Science desk
< March 27	<< Feb \| March \| Apr >>	March 29 >

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.

March 28

Silver Sulfide Decomposes At Boiling Point?

What does it mean that silver sulfide decomposes when it boils?Curb Chain (talk) 00:57, 28 March 2013 (UTC)[reply]

I assume it means that the vapour phase is unstable compared to liquid phase. Plasmic Physics (talk) 02:56, 28 March 2013 (UTC)[reply]

See thermal decomposition and chemical decomposition for our articles on the subject. Tevildo (talk) 10:21, 28 March 2013 (UTC)[reply]

Water and pressure related questions

1. Does evaporation of water take place at all temperatures? Does it take place at -200 and 0 degree Celsius?
2. Is it possible to convert water which is polar into non-polar by any means?
3. Suppose there is an empty bottle on a desk. Atmosphere is applying such a huge pressure on it, according to this situation it should be crushed into a small volume, but it doesn't happens. Why? Yellow Hole (talk) 03:43, 28 March 2013 (UTC)[reply]

(1) Evaporation occurs at all temperaures and pressures that allow water to exist in liquid or solid states. Evaporation is driven by vapour pressure. Vapour pressure exists because some water molecules continually leave the water (or ice) surface (and some on contact rejoin). If you enclose a quantity of water in a sealed rigid chamber, which contains nothing except the water, some of it will occupy the space left over by evaporation to a gas, to the extent that the gas pressure equals the vapour pressure.

The molecules that leave the water surface do so because heat energy is randomly distributed between molecules - thus some are more energetic than others, ans some of those have enough kinetic energy to leave and dart about on their own. Vapor pressure, and evaporation, sharply increase with increase in temperature. Vapour pressure and evaporation are very weak at 0 C but it does continue very slightly down to absolute zero.

(2) No. The way that hydrogen atoms each bind to the oxygen means that a water molecule can only be polar.

(3) Truely empty containers do get crushed, if their strength does not withstand atmosphereic pressure. To get a truely empty container, you have to pump out the air or whatever that is in it, and create a vaccuum. Otherwise whatever is in it will have the same pressure as the air outside. There was a televison personality, Julius Sumner Miller, who effected the manner, in his voice and mannerisms, of a mad scientist and who made a name for himself crushing all manner of containers from small tin cans to huge oil drums by filling them with steam, sealing them, and then pouring cold water over them to condense the steam and create a vacuum ( usually swearing under his breath if the drum didn't collapse on cue ). He's dead now, unfortunately. He used to drive high school science teachers nuts by inspiring school kids to ask questions that teachers cannot answer but know that they should. Teachers hated him.

Wickwack 60.230.200.148 (talk) 04:06, 28 March 2013 (UTC)[reply]

If there is a complete vacuum inside the bottle, will the atmospheric pressure be able to crush the bottle? If the bottle contains air, the air will prevent the bottle from being crushed. But what prevents a page from being crushed, when one holds it vertically or horizontally. If the bottle containing air is taken on the moon (where there is no atmosphere), will it expand and ultimately burst? Yellow Hole (talk) 05:46, 28 March 2013 (UTC)[reply]

What do you think? You have all the information you need to figure it out. If you are too dumb to figure it out from my first sentence in (3) above, get someone to show you a tube from an old TV set. See that big glass thing that you look at to see the picture? It's the tube - really a large bottle, and it has a very near perfect vacuum inside, so that the electrons can get from one end to the other without being impeded by anything else. Look how thin the glass is at the back end (about 1 mm or so), but notice it's round, which is the strongest shape. Wickwack 60.230.200.148 (talk) 05:55, 28 March 2013 (UTC)[reply]

Enough with the nastiness! Nothing is obvious if you look at it hard enough to really understand it in the first place. This question reminds me of [1] ... if you don't think something through in the simple case you'll only get tripped up in the more complex one. It is hard to grasp intuitively at first that air is under such dramatic pressure while it yet has so little weight. The difference between the top of a page held in the air and the bottom is small, the difference in pressure is small, and the air on either side increases in pressure equally. Wnt (talk) 13:30, 28 March 2013 (UTC)[reply]

