Wikipedia:Reference desk/Archives/Science/2009 April 22

From Wikipedia, the free encyclopedia
Jump to navigation Jump to search
Science desk
< April 21 << Mar | April | May >> April 23 >
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.

April 22[edit]


Ok; the idea of global warming is consistant with it being a natural process all we do as humans is speed up that process... Has there been thought on the idea of creating a large volcanic eruption which would place large amounts of what ever into the atmosphere and thus help cool the earth...Simlar effect in Victorian times when global temp was reduced due to an eruption. The Year Without a Summer 1800 and something? To control this even would be extremly difficult of course but would it work? —Preceding unsigned comment added by (talk) 08:41, 22 April 2009 (UTC)

Or maybe there could be a way to divert an asteroid (nuclear blasts?) so it falls into Earth, the impact of which will result in blocking out of the sun like it's frickin dinosaur extinction time. That might also achieve your goal.—Preceding unsigned comment added by (talk) 10:00, 22 April 2009 (UTC)
I am afraid that your second method is just silly. Life on Earth would not exist without sunlight and furthermore, the impact of an asteroid on the Earth's surface would result in many deaths. --PST 10:10, 22 April 2009 (UTC)
First, what do you mean by "natural process"? "Natural" like in "without human intervention"? We have caused this by releasing greenhouse gases into the atmosphere that disturbed the balance, and now the system is "readjusting", so the mean temperature will rise until a new equilibrium in energy received/energy lost is reached. It is true that in the past "natural" warmings occured (over thousands of years), but as far as we know nothing even roughly comparable to the situation today ever happened (warming the planet in centuries or even decades). We are to blame! Now coming to the question: What makes you think that we are able to trigger a volcanic eruption? I don't say it is impossible, but at least nothing like this has been tried before (for good reasons, I'd think), so it would at least be a really uncontrollable, dangerous experiment. But now to the main caveat: You are confusing climate change with the actual weather. Climate is something that happens on the scale of several years, but the cooling you mentioned mainly affected a single year and then quickly (in climate scales) disappeared. To the contrary, a much discussed theory is that vulcanic eruptions in the past could have triggered massive climate changes which added to or even caused mass extinctions, because many lifeforms could not adapt fast enough to the changing world. This is because vulcanos also emit greenhouse gases, and large vulcanos can put an awful lot of them into the atmosphere in a short time. So in summary: I think it would be a real bad idea to try something we don't understand (vulcanic eruptions) to cure something we don't understand (the climate). TheMaster17 (talk) 10:08, 22 April 2009 (UTC)
To the second question with the meteor: I hope this is irony, because the answer has to be nearly the same: we have no idea how to divert an asteroid, because of very many practical problems on the way. And even if we could, we would bring an incalculable element into an already incalculable situation. TheMaster17 (talk) 10:17, 22 April 2009 (UTC)
(Reply to both PST and TheMaster) I could be wrong, but it looks to me like those two anons are unrelated. I'm pretty sure 94 was trying to be sarcastic indicating that the first idea was likely to be very difficult to achieve and to have serious side effect and so was (almost?) as silly as the second idea which I guess he/she hoped the OP could more easily see as an idea which would never work. Nil Einne (talk) 13:52, 22 April 2009 (UTC)
Well, of course the Earth has warmed and cooled before, but the current process is, as far as we know, different in that a) it's primarily driven by greenhouse gases (instead of them acting as a feedback to amplify an otherwise triggered warming trend) and b) it's a lot faster that anything we have seen before. So calling it "a natural process" that we "only speed up" is a very arguable position. What cools the Earth after a large volcanic eruption are aerosols, mostly sulfur-aerosols in the atmosphere. We have indeed injected a large number of those via industrial processes, and they have masked the early warming trend. Warming only became very notable when Europe and the US installed scrubbers to reduce acid rain and other negative effects of sulfur emissions. There also have been some proposals to actively introduce aerosols to reduce the warming. However, there are serious technical challenges and ecological side effects - and, since it's a global problem, it will be very hard to find a politically acceptable solution. Just imagine Mexico emitting large aerosol clouds that rain out as acid rain over Yosemite National Park, or emissions in Germany causing respiratory diseases in Poland and the Czech Republic. A final problem is that CO2 accumulates over a long time, while aerosols have a short atmospheric lifetime. So to counteract the effect of inreasing CO2 concentration, one would need ever-increasing aerosol emissions. There is some discussion at Mitigation of global warming. --Stephan Schulz (talk) 10:23, 22 April 2009 (UTC)
I think your point b is pretty worthless. Saying that the temperature is changing faster then anything we have ever seen before completely ignores the fact that written human history runs for thousands of years, and thermometers that operate independent of air pressure have only been around for 400 years tops. And do we have documents showing a daily temperature record for hundereds or thousands of locations so that we have a basis of comparison? I would say we don't have anything like that until the 19th century. So saying we have never seen anything like this before does not mention that we had our eyes closed the other 23 hours of the day. (talk) 13:06, 22 April 2009 (UTC)
We don't have human recorded measurements but we do have quite good estimates of previous temperatures from ice core samples for example. Take a look at temperature record for more detail. We may have closed our eyes for the other 23 hours, fortunately there were cameras recording the 23 hours while our eyes were closed which we can now review. Nil Einne (talk) 13:52, 22 April 2009 (UTC)
But an ice core sample will only work at location that have old ice. Not places the majority of people spend their time. (talk) 13:57, 22 April 2009 (UTC)
In fact, the icecore registers the temperature at the place where the evaporation happened, not the place where the snow fell. Since there is a lot og mixing between one and the other, The ice core turn out to be an excellent proxy for the past global average temperatures. Dauto (talk) 19:42, 22 April 2009 (UTC)
But temperature changes exist all over the planet, not just at one location. If anything, the temps at the poles vary more than on the rest of the planet, so any global temp changes would be well-preserved in ice cores. Thus, Antarctic ice cores are a good indication of the global temperature. Also, we can go back a few thousand years by looking at tree rings. Rapidly warming weather would favor some trees and hurt others, and the tree rings from those years would reflect that. StuRat (talk) 14:36, 22 April 2009 (UTC)
But things other then temperature can have a big impact on tree rings, like drought or flooding or fire. Is there a way to differentiate? (talk) 15:11, 22 April 2009 (UTC)
Yes, but not perfectly. The general idea is that drought and flooding are local effects that last a small number of years. By looking at tree rings over wide geographic areas and over longer time periods you get plot a general trend. See Dendroclimatology. APL (talk) 15:56, 22 April 2009 (UTC)
The Dendroclimatology article says you can't use it reliably for more then about 1,000 years of history. (talk) 16:11, 22 April 2009 (UTC)
Yes, it's only useful for relatively recent history. I've read articles about attempts to use it to go back farther than 1000 years, but never spectacularly far back. 2 or 3 thousand at the absolute most. Still just a drop in the bucket compared to geologic time periods. APL (talk) 20:21, 23 April 2009 (UTC)
