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December 16

principal stress

"Hydraulic fracturing in rocks takes place when the ﬂuid pressure within the rock exceeds the smallest principal stress plus the tensile strength of the rock".

What does this mean? Does principal stress increase with depth? When fracture occurs -- is there negative feedback inhibiting further fracture? Or is there positive feedback (other than that there are now more cracks for fluid). elle _{vécut heureuse} ^{à jamais} (be free) 00:37, 16 December 2011 (UTC)[reply]

Does Stress_(mechanics)#Principal_stresses_and_stress_invariants help? --Jayron32 04:14, 16 December 2011 (UTC)[reply]

It's literally all Greek to me. I don't understand tensors. I'm a biochemistry student. Basically, what governs crack propagation in hydraulic fracturing deep underground? At what point does the rock stop fracturing? elle _{vécut heureuse} ^{à jamais} (be free) 04:44, 16 December 2011 (UTC)[reply]

Hey, if it were greek I may have understood more of it. I'm a chemistry teacher. I also never got to much of the higher algebras either. It was what I could find on the topic when I searched. Knowing what you are looking for, does the Wikipedia article Hydraulic fracturing or any references therein help? --Jayron32 04:49, 16 December 2011 (UTC)[reply]

It is basically just a fancy way of saying that a rock will break when the force applied to it is greater than the minimum force required to break the rock. Fractures propagate easily within a single crystal / rock, but geology is sufficiently heterogeneous that such advantageous fracture growth tends to be limited in extent. In general, when you apply a hydraulic overpressure to a geologic formation the effective overpressure will decrease with distance from the well (often as something around 1/r), which limits the distance over which hydraulic fracturing is generally possible from a single well. Dragons flight (talk) 08:53, 16 December 2011 (UTC)[reply]

Because of the way that rocks are laid down, there is often a direction of least stress. For example a rock might have been formed from sand and shale particles laid down in a prehistoric river. Over centuries the sand which would later form a rock would tend to become oriented in the direction of the river. After the sand in the now dry river bed is buried and compacted to form rock, secondary effects start to happen. The rock will may be tilted, or bent by faulting and mountain uplift. The tilting and bending will usually be in different directions than the river, so the rock ends up with a variety of stresses. Additionally, due to the weight of the rock above it, other stresses will be created. So, what that statement means is the rock will fracture in its weakest direction. If the rock has been bent enough by geologic forces, there will be millions of microfractures in the rock, which will also help start fractures.

When someone attempts to hydraulically fracture rock, they will first run sensors to try and determine the least stress direction and mechanical properties of the rock. They then create a model of the pressure required and the direction that the fractures should go. They use casing and perforations to control where the fractures began and then start pumping fluid downhole. When the pressure exceeds the strength of the rock, the weakest spot cracks open. The surface pressure will suddenly drop and the frac engineer will know that the fracture has started. They then play with pressure, pump rates and what types of sand that they pump downhole to try and extend the cracks. As Dragon flight says, the pressure decreases with distance, due to friction between the fluid and the rock and also due to an increasing surface area. Since pressure is force per unit area, if the area increases it takes more force to keep the same pressure. Eventually it becomes too expensive to open the fractures up any more, and the process stops.Tobyc75 (talk) 18:37, 16 December 2011 (UTC)[reply]

That quote originally comes from here, but not all of the relevant parts are visible (at least to me), so it lacks a full explanation. To produce a tensile fracture (that is a fracture in which the direction of opening is perpendicular to its length - also known as an 'opening mode' or 'Mode I' fracture in fracture mechanics) at depth in a wellbore (borehole), it is necessary to overcome the effects of the weight of the pile of rock sitting above, which produces what is referred to as a confining pressure. By raising the pressure of fluid it offsets the confining pressure until the rock eventually reaches the necessary condition for tensile fracture to happen. This happens naturally in the formation of mineral veins, where the high fluid pressure comes from metamorphic reactions that produce fluids, particularly water. The fracture orientation will be in the plane of the maximum and intermediate principal stresses and perpendicular to the minimum stress direction. Accidental or deliberate hyrofracturing is sometimes used in boreholes as a way of determining the orientation of the stress field at depth. Mikenorton (talk) 20:38, 16 December 2011 (UTC)[reply]

the meaning of isogene and examples are welcomed

Hi,everyone,

What i am puzzled was the exactly meaning of the term "isogene".