Sometimes it's better to challenge people to think, rather than just tell them the answer. If you think, you learn. If you fed an answer, you soon turn to something else and forget about it. This one was in fact really simple. He wasn't asking about paper, he asked would a bottle get crushed if it contains a vacuum. This OP has asked about very simple things (like under what cicumstance will air pressure crush things) obvious to anyone with ordinary schooling, mixed with significantly more advanced concepts (eg water being polar) which made me a tad suspicious, actually. Wickwack 124.182.145.28 (talk) 13:58, 28 March 2013 (UTC)[reply]

Challenging someone to think ==/== being rude and insulting. Those are not coincident activities. You can challenge someone without calling them dumb. --Jayron32 03:26, 29 March 2013 (UTC)[reply]

collapse bickering

The following discussion has been closed. Please do not modify it.

1) Note that evaporation directly from a solid is called sublimation. StuRat (talk) 16:05, 28 March 2013 (UTC)[reply]

Not right. Sublimation is the conversion of a solid into a gas by adding heat to raise it above the sublimation temperature. (Many substances, including H2O, go directly from solid to gas without passing through a liquid stage, if the pressure is right). In pure substances (ie not solutions or alloys), sublimation occurs sharply at a specific pressure dependent temperature and only that temperature, just as melting and boiling does. Sublimation is not evaporation any more than boiling is evaporation. Evaporation, regardless of whether it is off a solid or a liquid, occurs at any temperature below the sublimation point, melting point, or boiling point, whichever is applicable at the operating pressure. Stick to what you know or have checked, please Stu. Ratbone 121.221.4.38 (talk) 03:05, 29 March 2013 (UTC)[reply]

As usual, you're talking out of your ass, Ratbone. See Sublimation_(phase_transition)#Water for examples of water ice subliming. StuRat (talk) 03:14, 29 March 2013 (UTC)[reply]

And as usual, StuRat, you've come back with a ridiculous attempt to justify your initial error. There's nothing in the article you cited that supports your claim. Look in any standard physic textbook. I have several, all saying essentially the same thing. One, a standard univerity text, says "sublimation in the process by which a solid passes from solid phase to vapour phase without going through a liquid phase, and occurs at low pressures. Sublimation occurs at the sublimation line in the pressure-temperature phase diagram, between the origin and the triple point." This means, of course, that sublimation occurs at a specific temperature for the given pressure. See Physics for Scientist and Engineers, Douglas C Giancoli. There's several editions of this. In mine it is on page 474. However, very old texts and standard dictionaries often do not make the distinction plain. Now be good for once, and learn from your error. Sublimation, an abrupt process, can only occur below the triple point pressure. Evaporation off a solid can occur at any pressure below the vapor pressure, though for many materials the vapor pressure off solid will be below the triple point value at all temperatures and evaporation may not occur. That is why glass does not evaporate on the inside of vaccuum tubes, for example. For water, vapor pressure off solid runs above the triple point pressure and evaporation does occur. You cannot make a vacuum flask out of ice. I hope you can see for all this, at least that there are two distinct processes going on, just as there are two distinct processes involved in going from liquid to gas/vapour (boiling being the abrupt one, evaporation being the other). Ratbone 124.182.24.177 (talk) 04:08, 29 March 2013 (UTC)[reply]

Nothing you said in any way contradicts what I said. You're just trying to pick a fight, as always. Might I suggest you go to a pub and insult a man with a shaved head and multiple tattoos ? That's the most effective way to provoke a fight. StuRat (talk) 05:00, 29 March 2013 (UTC)[reply]

As usual, you are the one who started with the insult. But what I said most certainly is in contradiction to yourself. You claimed sublimation is evaporation of a solid. I have said sublimation and evaporation are two diffrent processes occuring under difernet conditions. Ratbone 120.145.25.156 (talk) 05:35, 29 March 2013 (UTC)[reply]

