Wikipedia:Reference desk/Archives/Science/2015 August 30

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August 30

Dealing with the polarity of a DC plug

Normally, DC connectors, contrary to AC plugs, force the user to connect in a certain way.

If it were not possible to assure physcially that the device gets connected to the power source at the right polarity, how could we deal with it?

Could an electric component just "guess" what pole is connected to what cable? Could a device just adapt cable A (or B) to the + or - that it's getting? That would imply that it does not matter whether the user connects A to + or A to -. --Scicurious (talk) 12:22, 30 August 2015 (UTC)

You could use a Diode bridge as it will force the polarity to the connected device stays constant even if the source is changing its polarity like AC or swapped DC cable 139.228.134.204 (talk) 12:43, 30 August 2015 (UTC)

Indeed. But then I've never seen a DC power connection designed for consumers that wasn't physically asymmetrical, if only in color. Greglocock (talk) 19:25, 30 August 2015 (UTC)

A bridge will only work if the device can use a floating supply - if the zero volt line doesn't need to be connected to ground anywhere. It's more usual to just put a single reverse-protection diode on the input; the device won't work if the polarity is wrong, but it won't be damaged either. Tevildo (talk) 22:43, 30 August 2015 (UTC)

Side comment: actually many AC plugs to do force the user to connect in a certain way. If the power supply has one "hot" wire (say at 120 or 240 volts) and one "cold" (maintained near neutral, for the return current), then a polarized plug allows the hot wire to be routed directly to the device's "on/off" switch and therefore minimizes the amount of wiring that is live when the switch is off. Designs that force polarization include North American plugs of the type with unequal flat pins, Australian plugs with flat pins at different angles, and various designs that also include an asymmetrically placed ground pin. See AC power plugs and sockets for further details. --65.94.50.17 (talk) 18:50, 30 August 2015 (UTC), copyedited later

ROC curves

Sorry about cross-posting, but I suspect that some of the contributors to the Science Desk would be able to help me with a question that I've posted at the Maths desk. Please answer at Wikipedia:Reference_desk/Mathematics#ROC_curves. Thanks. NorwegianBlue ^talk 14:36, 30 August 2015 (UTC)

Does a brown dwarf ever have a rocky core?

I'm thinking right now of cases where no deuterium has been fused, since I suppose if deuterium were fused, the core would be gaseous and the full convection would at least eventually disperse any heavy elements thruout the interior. Thanks.Rich (talk) 20:21, 30 August 2015 (UTC)

We wouldn't even say the earth has a rocky core, since "rock" implies solid minerals cool enough to crystalize, or at least form glass, like obsidian. In any case, the lead of the Brown Dwarf article itself says: "Brown dwarfs may be fully convective, with no layers or chemical differentiation by depth." μηδείς (talk) 20:33, 30 August 2015 (UTC)

• Well you are likely right in one sense, but rocky core apparently need not literally mean what you think it should in this context. For example, from the wikipedia article on Uranus, which should have pressure and perhaps temperature at the center that are much higher than Earth's:"The standard model of Uranus's structure is that it consists of three layers: a rocky (silicate/iron–nickel) core in the centre,...". So in my question, I just used the term that I thought was being used by the people apparently expert in the field. Also, my question did mention full convection, which is not known to always be present, expecially without deuterium fusion, which is the situation I stressed.2601:681:4902:31B8:85D6:C4DA:397F:4283 (talk) 05:47, 2 September 2015 (UTC)

This is not at all my field (address another if you want a better response), and I do understand what you mean about the sun's planets having rocky cores. But the three-million degree core temperature addressed below and the convection pretty much guarantee that all you will have is plasma, perhaps with some gasses and precipitates at the surface. μηδείς (talk) 16:34, 3 September 2015 (UTC)

• Look, what you have just said doesn't make any more sense that what you said the first time. Although you usually provide good answers in subjects you are more familiar with, I wish you would either do more research before answering when you aren't sure that you know, or just desist from answering in such situations.-RPeterson2601:681:4902:31B8:108D:A20D:4669:1153 (talk) 04:49, 4 September 2015 (UTC)

According to our article, brown dwarfs are defined by a core temperature less than about 300 million degrees. Clearly this is much, much more than rock can handle; it would be reduced to plasma. Brown dwarfs could contain some silicon, oxygen etc. but they are going to be utterly dwarfed by the amount of hydrogen present, which cannot be blown away by solar wind like on a rocky planet, and also the convection mentioned above would ensure whatever is present would stay distributed, I think. See rocky planet for the fairly loose definition of "rock". Wnt (talk) 20:41, 30 August 2015 (UTC)

