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sethoflagos

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Posts posted by sethoflagos

  1. Personally I take my tea strong, black and well stewed. The more astringent the better within reason.

    But it seems there are some folks who just can't get enough blandness into their lives. Salting tea sounds to me as doolally as decaf, but whatever floats your .

  2. 4 hours ago, Carrock said:

    As you've mentioned reciprocity a few times in this thread, just a clarification that reciprocity is not always valid.

    I mentioned it once in passing. It isn't a cornerstone of my argument.

    While grateful for your advice, I'm at a loss to see how it applies to the optical rectennas under discussion.

  3. 6 hours ago, exchemist said:

    OK, however I am  not clear now what the relative proportions of Na and K would have been in the Earth's crust , or crust + mantle, before the oceans formed. Without that information it seems hard to determine whether the reason for the difference in concentration of today's seawater is due mainly to the original composition of the rocks or to the differential leaching that we have been discussing.

    On reflection, I'm inclined to think that it's a bit of each. The salinity of the Hadean oceans would no doubt have reflected the mantle Na:K ratio which in turn is reflected in the Na:K estimated ratio of the solar system. So perhaps the similarity in current times isn't as much of a coincidence as I suggested in an earlier post.

    However, with the onset of plate techtonics in the mid-Archaean, the two following quotes, taken together indicate that both surface waters and their salt inventory are being continually recycled to and from the mantle:

    On 1/20/2024 at 12:27 AM, TheVat said:

    In fact, the oceans would be even more saline were it not for subduction.

     

    Quote

    Experiments have documented that olivine at high pressures (12 GPa, the pressure at depths of about 360 km (220 mi)) can contain at least as much as about 8900 parts per million (weight) of water, and that such water content drastically reduces the resistance of olivine to solid flow. Moreover, because olivine is so abundant, more water may be dissolved in olivine of the mantle than is contained in Earth's oceans.[17]

     

    I guess the turnover time is at least oto 1 billion years so it's a very slow process and almost certainly not at equilibrium. But it is clear that there is some continuous limited exposure to mantle cation ratios that would act as a negative feedback loop tending to restore primitive values.

    Which leaves us with having to deal with why continental crust is so markedly different with a near unity Na:K mass ratio.

    On 1/20/2024 at 12:51 PM, exchemist said:

    Surely both Na and K prefer granite to peridotite, the latter being ultramafic (Mg/Fe), don't they?

    Arguably so, but at constructive plate margins, there is no neighbouring granite for them to migrate to. Rather we have a fractionation process that might be roughly summarised as:

                                       3 Peridotite => 1 Dunite + 2 (Gabbro + Basalt)

    Dunite is over 90% olivine, offers very limited hospitality to Na (maybe in some residual pyroxene) but as far as I can tell none to K. So most of the Na and all of the K plus the rest of the more volatile components creates a gabbro melt that ascends to fill the gap between the separating plates.

    At this point I wanted to give some indicative Na:K ratios for oceanic crust gabbro/basalt. Only to find lots of field data showing Na:K ratios up to 50+ for gabbro and 2 for basalt. The common understanding is that basalt is simply rapidly cooled gabbro. Clearly this is somewhat of a simplification. On the face of it, the fractionation process is not the unitary division I've just described, but some multistage fractionation process that preferentially shunts Na into the gabbro levels and K into the basalts. I'd be interested in your views on this.

    Anyway, it appears that we have the K concentrated in the upper basaltic levels of the oceanic crust, possibly in the form of a feldspathoid such as leucite, the Na evenly spread between the basalt and lower gabbro horizons within clinopyroxenes such as augite, and below the crust mantle boundary a zone of depleted cumulate dunite.  

    At the other end of the conveyor hopefully the picture is a little clearer. As the old oceanic plate is subducted, it eventually reaches a region of high temperature, high pressure which (give or take a little controversy) is where it is converted to a highly metamorphosed rock type called eclogite, mainly comprising pyralspite garnet and a sodium rich clinopyroxene called omphacite. 