The question was "Does evaporation of water take place at all temperatures ?". I corrected their terminology, by pointing out that water going from the solid phase to gas directly is called sublimation. You come into every thread here looking for any possible way to misinterpret a response so that you can say it's wrong. Now go pick a fight elsewhere. StuRat (talk) 05:48, 29 March 2013 (UTC)[reply]

If I may step in: it appears to the layman outsider that what's really at work is that one of you is using the colloquial form of "evaporation" while the other is using the formal definition. Both are acceptable within the right context; certainly neither of you is unconditionally "wrong". Had either of you bothered to consider or acknowledge such, this whole mess could have been useful to other readers. — Lomn 14:38, 29 March 2013 (UTC)[reply]

Why do we use the word 'infinity' in the definition of potential energy?

In almost every definition of potential energy, whether electric or gravitational, I see the word infinity. While defining electric potential, we write "The electric potential at a point in an electric field is defined as the amount of work done in bringing a unit positive charge from infinity to that point." Same definition applies to gravitational potential energy. What is the significance of the word 'infinity', here? Thanks in advance! 106.216.102.21 (talk) 06:33, 28 March 2013 (UTC)[reply]

It's a hypothetical point infinitely far away. To define potential energy, you need some point of reference. The natural choices seem to be the point at the center of the field, a point on the surface of the body creating the field, and infinity. The first one results in a mathematical problem, since it represents a singularity. The second is only well-defined for spherical bodies, and it results in different potential energies for different sized bodies with the same mass or charge. The third is not quite as obvious, but it works consistently in all cases, it has nice mathematical properties, and its the one we use in physics. In reality, you are often interested in the potential energy difference between two points - that is independent of your choice of reference point. --Stephan Schulz (talk) 06:45, 28 March 2013 (UTC)[reply]

"Infinity" means "beyond the range of effect of the charges we're talking about". Given how willing physicists usually are to make approximations, I'm surprised they don't just say "far away". Wnt (talk) 17:13, 28 March 2013 (UTC)[reply]

The short answer is laziness. A proper definition makes it clear that potential energy is always defined with respect to an arbitrary reference point, but unfortunately people have difficulty understanding concepts that are relative in this way, and feel an urge to reword the definition in a way that makes it look less relative. I believe that this is a blunder, because a person who does not understand that potential energy is relative really doesn't understand it at all. The same issue applies to voltage (electrical potential), and I have repeatedly run into trouble in my efforts to maintain our membrane potential article from editors who try to reword the definition to remove mention of an arbitrary reference point. Looie496 (talk) 17:23, 28 March 2013 (UTC)[reply]

The actual answer is less laziness and more about reasonable precision. As you bring electric charges farther and farther apart from each other, the effect they have on each other decreases with increasing distance; that is the effect of moving a distance of 1 inch to 2 inches is much greater than from moving 2 inches to 3 inches, and MUCH MUCH greater than moving 15 inches to 16 inches, and so on and so on. Eventually you reach a distance where incremental changes to the distance betweens the charges stops making a measurable difference in their effect, so, for example, you can't actually measure any difference in effect no matter how far you move it. Mathematically, you can always calculate such an effect, but when you reach the point where such an effect is dozens or hundreds of orders of magnitude smaller than the finest measurement than any device could ever make, it's not necessary to make a distinction between those distances and "infinity". That is, bringing a charge "from infinity" to a given point just means "bringing a charge from a distance far enough away that the incremental changes in distance are insignificant" to that given point. Since functionally, any distance that large would work, infinity is as good as any word, and useful in the sense that it takes less syllables to say than "a distance far enough away that the incremental changes in distance are insignificant" --Jayron32 20:28, 28 March 2013 (UTC)[reply]

I must respectfully disagree. You can go clear to the other side of the universe, and still if you move the object next to a black hole, you'll get an enormous potential energy. That's a nitpick in a sense, but it touches the crucial point. I've taught this stuff a number of times, and I've come to believe that to teach it properly you must get the student to understand the concept of a reference point, and that the idea of "moving to infinity" only serves to obscure that concept. Looie496 (talk) 20:44, 28 March 2013 (UTC)[reply]