from the temperature record article,
"However, coverage of these proxies (tree rings and ice cores) is sparse: even the best proxy records contain far fewer observations than the worst periods of the observational record. Also, problems exist in connecting the proxies (e.g. tree ring width) to the variable of interest (e.g. temperature)." I am not saying global warming does not exist, I am just saying that the evidence supplied is less conclusive then most people are lead to believe. (talk) 16:15, 22 April 2009 (UTC)
I'm not sure what you are saying. Are you saying you don't believe global warming is happening or are you saying that because the record of the past is less accurate than the present there is a possibility that something like what is happening now might have happened before and therefore it cold be natural and not man made? By the way the Paleocene–Eocene Thermal Maximum only 55 million years ago looks like it had spikes of 3°C increases in less than a thousand years so there is a possibility that it was as bad as is happening now so that would bolster the second point of view if that is what you are saying. Dmcq (talk) 16:42, 22 April 2009 (UTC)
I am saying that there are holes in the theory that are not addressed well and thus it is not the scientific fact it is often purported to be. (talk) 16:51, 22 April 2009 (UTC)
There may be holes in the theory, but your atempts here fall in the naive category. Denial isn't a scientifically acceptable alternative. Dauto (talk) 19:45, 22 April 2009 (UTC)
And denial of the politicization of science is not helpful either. (talk) 15:14, 23 April 2009 (UTC)
Agreed. Lets leave science for the scientists. What have they been telling us about that matter? Dauto (talk) 17:52, 23 April 2009 (UTC)
It doesn't matter what they tell us if the evidence is insufficient. (talk) 18:58, 23 April 2009 (UTC)
Only the scientist themselves are in position to properly evaluate wheather the evidence is sufficient or not. Dauto (talk) 20:14, 23 April 2009 (UTC)
Scientists are not impartial, someone else is controlling their purse strings. Who watches the watchers? (talk) 20:32, 23 April 2009 (UTC)
Yes, indeed. Scientist are humans like anybody else and can be partial. But we are not talking about a handfull of scientists. We are talking about the whole world wide community of climatologists. What you are proposing is a conspiracy involving the vast majority of the climatologists of the world. Unless you have some strong evidence to suport that idea, I think I am going to file it as an irrational conspiracy theory. Besides, what is the motivation behind that conspiracy? There isn't more money to be made just by supporting global warming theory. Dauto (talk) 00:16, 24 April 2009 (UTC)
Just because coverage is more sparse doesn't mean that it is inadequate. The question is, would we detect the current rise from the ice core? If we would, then the fact that the record is more sparse is largely irrelevant. Our excellent modern observation is good because we can see more carefully what is going on, it doesn't mean it's necessary to detect massive global warming Nil Einne (talk) 03:50, 23 April 2009 (UTC)
These kinds of ideas are what is increasingly being called "Plan B" amongst climatologists. If it's truly too late to stop emitting CO2 - or if we can't get international agreement - or if the 'runaway' scenario has truly kicked in - is there anything we can do to make the planet cooler and thus circumvent the worst of the danger? Well, volcano's definitely aren't it because they are actually a major contributor of greenhouse gasses. While the smoke from the ash plume would reduce temperatures for days, weeks, months or even perhaps a year or two - the CO2 it emits will still be in the upper atmosphere in 10,000 years time. So that particular trick certainly won't work. But there are other possibilities. It's known that the contrails from the exhausts of high altitude airliners is reflecting heat away from the earth to a small degree (it was noted that after the 9/11 attacks when all North American aircraft were grounded - the temperature shot up a couple of degrees!). So maybe we could make some more big white clouds up there and block sunlight that way? The really big problem with all "Plan B" approaches is that we really don't understand the complicated inner-workings of the whole planet to know whether one of these "cunning plans" would actually do more harm than good. For example - if we reflect away sunlight to make the earth cooler, plants that use sunlight to perform photosynthesis would do less well - this might result in disasterous world-wide crop failures - and since plants consume CO2 and output oxygen during photosynthesis - we might easily make MORE CO2 than we do now! In fact, it's hard (or perhaps, impossible) to imagine any kind of "Plan B" that might not have disasterous consequences. Worse still, these things have to be world-wide events - we're modifying the atmosphere for the entire planet - and you really can't go around doing that kind of thing without (at least) the agreement of all of the major governments of the world. While we may ultimately have no choice but to attempt something that wreckless - it would have to be a last-ditch effort when we know for 100% sure that we're doomed if we don't do it. But there are LOTS of "Plan B" ideas out there...covering large areas of the ocean with white foam polystyrene beads to make it reflect sunlight away...launching massive sun-screens into earth orbit to block sunlight...there are MANY choices! SteveBaker (talk) 19:19, 22 April 2009 (UTC)
As for aerosols such as sulfur dioxide, a better alternative is to launch them up to the stratosphere, where it doesn't interact with our weather. As for warming/cooling being part of a natural cycle, yes there has been a period of warming since the Younger Dryas, but that warming is no longer a part of the trend seen today. As for rapid climate change in the past, some of those events may have been caused by methane clathrate collapses (which are starting to occur today), but now instead of 3C of warming in several centuries (under 1,000 years), it could be 3C in several decades (less than 100 years). ~AH1(TCU) 23:05, 22 April 2009 (UTC)
Our Mitigation of global warming includes a section on geoengeering and we also have an Adaptation to global warming. Fertilising the ocean is one proposal that hasn't been mentioned yet. I think Precautionary principle is highly relevant here. Classic examples of things which seemed an okay or good idea at the time may be rabbits in Australia and to a lesser extent in New Zealand, and the even 'smarter' idea of bringing in stoats to control the rabbits [1] in NZ Nil Einne (talk) 04:00, 23 April 2009 (UTC)
An ocean fertilization project has just failed - the algae bloom produced was just eaten by animals, which released most of the Carbon back into the atmosphere via their metabolic processes. [2] --Stephan Schulz (talk) 14:52, 23 April 2009 (UTC)
And see Law of unintended consequences. Intentional geoengineering in an attempt to offset our unintentional geoengineering that results from burning gigatons of fossil fuels is pretty scary. One should carefully review the history of previous engineering disasters, then scale up the risk to a global level. Engineering disasters occur when smart people get together and only anticipate some of the relevant consequences of their actions. See also Superiority (short story) and Wonder weapons. Technology can do some wonderful things, but it doesn't always deliver exactly the desired wonderful thing on the desired schedule. Technology develops best when people approach it incrementally, learning from their mistakes as they go. Humans cannot even write error-free software on the first attempt (or even after years of debugging by teams of skilled professionals). We don't have access to a "test planet" where we can try various geoengineering schemes to see how they will work, and to verify our models. --Teratornis (talk) 23:31, 24 April 2009 (UTC)

