Wikipedia:Reference desk/Archives/Science/2008 February 11

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February 11

Melting points of ice in cold

We're experiencing bitterly cold windchills where I live, and it got me thinking: when salt is applied to ice, it removes the ice because it lowers the melting point. So let's say that I have some calcium chloride salt that is effective in temperatures as low as -25°F. Now let's also suppose that the temperature outside is 0°F, but the windchill is -40°F. Will the salt work? --Emery (talk) 01:43, 11 February 2008 (UTC)

As far as I know, it shouldn't make it stop working. As you can tell from reading the article, wind chill is merely an apparent change in temperature, caused by changing the rate of heat transfer. It does not actually lower the temperature of a substance any more than still air would, it just does it faster (it only lowers your body temperature further than still air because cooling air has to fight with your body's heat-generating metabolism). Someguy1221 (talk) 01:56, 11 February 2008 (UTC)

He stole my answer! Just kidding. Someguy is absolutely correct. Wisdom89 (talk) 02:02, 11 February 2008 (UTC)

To say the same thing another way: wind chill does not describe a temperature; it refer to a temperature as an expressive way of describing a rate of heat loss from your body. --Anon, 03:40 UTC, Feb. 11.

"Perceived cold" is a good way of describing it methinks. Wisdom89 (talk) 03:41, 11 February 2008 (UTC)

Thank you very much, I figured as such but I just wanted to make sure. :) Emery (talk) 23:43, 12 February 2008 (UTC)

Orbits

Assuming you have a spaceship with an engine that can provide one gravity of continuous thrust, what's the minimum orbital period you can manage for orbiting Earth? --12.169.167.154 (talk) 01:51, 11 February 2008 (UTC)

Unfortunately, I don't think the question is well-formed. If you launch your spaceship from a point above the Earth's surface (so that the gravitational acceleration is slightly less than g) you will accelerate at an increasing rate, past escape velocity, and break any orbit. As long as you can produce continuous thrust, in fact, you will always be able to break out of any orbit - orbits only require an initial velocity. Anyway, you might be able to find what you're looking for if you note that for stable orbit, Gravitational acceleration must equal centripetal acceleration, or $GM/r^{2}=v^{2}/r$ and the period $T=1/2\pi \omega =r/2\pi v$. SamuelRiv (talk) 03:04, 11 February 2008 (UTC)

The question makes sense if you realize that it's not asking about an orbit in the usual sense of a freefall path, but one where the spacecraft is thrusting continuously downward, but is moving fast enough that it does not collide with the Earth. I have seen the term forced orbit used for this, although it doesn't seem to be known to Wikipedia.

As Samuel noted, in a conventional orbit $GM/r^{2}=v^{2}/r$. Assuming that the orbit is not so high above ground level that diminution of gravity is a significant factor, the left-hand side is simply equal to 1 gee. So if we thrust continuously downward at 1 gee, we double the total acceleration and therefore double the value of $v^{2}$. So the speed is greater by a factor of ${\sqrt {2}}$ and the period is smaller by the same factor. The answer is just about an hour. For a more accurate number, do the math based on a realistic value of $r$.

--Anonymous, safely at ground level, 03:52 UTC, February 11, 2008.

Ah, I see it now. Anon is correct, so we have $v={\sqrt {2gr}}$ or $T={\sqrt {r/2g}}/2\pi$, which will give minimum period for minimum r, so we must orbit right at the surface of the Earth, and we get a period of about a minute (87s), if I did this right. SamuelRiv (talk) 18:22, 13 February 2008 (UTC)

Well, I said "about an hour" (i.e. 1/sqrt(2) times the minimum period for an ordinary satellite), so one of us isn't "correct". I don't have time right now to look for the error. --Anon, 20:05 UTC, Feb. 13.

Okay, since no one else filled in, I'm coming back to this now. In short, I was right. Dimensional algebra confirms that ${\sqrt {r/2g}}/2\pi$ is impossible: it would have units of s/m, not m/s. Samuel's first equation is correct and equivalent to what I said before. But then the period is given by $T=2\pi r/v=2\pi r/{\sqrt {2gr}}=\pi {\sqrt {2r/g}}$, which is indeed about an hour.

More precisely, we are dealing with two different acceleration components: one due to the spaceship's power and one due to the Earth's gravity. The first is fixed at 1 gee, while the latter obviously depends on the ship's altitude. Taking the Earth's radius to be 6380 km and the lowest possible orbit to be 160 km above ground, we have r = 6540 km and the Earth's gravity at that height is (6380/6540)² = 0.95167 gee. So the total centripetal acceleration is a = 1.95167 gee = 19.1393 m/s². Replacing $\pi {\sqrt {2r/g}}$ with the more general expression $2\pi {\sqrt {r/a}}$ and substituting these values, we get T = 3,673 seconds or say 61.2 minutes. Okay?

--Anonymous, 07:45 UTC, February 17, 2008.

Subsidiary question for anyone interested: could one do even better by using a polar orbit with the perigee over one pole and the apogee over the other? Since the Earth's polar radius is shorter, this would allow a perigee less than 6540 km, but what apogee would be required? You may assume that the 160 km minimum height is measured from sea level and applies at all points in the orbit -- but this means that the curve which the orbit must stay above is not exactly elliptical, so things get tricky.

--Anon, 07:55 UTC, Feb. 17.

Another afterthought: the 160 km figure may actually be too high for this specific problem. If the spacecraft can thrust continuously, then it could vector that thrust to an ever so slightly forward angle and and so overcome a small amount of atmospheric drag. Of course at some point you get into serious frictional heating: what you want is an altitude just high enough that that won't happen. I can't find any information on how high that would be. But if, for example, a circular forced orbit is possible at r = 6480 km (100 km above sea level) instead of 6540 km, then by the formula I gave above, the period would be 3,628 seconds or almost a minute less.  ::::--Anon, 08:16 UTC, Feb. 17.

Force+chair

1. I can lift my own weight. (empirically tested at gym with weight machine)

2. I can lift the weight of the folding chair in my room. (again, tested it)

3. I can lift the chair's weight combined with my own (tested at gym again)

4. I can not lift the chair while standing on it.

Why? I understand that if I did manage it, I would be levitating, and I think it has something to do with...by lifting the chair, I'm pushing down ONTO it with an equal amount of pressure from my legs, neutralizing the lift. Is that right?