Dose anyone give me an answer? An detailed explanation will be appreciatedLiujem (talk) 04:33, 16 December 2011 (UTC)[reply]

There are several unrelated concepts which use similar terms:

Isogeny is a mathematical term from Algebraic geometry, that refers to a method of mapping one object to another.
Isogenicity is a synonym for Zygosity, which is a genetic term refering to the relationship between genes on the two homologous chromosomes in a genome.
Isogenic human disease models are simpler genomes used to study various diseases, especially cancer.

Does any of those help? --Jayron32 04:44, 16 December 2011 (UTC)[reply]

Isogene has two meanings: (1) a line on a map showing the distribution of a gene, by analogy to an isobar, isotherm, etc[1] (2) a copy of a gene that occurs multiple times in an organism's genome - this sense is common in biology[2][3] but I can't find a reference for a definition. --Colapeninsula (talk) 14:31, 16 December 2011 (UTC)[reply]

Gas cloud entering black hole in milky way

Would this be visible to any extent other than massive space telescopes; http://4.bp.blogspot.com/_YuR6V_Yr7Bk/S_0PvCTAelI/AAAAAAAAFF4/wNBkqw_INTM/s1600/black+hole.jpg Something like what the link looks like? — Preceding unsigned comment added by 109.224.25.14 (talk) 06:41, 16 December 2011 (UTC)[reply]

There's no fundamental reason why you couldn't see it if conditions were favorable. Such observation depends, particularly, on how far away the black hole is. Assuming that we're talking about something with the approximate visible-spectrum luminosity of the sun, it would have to be within 50 light years or so to be naked-eye visible. Mostly, though, such phenomena radiate in the x-ray spectrum, which you're not going to see except via specialized equipment. — Lomn 13:48, 16 December 2011 (UTC)[reply]

The centre of the Milky Way is heavily obscured by dust along the line of sight. At visible wavelengths (i.e. those that the human eye can register), essentially everything is absorbed. At infrared wavelengths, the absorption is much less severe which is why scientific observations of the centre of the Milky Way, such as the one that identified the gas cloud, are done in the infrared, which is essentially a matter for professional telescopes (not necessarily space-based - the interesting observations are made from the ground, notably with ESO's Very Large Telescope, and adaptive optics). In addition to X-ray emission, there may also be radio emission, presumably on a longer time scale, though. --Wrongfilter (talk) 14:53, 16 December 2011 (UTC)[reply]

how are promoter sequences discovered?

A rather urgent question-- thanks!

You mean, as (concisely) described here? --Ouro (blah blah) 12:45, 16 December 2011 (UTC)[reply]

Celsius vs. Kelvin

what ias thew relation bitwin Celsius and Kelvin degree — Preceding unsigned comment added by 77.28.22.143 (talk) 14:49, 16 December 2011 (UTC)[reply]

The Kelvin is exactly the same as a degree Celsius. See Kelvin#Use_in_conjunction_with_Celsius. The difference is where the two scales start. 0 Kelvin means absolute zero, but 0 degrees Celsius is the freezing point of water (at least historically, there are some minor modern modifications to the definition of 0 degrees C). Note that "degrees Kelvin" is not proper usage, a Kelvin is the unit. For example, we say "100 Kelvin", but "100 degrees Celsius". SemanticMantis (talk) 15:09, 16 December 2011 (UTC)[reply]

... so, just in case this is not clear from the linked articles, to convert degrees Celsius to Kelvin, just add 273.15 Dbfirs 17:13, 16 December 2011 (UTC)[reply]

Looking for details behind a Mr. Wizard science trick

One time when I was a kid, I was watching Mr. Wizard on TV and he did this thing where he took one clear liquid and poured it into a pitcher holding another clear liquid, mixed them around a little (the mixture stayed clear), and then had a kid assistant hold the pitcher high while pouring it into another pitcher, and he counted down "three, two, one..." and clapped his hands (for dramatic effect, not that that has anything to do with it), and all at once, the liquid in the bottom pitcher, the liquid in the pouring arc, and the liquid still in the pouring pitcher, all turned a dark purple in an instant. What were those two liquids? 20.137.18.53 (talk) 15:07, 16 December 2011 (UTC)[reply]

That's an iodine clock. Classic, performed it a few times myself :) Grandiose (me, talk, contribs) 15:11, 16 December 2011 (UTC)[reply]

Thanks!20.137.18.53 (talk) 15:14, 16 December 2011 (UTC)[reply]

Damastes--redirects to Procrustes, but recommends "Huntsmen Spider"

Hello!

