Wikipedia:Reference desk/Archives/Science/2011 November 17

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.

November 17

Gravity and Water

Now, this might sound confusing but I'll try to say it as simply as I can. Say that the Earth, same size and water intact, was a moon of Jupiter, and it was far away enough from it that the water was, I'm not sure if this is scientifically correct but let's just say, "ripped away" from the Earth (where Jupiter's gravity would be so strong water couldn't stay on it because of tidal forces.) Now, if the Earth was like ours, so that a magnetic field existed to protect the water on it from being wiped away, surely Jupiter's tidal forces would change the water somehow. So my question is, would Jupiter's gravity "stretch" the molecules in the water to make water lighter than it would be on our Earth? If not, what would change exactly? 64.229.182.199 (talk) 01:43, 17 November 2011 (UTC)

I don't really understand the question, but there is no place within the solar system where there exists a tidal force strong enough to stretch a water molecule significantly. Only close to the gravitational singularity at the center of a black hole would there be a tidal force strong enough to have any significant effect on something as small as a water molecule. Red Act (talk) 02:17, 17 November 2011 (UTC)

Earth's magnetic field isn't what keeps our water in place; it's the gravity. So if we were close enough "where Jupiter's gravity would be so strong water couldn't stay on it [the Earth]", it wouldn't stick around (nor would we for that matter). Clarityfiend (talk) 02:25, 17 November 2011 (UTC)

Well, yes, I just meant it had the magnetic field so that solar radiation wouldn't split the water into hydrogen and oxygen. 64.229.182.199 (talk) 02:44, 17 November 2011 (UTC)

I don't think you could put Earth close enough to Jupiter that tidal forces ripped away our water (and anything else not nailed down) without actually putting Earth inside Jupiter.

However, are you asking if tidal forces make things seem lighter on one side of the planet? (And heavier on the other?). Yes. You're infinitesimally lighter when the moon is overhead. Not enough that you could measure it, though. APL (talk) 04:36, 17 November 2011 (UTC)

So, what you'd actually get if Earth was in a close orbit around Jupiter, and wasn't yet tidally locked to it, would be massive tides, much larger than they are currently. You'd also get massive earthquakes and much more tidal heating (I'm not sure if this and the radiation from Jupiter would be enough to keep water liquid, though, as the reduced sunlight at that distance would make a major difference in surface temperature). StuRat (talk) 04:45, 17 November 2011 (UTC)

we can generalize this question to have best discussion. the moon absorbs water of oceans night times , so if there be any strong enough object to have tidal force to give huge tidal to oceans water what will happen?--Akbarmohammadzade (talk) 09:15, 17 November 2011 (UTC) solution :the moon can give tidal force to water to uprise about 1~2 meters , then if any massive planet or object which is about 30000 moon mass was placed there in moon position , absorbing water toward itself strongly ,and you think where this condition is happening  ? in binary stars ,one of stars is swallowing others body by gravitational force ,water will grow up and with air of atmosphere rise toward sky then will frizz in high cold atmosphere then will crack and leave earth with high speed ,in addition in thus condition, earth will be in Roche limit and all its body will explode--Akbarmohammadzade (talk) 09:15, 17 November 2011 (UTC) water came here to our eatrh when its surface was molten and very hot , it may happened about 3 billion years ago , this is a part of my article : " A most believed theory says that water was added to earth by any comet ,the comet might had 10^18kg~10^19kg weight and it was hunted by earth ,then the comet might be reached to Roche limit of earth gravity field then rotating of its exploded body casing long time raining for the duration of about 50~100thousand years continues raining, this event could case the surrounding of water with200 meters dept of water on earth , "--Akbarmohammadzade (talk) 09:40, 17 November 2011 (UTC)

"Absorbs" is the wrong word, perhaps "pulls" is better. The relative masses isn't what's important, but rather the gravitational attraction of the bodies. The Moon has 0.1654 g at the surface while Jupiter has a gravitational attraction of 2.528 g. That's a bit over 15 times as much. So, we'd expect the tides to be 15 times as high. Of course, the gravitational attraction of the Moon is far less on the Earth's surface than on the Moon's, and so would Jupiter's attraction on the surface of the Earth, if the Earth was in orbit around it, but hopefully the reduction in gravitation attraction is proportional in both cases. StuRat (talk) 17:23, 17 November 2011 (UTC)

Physics of the artificial trees

Hi. I am gearing up for a presentation on the topic, focusing on the physics principles behind the concept in addition to the thermodynamics of the atmosphere. I would like relevant links, citations and information, plus any new forms of carbon air capture (I know that lithium hydroxide works for small compartments), the logistics issues of storing the waste material, and especially any power generation techniques used in concert with the air capture technology, such as STEP, alongside any issues involved. Some equations on Kirschoff's Laws, ie. absorptivity and emissivity and explanations thereof would be helpful, as would any synthesis of existing equations, as I did above for the momentum/energy synthesis, thus enabling extrapolation from one data point, which is scientifically unvalid, but more equations are needed. Thanks. ~AH1 ^(discuss!) 03:12, 17 November 2011 (UTC)

Which office furniture / equipment would last the longest?