• Yes, but consider your words "less than". Some brown dwarfs might have temperatures considerably below 3 million, which is your revised figure you gave below. Also, the full convection may not happen for some brown dwarfs, especially for smaller ones which have mass not much greater than superjupiters.2601:681:4902:31B8:85D6:C4DA:397F:4283 (talk) 05:55, 2 September 2015 (UTC)

Actually 300 thousand degrees. Ruslik_Zero 05:39, 31 August 2015 (UTC)

D'oh! We're both misreading it. It has <= 3 x 10⁶, which is 3 million degrees. I need new lenses because I thought that was an 8, should have known it was wrong though. Wnt (talk) 22:16, 31 August 2015 (UTC)

Could extreme pressure increase the radioactivity of a radioactive element?

The extreme pressure could perhaps be from a diamond anvil, or in the core of a exoplanet larger than jupiter or a brown dwarf. Recently in Physics News it was reported that Osmium has been pressurized in a diamond anvil to the extent that inner shell electrons interact.(I wouldn't mind getting more information on how they interact, is it intra-atom of just between the inner electrons in an atom?) I'm wondering if very great pressures could cause heightened fission, rather than fusion, in the cores of large exoplanets and in brown dwarfs becoming an important source of heat and energy. Thanks.Rich (talk) 20:45, 30 August 2015 (UTC)

For this to happen the pressure should be comparable to that inside neutron star. Ruslik_Zero 05:34, 31 August 2015 (UTC)

Well, it depends entirely on what one means by "Radioactivity". Extreme pressures were used in early nuclear bombs to decrease the critical mass necessary to create the nuclear chain reaction and cause the big BOOM. Implosion_(mechanical_process)#Nuclear_weapons discusses it briefly. It doesn't actually change decay rates, which is what is normally meant radioactivity, but it does increase the rate at which neutron-induced nuclear fission occurs. --Jayron32 12:43, 31 August 2015 (UTC)

Your heading asks about "radioactivity", but your question seems to be about nuclear fission. The term "radioactivity", by itself, usually refers to radioactive decay, which according to current theories is completely random at the level of an individual nucleus. There's no known way to influence when it happens. Fission and fusion, on the other hand, can have their reaction rates changed by changing how close atoms of the fuel are to each other. In nuclear energy, this is called the "geometry" of the reactor/fuel assembly/etc. As Jayron32 pointed out, this is an essential part of nuclear weapon design: nuclear weapons work by extremely rapidly turning a sub-critical assembly of fuel into a critical mass. It's also central to how stars work. Stars are in a constant tug-of-war between the gravitational attraction trying to collapse the star inward, and the heat and radiation pressure from the nuclear reactions in their cores pushing the star's material outwards. During most of a star's life, this is a self-balancing equilibrium: if gravity squeezes the star inward, it increases the reaction rate in the core, which increases the forces pushing the star outward, and vice versa.

Anyway, back to your question. I suppose it's theoretically possible that fission could become an appreciable source of heat in a large planet's core, but I doubt it's that common. Fissile elements are rare in the universe, because they're only created in supernovas. For rocky planets like Earth, most of the core is iron and nickel. Now, the thing is, gas giants' cores are actually quite poorly understood. It's understandably kind of difficult to peer deep into them! Much of what we know about Earth's internal composition we gained from observing how seismic waves travel through the Earth. This was helped by the fact that we live on Earth. To do something similar on a gas giant we'd need to send a bunch of sophisticated probes that could observe pressure waves traveling through their atmospheres. But anyway, my non-expert answer would be that, based on looking at rocky planets' cores, fission just will never become a sizable contributor to the core's heat production. Like I said, fissile elements are rare in the universe, because their production consumes energy. Iron and nickel are the most prevalent "heavy elements" in the universe because they're at the peak of the nucleon binding energy curve. Fusing these nuclei into even heavier elements consumes energy rather than releasing it. In stars large enough to produce nickel and iron, these elements gradually build up at the core until the core no longer produces enough energy to keep the star from gravitationally collapsing, at which point it dies in a supernova. Only in supernovae are the heavy fissile elements created. So this is why rocky planets' cores are mostly iron and nickel: they're the most common heavy elements, which sink to the core when the planet is still molten. --71.119.131.184 (talk) 00:50, 1 September 2015 (UTC)

