Wikipedia:Reference desk/Archives/Science/2011 May 21

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May 21

Rat factoid I heard on TV the other day...

"Rats have been found living permanently in walk-in freezers with fur that has grown to a length of 30cm or more. They survive by eating frozen meat."

Just something I heard someone say on TV the other day (it was a documentary following a guy who inspects ships for vermin and diseased crew members before they are permitted to leave port).

Anyone able to confirm/debunk? Sounds a bit unlikely to me... --Kurt Shaped Box (talk) 01:28, 21 May 2011 (UTC)

Yes, mice and rats both can live in walk-in freezers. Dualus (talk) 04:31, 21 May 2011 (UTC)

The walk-in freezer part of that isn't surprising to me, but the fur length of 30 cm seems highly dubious. Are you sure you weren't mishearing, and it was just the body length that was 30 cm? That would be much more reasonable. The Mountain Giant Sunda Rat article, for example, lists a body length of up to 29 cm. And if you include the tail, even a brown rat can easily be more than 30 cm long. Red Act (talk) 05:51, 21 May 2011 (UTC)

There would be little survival value in a rat having 30 cm long fur dragging around behind it, even if many generations of rats had lived in the freezer. Rats which live outdoors do not somehow grow ridiculously long fur when it gets cold. Sounds very unlikely. Edison (talk) 19:49, 21 May 2011 (UTC)

That was what made me suspicious as to the veracity of the tale. How would a rat even be able to move with 30cm-long fur? Or see out? Or defecate/urinate without becoming so filthy as to eventually die of something nasty? Na, Red Act - the guy definitely said that the fur was 30cm+, I played it back at the time because I was all like 'Wha-?'. IIRC, he didn't specifically claim to have seen this himself, just that it was something that had happened sometimes... --Kurt Shaped Box (talk) 20:05, 21 May 2011 (UTC)

Obviously a bad edit. The rats must have grown to 30 cm or more. Maybe a square-cube law adaptation to help them live more efficiently in an arctic-ish environment. Wnt (talk) 09:07, 22 May 2011 (UTC)

Morganza spillway absorption and flow

Re Talk:2011 Mississippi River floods#Where is all the water going?, it looks like the Morganza Spillway is absorbing far more water than predicted. "The region had been suffering through a significant drought, so the ground and side waterways have been able to absorb more water than originally anticipated." My question is, basically: How can it possibly absorb that much more water? The prediction was that 20 miles on either side of Morgan City would be an estuary by the time of that satellite photograph. Plus, the pattern clearly shows water leveling into low regions while avoiding high regions after the first few miles. Are the low regions of the spillway wide and deep enough to handle the 114,000 cubic feet per second without flooding the entire width of the spillway as predicted? (If so, that would be fantastic, and suggests that a spillway channel should be kept maintained as this will probably have to become more common in the near future with increasing precipitation.) I understand that this question probably involves understanding the topography of the spillway at far more resolution than available maps can provide (which must be why the Corps of Engineers overestimated the flooding) so I don't have my hopes up for a clear answer. Dualus (talk) 05:37, 21 May 2011 (UTC)

They could be basing their hydrological transport model forecast on a worst-case scenario. It might not be very accurate model and probably hasn't been tested before - these things cost millions to develop and the gov. likes black projects more than flood management. Any actual water depths 5% or less than forecast would would be considered 'significantly' better than expected and the press will also quote these sort of things out of context and exaggerate. The model might not have taken into account the friction caused by the vegetation at this time of year, encouraging a greater proportion to go west into the Atchafalaya rather than southwards. If the water is not so deep the hydrodynamic gradient will be less and so will flow slower. All these things added together could make quite a difference. You will have to wait for at least a couple of years for the final analysis to be published. --Aspro (talk) 18:32, 21 May 2011 (UTC)

Don't be fooled into thinking that the flood is neatly partitioned into those blue areas; the entire area below the spillway is likely underwater, it's just that differences in land use are causing the water to show up much more clearly in farmland areas. The image on that website comes from ASTER, which is a radiometer, and can only see what is directly visible from above. If you look at this previous Earth Observatory post, the areas that we see in blue almost exactly line up with areas that were brown previously, indicating that these areas had little or no substantial vegetation (likely farmland). So, I do not believe the flood is as "scattered" as the image makes it seem; there likely is water across that whole area, it's just hiding under trees or other thick vegetation.

