Wikipedia:Reference desk/Archives/Science/2011 January 26

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January 26

What's the difference between van der Waals forces and London forces?

My chemistry textbook claims that they're different names for the same force, but Wikipedia has separate articles for them. --75.15.161.185 (talk) 00:28, 26 January 2011 (UTC)

Have you read the intros to the respective articles? The London dispersion force is a component of the van der Waals force, which is itself a sum of many forces. We note in the former article that "dispersion forces are usually dominant of the three van der Waals forces", with the other two being orientation and induction forces. Small polar molecules like water are a notable exception to this trend. — Lomn 00:42, 26 January 2011 (UTC)

Forces stronger than London dispersion forces that are also van der Waals include Debye forces, Keesom forces, hydrogen bonds, ionic bonds and induced ion forces; see intermolecular force. The "London forces" are dominant in nonpolar molecules that do not have the stronger forces, as they lack permanent dipoles, and increase in strength given larger molecules and thus more atoms for temporary charge induction. ~AH1^(TCU) 22:27, 26 January 2011 (UTC)

The amount of energy in houshold rubbish

How much energy would a typical household expect to get from burning all their burnable rubbish and using it for heating? Thanks 92.24.182.244 (talk) 02:02, 26 January 2011 (UTC)

More than enough to heat their home, in most locations. But the technology to do it cleanly enough is not feasable on a household scale. And in most developed countries, it would be illegal to try to do this in a crude way yourself. Some cities do this to some of their garbage not just for energy, but to reduce landfills. Some of the remaining matterial (black "nuggets" formed) that doesn't go up in smoke can be collected and used as a raw material (for road pavement is one common application). Roberto75780 (talk) 02:20, 26 January 2011 (UTC)

Well my household does not produce enough rubbish to provide even a twentieth of the heating cost, even with super-efficient combustion, but perhaps some households produce tons of rubbish. Dbfirs 08:18, 26 January 2011 (UTC)

The article Incineration has good information on waste incineration. It says one metric tonne of garbage can produce 0.67 MWh of electricity and 2 MWh of heating. A person produces on average 726 kg garbage per year[1] Taking the average US household size as 2.6[2] that's 1890 kg per household, which will give 1.3 MWh electricity and 3.8 MWh heating.

For comparison, domestic energy consumption is around 100 million BTU or 29 MWh per household [3]. Energy in the United States says 45% of domestic energy is used for heating water, which assuming the figures match would be 13 MWh. Big shortfall. --Colapeninsula (talk) 10:55, 26 January 2011 (UTC)

Energy in the United States actually says "Residential Living quarters for private households: 13% water heating" - not 45%. Living in a mild weather area in the U.S. Midwest which is currently on day 45 of 57 to fail to reach a daily high temperature above 0°C (32°F), 45% of energy just for water heating sounds like optimism. Rmhermen (talk) 14:45, 26 January 2011 (UTC)

For many years I had the job of burning my family's trash in an old 55 gallon drum. It flared up to a high temperature, but burned down quickly, and would have been far too little to heat the home in the winter. For a time, we used coal, so I had a direct comparison of the mass of coal versus the mass of trash, and the heat produced by the coal stove versus the trash fire. There was no trash collection service at that time in that location. Edison (talk) 18:17, 26 January 2011 (UTC)

Methane capture is another method of making energy from trash, and incineration often works best given combustable (ie. organic-based) garbage. ~AH1^(TCU) 22:20, 26 January 2011 (UTC)

Rubbish Island

a) It was a quite frequent trope of 1950s-1970s films set in New York, or elsewhere, to see as a finale the hero ending up on a large barge or 'scow' taking rubbish out to sea. Where did this rubbish in actuality go to? Does this practise still continue?

b) Is there any reason why it would not be a good idea to use rubbish for land reclamation? In Britain the dogger bank is crying out to have an island built on it, and would be a great place to put all the things that nimbys do not like. Thanks 92.15.10.209 (talk) 14:47, 26 January 2011 (UTC)

Fresh Kills Landfill is now closed but the location is used to repackage New York garbage for rail shipment to South Carolina. Also not an island. Rmhermen (talk) 15:03, 26 January 2011 (UTC)

Fresh Kills Landfill is not an island, but it is on an island -- Staten Island. Indeed, it forms a rather large portion of the island. --M@rēino 20:02, 26 January 2011 (UTC)

The trash heaps occupy 2.6% (990 acres) of the 37,400-acre island and the entire site occupies about 6%. Not "a rather large portion". 75.41.110.200 (talk) 21:41, 26 January 2011 (UTC)

