Wikipedia:Reference desk/Archives/Science/2013 July 5

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July 5

Electricity

when electric charges move in close loops, it produced what type of force — Preceding unsigned comment added by 196.46.246.59 (talk • contribs)

When the reference desk gets people's homework questions, it forces them to do it themselves. Try Electric current. 202.155.85.18 (talk) 06:01, 5 July 2013 (UTC)

...or Coil. SteveBaker (talk) 14:41, 5 July 2013 (UTC)

Can magnetic fields be like a laser?

Or at least, can be focus a magnetic field? I am aware that solenoids are somehow a way of concentrating a magnetic field, but it would still fall off at a rate of 1/r^3 as the distance increases. OsmanRF34 (talk) 11:43, 5 July 2013 (UTC)

You are making the common mistake of confusing laser light with collimated light; you are conflating focus with beam collimation. You can construct a device to collimate a magnetic field, but a static field cannot propagate as a wave, and the static magnetic field cannot be amplified as part of a lasing process. A laser operates to amplify light using stimulated emission. Nimur (talk) 14:55, 5 July 2013 (UTC)

Perpendicular recording shows one thing which is possible. One pole of a magnet can be smaller than the other, creating a field which is more concentrated near the small pole. - ¡Ouch! (^{hurt me} / _{more pain}) 16:04, 6 July 2013 (UTC)

Fern spore range

How far can ferns fire their spores, with a catapult like this [1]? I'm just interested in a ball park figure here, but can't find a number online. Cheers, Aaadddaaammm (talk) 12:20, 5 July 2013 (UTC)

The paper (PMID 22422975) says that the spores are ejected with a velocity of up to 10 meters per second. Taking into account air resistance, that's probably enough to hurl them a few feet -- although even a slight amount of wind could carry them much farther. Looie496 (talk) 14:38, 5 July 2013 (UTC)

I don't think we can answer this one.

According to our article, Polypodium aureum, the spores are mostly carried by the wind...the "catapult" may only be there to launch the spores far enough to get away from the wind-shielding effect of the frond it's attached to. At any rate, even with completely still air, the distance would depend crucially on the height above the ground and the angle at which it was launched - both of which are unknown. Even knowing the speed at which the spore is launched - the range is unknowable without a LOT more information about the nature of a specific launch event.

My guess is that since the spores are light, they could easily float in the wind for miles - that's true for the wind-born spores of many plants and fungi. SteveBaker (talk) 14:39, 5 July 2013 (UTC)

Many people study dispersal kernels of plants. That's a redlink, but see also Biological_dispersal#Quantifying_dispersal. I'd try searching google scholar for /dispersal kernel fern/ to get a feel for what's been discovered through empirical tests and theoretical models. SemanticMantis (talk) 16:08, 5 July 2013 (UTC)

Tension

I seem to have got into a muddle about this. If a string is in equilibrium, and there is a force F pulling in one direction at one end and a force F pulling in the opposite direction at the other end, then each point of the string has a force of F one way plus F the other, so why is the tension not 2F? Or is it? 86.176.210.160 (talk) 13:52, 5 July 2013 (UTC)

Think of it like this: That situation is identical to one in which a string is tied to a fixed object at one end with a force of F on the other because at the fixed end of the string, every action has an equal and opposite reaction. SteveBaker (talk) 14:31, 5 July 2013 (UTC)

Great answer. Often this problem is seen in finer clothing: a strongman stands on a runway between two planes trying to take off in opposite directions, with each hand holding a handle at the end of a rope tied to each plane. Surely this is harder than holding back just one plane with one comparable handle in one hand and one mounted to a wall in the other, right? :) (Of course, part of why our intuition on this is wrong is probably that we simply assume some kind of bracing, even when the premises of the problem rule it out) Wnt (talk) 15:11, 5 July 2013 (UTC)

Thanks, but I don't get it. That answer is just restating the same question. If a string is tied to a fixed object with a force F at the other end then there must be an opposite force F at the fixed end, just like I posited, otherwise the string would move. I understand that part fine. What I don't understand is why, when at each point on the string there is a force F in one direction and an equal and opposite force F in the other, why the tension is not F + F, or 2F. 86.176.210.160 (talk) 17:04, 5 July 2013 (UTC)

More to the point, "Why isn't the tension F + (-F) = 0", because forces are vectors, and forces of equal magnitude and opposite direction will exactly cancel out!