Who said anything about introducing black holes into the problem? If a problem doesn't mention them, what's the point of bringing them into the discussion? --Jayron32 03:32, 29 March 2013 (UTC)[reply]

Well, it seems we have two teachers disagreeing on how to teach. I agree with Jayron though. He's in good company, I have several engineering and physics texbooks that set out his very reason. However, Ralph Morrison's textbook Grounding and Shielding Techniques (don't fret over the name, it is a book about mostly about electrostatics and electromagnetics fundamentals) carefully avoids using points at infinite distance - it develops everything from two arbitarily charged points a distance x apart. Morrison's book in its various editions was a standard undergrad text for electrical & electronic engineers for 40 years. So Looie is in good company too, though his reference to black holes is a red herring. It is undertood that a pint at infinite distance is a point in the middle of nowhere. Wickwack 120.145.31.249 (talk) 03:26, 29 March 2013 (UTC)[reply]

More clearly, when we say "infinity", it implies that we are really taking a limit, as there is no such point.--Jasper Deng (talk) 03:51, 29 March 2013 (UTC)[reply]

If the OP is really asking about why physicists set the zero point at infinity, as opposed to why students are taught that, there's a number of compelling philosophical reasons. First, setting the zero point at infinity implies that a completely empty universe has 0 energy. There's an undeniable elegance about that, and it's certainly better than claiming that an empty universe has 5 joules--why 5, and not 5.01, or -151? Second, where else would you set the reference point? You can't set it at 0 distance from the point charge/mass, because the potential energy would be infinite there. If you set it at 4.3 meters, why are you choosing 4.3, instead of 4.8? Infinity is the only elegant solution.

Third, many equations become much simpler and more intuitive with the reference point set to infinity. For example, a Sun + comet system has positive energy if the comet can escape the Sun's gravity, and negative energy if it can't. The virial theorem says that in any bound system with no mean velocity, gravitational potential energy is -2 times the kinetic energy. Physicists are not in the business of making equations more complicated than they need to be--besides wasting time, that tends to hamper intuition more than it helps, and for no benefit. --140.180.254.209 (talk) 06:52, 29 March 2013 (UTC)[reply]

Note that in some cases you can neither use zero nor infinity, e.g. in two dimensions the Coulomb potential is proportional to Log(r). But even in these and other cases where you can't escape singularities anymore, you still want to use such points. A good example is quantum field theory, where it turns out to be much more convenient to define the theory at arbitrary small lenght scales and then use clever tricks involving counterterms that become infinite in the limit, rather than doing all the calculations such that everything stays finite. Count Iblis (talk) 15:24, 29 March 2013 (UTC)[reply]

Louie brought up a good point here. Realizing that there isn't anywhere "infinitely far away" from all the stuff in the universe - that any practically picturable point of "infinity" is an average of all sorts of stuff including the occasional star, planet, black hole, what have you - is the zero point truly infinity, or some kind of abstraction of what infinity would be if there were an infinitely large void in the universe it was in the middle of? (I think it's the latter, at least for the simple formulas I know; I don't know if anyone tries to use an actual average of stuff far far away) Wnt (talk) 03:25, 2 April 2013 (UTC)[reply]

How close must two particles meet for annihilation to take place?

--Inspector (talk) 08:26, 28 March 2013 (UTC)[reply]

See Cross section (physics) (not a very good article at the moment, to be honest). Calculating the cross section for a particular interaction is fairly complicated - see Bhabha scattering for an example, or this rather more accessible paper. Both require a reasonable university-level knowledge of particle physics to understand completely, I'm afraid, but science is sometimes like that. Tevildo (talk) 10:38, 28 March 2013 (UTC)[reply]

Flowing electrons

Electricity, which has invaded our lives from all directions, is nothing but the flow of electrons. One should think how these charged particles run a number of electrical equipment? What makes electrons so special that they produce electric current and light the bulbs, run the fans and motors, etc? Britannica User (talk) 13:36, 28 March 2013 (UTC)[reply]