I used to be in charge of recommending ergonomic furniture / equipment for offices, and wondered which item would last the longest under normal use. I came to the conclusion that metal in / out trays would surely be a winner. The ones on contract were steel, and only had to bear the weight of a single sheet of paper being pulled along its length once a day, if that. Asked by senior management how long such an item would last, I did some very basic calculations, and came up with the figure of 4 billion years. That meant that there was little need to renew the contract. Anyone think this correct, or can think of any other long-serving contenders? Myles325a (talk) 04:05, 17 November 2011 (UTC)

In/out trays get filled up every day in a busy office, or overflow if the desk occupant is not diligent. One sheet a day sounds like a definition of "redundant employee," unless almost all communications are electronic. In one office I had such a tray which was likely over 70 years old and still in fine condition, certainly not 10% worn out, presumably with daily heavy use. Edison (talk) 05:27, 17 November 2011 (UTC)

It seems you are making the mistake of thinking that an item will continue to be used until it breaks. Many items are replaced for other reasons. Those metal trays may be considered unsafe, if they have sharp edges, or might just be replaced with something that makes the office look warmer and more inviting. Or perhaps a different system is implemented for filing "ins" and "outs", say where they are divided up by date, priority, etc., and thus a file cabinet is needed. StuRat (talk) 05:40, 17 November 2011 (UTC)

You assume only wear and tear from normal use. What if they fall off the table and land on something hard, or somebody is trying to reach something high and accidentally or deliberately stands on them (even in the middle of the room there may be somebody trying to fix a ceiling panel, test a fire sensor, etc), or if there is office-party-related tomfoolery, or the desk collapses?

Doesn't steel tend to rust? Even stainless steel can tarnish or discolour. Certainly it will not last 4 billion years. --Colapeninsula (talk) 09:34, 17 November 2011 (UTC)

I have some antique, or at least very old solid wooden in trays. They were over engineered, and built to high quality, and so have lasted long past them dropping out of fashion and being replaced by cheap plastic ones that could not even stand being filled with paper. Of course these will not survive fire or being exposed to water for long periods. Over a long time you can expect an earthquake to destroy your work place or fire to burn it down. Graeme Bartlett (talk) 10:13, 17 November 2011 (UTC)

NMR and pi and pi/2 pulses

I don't get how the pi/2 pulse in nuclear magnetic resonance results in maximum magnetisation, or why the pi pulse puts the population of all the protons in an excited state. Say a pi/2 pulse (RF frequency dependent) lasts 2 microseconds. It doesn't take two microseconds to tip over the magnetic moment of a proton, 90 degrees right? 137.54.13.75 (talk) 05:11, 17 November 2011 (UTC)

Life on Europa and water under extreme conditions

Recent news stories say scientists wonder if Europa, a moon of Jupiter about the size of Earth's Moon, might have living organisms in what is believed to be an extremely thick saltwater ocean (perhaps 100 miles deep per the BBC article)under a thick blanket of ice ("a few km" to 30 km per the Wiki article). Some water temperatures in a BBC news article are way outside the range of common experience. Since the surface is very light colored (from the ice) and Europa is farther than Earth from the Sun, it is understandable that it would have a very low surface temperature (110K at the equator). There is supposed to be some heating from radioactivity of the core, and far more from tidal flexing of the ice and the core. The article says the water temperature might run -60 to -50C. What prevents it freezing, which Earth's ocean water would do at -2C at normal Earth atmospheric pressure? A physics website says that maximally saturated saltwater freezes at -21.1C at Earth's atmospheric pressure. What would the pressure have to be below the ice to depress the freezing point of saltwater to -50C? I could not extract this information from Water or Phase diagram, and my chemical thermodynamics class was a very long time ago. I am curious about the pressure a space probe would endure as it descended into the Europa ocean after penetrating the ice. Edison (talk) 05:22, 17 November 2011 (UTC)

Have you seen Don Juan Pond, it stays liquid below -21C and as low as -50C Perhaps Europa has a higher salt level than Earth's ocean. CambridgeBayWeather (talk) 07:59, 17 November 2011 (UTC)