Ok stupid idea or not..i wasnt actully suggesting doing it i asked a THEORETICAL question of WOULD IT WORK? IE planet gets colder glaciers reform gulf stream stays blah blah blah....sure why not blow up the world divert astaeroids...Guess i need to solve the problem and argue about who is to blame NOT what i wanted suppose we could stick with recycle your fairy liquid bottles rock on the scientists! —Preceding unsigned comment added by (talk) 07:26, 26 April 2009 (UTC)

People have already discussed why it may not work and the potential catastrophic side effects which could easily be worse then what you're trying to solve. There's not much more to say. As has already been mentioned there are a lot more ideas which are at least aren't quite as extreme which are being studied although even these already have many many difficulties and potential failings. No one bothers to study ideas which are so insane and inpractical that they aren't worth considering so you're not going to get much better then what has already been said. Also, I don't know of anyone who is arguing over who is to blame, except the denialists. Most people are arguing over how best to resolve it concentrating on things that are actually potentially possible. Nil Einne (talk) 01:53, 29 April 2009 (UTC)

B cell activation[edit]

Hi everyone just I have a couple of quick questions - Would I be correct in saying that B cells don't have to be activated by a helper T cell before they can produce antibodies? If they don't need helper T cells to activate them how are they activated in the absence of helper T cells - does another type of cell activate them into producing antibodies/forming plasma cells etc. or can they can activate themselves? If they can activate themselves (I don't know if they can or not)then wouldn't that render the helper T cell uneccessary in this sense? (I realise helper T cells probably have other functions aside from activating B cells.) Sorry if my questions are badly worded and thanks in advance to all who help! —Preceding unsigned comment added by (talk) 11:21, 22 April 2009 (UTC)