I believe your numbers are accurate. I'd still say that's a rather large portion of the island. --M@rēino 17:56, 27 January 2011 (UTC)

The annual volume and mass of NYC garbage must be impressive. How much trash (excluding recycled materials) has to be disposed of from that one city each year? Does SC or other victim states build Mount Trashmores out of it, or what? Edison (talk) 18:10, 26 January 2011 (UTC)

Environmental issues in New York City#Garbage disposal says the city produces 12,000 tons of (landfill) garbage per day (confirmed by this article). This totals more than 4 million tons annually. -- Finlay McWalter ☻ Talk 18:26, 26 January 2011 (UTC)

(edit conflict)New York has shipped its garbage to various places, including Fresh Kills Landfill, Philly, and North Carolina (cf Mobro 4000). Building islands is very expensive; regular garbage, and miscellaneous landfill in general, isn't a stable engineering material. High-value properties like Kansai International Airport have been constructed in shallow water, but it takes a lot of concrete to make something that doesn't just wash away. Moreover, post-consumer waste yields for lots of problems to anyone planning on building, farming, or living on top of it. Firstly it's not compact, so structures built on fill subside - the Japanese airport solution (which I think Changi uses too) is to build structures on jacks, and jack the sagging end of the building up as they subside. Secondly is methane offgassing due to biodegradation in the waste. And third is contamination - the waste stream contains (whether it should or not) all kinds of nasty stuff like paint, motor oil, medical waste, dioxins, asbestos, and heavy metals. So you'd spend a fortune constructing an island that couldn't be used. -- Finlay McWalter ☻ Talk 15:08, 26 January 2011 (UTC)

(edit conflict) The Mobro 4000 article is a very interesting read in this regard. --Jayron32 15:17, 26 January 2011 (UTC)

See also Great_Pacific_Garbage_Patch. Kittybrewster ☎ 15:35, 26 January 2011 (UTC)

I get the impression that some people think the sheer volume of garbage is a problem (see the opening of Wall-E, or of Sex, Lies and Videotape, where I think she's specifically reacting to the Mobro 4000 story). There just isn't that much garbage. Pollutants leaching from garbage into water is a problem, methane from rotting garbage is a problem, but the simple volume occupied by garbage is not a problem. If you want to reduce the environmental impact of garbage, dumping it in the middle of a body of water probably isn't the best idea. Also, if you're upset that Manhattan exports its garbage, remember that it imports the stuff that becomes the garbage. There are advantages to concentrating people in one location and putting the support systems elsewhere -- BenRG (talk) 20:52, 26 January 2011 (UTC)

Some floating islands could be constructed from garbage. ~AH1^(TCU) 22:19, 26 January 2011 (UTC)

The volume isn't a problem, until you run out of convenient places to put it. Some large cities seem to be in this situation.--Srleffler (talk) 02:16, 29 January 2011 (UTC)

Submarine speed

Why do submarines go faster underwater than on the surface? Example: German Type XXIII submarine. When underwater, would they not have more drag and more displacement of water to overcome? Thanks 92.15.10.209 (talk) 14:54, 26 January 2011 (UTC)

Turbulence. Cuddlyable3 (talk) 14:57, 26 January 2011 (UTC)

See diagram. The prop-shafts and therefore thrust is generally in-line with the central axis. If the vessel was to go too fast, the water level behind the vessels would drop further than it does at max surface speed, until the blades of the screws would experience rapid changes in the forces acting upon them as they first left, then re-entered the water. This would lead to early failure. As it is, some screws are only just covered by water when cruising on the surface. Also, and which needs to be mentioned. The deeper a sub goes, the less likely the screw is to experience cavitation but that is a bit secondary to the much faster damage caused by the atmospheric air ventilating the screw.--Aspro (talk) 15:37, 26 January 2011 (UTC)

A significant part of the drag of a surface ship is due to creation of a bow-wave and other signs of disturbance on the surface of the water. See Wave making resistance. A submerged streamlined body, such as a submarine, does not make any disturbance on the surface of the water so this form of drag is avoided.