Thanks76.118.184.59 (talk) 02:23, 11 February 2008 (UTC)

Yes. Because the amount of force with which you lift the chair is equal to the amount of force with which your feet press against the chair, the net force on the chair and you is zero, and you don't move. Wrad (talk) 02:32, 11 February 2008 (UTC)

But see cartoon physics. --Milkbreath (talk) 04:11, 11 February 2008 (UTC)

"Perceived cold" is a good way of describing it methinks. Wisdom89 (talk) 03:41, 11 February 2008 (UTC)

You meant to put that under #Melting points of ice in cold, right? --Anon, 03:55 UTC, Feb. 11.

heh, yep, slip up - thanks for pointing it out. Wisdom89 (talk) 04:57, 11 February 2008 (UTC)

You could always use a pulley to reverse the direction of the applied force.--Shantavira|^{feed me} 08:32, 11 February 2008 (UTC)

Yea, but the pulley needs to be attached to something which is fixed relative to the ground, and that thing is in effect holding up both you and the chair. --Spoon! (talk) 13:15, 11 February 2008 (UTC)

I have always found such levitation quite easy- while I am dreaming. Edison (talk) 03:23, 12 February 2008 (UTC)

What is Adjunctive Therapy and how does it work?

What is Adjunctive therapy and how does it work? Also, how does it apply to animals, especially exotics, and how does it effect them? —Preceding unsigned comment added by SBlondie8967 (talk • contribs) 04:49, 11 February 2008 (UTC)

Adjunctive therapy, according to the National Institute of Health is "Another treatment used together with the primary treatment. Its purpose is to assist the primary treatment." [1] For example: giving glycoprotein IIb/IIIa inhibitors to someone who has had angioplasty. The angioplasty is the primary treatment, but the IIb/IIIa inhibitors help improve the effectiveness of the primary treatment and are thus considered adjunctive therapy [2]. I'm not sure how to respond to your second question about animals, as I imagine that it would be the same or very similar is in humans. (EhJJ)^TALK 19:25, 11 February 2008 (UTC)

Can you think of any major new organs or adaptive behaviours which evolution might throw up in the next few million years?

Some researchers believe that the Cambrian explosion may have been due to the sudden development of vision. Organs for hearing similarly affected life on earth in profound ways. There are now countless methods whereby creatures walk, fly, mate, live and so on. But are there any other possible major developments that we could second-guess and say: “Here is another organ or sense or way of getting around or maintaining an existence that evolution has not yet found”? Can you think of anything, or do you think that basically nature has no more tricks up her sleeve. Myles325a (talk) 05:56, 11 February 2008 (UTC)

I doubt there will be anything that will create a new Cambrian explosion, but the ability to detect harmful radiation might be useful after humans have irradiated the planet. — Ƶ§œš¹ _{[aɪm ˈfɻɛ̃ⁿdˡi]} 06:54, 11 February 2008 (UTC)

Evolution will continue so long as life exists on this planet. Nature is certainly not done. In my opinion it's really not feasible to infer what other adaptations, phenotypic changes, or speciation events might occur in the next million years. Humans most likely will not survive that long. However, if forced to make some kind of a projection, human beings will not be around to observe any major changes. Allelic frequency changes will continue to occur, and new species of plants will continue to emerge, but other than that, I doubt we'd see much. Wisdom89 _{(T / ^C)} 07:52, 11 February 2008 (UTC)

For either genetic drift or natural selection to result in anything you would really consider "evolution," there would probably need to be some manner of reproductive isolation to arise (stir and let cool for 3000 generations). I don't see this as likely in a stable world. Someguy1221 (talk) 08:03, 11 February 2008 (UTC)

An organ capable of breaking down and expelling radioactive material/heavy metals from the body would be a useful adaptation - as would the ability to emit EMP (to defend oneself against those pesky wild, predatory nanobots). --Kurt Shaped Box (talk) 08:55, 11 February 2008 (UTC)

Who is to say that the nanobots won't evolve resistance to EMP? Advanced nanobots can evolve in just one generation, but nature takes a much longer time. Let's hope we don't invent anything like that. 206.252.74.48 (talk) 14:37, 11 February 2008 (UTC)

Is this a reference to Star Trek the Next Generation by any chance? Wisdom89 _{(T / ^C)} 20:48, 11 February 2008 (UTC)

I don't watch Star Trek, so no. Nearly every scientific thing ever discovered seems to be featured on that show, so it's hardly a coincidence. 206.252.74.48 (talk) 16:59, 12 February 2008 (UTC)

WHEELS!!

Eventually humanity will adapt to pavement, as rollerbladers have done... Wheels FTW! —Preceding unsigned comment added by 79.122.115.170 (talk) 08:40, 11 February 2008 (UTC)

I saw the future on TV once. It was land squid. --Kurt Shaped Box (talk) 08:48, 11 February 2008 (UTC)

Kurt Vonnegut's Galápagos posits that the best adaptation for everyone would be for the human brain size to shrink back down to a less troublesome size. --Sean 14:37, 11 February 2008 (UTC)

But there is the problem of biological wheels being nigh impossible. But there is a clever work-around in the book The Amber Spyglass. I think humans are more likely to develop cup-holders and storage pouches for snacks. We may also eventually secrete petroleum, which will solve all our problems, right? 206.252.74.48 (talk) 16:37, 11 February 2008 (UTC)

Biological wheels are nowhere near impossible- they're quite common, albeit on a small scale. See Flagellum. Friday (talk) 16:52, 11 February 2008 (UTC)

Anyway, it's not the wheels that are the hard part, but the axles! :) --Sean 18:27, 11 February 2008 (UTC)

Not really, see the structure of ATP synthase. It has a protein axle (gamma subunit) that connects the two wheels (F0 rotary motor and the F1 catalytic core) One difference is that the wheels are stationary and the axle spins but the general idea is there from a structural perspective. David D. (Talk) 22:23, 11 February 2008 (UTC)

AIDS

Are we any closer to a cure for AIDS than we were 25 years ago? —Preceding unsigned comment added by 195.188.208.251 (talk) 10:29, 11 February 2008 (UTC)

That's a very possible maybe. There have been some promising studies, although no absolute proof that an HIV cure or vaccine has been developed yet. HIV vaccine describes some of the research in this area, though I'll admit it's not the greatest article. There is also this research program that has (apparently) successfully cured HIV infected mice during the ~40 day duration of the animal testing. Interestingly, that press release completely leaves out about two thirds of what that treatment entails, although those two thirds may have been thought up only after that very old press release was made. There have also been treatments tested in humans that successfully lowered the viral load of infected individuals, though the true impact of such treatments is yet to be seen (I also don't have a link for the treatment I'm thinking of, offhand). Someguy1221 (talk) 11:03, 11 February 2008 (UTC)

I saw some information about this at this link --Dweller (talk) 13:35, 11 February 2008 (UTC)

Military Tattoos

I don't know if this is a big public misconseption but I think it is a common thing for military units to all get tattoos. And, at least here in Aus, you cannot donate blood if you have tattoos. And I would suppose that militaries around the world would use blood transfusions very regulary.