It is interesting that the predictions on the flood's progress have been that far off, but it should not be surprising; these predictions were made with flood models that have never been tested before, so they must rely on a huge number of assumptions, and minor differences in the quality of the soil can make a big difference in how much water it can absorb. As Aspro mentions above, they likely issued a worst-case scenario prediction in order to enable residents to prepare for a worst-case scenario; and when you have large uncertainty the worst case scenario can be very far from the actual solution.-RunningOnBrains^(talk) 20:19, 21 May 2011 (UTC)

I thank Running-on-Brains for giving my hypothesizing credence, so now I feel awkward in asking how infra-red at these wave lengths can hide under foliage? Mind you! If the early pioneers had had the benefit of his education, they would have realised that building a levée here and building a levée there - as in this example was unnecessary, [1] What a wasted effort. As (R.O.B. suggests) a barb wire fence would be all that was necessary to keep the flood waters “neatly partitioned into those blue areas” ;-) Drink lots of coffee when you first get up in the morning :-)--Aspro (talk) 21:23, 21 May 2011 (UTC)

Foliage is actually very reflective at near IR wavelengths. Water is highly absorptive, but since the leaves will be above the water they will reflect before the IR penetrates to the water and will dominate the appearance. Graeme Bartlett (talk) 22:45, 21 May 2011 (UTC)

I'm not sure about the exact properties of vegetation vs. water at infrared, but Graeme's reply is along the right lines. The satellite is merely measuring the intensity of light at certain wavelengths, and then an algorithm is applied to the data to determine what is emitting that light, since different objects typically emit differently at different wavelengths (it's essentially looking at the "color", but in a non-visible spectrum). Vegetation is opaque to infra-red, so the signatures the satellite will be detecting will be from the vegetation, not the water beneath. The satellite can't "see" through solid matter any more than we can. -RunningOnBrains^(talk) 20:26, 22 May 2011 (UTC)

If you look at monochromatic near-infrared photographs, you'll notice two distinct differences from the equivalent greyscale visible-light photograph: 1) Grasses, leaves, and similar plant structures are a brilliant white, and 2) water is dark grey to black. It really is that easy for plants to mask underlying water. --Carnildo (talk) 00:32, 26 May 2011 (UTC)

Gaseous Ions

1.Can gaseous Ions be stable at room temperature?

2. Does ordinary air contain any gaseous Ions (If the answer to 1 is yes)

3. Can air contain enough gaseous Ions tomake it cunductive?(like an electrolyte)

4. If the answer to 4 is yes, does it have any uses?--Irrational number (talk) 05:59, 21 May 2011 (UTC)

Disapointingly, Gas-phase ion chemistry deals mostly with mass spectrometry and related fields, but yes you can generate ions at room temperature. Broadly speaking, an ionized gas is a Plasma, and they are known to exist down to near absolute zero. --Jayron32 06:20, 21 May 2011 (UTC)

Continuing on, the plasma can be formed by an electric discharge, and that resulting ionized air becomes highly conductive, which allows the discharge to continue. It's hard to get started (have to overcome the breakdown voltage) because there are ordinarily few if any ions in normal air or most other gas samples. But the results are things like lightning bolts and neon signs. I would suspect that any ions that do exist naturally (i.e., not induced with electric or other means) would cluster together (ionic attraction is strong!), so there might be a small area with lots of ions rather than an even distribution throughout (as in solutions or other mixtures). Would they stick so close together that they would technically be nano-droplets of liquid or nano-particles of solid rather than a gas? DMacks (talk) 06:39, 21 May 2011 (UTC)