You are correct, B cells can be activated without T cell help. But you could have found this out yourself, by trying the B cell article. TheMaster17 (talk) 11:32, 22 April 2009 (UTC)

Speed of Light and electricity[edit]

It seems that the speed of light and the speed of electricity are about the same. Are these two related? --LordGorval (talk) 12:10, 22 April 2009 (UTC)

May I point you to the appropriate article: Speed of electricity. The search box is your friend. TheMaster17 (talk) 12:24, 22 April 2009 (UTC)
Did you mean Speed of electricity or electromagnetic wave? The latter travels at the speed of light in the vacuum. The former is much slower. Mr.K. (talk) 12:34, 22 April 2009 (UTC)
Our article on List of common misconceptions says:
"Some textbooks state that electricity within wires flows at nearly (or even exactly) the speed of light,[1] which can give the impression that electrons themselves move almost instantly through a circuit. The electrons in a typical wire actually move at a drift velocity on the order of centimeters per hour[2] (much slower than a snail). The random thermal motions of the electrons are much faster than this, but still much slower than light, and with no tendency to occur in any particular direction. It is the electrical signal that travels almost at the speed of light. The information that a light switch has been turned on propagates to the bulb very quickly, but the charge carriers move slowly.[citation needed]"
But the article is sorely in need of cites so I don't know if the above is correct. A Quest For Knowledge (talk) 13:07, 22 April 2009 (UTC)
Wow, misconceptions are everywhere, including the Speed of electricity article and most of the things on the web that discuss this. When somebody asks for the "speed of electricity", what they usually mean is something like "If I switch on a light, how quickly does the signal travel from switch to light", or "if I send a signal along a wire, how quickly does it reach the receiver". To answer these questions requires cable theory. The answer you get depends greatly on the conductivity, boundary impedance, and capacitance of the wire, but it is always far slower than the speed of light. This article, for example, calculates the speed of signal transmission for a telegraph cable across the Atlantic -- a very fast carrier -- and gets a result of about 2x106 m/sec, or about 1/10000 of the speed of light. In other words, very fast in human terms (two seconds to cross the Atlantic), but very slow compared to light. The wires in your home are typically hundreds or thousands of times slower. For computer equipment, conduction speeds along wires are often an important design factor. Looie496 (talk) 18:21, 22 April 2009 (UTC)
The calculation for transatlantic cables may have been accurate in the days of the telegraph, but is not even close when most telephone cables are considered. Up until the mid-70s, transatlantic telephony used either satellite or copper cable. Satellite experienced noticeable delay, and cable did not. We can therefore estimate that the round-trip time was less than 100ms. Therefore for the signal to cross the atlantic, we can estimate a time of 50ms maximum, which is a speed of about 1x108 m/s, or about 1/3 the speed of light. --Phil Holmes (talk) 09:12, 23 April 2009 (UTC)
In a physics lab, we would send pulses down 200' coax cables and measure velocities of 2/3 c. Of course the pulse got smeared some and there are other issues to consider, but Looie's assertion that most wiring would be many orders of magnitude less than c seems unlikely to me too. Dragons flight (talk) 09:36, 23 April 2009 (UTC)
The way to imagine this (crudely) is to imagine an almost frictionless 200' long garden hose and some half-inch ball-bearings. If you stretch out the hose on level ground and roll a ball-bearing down the length of it - it might take 30 seconds to pop out of the other end. But if you fill the hosepipe full of ball bearings from end to end then if you try to push one ball in at one end - then another one pops out of the other end almost instantaneously. So even though the individual balls move fairly slowly - the time between stuffing one in one end and a DIFFERENT ball popping out of the other is quite short. The hose is a wire, the balls are electrons. The electrons move incredibly slowly - like watching the minute hand move on an analog clock. But when you push an electron into one end of the wire - a different one pops out the other end with a delay that's about equal to the speed of light. SteveBaker (talk) 19:06, 22 April 2009 (UTC)
Neat! Thanks for answers. The one I understand the best is from SteveBaker. That one I grasp the best and it makes sense on how the "speed of electricity" happens. --LordGorval (talk) 21:15, 22 April 2009 (UTC)

wikipedia article on E.D.T.A.[ethylene diamine tetra acetic acid][edit]

in the above article,Uses in Medicine,Last entry is Use in Thalassemia Major to remove excess iron.Pl.indicate the reference of source i.e journal etc.thanks —Preceding unsigned comment added by (talk) 12:59, 22 April 2009 (UTC)

Try searching [3] if you are interested in a publication. Wisdom89 (T / C) 13:15, 22 April 2009 (UTC)
Well to be fair to the OP, if our article says something (and it does "This therapy is used to treat the complication of repeated blood transfusions, which used in cases of severe thalassaemia") then it should be referenced (which it isn't). However that should be dealt with on the article talk page (which I've copied it to), not here Nil Einne (talk) 13:42, 22 April 2009 (UTC)
Yes, I know all challenged claims require a citation and are subject to removal if none is provided - but I got the impression that the anon was also interested in the veracity of the statement apart from just the quality of Wikipedia. Wisdom89 (T / C) 14:53, 22 April 2009 (UTC)