So...can people in the military donate blood? I have seen this happen in movies and such (Pearl Harbour and Band of Brothers comes to mind) but do they (now) enforce the same rule of no donating if you have tattoos? If not why not? (I can't think that where they get their tattoos from is any more sanitary than the general populution). If they do not allow the donating where do they get the blood from?

Shniken1 (talk) 12:44, 11 February 2008 (UTC)

Why wouldn't they let you donate blood if you have tattoos? I have never heard of any such rule in the U.S., so I think this is unique to where you are from. 206.252.74.48 (talk) 14:33, 11 February 2008 (UTC)

The UK National Blood service also has restrictions based on tattoos and piercings [3]. It's possible (probable, in my opinion) that the military have their own rules regarding who can donate. Travel to malarial areas would also be an issue affecting the military. 81.174.226.229 (talk) 14:44, 11 February 2008 (UTC)

From the American Red Cross ([4]) guidelines for deferral of donations:

Wait 12 months after a tattoo if the tattoo was applied in a state that does not regulate tattoo facilities. This requirement is related to concerns about hepatitis....

Acceptable if the tattoo was applied by a state-regulated entity using sterile needles and ink that is not reused. Only a few states currently regulate tattoo facilities, so most donors with tattoos must wait 12 months after tattoo application before donating blood. You should discuss your particular situation with the health historian at the time of donation.

I have seen other jurisdictions with a similar deferral period (12 months) following a tattoo or piercing. As noted, the chief concern is about diseases – particularly strains of hepatitis – that may be transmitted via improperly-cleaned piercing or tattooing equipment.

In wartime, and while experiencing severe blood shortages, it is conceivable that blood-collecting agencies might relax some of their deferral criteria. I cannot comment on how or when this may have occurred in the past. TenOfAllTrades(talk) 14:57, 11 February 2008 (UTC)

RD stress

(See my associated post at the talk page) How often do periods of RD angst occur and how long do they tend to last? I'm depending on you bespectacled intelligent folk to derive some cast-iron objective criteria for determining when these periods are, and then generating some lovely charts and graphs to map it. To help you, I'd suggest that posts to the talk page proliferate at these periods. I'm looking forward to some very informative and comforting responses. If you do really well, you might even prompt me to reestablish the neglected thread of the week award. --Dweller (talk) 13:05, 11 February 2008 (UTC)

You'd better re-establish that reward. In a bizarre coincidence I just discovered it today and was wondering if it will ever come back. 206.252.74.48 (talk) 16:23, 11 February 2008 (UTC)

One thing I agree on.. looking at the dreaded 'discussion' page is a sure sign that things are getting bad or going to get bad soon. The thing has 'bad vibes'>87.102.79.203 (talk) 19:15, 11 February 2008 (UTC)

It seems to me that reference-desk stress is something that resides in the individual, not the desk itself. My personal motto for years (after a brush with severe depression caused by stress at work) is "avoid all stress". Other than a brief fling with a medical advice question, I have simply ignored those questions that look like they are going to cause strife. After all, if we all ignored such questions (whether innocent or trollish), stress for all would be avoided. Another general rule is never try to answer a question that you really don't know much about, especially if you know some other reference-desk regular knows more that you do. I'm certainly not advocating a humorless reference desk, but after reading the desk for a while, it is perfectly obvious what kind of humor is acceptable and what is not. Indeed I love the clever puns and reductio-ad-absurdum arguments that some of you are so good at. Also, Dweller, I've missed you--I really enjoyed your thread of the week picks!--Eriastrum (talk) 18:55, 13 February 2008 (UTC)

I second just about everything Eriastrum wrote, especially the last sentence. Having Dweller dangle the award over our heads makes it tempting indeed to crawl through wikidashboards and stress analysis, but I'm afraid that re-reading the screaming contests in the archives might cause me too much stress for my own good. :-) Have mercy, Dweller, and give us our prized prize back. ---Sluzzelin talk 13:19, 14 February 2008 (UTC)

Looking for northern lights

I have vacation time at the end of August. I decided that I'd like to use it to go see the northern lights. Any suggestions on where to go? I'd want to go to a rather metropolitan area. Tromsø appears to work - but I don't know if it is far enough north. -- kainaw™ 13:32, 11 February 2008 (UTC)

Have you been keeping an eye on http://www.spaceweather.com/ ? There's a wealth of information there, and the site will send you an email (for free) or a phone call (for cash) every time something interesting is happening in the sky. TenOfAllTrades(talk) 15:05, 11 February 2008 (UTC)

One thing to keep in mind is that it will be summer up there, and thus the sun will be shining for most of the night, effectively making the northern lights impossible to see unless you stay up very late. Being in a city won't help much either, as city lights caan block all but the most startling northern lights. If you're lucky and you do your homework, though, you might be able to see them. Wrad (talk) 15:15, 11 February 2008 (UTC)

Tromsø in August is absolutely beautiful, but it's not the place (or time) to go to for watching polar lights - at that latitude, the sun sets for an hour or two in August, but you don't get to see a dark night sky - it's more like a prolonged dusk immediately followed by the dawn. If you can brave the weather, visit Tromsø in December or January for spectacular polar light displays. In August, you probably have better chances at Patagonia (although I don't know if that is far enough to the south) -- Ferkelparade π 15:30, 11 February 2008 (UTC)