After a diving entry, competition swimmers remain underwater as long as possible in the hope of maximising their average speed. It seems to work. Dolphin (t) 21:31, 26 January 2011 (UTC)

A Discovery Channel TV documentary I just happened to watch attributed the speed increase to water density. Go deeper, denser water, more stuff for the propeller to push against. 88.112.59.31 (talk) 22:32, 26 January 2011 (UTC)

Untrue! The pressure of water increases with depth but the density is almost constant throughout the ocean. The density of the atmosphere changes significantly with height above the Earth's surface because air is a gas. Water is a liquid and its density does not change (significantly) with changes in pressure. Dolphin (t) 01:41, 27 January 2011 (UTC)

Water does not compress as gasses do, so the density is mostly the same (as the appropriately named Dolphin51 mentioned), and even if it did increased moved mass would be offset by increased density of the medium you were pushing through, to a point. I would also suspect that a significant factor is that submarine hulls are optimized for travel at depth, not surface travel (this is especially true of the modern hull, less so I'd imagine of WWI and II-era hulls that were more closely adapted from surface ships). The article on Teardrop Hulls confirms this. Oddly the article makes no mention of the modern "Albacore" Hull, so named for the ship that first used it, which is the standard more or less today. This hull is strongly optimized for travel sub-surface and highly suboptimal for surface travel. 65.29.47.55 (talk) 09:20, 27 January 2011 (UTC)

First lets dispel the landlubber's myth that submerged vessels do not create bow waves. The sea is a viscous medium and as such, the hydrodynamic force will raise the sea's surface, above the vessel, to a hight over the mean. Although, this hight diminishes with the vessel's depth, its surface area increases. The American Fleet understood bow waves and so by the 1970's had become alarmed to discover the capabilities of synthetic aperture radar satellite technology and its ability to resolve alimentary delta information to millimetre resolutions and thus its possible potential to expose the position of its nuclear launch subs. (Note: and the difference in most cases is only in the order of millimetres.) Quote: The bow wave from submerged submarines could thus show up in radar images of the ocean's surface unquote. Page 97 Therefore, the 'wave making resistance' can not account for the slower surface speed because it exists in both situations. Indeed, if we go here to calculate the approximate displacement hull speed, we find that for this submarine's length it is about 14.3 knots rather than 9.7 or almost half as fast again and this is without the added frictional increases when submerged. Therefore, perhaps a better understanding of the danger of the screw ventilating and the way chosen to prevent it, can be had by looking at photographs of this sub on the surface, both in the article itself and again here. Compare those to the line drawing of this sub and you will see that is surface attitude is trimmed to keep the bow high. The line drawing also shows that if it was to sail without this raised forward pitch, although it would go faster, the trough formed at the rear would quickly allow the screw to suck down air as the speed increases. This would hammer the bearings, fatigue the blades of the screw and create vibrations throughout the vessel. Marine architecture is, like many other engineering structures, an assemblage of compromises. --Aspro (talk) 18:33, 27 January 2011 (UTC)

I'll just note that there have been submarines that ran faster on the surface -- for example, the German types VII, IX, X, and XIV. I think there wasn't much point in optimizing the speed underwater when they couldn't store enough electricity to stay underwater all that long anyway. --Anonymous, 11:22 UTC, January 28, 2011.

Endosome/endocytic vesicle

Is there any difference between these two terms? I have read endosome and the Gene Ontology entries [4][5] but I'm still not wiser. Is an endosome an organelle and an endocytic vesicle not (as GO suggests)? But why, both have a lipid bilayer. Can you help? -- Ayacop (talk) 16:45, 26 January 2011 (UTC)

It gets a bit semantic, but the term "endocytic vesicle" tends to be used to describe the structure that pinches off during endocytosis. This vesicle then fuses with an endosome where sorting of different receptors takes place and then allows for trafficking of those consituents to different locations. One way to think of it is that the endocytic vesicle comes directly from the plasma membrane and carries something from outside to inside. The endosomal compartment (not one organelle but a very dynamic collection of membrane-bound structures) is a bit more murky, with different compartments handling flow of materials from inside to outside, outside to inside, into the lysosomes for degradation, etc. --- Medical geneticist (talk) 16:57, 26 January 2011 (UTC)

Ah, thanks. Followup: so a phagosome would be a kind-of endosome (both can mature)? -- Ayacop (talk) 17:01, 26 January 2011 (UTC)

Why the "Coma" Cluster?

The article on the Coma Cluster gives no hint of why it is called that. Was it named after a person or a place? Was it named after the medical condition Coma, or did it somehow look like the galaxies were in a deep sleep? Is it pronounced like the medical condition? Edison (talk) 18:04, 26 January 2011 (UTC)

It's named after the constellation Coma Berenices. Info found on the Coma supercluster article. 90.193.232.5 (talk) 18:26, 26 January 2011 (UTC)

Coma is also a type of optical aberration in telescopes. That could possibly be related to the etymology of the Coma Cluster. On the other hand, coma is also used to refer to any sort of nebulous blurring, whether an artifact of instrumentation or actually due to the characteristics of the astronomical object. The word root is "coma (from the Greek κόμη, "hair")" - not from Ancient Greek κῶμα (kōma, “deep sleep”). See the wikt:coma etymologies. Nimur (talk) 18:28, 26 January 2011 (UTC)