The descriptions above were all very good, qualitatively; but they have missed out on one piece of the explanation, which is that we define tension relative to the force that we can measure; and we can measure it only at the boundary condition of the string. We could have defined tension to equal twice the static force at the boundary condition; or π times the static force at the boundary condition; but those definitions don't help make the mathematical model any simpler.

If you should ever want to pursue a more advanced study of strings using calculus, you will use a new mathematical model that replaces the string tension - which is a scalar quantity that is just a statistical average over the whole string - and replace it with an infinitesimal restoring force at every point on the string, whose direction and magnitude is related to the infinitesimal deviation from a perfectly taut string; and whose period of oscillation is determined by the infinitesimal mass density of the string and the elasticity of the string. And you will have to integrate over the infinite number of infinitely-small strings to calculate the force on each differential element; the only constraints are the physical realities like the string's mass and its propensity for elastic deformation. That is how we mathematically derive the force on a piece of material that is inside the string - we can't directly measure it, because if we cut the string to put in a measurement tool, by definition we'd only be measuring the force on the end of the string we just cut! Suddenly a simple, scalar quantity that represents "tension" is looking pretty friendly, especially when it works out so well for statics! Nimur (talk) 17:24, 5 July 2013 (UTC)

So, defining tension as F is just a convention? I disagree, by the way, that defining it as π times the force at the end makes as much sense as defining it as twice the force at the end, but that's a secondary issue... if it's just a matter of definition then my main concern goes away. 86.176.210.160 (talk) 17:30, 5 July 2013 (UTC)

It would not be the first time physicists threw a pi at the universe just to make one specific form of their equation look prettier. Nimur (talk) 17:32, 5 July 2013 (UTC)

Well, think of how you measure tension, e.g. with a spring scale. You can attach the spring scale to one end of the line, and measure off the reading of the force you're putting on that end only relative to the force of a kilogram weight under Earth gravity. Or, you can cut the line and tie the spring scale in the middle of the line with some rope on either side. Either way, the spring scale itself has two ends (won't stay still without them!), and it measures the same result. For that matter, notice that even with your kilogram weight the same duality applies: the Earth is pulling down on it with that force, so why isn't it moving? Because the Earth is falling toward the weight's far punier gravitational field, but with far more mass, to have the same reaction as the spring! So ultimately there are two "ends" to any force you could put on something, provided it isn't actually accelerating. Wnt (talk) 21:12, 5 July 2013 (UTC)

polymers

Are most or all polymers solid at room temperature?--99.146.124.35 (talk) 17:46, 5 July 2013 (UTC) 10–1000 g/mol for simple chemical compounds;

You might want to read about melting polymers. It's a little bit unusual to think about a fluid of polymers. Nimur (talk) 17:48, 5 July 2013 (UTC)

Also, do most molecules with molar masses equal to or greater than 400g/mol tend to be solids at room temperature? What about all molecules >1000g/mol?--99.146.124.35 (talk) 18:11, 5 July 2013 (UTC)

Polymers occupy a region of solids between that of molecular compounds and network solids. Compounds composed of small molecules have relatively weak intermolecular interactions, and so tend to melt rather readily, often near or even below room temperature. At the other end of the spectrum, network solids are composed of massive networks of very strong covalent bonds (the same type and strength of bond holding individual molecules together in a molecular compound like water: that is the force holding the "H" to the "O" and not the force holding the individual H₂O molecules together, which is quite weak comparitively). These network solids like diamond and carborundum don't really have proper "melting" points as they don't really "melt" per se. They do break up into smaller bits, but this is really more of a Chemical decomposition than proper melting. Polymers lie somewhere between these two types of substances. Polymers are composed of discrete molecules, unlike network solids, but the molecules are so large and intertwined that they don't readily break apart as a simple molecular compound does (like water) so they don't really have a discrete melting point, rather they have a temperature where they get soft enough to flow, similar in some ways to the Glass transition that occurs in glass, which is also a type of material that doesn't fit neatly into the elementary school "solid-liquid-gas" classification scheme. --Jayron32 18:58, 5 July 2013 (UTC)

Affinity constants

I've lost track of the scale of association/dissociation constants for medications. There are so many. For instance, the binding profile of risperidone is:

D1 dopaminergic receptors (Ki = 536 nM)[20] D1 receptor antagonist D5 receptor antagonist D2 dopaminergic receptors (Ki = 3.13 nM) D2 receptor antagonist D3 receptor inverse agonist D4 receptor antagonist

This information comes from a wikipedia article on the medication.