Basically, seeing the northern lights is all about probabilities, and by going in the summer and staying in a city, you are drastically decreasing those probabilities, approaching impossibility. Wrad (talk) 15:35, 11 February 2008 (UTC)

Thanks. That all makes a lot of sense. I have seen the lights in Norway before (Bardufuss is the phonetic spelling of whatever city I was in - I don't know the Norwegian name). That was in February and I was on top of a mountain above the snowline. I guess my best bet is to try to head south instead of north. I've noticed on astronomy sites that you can see the band of the Milky Way in August if you head south as well. Now, it is just a matter of deciding where to head to. -- kainaw™ 16:06, 11 February 2008 (UTC)

Two points. First, in terms of being where the aurora is, it's not your latitude that matters, but your position in relation to the magnetic pole. Therefore the aurora borealis can be seen farther south in Canada than in Norway. This particularly matters in summer since the lower latitude in Canada means you get more darkness at night. I haven't looked into the question of where in Canada is best to go, and for the southern hemisphere I have no idea. I also note that if you get too close to the magnetic pole, the aurora dimishes again (hence the term "auroral oval"); but this isn't likely to be a problem in practicce.

Second, we are currently nearer the bottom of the solar cycle (2006) than the top (about 2012). There should be much better auroras available if you wait a few years.

--Anonymous, 22:22:22 UTC, February 11, 2008.

Should I be the smart-ass who points out that you can't see the Northern Lights from the Southern hemisphere? No, I shouldn't, carry on. Mad031683 (talk) 16:56, 12 February 2008 (UTC)

Hi. By the way, during occasional intense solar storms, the northern lights can reach pretty far south. One time, a few years ago, I believe it reached down to southern Ontario (but it was cloudy. By the way, if you're lucky and the sky is clear and you're in the place you were mentioning at the time, on August 1, there is a 75% solar eclipse at that location. Hope this helps. Thanks. ~AH1^(TCU) 22:47, 12 February 2008 (UTC)

re:thermoacoustic refrigerator

as mentioned earlier im working on thermoacoustic refrigerator for demonstration.in my final experiment my results were not that good as mentioned in the article.rather i m unable to interpret the results please help:

1. room temperature:25 cel {these are temperatures of cold end of stack) 2.temperature inside the resonator:27 cel 3.temperature reduced by 1 cel in 1 min at 385Hz freq and length of wave was 92: 26 cel 4.temperature reduced by 1 cel in 1 min at 385 Hz freq and length of wave was 92:25.5 cel 5.further temperature became constant at 25.5 cel for next 10 mins.

is the temperature reduction due to the stack.or it is due to a push of external air(at 25 cel ) into the resonator. ive used the function generator: http://www.edn.com/contents/images/di2409setup.exe

kindly help, regards sam Reveal.mystery (talk) —Preceding comment was added at 14:03, 11 February 2008 (UTC)

Vacuum in space

Who figured-out/proved that space is mostly empty between physical objects? In particular, that "the heavens" are not full of air like our own atmosphere? Note that I am specifically *not* talking about luminiferous aether or any such medium for EM radiation; just air. Thanks. --Sean 14:29, 11 February 2008 (UTC)

I don't know that this discovery can be attributed to a single person. In the early 17th century Francis Godwin wrote The Man in the Moone in which his hero travels to the Moon in a chariot towed by trained geese - so Godwin clearly believed that interplanetary space was filled with air [5]. By the late 17th century Robert Boyle was experimenting with the properties of a man-made vacumm and conjecturing that the interstellar medium could be a similar vacuum. I imagine that an evolving understanding of the implications of Newton's law of universal gravitation and Newton's laws of motion in the late 17th/early 18th centuries convinced scientists that interplanetary space must be mostly empty, otherwise the planets would quickly lose energy and spiral in towards the Sun. I don't know whether Newton himself ever explicitly pointed this out. Gandalf61 (talk) 16:31, 11 February 2008 (UTC)

Before that, Aristotle (and those who followed him) would not have countenanced the idea that the heavens might be full of air. For him, the cœlestial realm was utterly and profoundly different from the terrestrial/sublunary, cœlestial objects being made of æther. Algebraist 17:09, 11 February 2008 (UTC)

Bœnus pœints for þe use of archæic ligature! SamuelRiv (talk) 21:32, 11 February 2008 (UTC)

Um, doesn't that change the pronounciations completely? Such as it being pronounced "bee-nus peeints"? —Preceding unsigned comment added by 206.252.74.48 (talk) 21:56, 11 February 2008 (UTC)

Depends what century you're in. Originally, œ had the value [oi̯], but this had shifted to [e] by the middle ages. But what do I do with these bonus points? Is there a prize of some sort? Algebraist 23:42, 11 February 2008 (UTC)

The discovery of the barometric formula was a clue as well. Icek (talk) 15:47, 12 February 2008 (UTC)

plastic beads

How are solid plastic beads made? —Preceding unsigned comment added by 71.100.0.31 (talk) 15:36, 11 February 2008 (UTC)

This may be of use (http://www.wisegeek.com/what-is-plastic-extrusion.htm) perhaps? ny156uk (talk) 19:04, 11 February 2008 (UTC)

The material is probably sprayed into a cooling medium. The size of the droplets controls the size of the produced beads.

Atlant (talk) 20:14, 11 February 2008 (UTC)

But how do they get the eyes for the thread? David D. (Talk) 22:29, 11 February 2008 (UTC)

Well, I agree that any that are basically prismatic are probably extruded. Pierced spheres, I don't know about; I could imagine compression molding followed by tumbling for flash removal.