Electric current is not solely comprised of moving electrons. It can be other sorts of charge carriers, such as ions. What makes all these things comprise electric current, when they collectively move, is that each one carries an electric charge. What is an electric charge? Something that provides an electric field - that is it can exert a force wrt another charged thing. Wickwack 124.182.145.28 (talk) 13:51, 28 March 2013 (UTC)[reply]

Electrons are quite light yet carry a large electric charge, and are located on the outside of each atom. The electrons in the valance shell (outside edge) are particularly likely to "break off" and are therefore the most mobile. StuRat (talk) 16:11, 28 March 2013 (UTC)[reply]

It is kind of a boring answer but the fans and motors in question are designed to run via electric current (which is translated, usually through magnets and so forth, into kinetic motion). You could make fans and motors that ran off of diesel fuel or even just falling water. There is nothing "special" about electrons in this way; they can be conveniently used as a means of transmitting energy relatively efficiently, and that is why we've built a world around them. They are not so much "special" as "convenient"; we could have a world built around falling rocks as well, but that would be much more costly. Electric currents are very easy to produce from mechanical force (e.g. using a dynamo), can be transmitted over long distances, and can be tuned pretty efficiently for the task at hand — this is what makes them so common in our current world. --Mr.98 (talk) 16:23, 28 March 2013 (UTC)[reply]

I think much of it has to do with the conduction band of metals and other conductors being such a remarkably easy place to move around in. Wickwack speaks of other ion carriers, but you couldn't pump sodium ions hundreds of miles through a cable. Maybe you could build a machine run by virtue of a proton beam in an evacuated space like in a particle accelerator? But I don't know if it has ever been done. It really isn't that obvious to me why electrons are able to move through a vast matrix of positive and negative charges with so little overall resistance - I suppose delocalization is somehow at the root of it, maybe chemical resonance in a way, all ultimately due to the low mass of the electron and its long Compton wavelength? Wnt (talk) 17:08, 28 March 2013 (UTC)[reply]

Google

In responce to what is Google I was surprised that the word only had the mathematical definition of 10^100. For us early 1960's engineering and science students the word was then used (may have had slight difference in spelling like googel) to represent the number of atoms in the universe which at that time was 10^85. It has since been lowered to around 10^83. How come in the discussion of Google or the answer to the number of atoms in the universe there is no reference to this association. The math definition (Wikipedia response to Google)refers to the limit of calculators being 10^99 which occurred after the 1960 time period.

Is my recollection of old class notes incorrect?

Thank You Larry Oliva — Preceding unsigned comment added by 151.190.0.1 (talk) 14:06, 28 March 2013 (UTC)[reply]

Note that the number is spelled googol; "Google" is ~~an alternate corporate spelling for (I assume) trademark purposes~~ a corporate misspelling. A googol has always been defined as 10¹⁰⁰, without specific reference to any physical phenomena, but we note that it has frequently been used as a mental reference point for other very large quantities such as the number of subatomic particles in the universe. 10¹⁰⁰ isn't really a very good approximation for 10⁸⁵ (being off by a factor of, what, a quadrillion?), but it's a better approximation than any other number that you're likely to know a convenient name for (as illustrated by my uncertainty above regarding 10¹⁵, much less 10⁷⁰). — Lomn 14:14, 28 March 2013 (UTC)[reply]

For those who are interested: 10⁷⁰ is known as ten duovigintillion on the shotscale, and ten undecilliard on the long scale. Plasmic Physics (talk) 18:50, 28 March 2013 (UTC)[reply]

A googol is known as ten duotrigintillion and ten sexdecilliard respectively. Plasmic Physics (talk) 18:55, 28 March 2013 (UTC)[reply]

How do certain foods interfere with Levothyroxine?