Neat article! Though I wish it had more about how the lake was discovered. I'm imagining the first pilots to fly over and radio back about the lake they saw must have found drug test kits waiting for them at the airfield... ;) Wnt (talk) 15:09, 17 November 2011 (UTC)

I'm no physicist, but I don't think the article actually says that liquid water is 223 to 213 K (−50 to −60 °C). This paper theoretically puts the temperature at the base of the ice shell at 260 K (−13 °C), above the melting point of water, warmed by convecting liquid water below it from volcanic activity (and it should be considerable, given that Europa's next-door neighbor is the melted hell Io).-- Obsidi♠n Soul 09:56, 17 November 2011 (UTC)

The BBC article says "And in Europa's ocean it could be -50C, -60C. " They seemed to be discussing if the liquid ocean was too cold for microorganisms. I saw no ref for the possible temp at the bottom of the ocean, near the core, probably since they have no idea, but they posit thermal vents like in our ocean, which would imply warm water near the core, wouldn't it? What would be the pressure at the bottom of a 60 or 100 mile deep ocean on Europa? The pressure build up with depth would be far slower than on Earth.

Googling for Charles Cockell, I found this book, where he mentions a Tiréz Lake in Spain with hypersaline conditions and the subglacial lake in Antarctica, Lake Vostok. Both are liquid.

Pressure in Lake Vostok is 400 bar and remains liquid at −3 °C (270 K) because of pressure and geothermal vents. Maximum pressure estimated at the bottom of Europa from here is 200 to 700 bar (compared to 230 to 500 bar in Earth's poles). Tidal heating of Europa from models is estimated at 9-10¹² W (0.3 W/m²), compared to 10¹⁴ W for Io. Perhaps pressure and hypersalinity are cumulative? Dunno what to make of that, perhaps someone with a better grasp at physics can calculate? Heh. -- Obsidi♠n Soul 02:08, 18 November 2011 (UTC)

Diode circuit solution

In the attached image, I seem to be having a problem in question number 3.9 a and b. The attached image contains the solution, however I still dont understand how it can be done. The diodes are assumed to be ideal with no voltage drop across them. The values provided in the question are the resistances and the voltages 5 and -5. Could someone explain how it is done? Thanks. http://htmlimg4.scribdassets.com/48cpere7lsffb2a/images/63-7bc71d6790.jpg --115.167.124.240 (talk) 08:22, 17 November 2011 (UTC)

Well in a, since the left hand diode is conducting, the voltage is 0. D2 is conducting as well, so the voltage level is the same = 0V. So we have 5 V drop across the 10kOhm resistor, which will take 0.5 mA I=V/R. 5 volts across the 5KOhm resistor gives 1 mA. Since this is bigger than the 0.5 mA there is no proble with the D1 conducting and taking the rest of the current. Graeme Bartlett (talk) 09:56, 17 November 2011 (UTC)

How do you assume the first diode to be conducting in the first case and not in the second case? --115.167.124.240 (talk) 10:00, 17 November 2011 (UTC)

In each of the two cases, make an initial hypothesis that D₁ is not conducting. If no current goes through a component, a circuit will behave as if the component was simply not there, so to determine the voltages and currents in the circuit, analyze the circuit as if D₁ simply wasn't there.

In circuit (b), the voltage across where D₁ would have been will be -5/3 V. So sticking D₁ back into the circuit will cause D₁ to be reverse biased, which is consisted with the hypothesis that D₁ isn't conducting, so you accept that initial hypothesis.

But in circuit (a), the voltage across where D₁ would have been will be +5/3 V. So if you insert D₁ back into the circuit, D₁ will be forward biased, which is inconsistent with the initial hypothesis that D₁ isn't conducting. So reject that initial hypothesis, and re-analyze the circuit assuming that D₁ is conducting. Red Act (talk) 15:26, 17 November 2011 (UTC)

Also, in the first case, there should be no current flowing across the first diode as the potential difference across the diode is 0. --115.167.124.240 (talk) 14:49, 17 November 2011 (UTC)

If a component has a nonzero resistance, then a zero voltage across the component implies that there is zero current through the component by Ohm's law: I=0/R=0. But if the component has zero resistance, then if there is zero voltage across the component, Ohm's law says that the current through the component is I=0/0, which is an indeterminate form, so other means must be used to determine what the current is through the component. Consider a wire, for example. A wire can certainly have a current flowing through it, but the voltage across a wire is always zero (ideally). The same is true of a forward biased diode, which (ideally) has zero resistance. Red Act (talk) 15:40, 17 November 2011 (UTC)