Atlant (talk) 23:36, 11 February 2008 (UTC)

Solid beads are different from pierced beads. pierced beads are extruded pipe or tubing that is cut and then heated until the edges melt and disappear. The spray thing with molten plastic would be like how lead shot is made by pouring liquid lead into a container that has holes at the top of a tower and as the droplets fall they turn into spheres and solidify before hitting the ground. Maybe plastic is done the same way. 71.100.0.31 (talk) 01:27, 12 February 2008 (UTC)

Big-Bonedness

Is there such as condition as being 'big-boned'? --Ouzo (talk) 15:57, 11 February 2008 (UTC)

Yes, ICD9 278 covers the many variations of obesity. If you are referring to conditions in which the bones themselves are deformed and larger than normal, see the ICD9 710 to 739 range. -- kainaw™ 16:11, 11 February 2008 (UTC)

Thanks for the link. ICD-731 listed as Osteitis deformans describes what I'm looking for (articled as Paget's disease of bone). --Ouzo (talk) 20:46, 11 February 2008 (UTC)

Just out of curiosity, in what way does Paget disease of bone meet your criteria of big-bonedness? I would've thought that something like acromegaly would have been more up your alley. --David Iberri (talk) 03:45, 13 February 2008 (UTC)

Chemistry helper software

Hey everybody. I'm looking for some kind of (simple!) chemistry software, preferably for Mac OS X or Linux. I'm not quite sure what functionality I need, except I need it to help me draw Lewis structure diagrams. By 'help' I don't mean that it should do any kind of actual work (i.e. I want to place everything myself to learn) but I'm tired of using drawing programs! I found a huge list (and another similar one) of chemistry software for OS X, but I want something pretty small and simple (and free) since I'm just starting out. Also, I figured to ask here rather than at the computing reference desk, since the science geeks are over here! Thanks in advance. -- Aeluwas (talk) 16:23, 11 February 2008 (UTC)

Category:Chemistry software has a list of ones that have Wikipedia entries. Molecule editor has external-links to the homepage of many standard ones (including ones with no WP page). BKchem, ISIS/Draw (Windows-only), and XDrawChem are all free and make it easy to do nice organic structures and save or export them in standard graphics formats, without a steep learning curve. DMacks (talk) 19:10, 11 February 2008 (UTC)

I remember using 'chemdraw' on a 'very old' (68000 processor mac) - no idea what it is like now but at the time it was very simple to use.87.102.79.203 (talk) 19:09, 11 February 2008 (UTC) My god that's expensive for what you get - maybe 'xdrawchem' is the free alternative. Good luck. The price of 'chemdraw' is simply outrageous.87.102.79.203 (talk) 19:13, 11 February 2008 (UTC)

There's a very deep academic discount, but if something that's free suffices, that's even cheaper still:) DMacks (talk) 19:20, 11 February 2008 (UTC)

Hmm, thanks guys. I've looked through a couple of these, but haven't found anything that does what I need (i.e. drawing simple lewis structures). I'm sure they'll come in handy later, though. I'd like something to help me create pictures like these (the ones on the left). -- Aeluwas (talk) 12:42, 12 February 2008 (UTC)

Oh sorry, it should be trivial to take a program that does line structures and make it do lewis structures. Maybe you could e-mail some of the free software people and ask if their software can do lewis structures as well. I found http://www.acdlabs.com/educators/cincinnati.html quote:

ACD/ChemSketch is ACD/Labs' chemical structure drawing and visualization application for the Microsoft Windows (95, 98, NT, & 2000) operating system. The software enables you to draw virtually any chemical structure within the scope of your imagination and provides the ability to add chemical structures and drawn pictures, such as lab equipment, Lewis structures, bond ....

it does a lot more as well - here's the main page http://www.acdlabs.com/download/ Note I've never used this program myself.(looks good though) 87.102.81.140 (talk) 13:26, 12 February 2008 (UTC)

Seems to be what I'm looking for, thanks! Way more advanced than I need at the moment, but I suppose the more advanced functions will come in handy soon enough as well. :) -- Aeluwas (talk) 16:31, 12 February 2008 (UTC)

Juxtacapillary (J) receptors

I randomly found this article. It claims that these receptors "sense" oxygen levels (actually it isn't clear from the article how). But I found no mention of them in the Internal senses section of the Sense article. Is that an oversight or is the J receptors article misleading? Rmhermen (talk) 16:56, 11 February 2008 (UTC)

There are literally thousands of chemical receptors like that. Calling them all "internal senses" would be unwieldy and pointless. It is unclear whether the concept of "internal senses" itself is meaningful. --169.230.94.28 (talk) 22:13, 11 February 2008 (UTC)

I can't recall from memory regarding their exact nature, but the stub seems to describe a mechanoreceptor response, not chemical. I could be wrong about that, but this is what I understand from the wording in the article. Wisdom89 _{(T / ^C)} 22:18, 11 February 2008 (UTC)

Using the Hall Effect to Find the Speed of a Flowing Ionic Liquid

Hi, can somebody please give me an indication of how to go about measuring the speed of an ionic fluid using the Hall Effect. Is there any way of doing this theoretically i.e. without simply calibrating the meter by measuring the Hall voltage of a certain liquid at various speeds and plotting a graph?

90.241.222.228 (talk) 17:05, 11 February 2008 (UTC)Shauno

You could just treat the fluid in the same way as an electric current (knowing the charge carriers) as long as you know the magnetic field strength..

There are numerous problems with this - including ion-pairing (reduces effective charge amount) , shielding by the fluid of the magnetic field (reducing effective field strength) and maybe electrode polarisation as well as the physically resistive effect of the charge polarisation..

So simply - you'd need to calibrate.

You could try to build up a model using all these factors (if relevent) - and use that.. (Beyond me though)87.102.79.203 (talk) 19:05, 11 February 2008 (UTC)

• The Hall Effect is when charged particles (in your case, the ions of your ionic fluid) move perpendicular to a magnetic field, they experience a force in the third direction (i.e. perpendicular to both the fluid velocity and the magnetic field). Now if you allow the ions to collect at the edges of the fluid channel, eventually an electric field will build up that will oppose the motion of more ions in that direction. A simple theoretical approach would be to use the equilibrium: force from electric field balances force from Hall effect to find the linear coefficient relating velocity and voltage. JohnAspinall (talk) 19:07, 11 February 2008 (UTC)

Bouncing Squash Balls

If i knew the volume of a squash ball and the temperature, could i work out how high it would bounce from a drop, using gas laws and kinetic energy equations? —Preceding unsigned comment added by 81.151.74.117 (talk) 19:53, 11 February 2008 (UTC)

I doubt it, because most of the energy loss is in the rubber of the ball, and you haven't claimed any knowledge about that factor.

Atlant (talk) 20:13, 11 February 2008 (UTC)

There's an alternative method - ignore the rubber and assume energy loss comes from loss of heat from the gas inside. (this is probably simpler than working out how the rubber deforms and loses energy)

maybe - ignoring the effect of the skin - you could assume that the bit in contact is squashed flat.. - squashing it would cause an increase in pressure and hence an upward force on the squashed bit. Without loss of energy it would bounce just as high.