How to insulate an old home ?[edit]

My current house was built in the 1920s (I know, that's not old by European standards). The windows have all been replaced with triple-pane insulated windows. Now the problem is the walls. They are cold all winter long and hot all summer long. I don't believe there's any insulation at all (other than the bricks and wood of which the house is constructed, which offer minimal insulation value). The obvious answer is blown-in insulation. However, the wiring in the walls is sub-par and gets warm now. If it was buried under layers of insulation it might get hot enough the cause a fire. So, is there any way to insulate the walls, short of ripping all the walls apart to replace the wiring ? One thought we had was to disconnect the existing wiring, leave it in place, and install exterior mounted wiring to replace the wall wiring. This would then allow for blown-in wall insulation. Is this idea feasible ? Has anyone done this ? Any other ideas ? StuRat (talk) 14:11, 22 April 2009 (UTC)

Cladding? May not have a huge impact. Alternatively you could have the place re-wired (not the worlds most expensive job) and make it clear that post-rewiring you want to be able to install insulation. Beyond this there is this 'thermal wallpaper' ( and you can get 'insulation wallpaper' ( or 'insultating plaster' ( (talk) 14:17, 22 April 2009 (UTC)

Those sound like some possibilities. Do you know how much of an insulation value (R-factor) the thermal wallpaper provides ? StuRat (talk) 20:11, 22 April 2009 (UTC)
If your wiring is getting hot, you should definitely replace it. (5-10% of all deadly house fires in the U.S. involve electrical causes[4]) Wiring from that era probably has degraded insulation (mine certainly did) but more importantly as code standards and appliance usage have changed, the system may not be laid out to properly handle the much higher electrical loads we now use leading to overheating of the wires. Rmhermen (talk) 16:02, 22 April 2009 (UTC)
It should definitely be rewired to start with. The old wiring can usually be used to pull through the new wiring; it certainly won't mean ripping any walls apart, and your house will be a lot safer.--Shantavira|feed me 15:59, 22 April 2009 (UTC)
My thoughts too exactly. The insulation on ancient wiring can quite easily crumble and cause a fire, never mind the possibility of shocks. Do it now before it does you in. Dmcq (talk) 16:07, 22 April 2009 (UTC)
When you do get to the insulation I think your idea of blow in is best to start with. The problem with having thin insulation trying to do the whole job is with any bits which have to stick through may tend to get condensation at times, but they can be good supplementing the main business. Dmcq (talk) 16:18, 22 April 2009 (UTC)
To do a good job of blowing in insulation, you will need to make holes in the plaster or wallboard anyway, and patch and paint as part of the job. Therefore, the incremental cost of putting the new wiring inside the walls rather than outside the walls is much lower: the incremental cost is mostly in the wall repair. Depending on where you live, you may want to make the walls thicker to hold more insulation. This involves removing the plaster or wallboard and adding studs another set of studs, which should be offset from the existing studs. Of course, with the walls open you are no longer restricted to blown insulation. -Arch dude (talk) 20:13, 22 April 2009 (UTC)

Everyone seems to think new wires can be pulled through the walls using the old wiring. I'm far more skeptical that thin, 90 year old wires with deteriorating insulation wouldn't just break. Also, the builders may well have attached the wiring to studs using staples. And the walls are made of wet plaster, so the thought of tearing them open is horrifying. What about my idea of leaving the old wiring in place, disconnecting it, and putting new surface mounted wiring in place ? Has anyone done this ? StuRat (talk) 20:08, 22 April 2009 (UTC)

In addition to the fragility, a 90 year old home with the original wires would likely have knob and tube wiring instead of free-floating Romex-style power cable. (Applicable words from the article "Currently the United States NEC forbids use of loose, blown-in, or expanding foam insulation over K&T wiring. This is because K&T is designed to let heat dissipate to the surrounding air.") -- (talk) 22:29, 22 April 2009 (UTC)
There are ways of "snaking" wire through walls without pulling on the existing wire (which being metal should not have gotten any weaker - if it has it is an even greater fire hazard) Another problem with knob and tube (which was less common already by the 20s) is that some insurers refuse to write policies for houses that still use it. Rmhermen (talk) 00:49, 23 April 2009 (UTC)

My house is also of that period, but the previous owner did a major renovation. Where the original double brick walls are still in place, they are now lined with another layer of wall of standard frame construction, which allowed space for insulation and new wiring. All wiring and plumbing in the house was replaced. That's the sort of job you'd need to do things properly, but as you can imagine, it can be awfully expensive. I think you need to talk to a renovation contractor (or rather, several of them) to talk about what is possible and what it might cost. --Anonymous, 23:23:23 UTC, April 22, 2009.

With renovations on that order, it might be cheaper to tear the house down and rebuild. StuRat (talk) 03:00, 23 April 2009 (UTC)
That would let you build a monolithic dome and become tornado-proof. --Teratornis (talk) 23:18, 24 April 2009 (UTC)

Minimum particle separation - where to start?[edit]

Hi there, I've been revising Dynamics for a few days now and come across this question, and I'm not too sure where to go for the final part.