My questions are: Which affinity is tighter (stronger): Ki = 3.13 nM for D2 receptors or Ki = 536 nM for D1? I think the lower number represents the stronger bond, correct?

Also what does "i" stand for?

Thanks, - Alex168.178.74.86 (talk) 20:34, 5 July 2013 (UTC)

Lower values indicate stronger binding. You may find Receptor_antagonist#Affinity and IC50#Competition_binding_assays useful. The dimension of K_i is a concentration; one crude way to remember this sort of thing is that the K_x is the concentration (of something) at which its x is half-maximal. While this mnemonic may be imperfect, it's pretty useful. In this case, K_i is the concentration of an inhibitor (or binding antagonist/competitor) at which inhibition (binding) is half-maximal. With this relationship in mind, it is intuitive that 100-fold lower K_i would indicate a much more potent inhibitor. -- Scray (talk) 21:38, 5 July 2013 (UTC)

Here is a paper with some useful math, though the online calculator to which it links seems to be down. -- Scray (talk) 21:42, 5 July 2013 (UTC)

Thank you for the comprehensive answer. - Alex168.178.74.86 (talk) 21:53, 5 July 2013 (UTC)

Is Neptune habitable?

This does have the sound of a stupid question, yet... according to our article, Neptune has a reasonably comfortable equatorial gravity around 1.14 g. It has a core at up to 5,400 K, lying under a vast mantle which is variously described as "icy", "hot, highly dense fluid", and "water-ammonia ocean". True, at Earth atmospheric pressure its temperature is about -346 degrees Fahrenheit. [2] But so what? Lots of stuff on Earth lives at more than a few atmospheric pressures.

The problem is, I'm seeing stats in thousands of degrees for some unspecified point in the mantle, which is apparently 10GPa of pressure or (much much) more; but not quickly finding statistics for what the temperature is at pressures more like 1 MPa (Mariana Trench level). Can someone point at a pressure-temperature curve?

I don't know how feasible it is to try to test (or even breed) abyssal Earth lifeforms at super-Mariana pressures, in the presence of ammonia and other strong reducing agents... Wnt (talk) 21:58, 5 July 2013 (UTC)

The gas giants don't have surfaces to inhabit. I assume that means uninhabitable in your book. μηδείς (talk) 23:38, 5 July 2013 (UTC)

He probably asks whether there is liquid water between this gas surface and the core. OsmanRF34 (talk) 23:41, 5 July 2013 (UTC)

Check [| Liquid Water Oceans in Ice Giants]. If there is liquid water on Earth somewhere, there there is life. So, if there is liquid water in Neptune, then, life could be possible. However, liquid water oceans are not very probable there. So, try better a satellite of Saturn or Jupiter for a better chance. OsmanRF34 (talk) 23:41, 5 July 2013 (UTC)

There may be liquid water, but there will neither be a solid nor a liquid surface. Someone better at the physics can explain that, since my understanding of it is only intuitive. Most theories of life have it originating on a solid surface either at sea level or by hydrothermal vents. Think Yellowstone. Neptune will lack such environments. μηδείς (talk) 02:24, 6 July 2013 (UTC)

There are differentiated oceans on the all gas giants, however, these oceans are supercritical oceans. As you descend through the supercritical ocean of a gas giant, the composition gradually varies, perhaps even containing supercritical water at a certain depth range. Plasmic Physics (talk) 02:58, 6 July 2013 (UTC)

Yes, and supercritical means non-liquid, no? Just a gradual transition from a gas to a solid state as you descend, never reaching a discrete surface like the interfaces of land, sea and air on the Earth. (I am not even sure if the concept solid applies either--someone can qualify.) μηδείς (talk) 03:04, 6 July 2013 (UTC)

No, the supercritical phase is part-liquid, part-gas, not solid, the supercritical-solid phase boundary is distinct not gradual. While the s.c. phase is not the same as the liquid phase, it shares many of its liquid properties. Plasmic Physics (talk) 03:10, 6 July 2013 (UTC)

So you are saying that there will be a solid surface, somewhat like one you could imagine between the earth's crust and the bottom of a boiling ocean? μηδείς (talk) 03:23, 6 July 2013 (UTC)