To take account of the loss of energy you could say that the gas is being heated as it is squashed. If this increase in temperature causes loss of heat to the surrounding then the ball will not rebound as much - however since the heating is related to the rate of change of volume and the pressure you'd certainly end up with a differential equation.

These are the factors..

ʃpressure_t x volume_t dt = energy supplied

Rate of energy loss = k x (T_ball-T_external)

Volume of ball =4/3 pi r³ (1+cos a_t) .. a is the angle of the flattened bit (think this is right see Spherical cap for correct equation also try http://mathworld.wolfram.com/SphericalCap.html (eqaution 9) note the alpha is 90-a the angle I used below . sorry in a rush)

r(1-cos a) = distance of impact (you might need/want to reverse this to get a)

pi x (r sin a_t)² = A_t area of sqaushed bit

A_t x P_t - P_{external atmosphere}= F_t force upwards - note that here I've assumed that the original internal pressure was the same as the atmospheric pressure eg 1atm since I'm ignoring the skin

F_t-mg = overall force upwards (m is ball mass)

Pressure x Volume =2/3 Energy of gas

etc..

There's a bit more work to do here - including the above equations as well as newtons laws of motion I think there is all you need to at least construct a differential equation..(If you don't like differential equations you could use a computer to model this bouncing behavior in small steps of time.)

If you want/need more help please ask.87.102.79.203 (talk) 20:48, 11 February 2008 (UTC)

If you get that far and get stuck the maths reference desk might be your next step. Good luck87.102.79.203 (talk) 20:28, 11 February 2008 (UTC)

(( What follows is irrelevent to the original question but is added anyway 'just in case' .... Alternatively you could ignore any energy losses from the gas and calculated the energy required to flatten a rubber skin - here you might need to decide if there is a gas is in the ball or if the springyness comes entirely from the rubber itself - you could assume that any energy required to bend/compress the rubber is entirely lost - ignoring the bending force the energy is simply the change of length of a part of the skin times the distance it is moved. You might want to assume that the skin is flattenend vertically and does not move horizontally since that simplifies the eqautions..

In practice solid rubber balls bounce because of the springyness of the rubber, but hollow rubber balls bounce because of the springyness of the gas..

As a final option (and easiest) option you could just say that a certain percentage of the kinetic energy is lost on contact - but that wasn't your question.87.102.79.203 (talk) 20:44, 11 February 2008 (UTC) ))

We can do this far more simply. $PV=nRT$. Note that temperature is constant assuming perfect conduction into the outside reservoir (the outside air), as is the number of moles of gas. So $P=nRT/V$, and note that the energy in an ideal gas deformation is exactly $-\Delta U=\int PdV=nRT\int dV/V=-nRT\log V_{i}/V_{f}$. So we now know what kind of change in volume a given amount of energy will induce. Now we have to fudge things a little to make this work, and I don't think this is valid, but with what you are given, it's the only simple approximation I can think of. Let's now assume that the transformation when the ball hits the ground is isobaric - that is, of constant pressure. Then there must be some temperature change such that $V_{f}/V_{i}=T_{f}/T_{i}$, which will result in a heat loss of $\Delta Q=mc\Delta T$. Note there is a compression and then re-expansion of the ball, so we must double our heat loss value to find the total heat lost in the bounce. Find the kinetic energy from $U-2Q$, and you're home. I'm intrigued enough to try plugging in numbers on my own and seeing if this fudge works, so I'll report the results in a later post. SamuelRiv (talk) 21:28, 11 February 2008 (UTC)

The final equation I get is $h_{2}=h_{1}-2T(c_{air}/g)(\exp(-M_{air}gh_{1}/RT)-1)$. Plugging in the relevant numbers gets me a higher height than what I started with, suggesting this equation is wrong. I think my fudge was too fudgey. Back to the drawing board! SamuelRiv (talk) 22:41, 11 February 2008 (UTC)

How dangerous are Moka express machines...

...and other similar cookware that work with pressurized boiling water?Mr.K. (talk) 20:18, 11 February 2008 (UTC)

Moka Express and common pressure cookers have relief valves to prevent dangerous build-up of pressure. The temperatures are only a few dozen degrees above normal boiling point of water, so that on its own isn't a substantially worse scalding hazard than a normal tea-pot. I'd be most concerned about accidentally opening the vessel when it's pressurized...water would flash-boil coffee-grounds go flying, etc. DMacks (talk) 20:50, 11 February 2008 (UTC)

I think that would be difficult to do "accidentally". The pressure will push against the threads, so it should take an awful lot of torque to overcome the friction and unscrew the bottom from the top -- plus the bottom will be very hot, so it's not like you can grab it with your bare hand. Not sure what would happen if you didn't really get them threaded together right in the first place, but my guess is that the water would just bubble out from the joint and no high pressure would ever build up.

It has occurred to me to wonder what happens if the relief valve gets clogged with lime or something. Has anyone ever heard of an actual dangerous explosion from a moka? --Trovatore (talk) 21:01, 11 February 2008 (UTC)

I can't say for certain about Mokas, but the safety valve on my pressure cooker doesn't come into contact with the cooking liquid and isn't even used for steam venting except in failure, so it's not like to get scaled up. AlmostReadytoFly (talk) 09:25, 12 February 2008 (UTC)

I don't know about modern pressure cookers, but a traditional pressure cooker has two separate valves: the normal weighted pin valve that keeps the pressure at the nominal "correct" pressure by venting steam , and a safety relief valve. In normal operation, the regulator valve vents about the same amount of steam as a boiling teapot. If that valve ever gets clogged, the pressure will build until the safety valve blows out leaving a half-inch diameter hole in the top: This is a dangerous mess and should never happen. If it does, most of the contents of the pot are expelled violently all at once: you usually end up needing to clean food off of the ceiling of the kitchen, and a cook can be seriously scalded if in the wrong place at the wrong time. This is however safer than what would happen if the safety valve did not operate: the cooker could explode and send heavy pieces of metal (such as the lid) flying. In 1955, my mother was pressure-cooking a whole chicken when the safety valve let loose. She vividly recalls that nearly the entire chicken, bones and all, were pureed through the valve and spewed onto the ceiling. -Arch dude (talk) 21:42, 12 February 2008 (UTC)

Does reading ruin your eyes?