Two particles of masses m1 and m2 move under their mutual gravitational attraction. Show from first principles that the quantity is constant, where r is the position vector of one particle relative to the other and M = m1 +m2 .

The particles are released from rest a long way apart, and fall towards each other. Show that the position of their centre of gravity is fixed, and that when they are a distance r apart their relative speed is .

When the particles are a distance a apart, They are given equal and opposite impulses, each of magnitude I , and each perpendicular to the direction of motion. Show that subsequently , where ω is the angular speed of either particle relative to the centre of mass and µ is the reduced mass of the system.

Show further that the minimum separation, d, of the two particles in the subsequent motion satisfies .

That's all okay until I get to the minimum separation bit, at which point I get a bit confused - could anyone let me know how to go about finishing this part of the question please? Thanks! :) Otherlobby17 (talk) 14:44, 22 April 2009 (UTC)

It is possible to rearrange the angular momentum as an Effective Radial Potential. This is a "pseudo"-potential energy field. When added to the gravitational potential, there is a minimum of potential energy (this is the optimal radius for stable circular orbit); and another point at even lower radius, where effective potential is equal to the gravitational potential at infinite radius (this is the minimum separation for the scenario you described). This derivation is detailed in Marion & Thornton's dynamics textbook. I will try to dig out my notes on the subject. Nimur (talk) 15:56, 22 April 2009 (UTC)
No need - here's Effective potential (which should probably be renamed to something less general, like "Effective potential due to angular momentum"). To find the minimum separation, just locate the point where initial energy equals the potential energy. The particle can go no further (so it is at the minimum radius) unless new energy is added from another source. Nimur (talk) 16:00, 22 April 2009 (UTC)
While the effective potential idea works, it is not the simplest way to solve that problem. Before the Impulses, the system had both energy and angular momentum equal to zero. The Impuses transfer energy and angular momentum given by the formulae:
and both are conserved thereafter.
At the point of minimun separation, the velocity has no radial component
, and
which gives us energy and anguloar momentum
Solving the system of equations we get from this it is easy to show that
Dauto (talk) 19:10, 22 April 2009 (UTC)

Toilet germs[edit]

Adverts for toilet cleaning products always boast about how they kill "99.99% of all germs" in the toilet (in the UK, at least). Now, correct me if I'm wrong, but people don't go around licking toilet bowls regularly, do they? Exactly what would be the problem with germs inside a toilet? Is this just more advertising BS designed to sell products to ignorant consumers, or do germs inside a toilet pose a genuine threat of which I'm ignorant? Vimescarrot (talk) 14:50, 22 April 2009 (UTC)

I am reminded of a study on household germ contamination I read. They found that the most contaminated places in most houses were the cloth or scrubber used to clean kitchen surfaces, and the chopping board; the latter having some particularly nasty germs. The head of the study was quoted as saying "In the average house if I was forced to lick either the toilet seat or the chopping board, I'd choose the toilet seat every time". DJ Clayworth (talk) 15:43, 22 April 2009 (UTC)
And I would answer that he obviously does not know what he is even doing with this study. Just because you find bacteria on a surface, does not mean that anybody will become sick from licking it. I, as a biologist with training in immunology, would rather stick with Vimescarrot's explanation: Nobody eats out of the toilet, but everybody prepares food on kitchen surfaces. To what germs will our body be more resistent? And the problem, in reality, is the other way round: By cleaning all those bacteria which normally live there, you pave the way for bacteria that normally cannot grow there (because the "normal bacteria" outcompete them) and which could be "real" pathogens (meaning they can really make you sick, because your body has no immunity for them). There are few surfaces, in a hospital for example, that really need to be sterile. Most other things are perfectly "healthy" in everyday live even when they are covered with germs, unless your immunesystem is severely compromised. TheMaster17 (talk) 16:02, 22 April 2009 (UTC)
So microwaving my dishcloths to kill the germs on them is a bad idea? What about the fact that my mother wipes bloodstained hands on a cloth meant for drying hands that have just been washed? =p Vimescarrot (talk) 17:44, 22 April 2009 (UTC)
The chopping board may be a special case. The expert may have been concerned about getting germs from raw poultry. I wonder how he'd feel about a dry counter top? (compared to a dry toilet seat) APL (talk) 17:46, 22 April 2009 (UTC)
The stuff inside the toilet bowl can contribute to foul smelling odors. Also, doesn't flushing release a small aerosol of bacteria in to the air above the toilet? I remember reading that somewhere, I'm sure someone will prove me wrong. Livewireo (talk) 18:05, 22 April 2009 (UTC)
I've heard the same thing. --Tango (talk) 18:12, 22 April 2009 (UTC)
Don't let that stop you from flushing. :-) That will get rid of the first 99%. Graeme Bartlett (talk) 21:11, 22 April 2009 (UTC)
Although, it might be a good idea to close the lid first. --Tango (talk) 21:29, 22 April 2009 (UTC)