Yes, ideal wires and ideal diodes don't exist in real life, of course, so an alternative to dividing by zero is just to ignore Ohm's law for these components. In practice, of course, there is always a tiny potential difference across any current-carrying wire or diode, so Ohm's law does apply, but tiny potentials are difficult to measure and just complicate the arithmetic, so they are ignored in "ideal" circuits. Dbfirs 21:24, 17 November 2011 (UTC)

Heat loss and gain

We have a building at work with four windows. Two face south and, obliviously not in December and the first 1/2 of January, get the sun all year round. One faces east and starts to get the sun in the very early spring. The west window does not get the sun until later in the year. From 13:30Z to 22:30Z and from 01:30Z to 10:30Z the building is unoccupied. Now from 13:30, our "day" the blinds must remain open and from 01:30, our "night" the blinds must be closed. This is done to save heat with the idea that the sun will heat the building and conserve fuel. This is a good thing and we do the same at home, except we use curtains that are somewhat more thermally efficient than cheap plastic blinds. The problem at work is that there is no allowance for common sense. For example as of 31 January sunrise is 16:37Z and sunset 21:51Z and the temperature is likely to be around -40C. My feeling is that to a certain extent we are wasting our time. So, is there any way to calculate how much heat is lost through a window? Also is it possible to figure out how much heat is obtained from the sun through the same window. And finally the same thing but the window is now covered by a cheap plastic blind. CambridgeBayWeather (talk) 08:37, 17 November 2011 (UTC)

I think we'd need to know more about the window. A good window for such a climate should impose a very strong greenhouse effect, reflecting all the infrared radiation emitted from within and providing perfect insulation against direct transmission of heat. In which case, the light is all good and there's no problem. Of course, if your building were made of a perfect insulator then your body heat would be enough to keep it at a nice cozy temperature and every once in a while you'd have to open a window to cool it down. ;) So it's all a matter of the specifics of the material. Wnt (talk) 15:18, 17 November 2011 (UTC)

These are generic small trailer type buildings used across Canada and I don't think the windows are anything special. I don't seem to have uploaded a picture of the building and can't find one to show you. CambridgeBayWeather (talk) 17:06, 17 November 2011 (UTC)

I would say that the blinds can make a noticeable difference, but that they should be opened at each window when the Sun shines in that window and closed the rest of the time (maybe somebody needs to sell a device with a light sensor to do that automatically, or to darken an LCD panel in front of the window to block out the light). This pattern should then be reversed in summer, to help to keep the building cool. Opening and closing all of them at the same time each day doesn't make much sense. StuRat (talk) 17:00, 17 November 2011 (UTC)

leonardo da vinci laminate adhesive

Your article on laminates credits some dentist with that invention/discovery, but a few years back, Discover Magazine had a sidebar that credited Leonardo da Vinci with discovering the technique and potency laminated materials. They demonstrated his discovery with a fork handle made of cabbage leaves. I came to wiki to find out what adhesive Leonardo had used and was disappointed that this incredible invention was credited to someone centuries later. No mention of the sort of beautiful and engineeringwise work of Leonardo at all.

Surely this needs someone's engineering-history research effort as the technique seems so credibly usefulCighe (talk) 14:35, 17 November 2011 (UTC)

It should of course refer to dental laminates. Yet the Egyptian's used laminates (Plywood) long before Leonardo.--Aspro (talk) 16:51, 17 November 2011 (UTC)

As a general observation on Leonardo: the problem with attributing inventions to him is that he never published or communicated a great many of his ideas, and even today we are still deciphering more from his preserved private notebooks. To be useful an invention has to be communicated and brought into use (perhaps by others), so while Leonardo might be acknowlegeable as having himself privately had various ideas first, other people subsequently had to come up with the same ideas without his influence, and put them into practice. {The poster formerly known as 87.81.2301.95} 90.197.66.145 (talk) 17:48, 17 November 2011 (UTC)

PC monitor bugs (the 6 legged kind)

This time last year, I started to get some little bugs on the screen of my TFT monitor. They are really small, about the size of one of the pixels, and if you look through a magnifying glass they look reddish in colour. They aren't actually visible unless the background is white (like wikipedia) and you only see them when they move. At first I thought I was seeing things! It's very off putting when you are trying to read the screen. A quick squirt of fly spray (which I don't like to use) and I was troubled no more. Well today they came back! My questions are : What might they be ? (I can't post an image as I don't have a microscope that I can photograph through). Why do they like my monitor? (I guess they could be in the various fabrics around the room, but I can't see any). How can I control them without poison? We have no pets, so they aren't fleas. As far as I can tell, they look like this http://jenny.tfrec.wsu.edu/opm/opmimages/BRWf2.jpg but as I said they are very small and it's hard to make out details. I'm in the middle of England if that helps. Thanks in advance. --TrogWoolley (talk) 15:38, 17 November 2011 (UTC)

