rube goldberg machine
I'm going to be building a Rube Goldberg machine. The chain will be initiated by a falling domino and the 'objective' of the machine is to have a marble hit a domino at the end of the whole process. Can anyone give me some ideas on what to build? -- hello, i'm a member  |  talk to me!  06:34, 4 April 2008 (UTC)


 * The simplest design might be to have a row of dominoes hit a marble hanging on a string which then strikes the last domino. Add complexity from there. StuRat (talk) 07:19, 4 April 2008 (UTC)


 * At one stage you could whisk aside a curtain to reveal a picture of Hillary Clinton taking the oath of office, which scares a Tasmanian devil so badly that it runs down a plexiglass pipe to the other end of a seesaw. --Milkbreath (talk) 10:23, 4 April 2008 (UTC)
 * Use a metal domino to make an electrical connection and start a motor? 81.174.226.229 (talk) 10:41, 4 April 2008 (UTC)


 * My suggestion is to look at the sorts of things that other people have built, and take inspiration from there. I believe that there are a number of Rube Goldberg competitions, some of which get very "creative". You can also alway find examples on video sharing sites (YouTube, Google Video, etc.) -- 128.104.112.85 (talk) 15:48, 4 April 2008 (UTC)
 * Just saw this on the news this morning: results from the national Rube Goldberg Machine Contest -- 128.104.112.85 (talk) 13:23, 7 April 2008 (UTC)

In the very crude movie "Waiting", in the restaurant that they work in is a really neat Rube Goldberg contraption on their wall. Every now and then in the film they show parts of it, but at the very end after the credits, they show it work. An empty glass is put on a plate that starts the contraption going. In the end, a bottle of beer is poured into the glass. You might consider renting that movie and look at some of the events. But, I will warn you again, the movie is very crude.--Wonderley (talk) 08:05, 7 April 2008 (UTC)

Papadums
When I cook commercially made papadums in the microwave, it takes 30 seconds for three, 40 seconds for four. But if I put one in, it needs 15 seconds. What's the reason? Julia Rossi (talk) 07:39, 4 April 2008 (UTC)
 * Perhaps something else (the glass plate or whatever container is in the microwave) is heating up as well. Or it could be rounding error.  &#x2013; b_jonas 09:05, 4 April 2008 (UTC)


 * The magnetron that is the heart of the microwave oven is a vacuum tube and like most vacuum tubes, it includes a heater that must heat up before thermionic emission of electrons can begin inside that vacuum tube. This takes several seconds. So during those first few seconds while the heater itself is warming up, electrons aren't flowing in the magnetron tube so no microwaves are being generated so the oven isn't doing any cooking. If you listen carefully as the microwave oven starts operating, you'll often be able to hear its sound change as the heater reaches operating temperature. Because it is only at this point that the microwave oven starts drawing a lot of power from the power line (mains), you may hear:


 * The hum of the oven increase


 * The cooling fan, stirring fan, or turntable motor slow down a little


 * The famous "incinerate a CD" experiment will provide a very-dramatic exhibition of this effect.


 * Atlant (talk) 11:40, 4 April 2008 (UTC)

Proventil
DOes the effectiveness of Proventil decrease over time? I've read the article, and I couldn't find anything there.--AtTheAbyss (talk) 12:13, 4 April 2008 (UTC)
 * Proventil is a Beta2 adrenergic agonist. It is not a class of drugs considered to be resistance-building in the human body.  However, there are concerns with long-term use in asthmatics, a fear that they may make asthma uncontrolled.  A quick Google search on   will surely turn up a few studies on the subject. --  k a i n a w &trade; 15:28, 4 April 2008 (UTC)

Humans surviving in space
Can humans survive in an area of space where we se massive amounts of dust forming new suns?


 * As long as they're not too close to one of the stars and in a space-suit or ship, then I'd expect so. I think gamma ray and x-ray bursts occur when stars die, not when they form, so that should be OK.  You wouldn't want to be moving too fast relative to the dust, or that could damage the space-suit or ship, which would be needed to provide air, water, food, and regulate the temperature.  Although, for temp, you could probably find an ideal location where light from the star(s) would provide just the proper amount of heat, but you'd still need to distribute the heat from the part facing the star(s). StuRat (talk) 14:13, 4 April 2008 (UTC)


 * The most massive stars have short lifetimes (on the order of 1–10 million years), and so may die while the star-forming region is still active. -- Coneslayer (talk) 14:29, 4 April 2008 (UTC)

Finding out who cited a journal article
I have a journal article: and I am interested in which other journal articles have cites this one. How is it possible to check? --Seans Potato Business 15:08, 4 April 2008 (UTC)
 * Thomson Scientific's Web of Knowledge product has the ability to do citation searches in both directions. It is a commercial service, however, so you will likely have to find a university which has a subscription. -- 128.104.112.85 (talk) 15:41, 4 April 2008 (UTC)
 * Google Scholar offers a list of citations. For this article, check . Unfortunately, no citation is listed so far. This is not too surprising for a 2007 article, though. --Stephan Schulz (talk) 15:45, 4 April 2008 (UTC)
 * Thank you both. :) --Seans Potato Business 17:13, 4 April 2008 (UTC)