The Latin term "coma", according to the OED, derives from the Greek κόμη, meaning "hair of the head, also applied to foliage, etc., and to the tail of a comet". As the Coma Berenice article points out, in Latin the term means "Berenice's Hair". It's of note that the use of the term "coma" for a comet's tail dates only from the late 18th century, and the use of "coma" to mean aberrations caused by lenses is from the 1860s. So there's the same metaphor here, but it's not that the name of the constellation came from the previous astronomical terms. --Mr.98 (talk) 18:45, 26 January 2011 (UTC)

I've added that piece of information to the article - many nearby clusters are named after the constellation they're in, e.g. Virgo, Fornax etc. --Wrongfilter (talk) 20:52, 26 January 2011 (UTC)

Thanks for the explanation. Someone laughed when I was talking about late 20th century cosmology/astronomy and the importance of the "Coma Cluster" observations, and asked where the name came from, and Wikipedia provided no guidance. In American English, how is this "Coma" pronounced? Like "Coma," or like "comma," or otherwise? The responses above list two differently spelled Greek words, but Wiktionary has one pronunciation. The article Greek alphabet shows the letter ο or Omicron but the one shown above was "ό." The article Omicron says nothing about accents on the letter. Apparently F. Scott Fitzgerald's story Bernice Bobs Her Hair (1920) derives from this ancient Coma Berenices legend of Berenice II cutting her hair off. Edison (talk) 05:09, 27 January 2011 (UTC)

KBOs and TNOs

KBOs, a detached object and the inner Oort Cloud.

Is there any difference between Kuiper belt objects and Trans-Neptunian objects? Or are KBOs a subclass of TNOs? --T H F S W (T · C · E) 20:00, 26 January 2011 (UTC)

Our article Transneptunian object says

"The Kuiper belt, scattered disk, and Oort cloud are three divisions of this volume of space [where TNO reside]."

As I understand this quote, all KBOs are TNOs. But there is a difference between the classes: members of the Oort cloud are TNOs but not KBOs. SemanticMantis (talk) 20:45, 26 January 2011 (UTC)

(edit conflict) Other members of the TNO class that are not KBOs include Scattered disk objects and detached objects. The Oort cloud is thought to lie much farther than the Kuiper Belt and even objects that have great eccentricities in non-parabolic orbits are thought to be unable to reach it, however. Also, the Kuiper Belt seems to cut off almost abruptly at 50 AU. ~AH1^(TCU) 22:15, 26 January 2011 (UTC)

Proton spheres

I have a question which has been left unanswered by our article on photon spheres: are photonic orbits around neutron stars more stable than photonic orbits around black holes? Why/why not? Thank you in advance. --Leptictidium (mt) 22:13, 26 January 2011 (UTC)

No difference because the space around a neutron star has the same metric (space-time shape) as the space around a blackhole. 71.101.41.73 (talk) 02:46, 27 January 2011 (UTC)

Is that totally true? A photonic orbit around a neutron star must be very close to its surface, and so far as I know a neutron star rotates as a solid ball - so matter near its poles moves in a circle that is not an orbit. But a rotating black hole has all its matter stuffed in a ring singularity or the like somewhere far from any orbiting photon. Shouldn't this mean that spacetime is dragged in a different way around a neutron star, and photons should orbit in some weirder way, at least if they aren't on the equator? (Also curious if there would be a difference if the fuzzball theory is true rather than a ring singularity...) Wnt (talk) 19:15, 27 January 2011 (UTC)

It is certainly true for a "spherical cow" non-rotating neutron star -- the Schwarzschild spacetime is the only stationary spherically symmetric vacuum solution to GR where the central body has no electromagnetic field, so it must equally describe the situation outside a black hole and that outside a neutron star.

However, there's no corresponding uniqueness property for spacetimes that are merely rotationally symmetric. (Even with zero angular momentum, surely a long dumbbell-shaped object will have a gravity field that differs from the Schwarzschild/Kerr one).

And real neutron stars can also have strong rotating magnetic fields whose axis need not coincide with their rotational axis (as for pulsars). This situation is forbidden for black holes by the no-hair theorem. Since a magnetic field shows up in the electromagnetic stress-energy tensor and so influences spacetime curvature, that would make the spacetime outside a neutron star different in principle from that outside any black hole. I don't know how that difference affects the stability of photonic orbits, though. –Henning Makholm (talk) 21:10, 27 January 2011 (UTC)