They may be thrips. Googling /thrips monitor/ yields several interesting results. SemanticMantis (talk) 16:22, 17 November 2011 (UTC)

I don't think bug spray is needed, or wise, given the exposure you would get from inhaling all those fumes as you sit in front of the monitor. Just use a window cleaner with ammonia, like Windex, and wipe them off with a paper towel, then flush the paper towel, in case they aren't all dead. Leave the room for a few minutes until the ammonia dissipates. I often get gnats attracted to my screen, although in my case they seem to prefer my CRT monitor over my LCD. Perhaps it's the static electricity of the screen they like ? It's even possible that any gnat flying by gets sucked onto the screen. I actually like this effect, as it allows me to kill the li'l bastards before they fly up my nose. StuRat (talk) 16:43, 17 November 2011 (UTC)

I assume that your bugs can't fly ? If so, perhaps they are spider mites ? Those feed on plants, so I'd move any plants away from your monitor. StuRat (talk) 16:53, 17 November 2011 (UTC)

Yes, I was going to say red spider mite too. I think in a thread in the archives somewhere we established that there are actually many kinds of animal that look like tiny red dots, some of which are predators and prey on the others. (I see mentioned red velvet mites and clover mites.) None of them prey on computer monitors, though, so I can't say what they're doing there. Perhaps they are confused, like the confused flour beetle.  Card Zero  (talk) 17:02, 17 November 2011 (UTC)

Resolved

Thanks for the suggestions. I'm still none the wiser as to what they are, but window cleaner has gotten rid of them and as an added bonus I can read the text on my screen easier too! --TrogWoolley (talk) 17:53, 18 November 2011 (UTC)

Simple thing.

What is the most "simple thing" that science has no explanation for? i am reading an article on cracked.com that science cant explain why ice is slippery, is there any other things like that? Answer on your own definition of a "simple thing". — Preceding unsigned comment added by Arah18 (talk • contribs) 22:05, 17 November 2011 (UTC)

That site must be outdated, it's already been explained. Here is an question unsolved by science, what caused everything to exist? Plasmic Physics (talk) 22:54, 17 November 2011 (UTC)

Ice is slippery, because it is autolubricating, due to a pertual molecular monolayer of liquid water existing at the ice-air/vacuum interface, even at temperatures approaching 0 K. Once an ice-solid interface is created, the monolayer starts to solidify into ice creating a weld. The colder the ice is, the faster this weld forms. Plasmic Physics (talk) 00:12, 18 November 2011 (UTC)

My answer would be turbulence. Looie496 (talk) 23:15, 17 November 2011 (UTC)

Turbulence:

Big whirls have little whirls that feed on their velocity,

and little whirls have lesser whirls and so on to viscosity.

Lewis Fry Richardson

AndyTheGrump (talk) 23:26, 17 November 2011 (UTC)

The OP's question would be better written as "What is the most "simple thing" that science has no explanation for YET?" HiLo48 (talk) 23:39, 17 November 2011 (UTC)

You really believe in science dont you Arah18 (talk) 00:57, 18 November 2011 (UTC)

Science doesn't require belief. HiLo48 (talk) 06:57, 18 November 2011 (UTC)

Science requires belief in the truth of basic precepts. Plasmic Physics (talk) 08:14, 18 November 2011 (UTC)

Here is the article in question: http://www.cracked.com/article_19442_8-simple-questions-you-wont-believe-science-cant-answer.html and this is their reference: http://www.nytimes.com/2006/02/21/science/21ice.htm I read Cracked.com, too. Mingmingla (talk) 01:12, 18 November 2011 (UTC)

yeah thats it, but i was actually looking for something that's not included in that article. MaHadik usap 01:21, 18 November 2011 (UTC)

We have an article List of unsolved problems, which goes into a number of scientific fields. APL (talk) 09:56, 18 November 2011 (UTC)

I'm not exactly sure what counts as a "simple thing" but Existence of the Universe and Ultimate fate of the universe are kind of fundamental. The existence of two sexes (Evolution of sexual reproduction) is also quite an important fact to not yet have an explanation. --Colapeninsula (talk) 11:32, 18 November 2011 (UTC)