• But I have often read what I thought was saying that the Earth's core is still much hotter than it would be otherwise, after 4 billion years, if not for the heat created by fission of uranium, radium etc?-Rich Peterson2601:681:4902:31B8:85D6:C4DA:397F:4283 (talk) 06:05, 2 September 2015 (UTC)
  • Earth's internal heat budget says about half the earth's internal heat comes from radioactive decay. DMacks (talk) 06:22, 2 September 2015 (UTC)

• • I probably should have stressed in my question that I meant some new phenomena for fission, related to pressures, aside from "critical masslike" or "chain reactionlike" phenomena due to random variations in elemental concentrations in a core(or mantle), which for example I have read may occur within the Earth.2601:681:4902:31B8:85D6:C4DA:397F:4283 (talk) 06:17, 2 September 2015 (UTC)

Fission and fusion are not the only nuclear reactions that might be affected by density (so indirectly affected by density). Any nuclear absorption process may be affected by the density of the material, even those that do not lead to fission/fusion events, such as neutron capture and the p-process, due to changes in the concentration of nuclear reactants. High temperature can also influence the emission of electrons and positrons from nuclei, through a spontaneous pair production process, but only at temperatures so high the black body spectrum peaks in the gamma range (you won't find such temperatures inside a planet, but they can happen at the cores of some stars). Someguy1221 (talk) 02:16, 1 September 2015 (UTC)

• As perhaps in a brown dwarf star, or a superjupiter? Or in some future version of a diamond anvil? -Rich Peterson2601:681:4902:31B8:85D6:C4DA:397F:4283 (talk) 15:08, 2 September 2015 (UTC)

According to the multiply-cited content in Radioactive decay#Changing decay rates, there are some simple radioactivity processes that are indeed influenced by certain environmental factors. DMacks (talk) 02:34, 1 September 2015 (UTC)

The actual article underlying the news report was presumably:

L. Dubrovinsky; N. Dubrovinskaia; E. Bykova; M. Bykov; V. Prakapenka; C. Prescher; K. Glazyrin; H.-P. Liermann; M. Hanfland; M. Ekholm; Q. Feng; L. V. Pourovskii; M. I. Katsnelson; J. M. Wills; I. A. Abrikosov (2015). "The most incompressible metal osmium at static pressures above 750 GPa". Nature. doi:10.1038/nature14681.

And it's indeed the inner-core electrons within an atom. The question "is it intra-atom of just between the inner electrons in an atom?" doesn't make sense--maybe it was meant to be inter-atom? DMacks (talk) 02:30, 1 September 2015 (UTC)

• Yes I should have written "inter-atom, or", not 'intra-atom of'.2601:681:4902:31B8:85D6:C4DA:397F:4283 (talk) 06:10, 2 September 2015 (UTC)

The authors mention that the anomalous interaction between core electrons within an atom influences the chemical bonding between adjacent atoms, but they are not direct participants in these bonds. Someguy1221 (talk) 02:39, 1 September 2015 (UTC)

Hmmm, just an observation that the non-fusion objects in the solar system are full of higher order elements while the sun is not. Is there Iron in the sun? If not, why not? --DHeyward (talk) 03:53, 1 September 2015 (UTC)

Our Sun is 0.2% iron. DMacks (talk) 04:23, 1 September 2015 (UTC)

I believe it's more an issue of the planets not retaining so many of the lighter elements like hydrogen and helium. The sun contains over 99% of the mass of the solar system, so the elemental composition of the solar system is essentially the elemental composition of the sun. All planets would have started with the formation of a rocky core, as heavier elements will sink to the center, but the inner planets were never heavy enough to gravitationally bind a significant helium/hydrogen atmosphere. As a result, all of that gas in the inner solar system eventually got blown away by the solar wind, leaving behind the rocky planets with minimal atmosphere. The outer planets have a composition much closer to that of the sun and thus the original gas cloud that formed our solar system as they were large enough to gravitationally bind their light-gas atmosphere. There is more information at formation and evolution of the solar system. Someguy1221 (talk) 04:25, 1 September 2015 (UTC)

Pressure could affect atoms decaying via electron capture. ⁷Be decay rate could have a measurable effect if the environment changed. Graeme Bartlett (talk) 02:52, 5 September 2015 (UTC)