Warnings on the bottle, our article, and everything I can find on the net indicates that levothyroxine should be taken on an empty stomach, half an hour before meals, because of interference with its absorption. I am wondering what the exact mechanism(s) of that interference is. Is, for example, the proper acidity of the stomach or intestine important? Does the drug have an affinity for certain foods they way fiber can absorb certain lipids?Is it only absorbed in the stomach or upper small intestine, with the presence of food preventing it from getting fully absorbed before it passes down the line? Does it react chemically with other substances, denaturing it? I can speculate, but am hoping for a referenced or authoritative answer, if anyone has one. Thanks. μηδείς (talk) 17:51, 28 March 2013 (UTC)[reply]

The fact that this drug binds to fibre etc., that can pass thought the human gut undigested is all perhaps the medic need to know (it is not explained in my pharmacology books either). Your only recourse (and perhaps a quicker one) might be to email or phone [2] and ask them: “what is it about this molecule that that inhibits it from being broken down once it has bonded to fibre”. I can guess, that enzymes can't get their hooks into the middle of it and release the iodine hormone due to to bonding strength, but as you say, you don't want a guess.Aspro (talk) 19:21, 28 March 2013 (UTC)[reply]

For some reason I couldn't open your url, but if you are telling me it binds to fiber that is fully comprehensible and the sort of answer I am looking for. I have also seen remarks about it interreacting with coffee and grapefruit juice. I am wondering if the drug itself is a peptide or peptide analog, which I assume would mean it's absorbed in the small intestine, or what other class of drug/nutrient it might be classified as or analogous with, such as an MAO inhibitor. μηδείς (talk) 19:28, 28 March 2013 (UTC)[reply]

Here is the link in long hand http://www.forestpharm.com/mic.aspx It states that For Product Inquiries Forest's Medical Information and Communication department can be reached at: 800-678-1605 ext 66297. According to this http://medlibrary.org/lib/rx/meds/levothroid-2/ Quote:”Synthetic T4 is identical to that produced in the human thyroid gland.” unquote. So, it looks like an exact peptide copy.--Aspro (talk) 20:02, 28 March 2013 (UTC)[reply]

Thanks, again, exactly what I was looking for. μηδείς (talk) 20:14, 28 March 2013 (UTC)[reply]

Resolved

What lifeform can regenerate the fastest?

^Topic. ScienceApe (talk) 19:29, 28 March 2013 (UTC)[reply]

The Doctor. Whoop whoop pull up ^{Bitching Betty | Averted crashes} 05:56, 29 March 2013 (UTC)[reply]

James Bond 007. 24.23.196.85 (talk) 06:04, 29 March 2013 (UTC)[reply]

I think yeast would be hard to beat. Looie496 (talk) 19:47, 28 March 2013 (UTC)[reply]

My vote goes to some thermophilic anaerobics [3]. Even band Wikipedia editors, trolls and Madonna can't regenerate that fast. --Aspro (talk) 20:10, 28 March 2013 (UTC)[reply]

From [ http://www.britannica.com/EBchecked/topic/48203/bacteria/272364/Growth-of-bacterial-populations#ref955453 ]: ", Clostridium perfringens, one of the fastest-growing bacteria, has an optimum generation time of about 10 minutes; Escherichia coli can double every 20 minutes; and the slow-growing Mycobacterium tuberculosis has a generation time in the range of 12 to 16 hours." --21:01, 28 March 2013 (UTC)

What do you mean by "regenerate"? The previous (good) answers are focused on population growth/ generation time. But if you're interested in healing or regeneration of lost organs or tissue, things like ~~planaria~~ planarians can grow new heads and tails if they need to. SemanticMantis (talk) 23:08, 28 March 2013 (UTC)[reply]

Wouldn't viruses be among the leaders in rapid replication? ←Baseball Bugs ^{What's up, Doc?} carrots→ 13:38, 30 March 2013 (UTC)[reply]

The OP said "lifeform". Whether viruses count as lifeforms is debatable. Bramble claw x 20:14, 30 March 2013 (UTC)[reply]

Do they replicate faster than what you would consider "true" lifeforms? ←Baseball Bugs ^{What's up, Doc?} carrots→ 22:20, 30 March 2013 (UTC)[reply]

Well, according to [4], 8 hours replication time is considered very quick, compared to E. coli which takes 20 minutes. But I wouldn't consider myself an expert on this. Bramble claw x 03:23, 1 April 2013 (UTC)[reply]