Does reading books, ruin a persons' eyes? If so can vision be preserved by not reading? How about reading on the computer?

Which is worse for vision, reading on the computer or books? --Frsoroad (talk) 23:05, 11 February 2008 (UTC)

Straining in the dark by reading books can be damaging, yes. IMHO, working late at night in front of the computer is more deleterious than reading books in normal warm light. Seriously, using wikipedia in the dark is like staring at white light. Wisdom89 _{(T / ^C)} 23:10, 11 February 2008 (UTC)

A recent article in the British Medical Journal said that idea that "Reading in dim light ruins your eyesight" is a myth. [6][7] Although low light levels can lead to temporary eye strain, there is no evidence that inadequate light while reading will result in permanent vision damage, or so says the article. -- 128.104.112.12 (talk) 23:34, 11 February 2008 (UTC)

Several studies have found myopia to be rare among illiterate cultures, and to increase dramatically in frequency in the first generation of children in those cultures to receive schooling. (See for instance Figure 1 of [8], originally from Morgan RW & Munro M (1973): Refractive problems in northern natives. Can J Ophthalmol 8: 226–228). These observations are widely accepted as evidence that spending a large fraction of ones time during childhood focusing at close distance -- e.g., reading -- strongly increases the likelihood of developing myopia. This conclusion is supported by experiments that have shown that chickens consistently develop severe myopia if they are raised in an environment without anything distant to focus on, or given eyeglasses for the same effect. Chicks, however, apparently unlike many people, only need a brief daily exposure to distant scenes to fully prevent this development of myopia. --mglg_(talk) 01:17, 12 February 2008 (UTC)

I always thought that my chicken tasted rather nearsighted. — Ƶ§œš¹ _{[aɪm ˈfɻɛ̃ⁿdˡi]} 02:10, 12 February 2008 (UTC)

Your argument makes perfect sense, mglg. I always did read much more than my peers back in grade school, but even though my lenses are literally an inch thick, I think the rewards outweigh the disadvantages. 206.252.74.48 (talk) 13:36, 12 February 2008 (UTC)

Absolutely. Just look up from the book now and then to gaze dreamily into the distance... --mglg_(talk) 21:26, 12 February 2008 (UTC)

A lot of straining of the eyes will develop bags under them as well. 200.127.59.151 (talk) 13:38, 12 February 2008 (UTC)

HCl Crystal Structure

I cannot find confirmation of this but when frozen does HCl have the same crystal structure as NaCl? i.e. cubic close packed? I assumed that it has this structure because the bond is very ionic, but the proton is very light so this may cause something to happen to the crystal.

Oh and I'm taking about at liquid nitrogen temps ~79 K

Shniken1 (talk) 23:39, 11 February 2008 (UTC)

See doi:10.1038/213171a0. Sounds like some interesting content to add to the HCl article. DMacks (talk) 01:43, 12 February 2008 (UTC)

Thanks for that, it is wierd, it doesn't have HCl written anywhere, hence I couldn't find it. :P (Lesson 1.a try all possible chemical names!)Shniken1 (talk) 01:51, 12 February 2008 (UTC)

Circumcision Controversy

I was just noting today how often this crops up. Why is a minor medical/cultural procedure tinged with so much partisan fervour? Web sites with such fanatical language and claims are myriad. It's not the same for other such things. If one gets an eyebrow piercing there are not fanatical pro and anti camps who will bend logic to their respective ends. Or a pro and anti appendectomy "lobby". I'm guessing it's because it's our "intimate" parts and we hope we have the "right" configuration. I'm not sure if there is any research as to the psychology of circumcision opposition/advocacy, but arrogant opinion is welcome ;-) Fribbler (talk) 23:51, 11 February 2008 (UTC)

Well an appendectomy procedure is designed to prevent the death of the patient. Circumcision is usually just slicing off a bit of someone’s genitals without pressing medical need. Eyebrow piercing would be just as controversial if it was performed on babies who lack the ability to give consent. --S.dedalus (talk) 00:00, 12 February 2008 (UTC)

Edit conflict, ARGH! But as Sdedalus says, the main point is that babies cannot give consent for a precedure that is not needed —Preceding unsigned comment added by Shniken1 (talk • contribs) 00:04, 12 February 2008 (UTC)

Just to clarify: babies don't give consent for ear-piercing, and it is quite common in some cultures that have brought the practice to North America. I don't agree with it, personally, but I don't know of any lobby against it. ៛ Bielle (talk) 00:08, 12 February 2008 (UTC)

I didn't know they did that on babies... But anyway if you don't wear ear rings for a while don't the piercings close up? i.e it is essentially reversable?Shniken1 (talk) 00:10, 12 February 2008 (UTC)

I'm against circumcision myself, certainly in infancy. But then why such vehement pro-circumcision argument? It seems almost like a religious/personal/political type position in that people attach much emotion to their slant without science. Which I believe would generally not favour it. Fribbler (talk) 00:26, 12 February 2008 (UTC)

It has nothing to do with science. It has to do with parents who don't think anyone has the right to tell them they can't have what they see no more harmless than cosmetic surgery done to their kids. Someguy1221 (talk) 00:31, 12 February 2008 (UTC)

To respond to Shniken1, yes, the holes in the ears will close over if left alone, as would the eye-brow piercings referred to by S.dedalus. Both piercings and circumcisions are painful procedures done on infants, when there is no medical necessity to hurt them and no ability for them to give "informed consent". My own vote would be for child abuse, but I doubt I will find many kindred souls in this. (You did ask for arrogant opinions, did you not?) ៛ Bielle (talk) 00:39, 12 February 2008 (UTC)

I did! And agree with you aswell. But I still wonder just why it elicits such emotion. The websites say things like, and I paraphrase for emphasis, if pro: "If you don't get it done your child will be austracised as a FREAK!!!" (in the US I imagine, sure we dont really do it here) and if anti "Your child will be scarred for LIFE!!!" I just wonder if there are reasons for the emotional nature of the arguments. Fribbler (talk) 00:49, 12 February 2008 (UTC)