The dishcloth one always gets me. Is that measured before you place it in hot water (to get it wet ready to wipe a side), just after and before using to clean or after cleaning? I guess my point is - the circumstances will matter as much to the number of bacteria as the place you are measuring. I would expect that the reason for most people to want rid of germs from toilet seats is that they A) sit on them with a part of their body they otherwise only wash daily (or potentially less) compared to touching something with your hands (which would be washed numerous times a day by most). Also toilets are a place where people go to get rid of their 'waste' so they associate them with being dirty/unhygienic as a result it'd make perfect sense for a marketing/branding dept to make a point about their product killing germs there. After all that it still matters whether the bacteria is harmful or not. Numbers alone are a bit of a con in comparison to the type of bacteria? ny156uk (talk) 19:11, 22 April 2009 (UTC)

It's true that in the UK these adverts make us feel like we are surrounded by monsters, and that we should get rid of them before it's too late. However, if they wanted to be honest, they'd say "kill 99.99% of germs (99.9999999% of those being harmless)". Laurent (talk) 19:28, 22 April 2009 (UTC)
There's no such thing as a harmless germ. A germ is defined as a microorganism that can cause disease. A harmless bacteria is not a germ. APL (talk) 20:12, 23 April 2009 (UTC)
Toilet bowls normally contain far more "germs" than toilet seats. And they can be harmful to you if someone else in your household has a communicable disease which you don't want to catch. They also look and smell bad when you get ugly growths on the sides of the bowl. It's true that you aren't likely to be exposed to those germs though, with exceptions for pets or small children which happily drink from and play in the toilet bowls. However, nothing fancy is needed to kill those germs, bleach will do the job, although you need to scrub the bowl to get the bleach on all the microbes. Bleach in a thick gel would be even better, as that can stick to the sides of the bowl long enough to kill whatever needs killing. Many toilet bowl cleaners are just that, bleach in a thick gel in a bottle with an angled neck for ease of delivery. Note, however, that bleach and other cleansers may be worse for the children and pets than the original germs were, so keep them out of the bathroom during cleaning. StuRat (talk) 19:45, 22 April 2009 (UTC)

Teatowels are full of germs, because they get damp and warm. Little known fact. (talk) 20:53, 22 April 2009 (UTC)

Products that kill "99.99%" of germs are likely to leave behind ones that are now more resistant to the antibiotic spray, creating what are sometimes called "superbugs". Also, humans don't usually drink from a toilet bowl, but a pet dog might. Another thing, MythBusters confirmed that flushing the toilet brings a small number of the fecal germs up into the air, thus landing on other items such as your toothbrush (in very detectable amounts, even if placed in enclosed containers). ~AH1(TCU) 22:48, 22 April 2009 (UTC)
Such products are not antibiotics and thus there is minimal risk of inducing the formation of resistant strains of anything - Disinfectants such as benzalkonium chloride generally do not produce tolerance. Wisdom89 (T / C) 22:56, 22 April 2009 (UTC)
I've noted much more guarded language from advertisers of these products of late - one I noticed a couple of evenings ago said "Kills up to 99% of known germs that may do you harm"...which is pretty much saying "It might kill some bacteria"! The word "germ" is carefully vague - is this bacteria? fungi? viruses? The advertiser's favorite get-out-clause "up to" is a good one. If there are some really virulent bugs, then 1% of them will plenty enough to harm you. Evidently it only kills "known" germs (known by whom?) - and it might only kill the ones that would do me harm...what about my pets or whatever? SteveBaker (talk) 00:08, 23 April 2009 (UTC)
Are you really serious about microwaving a dishcloth to kill bacteria? What makes you think that this will work? As far as I know, microwaves only really heat water, and most spores of bacteria are water poor, plus they have a small volume, are very damage resistant and repair capable. I would assume microwaving for a long time will reduce the number of viable bacteria, but kitchen microwaving is not capable of sterilizing something. My main argument stays the same: For 99.99999999... % of time that life on earth existed, there were no artificial antibiotics and no really sterile surfaces. Even considering that we have changed the rules lately (moving around the globe faster and changing the patterns of germs that can infect us, by changing the environment we live in), I'm totally fine with my immunesystem coping with the "usual" suspects like bacteria dwelling in my home, the soil, the food, the water or whatever I encounter on a regular basis. I think cleaning your home from 99.99% of bacteria on a regular basis does more harm than good, because you disturb the established pattern of microorganisms and so create a situation where really nasty things can creep into your home, that in addition are new to your immunesystem. I'm not saying that you shouldn't clean your home, but trying to keep it near-sterile every week is really overdoing it, without a clear justification why those things you try to eradicate should not peacefully coexist with you. TheMaster17 (talk) 08:12, 23 April 2009 (UTC)
I would assume that the idea is to heat the water in a damp dishcloth to boiling point (or so) and to keep it there long enough to sterilise it. But I'd be pretty surprised if that actually worked. If you merely warm it up to a nice comfy body-heat, you may actually be encouraging the bacteria to multiply. I strongly agree that an overly sensitive person can do more harm than good. Continually killing only 99% of bacteria simply encourages the remaining 1% to evolve to the point where you can't kill them that way. Better to reserve that killing capability until you actually need it for something. Also, it's been well documented that children NEED exposure to a wide variety of bacteria in order to develop immune responses to them. Children who are born into families of "neat freaks" get sick far more often than those of people with a more relaxed attitude to cleanliness. So, keep things clean - yes. Pay a heck of a lot of attention to places where you prepare chicken and such - but otherwise, chill out. We are able to withstand most of what the bacteria can throw at us without even noticing that we're doing it - and when we do get sick, modern medicine is there to help. Of course different rules come into play if you're actually immuno-compromised in some way (eg with AIDS or something). SteveBaker (talk) 11:53, 23 April 2009 (UTC)