The portion of Neptune that is made of supercritical water/ammonia would have a combination of properties found in both solids and liquids. It could certainly be described as "fluid", and is capable of solvating other compounds, just as liquid water can. However, this fluid exist at extremely high pressures and temperatures (thousands of degrees). So the temperature is certainly too high for "life as we know it". Not sure what the pressure is in the "ocean", can't find a number on it. For comparison, and in response to an earlier question, the pressure at the bottom of the Marianas Trench should be about 100MPa. And in response to Medeis, yes, there is a solid core beneath the fluid ocean (suspected). It would probably be made of rock and iron, be extremely pressurized, and very hot. Also, to one of Wnt's original comments, "icy" does not mean "cold" in planetary science, in case that's what you thought. I don't know why. Someguy1221 (talk) 04:08, 6 July 2013 (UTC)

Would not the "solid" core go from a molten to a crystalline phase? There certainly wouldn't be what we think of as "rock", would there? (I.e., by "rock" you mean silicate, but not minerals in the form we are familiar with on the Earth's surface.) Do we have a schematic image we can link to? μηδείς (talk) 04:47, 6 July 2013 (UTC)

I think there's some confusion here - so far as I know, there shouldn't be anything solid about supercritical water; it's only a question of whether it is liquid or gas. As far as I know, supercritical fluids tend to be less viscous than real liquids. Nonetheless, whether you call it liquid or gas, a living organism might plausibly float in liquid water, if it could take the heat. Wnt (talk) 06:53, 6 July 2013 (UTC)

On another note, does Neptune produce strong ionising radiation like Jupiter and Saturn? Their levels are far too high for Earthen life to survive, even for microbes. However, if we throw Earthen chauvinism out the door, then nearly anything is possible I suppose. Plasmic Physics (talk) 03:17, 6 July 2013 (UTC)

I'm all for throwing Earthen chauvinism out the door, but not the laws of physics. The problem is that almost by definition, life is made up of large molecules, and large molecules can't exist at high temperatures because the high-energy collisions between molecules would rip apart chemical bonds in a fraction of a second. --Bowlhover (talk) 06:45, 6 July 2013 (UTC)

Osman: great link. I gotta say, I thought I was going out on a limb asking if Neptune was habitable, but believe you me, the answer I was expecting was not to read that Neptune is too hot and dry for liquid water. I mean, I started out making a little edit on Gliese 667 C about the habitable zone, and getting skeptical... but if Neptune could have a liquid water ocean if it were but a bit cooler, what does that say about "super-Earths" that can go up to more than half its mass? There's something kind of wonderful about it really, if this is true - it means that the universe, so dourly pigeonholed for so long into warm and cold, rock and gas giant, has finally struck back with a declaration of independence. The way they speak of the planet possibly gaining one if it cools to 30 K ... I wonder if even the rogue planets wandering the trackless voids could have oceans of life.

Adapting bacteria to a salty water-ammonia ocean at thousands of degrees would be difficult to imagine, I admit. Nonetheless I should point out that hyperthermophiles can survive autoclave conditions, at least ... and I wonder how well such conditions have been sampled. In subduction deposits of water reach the supercritical point, very slowly ... I wonder if sufficient exploration could find bacteria that had evolved to deal with that? Wnt (talk) 06:49, 6 July 2013 (UTC)

The term "habitable" is to undefined to answer the Question. Befor 1980 any Biologist would have told you that 200 °C cooking water is unthinkable as Habitat for higher lifeforms and then they found life around Black smokers in deep sea capable of staying alive even in 400 °C cooking water. --Kharon (talk) 09:28, 7 July 2013 (UTC)

So far as I know the life around the black smokers is further out, at much colder temperatures. However, I don't regard this as proof that adaptation could not occur, because smokers are transient on a geological/evolutionary time scale, and dispersal (survival) of adapted organisms through normal cold seawater might be difficult. That said, there probably ought to be some kind of microbe observable flowing out from the deeper reservoirs of supercritical water with some low frequency, if life can adapt to those conditions, and so further research into these formations ought to find it out sooner or later. I think. Wnt (talk) 17:05, 7 July 2013 (UTC)

I should add that the thermal stability issue Bowlhover pointed out is, of course, central. Taking a random protein (RNase A) and subjecting it to 205 C leaves some hydrophobic pentapeptides untouched. [3] There's a reasonable catalogue of compounds from black smokers [4] It is perhaps plausible for life to evolve under such conditions, but clearly no easy task! Wnt (talk) 17:18, 7 July 2013 (UTC)