I think it's not so much a matter of circumcision there as it is that a fair proportion of humans are insane. And wherever a few crazies find an issue to go nuts over, more crazies will go nuts in response. Oh yeah, basic human nature ;-) Someguy1221 (talk) 00:51, 12 February 2008 (UTC)

It's really too bad, for many reasons, but for this one in particular, that User:SteveBaker is no longer active on the Ref Desks. He had, as I recall, very strong, scientific views against circumcision. ៛ Bielle (talk) 01:52, 12 February 2008 (UTC)

Since you asked nicely (and I care a lot) - I've copied my response from my Talk page below. SteveBaker (talk) 01:46, 19 February 2008 (UTC)

My belief is that people are tired of complaining about things that matter, that they decide to complain about something trivial just so they have something to complain about. You pick just about anything (e.g. how to hang toilet paper (there was a RD vote on this!), how much sugar to give to children, fast food, placement of water towers, Star Wars vs. Star Trek) and people will debate it to the end of time. It is mankind's nature to argue and whine and such things. Of course, you can debate whether or not I'm right about that. 206.252.74.48 (talk) 13:48, 12 February 2008 (UTC)

To Someguy: although there is no pressing medical need for circumcision, (indeed I find it frankly barbaric myself), it is not something which "has nothing to do with science". Circumcision eliminates the possibility of phimosis, a risk factor for penile squamous carcinoma and infections, and also of paraphimosis, which can be a medical emergency. I have also heard that circumcision may reduce HIV transmission rate (in some men, in some situations, etc.). In times past when personal hygiene practices were less stringent, the difference in anatomy may have led to a difference in cleanliness and been more important to penile and general health. Thus, although I wouldn't argue that, today, this choice is probably based more on the fact that daddy wants his kid to look like himself, I think it is presumptuous to say that circumcision has nothing to do with science. Tuckerekcut (talk) 00:22, 14 February 2008 (UTC)

One could use that argument to say that the amputation of the arms at the elbow eliminates the possibility of carpal tunnel syndrome. Sure, in those cases where a medical condition is present and needs to be addressed, such as phimosis, circumcision might be the best solution; and that's scientifically valid. But circumcising a perfectly health boy in this day and age because one day, maybe, perhaps, he might fail to clean himself properly and might get a nasty infection, so let's nip it in the bud before it ever gets to that stage - that's in no way scientific. Some religions and cultures require circumcision - and if that's the reason it's done to a particular boy, let's be upfront about it and say so. Not that I agree with such religious and cultural requirements, but they are a factor and it's pointless to prevaricate about it and try to justify them on scientific grounds, as I've heard various people do. If their religion or culture didn't require circumcision, it would never occur to the parents to consider it for a second. -- JackofOz (talk) 07:16, 15 February 2008 (UTC)

It's an issue of religion and of "the correct way to bring up a baby". That's two sure-fire ways to get serious arguments. AlmostReadytoFly (talk) 09:18, 12 February 2008 (UTC)

Toilet paper should obviously be fed OVER the roll. :P Emery (talk) 23:41, 12 February 2008 (UTC)

FFS I am so sick of you over the roll people. Without a doubt the best way is under the roll. GET IT THROUGH YOUR THICK HEADS YOU IGNORANT SO AND SOS!!!!Shniken1 (talk) 22:47, 13 February 2008 (UTC)

Look, Fribbler, it's not going to grow back, and the sooner you realise this and accept having 30% less gloriously erotic tissue compared to me, the sooner you will be able to appreciate what little remains to you and move on. Myles325a (talk) 01:07, 15 February 2008 (UTC)

Evolution is not wasteful. When creatures are trapped in a dark cave, within an amazingly few generations their offspring evolve to lose their eyes. If human males didn't need the foreskin then we'd undoubtedly have evolved without one...if (as often claimed) there are positive benefits to removing it then we'd most certainly have evolved without it. The fact that we still have it is absolute scientific PROOF that it has a function - even if we don't know what that function is.

The truth of it is this - people who were circumcised at a very young age have no choice in this matter and because that particular part of the male anatomy is something we're all terrified of being found wanting in - they are CERTAIN to claim they are better off - no matter what. The same thing applies to those of us who remain 'intact'. There is absolutely zero serious, peer-reviewed scientific evidence of health benefits either way - so all we're left with are claims of greater or lesser sensitivity and/or enjoyment during sex. But the trouble with that claim is that only a VERY few healthy men get circumcised after they become adults (the very thought makes my blood run cold!) - so only a very few people can testify after having sex under comparable conditions both with and without the foreskin...and those that are in a position to do so have ample reason to lie about the results because they don't want to seem like idiots for making the decision! So we don't have any proof of health benefits and we don't have proof of advantages or disadvantages during sex.

All we're left with is the sure and certain knowledge that we wouldn't have evolved to be the way we are if the foreskin had no purpose whatever. I'm personally pretty sure that the purpose is to cover the super-sensitive area of skin beneath in order to allow the nerve endings there to be protected until the act of intercourse when the foreskin tends to be rolled back to expose the most sensitive patch of skin on the entire body. Nerves that are exposed to daily wear and tear after circumcision are going to lose much of that extreme sensitivity.

So I strongly suspect that circumcised men are losing out on the experience - but I can't prove that.

From an evolutionary standpoint, we might imagine that men who have more fun during sex are more motivated to do it more often - that would imply that they would produce more offspring and therefore those men would be more prevalent in the gene pool. Hence the foreskin may well be a carefully evolved device that most certainly isn't meant to be chopped off at birth!

Sooner or later, science will figure out the truth - and then the debate will really begin. Right now we don't KNOW. But there are other parts of the body that we don't know the purpose of...do we go around routinely chopping off people's earlobes soon after birth? Are appendices removed "just in case" because we don't understand their function?

For parents (PARENTS!!) to go around mutilating the genitals of tiny helpless babies is one of the most disgusting and brutal practices imaginable to me. If parents went around removing the earlobes of their newborns in bizarre religious rituals they'd wind up in jail for sure! Thank goodness the world has realised this in the case of female genital mutilation and is working hard to outlaw the practice - let's just hope that as a society we can see the light in the case of the hundreds of millions of little boys whom their parents have so brutally mutilated in the name of some ancient barbaric tradition.

SteveBaker (talk) 01:46, 19 February 2008 (UTC)