I am just wondering why "Damastes," one of the names for Procrustes, is suggested in addition to "Procrustes."

What is the connection between the word "Damastes" and the Hunstman Spider?

Or between proctrustes and the Huntsman Spider?

THANK YOU SO MUCH!

Jennifer — Preceding unsigned comment added by 98.97.183.38 (talk) 17:41, 16 December 2011 (UTC)[reply]

If you see the Huntsman spider#List of genera section, you can see that Damastes is also a genus of Huntsman spider. We don't yet have an article about the spider genus. -- Finlay McWalterჷTalk 19:01, 16 December 2011 (UTC)[reply]

If you're curious as to why there's a spider genus called Damastes (which seems to contain at least the species Damastes nossibeensis), we don't appear to have that information (and what little non-wikipedia info I can find on the spider online doesn't help either). -- Finlay McWalterჷTalk 19:10, 16 December 2011 (UTC)[reply]

It's a good question. The opening of the Procrustes article is a bit garbled and hard to follow, and could probably do with rewording. The Damastes page should possibly be made a Disambiguation page rather than a Redirect, and then the Damastes redirect template in Procrustes could be removed. Might do this later. --jjron (talk) 05:28, 17 December 2011 (UTC)[reply]

I've started the page for the genus in Damastes (spider) and tweaked the hatnotes for the articles.-- Obsidi♠n Soul 17:58, 17 December 2011 (UTC)[reply]

As for etymology, I think it's pretty obvious that the spider is named after the greek mythological figure. Happens a lot in scientific nomenclature. Procrustes itself is also a subgenus (previously a full genus) of the carabid (ground beetle) genus Carabus. However, subgenera do not generally get articles so a hatnote is not needed for it. -- Obsidi♠n Soul 18:09, 17 December 2011 (UTC)[reply]

My last two sentences read like tongue-twisters. :D -- Obsidi♠n Soul 18:56, 17 December 2011 (UTC)[reply]

Thank you, everyone. I am very grateful not just for the information, but also for the conversation! Do you think there is any way to find out who named the spider after Procrustes? I am writing a novel about Shakespeare that uses the metaphor of the Procrustean Bed . . .

Extracting work from a positive current

So in a fuel cell, hydrogen is ionized into a proton and an electron and the electron then passes through a circut and does work. Ok fine, but the proton passes through an electrolyte and doesn't do any work. Why not? Why can't we extract work from the proton? Is it possible? ScienceApe (talk) 18:02, 16 December 2011 (UTC)[reply]

You can extract work from such an electric current. It just so happens that in the practical setups you're typically thinking of, the efficiency would be very low. This is because in most configurations, the maximum current density that can be carried by positive ions is very low.

If you could construct a large current of positive ions flowing through an electrolyte, you could generate heat (for the same reasons that electron flow produces waste heat), and you could, e.g., drive an incandescent light. You could use positive ions as carriers of an electromagnetic wave in an antenna, and propagate a wave (converting energy from the current into electromagnetic energy carried away as a propagating wave). You could, with some effort, build devices that operate with positive ions flowing through wet electrolytes, but otherwise behave analogous to the ways that we extract energy from electron currents in copper wires. You could even create a terrifically inefficient wet-chemistry semiconductor, limited only in practicality by its poor noise, power requirements, and frequency response - but not fundamentally very different from a solid-state semiconductor.

In most cases, including most plasmas and most wet electrolyte solutions, positive charge carriers are less mobile than negative charge carriers; so for optimum efficiency, we use electrons to carry current. This invariably just comes down to the fact that protons are more massive than electrons. Nimur (talk) 18:25, 16 December 2011 (UTC)[reply]

It's a little disingenuous to say that you extract work from the electron current and not from the proton current. You can't have the electron current unless you also have the proton current. In reality you're extracting work from the entire system (the complete circuit). There's only a limited amount of energy to extract from the conversion of a set amount of hydrogen, so what you're likely to find is that if you try to extract energy from the proton current directly, the current and/or voltage of the electron current will go down, reducing the amount of work you're able to extract from it. Given the difficulty of extracting from the proton current directly, you'll likely find the best way to extract the greatest amount of energy from the system is to place the load in the electron current, and minimize the resistance in the proton current path. -- 140.142.20.101 (talk) 19:49, 16 December 2011 (UTC)[reply]