Wouldn't ions of the same sign (positive or negative) repel each other ? StuRat (talk) 04:33, 22 May 2011 (UTC)

Yes, but a material wouldn't spontaneously ionize into "just positives"...electrons gotta wind up somewhere too. My consideration was water, which does contain a small ionic component...but they are "equal and opposite" ions, not just one charge. It's definitely not a plasma, but plasma's aren't so stable at terrestrial (temperature, pressure, electromagnetic) conditions because they are so ionic and either recombine, or react due to interactions with other stuff nearby rather than just sitting there. I can't figure out a way to have a sample of "just cations" enough to study without using exotic containment or isolation tricks. DMacks (talk) 16:45, 22 May 2011 (UTC)

We seem to get unbalanced ionization of the air during a thunderstorm. StuRat (talk) 18:01, 22 May 2011 (UTC)

Again, not stably so. DMacks (talk) 18:42, 22 May 2011 (UTC)

Phosphorus pentafluoride

What are the molecular orbitals like in Phosphorus pentafluoride or other molecules that have a structure other than linear, trigonal or tetrahedral? thanks.--Irrational number (talk) 06:51, 21 May 2011 (UTC)

The starting article is Orbital hybridisation, where you can see the various simplistic approaches (and consideration of whether they are correct or not). Then move on to Hypervalent molecule for more in-depth discussion. DMacks (talk) 07:31, 21 May 2011 (UTC)

Black semiconductor

Top view of LED

I purchased an orange LED that has a black semiconductor in it. What is the semiconductor's chemical composition? The picture in the gallium arsenide article shows a black square similar to mine, but the article's description says that GaAs is gray. --Chemicalinterest (talk) 15:16, 21 May 2011 (UTC)

Probably AlInGaP. If you know the manufacturer, the semiconductor material should be listed on the datasheet. Tevildo (talk) 20:12, 21 May 2011 (UTC)

It didn't come with a datasheet. I just checked where I bought it from and there are no specs. --Chemicalinterest (talk) 20:52, 21 May 2011 (UTC)

@2 I must say that it is possissible.If we can calculate the number of photons,then it is possible to construct the device producing mass from a source of very high frequency.I could definitely prove it.If there anyone is interested,then contact me through this article. —Preceding unsigned comment added by 59.93.255.6 (talk) 05:19, 22 May 2011 (UTC)

An easier way would be just to add the semiconductor descriptions to the articles. :) --Chemicalinterest (talk) 12:43, 22 May 2011 (UTC)

Wheatstone Bridge - infinite current between legs of circuit?

Hello, I was looking at the Wheatstone Bridge circuit wiki: Wheatstone_bridge and I'm a little confused. The article, and other sources, indicate there are two types of the circuit: one where a voltmeter measures the potential between the two legs, and another 'classic' Wheatstone Bridge where a galvanometer or ammeter measures any current between the legs.

I understand the voltmeter type, but with the galvanometer (which ideally has zero resistance) if the bridge is unbalanced then there is a potential difference between the legs - and so a potential difference across the galvanometer. The ideal galvanometer has no/little resistance so surely a huge current flows?

Have I misunderstood, is there an implicit resistor, not on the diagram, protecting the galvanometer? I can see this as being reasonable, given that when the circuit is balanced this 'implicit resistor' would have no current through, hence no p.d. across it and so wouldn't affect calculations. Cheers 77.86.89.204 (talk) 16:10, 21 May 2011 (UTC)

The resistance in the legs of the bridge will limit the current flow through the galvanometer, and the limited current which flows through the galvanometer will cause the two measurement points to be at equal potential. When the bridge is balanced, the two measurement points will be at equal potential without any current flowing through the galvanometer. Jc3s5h (talk) 16:20, 21 May 2011 (UTC)