A news report (this isn't it but it makes reference to it) recently recommended 2 minutes of microwaving as a way to sterilise dishcloths. MyhtBuster actually confirmed that there is no discernible difference between a toothbrush stored near a toilet in the bathroom and one stored in a seperate room, so I'm not sure which episode you saw...Anyway, thanks for this info, guys. :) Vimescarrot (talk) 18:05, 23 April 2009 (UTC)

I was surprised by that too, because I remembered reading often that microwaving was reported as being an insufficient treatment for reducing pathogens. (e.g. in reheating leftover hamburger.) This is the study you were referring to, I think [5] If you want something really germy forget toilets and cutting boards, women's handbags tested as being really icky on the outside. (talk) 18:29, 24 April 2009 (UTC)
And then there are the handles on the doors of "restrooms" that are used by people who've had their hands up their arses or all over their family jewels (or whatever the equivalent term for the female variety is) and don't wash. Telephone mouthpieces too. -- JackofOz (talk) 20:59, 25 April 2009 (UTC)


Is white vinegar the same as white wine vinegar? Simple question but I can't find a definite answer.--Shantavira|feed me 16:00, 22 April 2009 (UTC)

No. White vinegar is distilled, and is basically pure, dilute acetic acid. White wine vinegar is vinegar made from white wine, and it retains some of the complex flavors associated with wine. IMO, white vinegar is not very useful for cooking due to its simple flavor (but can be useful for cleaning coffeemakers, etc.), while white wine vinegar is quite useful for sauces, vinaigrettes, etc. -- Coneslayer (talk) 17:05, 22 April 2009 (UTC)
I would not recommend substituting one for the other. Livewireo (talk) 20:44, 22 April 2009 (UTC)
White (distilled) vinegar can be used for cooking, but would be used in pickling or acidifying something (like the water used to cook poached eggs) when you don't want to add any additional flavors. I wouldn't use distilled vinegar in something like a vinaigrette or sauerbraten, where the flavor of the vinegar is a major component of the dish. -- (talk) 22:23, 22 April 2009 (UTC)
White vinegar is the only vinegar perfectly suited to chips Polysylabic Pseudonym (talk) 13:25, 23 April 2009 (UTC)
Really? Better than malt? I would have to disagree... --Tango (talk) 17:30, 23 April 2009 (UTC)
I've just added a section on distilled vinegars to the Vinegar article. --Heron (talk) 21:34, 22 April 2009 (UTC)
Thanks. That's what was needed, and I've added a redirect to that from white vinegar.--Shantavira|feed me 08:49, 23 April 2009 (UTC)

Birds collecting things[edit]

I think I am probably just not trying the right search terms, but where can I learn more about the tendency of some birds to collect (e.g. steal) shiny things and other man made objects for use in their nests? I'd like to know more about which birds do this and what types of things they go after. Dragons flight (talk) 23:38, 22 April 2009 (UTC)

Bowerbird and European Magpie come to mind - but the Magpie article tells you nothing of use in this regard. SteveBaker (talk) 00:03, 23 April 2009 (UTC)
Magpies hoard food; that behavior is well documented. I know magpies are also said to steal and hoard shiny objects, but I'm not sure there is any rigorous scientific study of that. If they do that at all, I guess it must be an extension of the food hoarding behavior. Now the bowerbird that Steve mentioned is a different story altogether. In bowerbirds, collecting colorful objects is a part of a (pretty damn impressive) courtship ritual. --Dr Dima (talk) 05:14, 23 April 2009 (UTC)
I'm not aware of a reputation magpies have for stealing. However, jackdaws (another member of the crow family) are well known and documented for this behaviour. --Phil Holmes (talk) 08:54, 23 April 2009 (UTC)
La gazza ladra. Deor (talk) 13:12, 23 April 2009 (UTC)
@Phil I ran out of google patience for a scientific study on magpies stealing, [6]. But if you get a chance to look inside a nest you can usually see some "evidence". If you do have magpies nesting nearby you could also leave a small piece of chrome plated metal out. (OR: The biggest thing I've seen one make off with was a make up mirror. The one's I encountered didn't go for aluminum foil but would take paper chewing gum wrappers that were aluminum coated.) The members of the crow family (which include Magpies and Jackdaws) have been found to be quite bright and they do get bored and play. The Kremlin used to have trouble with some crows claw-scating down their dome and peeling the gold off the roof. (talk) 18:02, 24 April 2009 (UTC)
  1. ^
  2. ^ hyperphysics