A competitor to Web of Knowledge is Scopus which offers a similar service (with subscription). ike9898 (talk) 19:03, 4 April 2008 (UTC)

Where can I get heavy water?
After reading the article about heavy water, I was intrigued and would like to do a photography shoot of heavy ice cubes that sink, not float, in water. But where can I get heavy water? Is it available for sale somewhere? J I P | Talk 15:27, 4 April 2008 (UTC)
 * It should be available from most places which sell isotopic materials to scientific researchers. For example Sigma Aldrich sells it (as deuterium oxide) for about $1/mL (Enough water to get a couple of ice cubes should cost you under a hundred bucks). Be advised, however, that due to it's potential use for nuclear purposes, there may be (I can't say for certain) extra regulatory hurdles to jump through before they will sell it to an average person off the street. -- 128.104.112.85 (talk) 15:37, 4 April 2008 (UTC)
 * You'd need tons of the stuff to use it for real nuclear purposes. Not to mention a few other things as well. I'm pretty sure that in the amounts he is talking about there are not even export restrictions. --Captain Ref Desk (talk) 14:44, 6 April 2008 (UTC)
 * (ec) You can order it from just about any large chemical supply house. Sigma-Aldrich sells 99% pure deuterium oxide for about one U.S. dollar per gram (plus shipping).  I don't know if it's possible to have chemicals – even harmless ones like heavy water – delivered to a residential address; you may need to have it delivered to your workplace, or even have a friend in a chemistry or biology lab buy it for you.  For what it's worth, you don't need to worry about any nuclear safety regulations; it's non-radioactive and not regulated in any special way.  TenOfAllTrades(talk) 15:43, 4 April 2008 (UTC)


 * $1/mL seems like a high price to pay for 99% pure deuterium oxide. United Nuclear offers 20 mL of Ultrex grade, 99.999% pure heavy water for $17.  There's nothing to indicate that they don't ship to individuals, so the same should be true for other chemical dealers.  Obviously try to find a local store to avoid paying shipping and handling costs, but the price should be less than $1/mL for 99% pure heavy water.  --Bowlhover (talk) 00:36, 5 April 2008 (UTC)
 * Actually, that United Nuclear product is 20 grams, which is only 18 ml. Remember, this is heavy water... --169.230.94.28 (talk) 02:02, 5 April 2008 (UTC)


 * Sorry, I misremembered the page and thought it used mL. Since the Sigma-Aldrich price was cited in dollars per gram, however, the conclusion of my post is still true.  --Bowlhover (talk) 02:40, 5 April 2008 (UTC)


 * If you manage it, I'll be interested too see what it looks like! Perhaps you can take a shot for the heavy water article? Seans Potato Business 17:05, 4 April 2008 (UTC)


 * And don't forget to do the opposite too: a chunk of normal ice or some other almost-as-dense-as-water object floating in normal water, and then the same object floating in heavy water. --Anonymous, 22:07 UTC [Insert joke here about daylight saving being "heavy time"], April 4, 2008.


 * They made tons of it at Norsk Hydro for the Nazis, under the direction of Dr. Seuss. But there were some problems with production and shipping. In 2005 a drum of it was found at the bottom of Lake Tinnsjå. Might look there.Edison (talk) 00:10, 5 April 2008 (UTC)


 * A sunken ice cube image from the site that Bowlhover cites above. --hydnjo talk 03:56, 5 April 2008 (UTC)

United Nuclear doesn't sell outside Merka. I'll try my chances at Sigma Alrdich though, as they already seem to have sales offices in Finland. For $100, I could get apparently get a whole decilitre of heavy water, which would make for many heavy ice cubes. I'm already starting to design pictures of cocktails with the ice at the bottom of the glass, not at the top. J I P | Talk 18:41, 6 April 2008 (UTC)
 * The Finnish Wikipedia referred me to Onemed Group. I asked them, and the reply was that they don't sell to private persons. I could try that Sigma-Aldrich place but I'm beginning to think I won't have much luck there either. Damn, this stuff is difficult to get hand of. I'd understand it if I were trying to buy tritiated (super-heavy) water, which I understand is extremely dangerous, but normal heavy water shouldn't be. J I P  | Talk 17:42, 10 April 2008 (UTC)

Lighting in photographs
Is it possible to describe the lighting observable here and here? What are it's features and does the technique, if any applies, have a name? Seans Potato Business 17:23, 4 April 2008 (UTC)


 * The first image is an example of backlighting: two colored lights behind a set of test tubes filled with water. The second image (the computer keyboard) is probably lit by two diffuse light sources (umbrella reflectors or similar): a blue light above the object, and a yellow light to the left. --Carnildo (talk) 20:38, 4 April 2008 (UTC)

electrical coil
What effect does the diameter of the coil have on its electrical characteristics? 71.100.173.69 (talk) 18:39, 4 April 2008 (UTC)


 * Inductance. Detailed explanations appear below the graphs, and read the article proper for information on inductance itself. Someguy1221 (talk) 18:53, 4 April 2008 (UTC)