With an ideal zero-resistance galvanometer, it's basically as if the galvanometer has been replaced by a wire. Even if the bridge is unbalanced, there will ideally be zero voltage across the galvanometer, even though there's a nonzero current through it. The current flowing through the ideal galvanometer is the same as the current that would be computed as going through the wire, if the galvanometer were replaced by a wire. Red Act (talk) 16:34, 21 May 2011 (UTC)

Thank you for your responses, I was thinking about that - replacing the galvanometer with a wire and asking what would happen. So we're replacing two nodes with a galvo between them with two nodes with a perfect conductor between them, so we could continuously deform the circuit until those two nodes merged (as there is now nothing between them). This circuit now looks like two resistors in parallel, a node, then another two resistors in parallel - so it's no longer a bridge? Sorry it's probably me needing to think about this more, thanks again for your replies. —Preceding unsigned comment added by 77.86.89.204 (talk) 17:04, 21 May 2011 (UTC)

The thing to keep in mind is that with a bridge, you do not try to get any quantitative information from an unbalanced bridge. You only use the galvanometer or voltmeter to indicate "balanced" or "not balanced". When it is balanced, the potential at the measurement nodes would be the same if the balance indicator were removed, so it does not make any difference that the galvanometer acts like a short circuit, because no current will flow. When the bridge is unbalanced, the galvanometer does disturb the operation of the circuit, but we don't care because we are not making any quantitative measurement while the circuit is unbalanced. Jc3s5h (talk) 18:17, 21 May 2011 (UTC)

It's bad for a battery to connect its terminals together because a high current flows, but not an infinite current because every real voltage source has an internal resistance. The midpoint of each side of a Wheatstone Bridge is a voltage source with an internal resistance of (on the left side) (R1 x R2)/(R1+R2). This (and the corresponding internal resistance on the right side) is the "implicit" resistance that the OP correctly reasoned. Cuddlyable3 (talk) 12:51, 22 May 2011 (UTC)

Taste of seafood

Hi. Could exposure to radiation in the sea cause seafood to taste bitter and otherwise unusual? If so what is the process that causes this? Not asking for medical or legal advice here. Thanks. ~AH1 ^(discuss!) 21:39, 21 May 2011 (UTC)

I highly doubt it. A change in taste after exposure to radiation would indicate that a significant amount of some substance in the fish has been transmuted. Such an extreme exposure seems unlikely except in some sort of extreme radiation event such as nuclear test Baker from Operation Crossroads, and would certainly result in the death of the fish (or shrimp or what have you).

Now what you have not asked is whether radioactive materials the fish might have absorbed could have a bitter taste. This seems more possible, but I suspect still quite unlikely, since a material that seafood readily absorbs will be absorbed whether or not it is radioactive.-RunningOnBrains^(talk) 01:00, 22 May 2011 (UTC)

Anything is possible in biology. I doubt it, but if you have some anecdotal evidence to go by, an experiment may be worthwhile. To give a purely hypothetical mechanism (I have no evidence for this), consider that Ramsar, Mazandaran has a just incredible level of natural background radiation, yet there is no increased incidence of cancer there. One might hypothesize (there doesn't seem to be as much evidence as I'd expect) that people can accustom themselves to a high level of background radiation by some means, e.g. induced DNA repair mechanisms. (There have likewise been varying signs from Chernobyl, which is said to look surprisingly like a nature park in places) Now if large amounts - comparatively speaking - of radiation penetrating an organism might change DNA repair enzyme expression, then it is possible that the flavor of a person, or lobster, might change due to some aspect of the proteins or the biochemistry they mediate. This might even occur simply due to increased radiation penetrating the body, with no radioactive material ingested at all - or it might occur when a trace element like uranium is sensed by some specific enzyme, and not function in the case of other environmental exposure. Alas, I don't know of any hard information at all on the topic. Wnt (talk) 08:47, 22 May 2011 (UTC)

An unusual bitter taste probably indicates that the seafood has gone bad or off. Mold, I understand, often creates a bitter taste. 92.15.21.174 (talk) 11:53, 22 May 2011 (UTC)