    With the K being concentrated in the contact zone of the upper levels of the oceanic crust, and an incompatible mineral assemblage forming beneath it, the path for transport of the vast majority of it into the continental crust appears non-problematic. Na on the other hand has the option of joining with the K to progress upwards, or staying in situ within a compatible mineral assemblage. The nett result seems to be a roughly equal mass diffusion of sodium and potassium into the lower levels of continental crust.

     I basically have zilch documentary evidence to support the overall picture of this mass balance, but it makes some sort of sense to me and my researches haven't yet thrown up anything that cotradicts it significantly. Hope it helps.

    6 hours ago, exchemist said:

    By the way, the poster who asked the original question does not seem to have returned. Perhaps he is watching and chuckling to himself as we struggle with it.😁

    I'm not going to lose sleep over that 🙂 

  4. 34 minutes ago, exchemist said:

    I'm not sure I follow this. Surely both Na and K prefer granite to peridotite, the latter being ultramafic (Mg/Fe), don't they?  

    Sodium is relatively happy in some mafic minerals, particularly clinopyroxenes. I suspect potassium simply won't fit in that lattice.

    In a feldspar environment, the alkali metal hole is clearly big enough for potassium and maybe this gives a little more stability to the potassium version under most surface conditions. However relative weathering rates show a high pH and temperature sensitivity so perhaps there are multiple mechanisms at play.

  5. 13 hours ago, exchemist said:

    Aha, that makes a difference, certainly!

    But there is a factor of 30 to account for.

    @swansont's reference is dominated by the composition of the earth's mantle. 

    Quote

     

    Composition of the Earth's upper mantle (depleted MORB)[16][17]
    Compound Mass percent
    SiO2 44.71
    MgO 38.73
    FeO 8.18
    Al2O3 3.98
    CaO 3.17
    Cr2O3 0.57
    NiO 0.24
    MnO 0.13
    Na2O 0.13
    TiO2 0.13
    P2O5 0.019
    K2O 0.006

     

    This is indeed the ultimate source of sodium and potassium at the earth's surface, and the sodium:potassium ratio is strikingly similar to the marine ratio. However, I think this is coincidental. Except for odd cases such as a notable ocean bed exposure off the Cape Verde islands, surface waters and upper mantle rocks are not significantly in contact. 

    Rather, there are a number of fractionation processes in the vicinity of the crust-mantle boundary that enrich the alkali metal content and deplete the mafic materials (Mg, Fe predominantly), starting with emplacement of basalt/gabbro at constructive plate margins and culminating with the granitisation of the base of the continental crust at destructive plate margins. 

    Even this is quite a simplification as there are a number of other processes involved most of which are not fully understood, so I tend to take a first order engineering approximation of the mantle injecting granite into the continental crust. It's easier on my head and the overall mass balance still works.

    The concentration factors of 30 for sodium and 600-ish for potassium reflect their relative preferences for a granitic environment over a peridotite environment.

  6. 1 minute ago, exchemist said:

    The latter point is what I too suspect, to do with ion sizes. But I’m not a mineralogist. 
     

    We need to check what these abundance numbers mean. Do they include the oceans or not?

    If this helps, the primary route for transport of sodium and potassium into the earth's crust from the mantle is via emplacement of granitic intrusions which occurs in roughly equal weight proportion. From https://en.wikipedia.org/wiki/Granite

    Quote

    Chemical composition[edit]

    A worldwide average of the chemical composition of granite, by weight percent, based on 2485 analyses:[14]

    SiO2 72.04% (silica)
     
    Al2O3 14.42% (alumina)
     
    K2O 4.12%
     
    Na2O 3.69%
     
    CaO 1.82%
     
    FeO 1.68%
     
    Fe2O3 1.22%
     
    MgO 0.71%
     
    TiO2 0.30%
     
    P2O5 0.12%
     
    MnO 0.05%
     

    Chemical weathering of rock minerals follow the Goldich Dissolution Series 

    Quote
    Discontinuous
    Series
      Continuous
    Series
      High
                         
      Olivine   Plagioclase
    (Calcium rich)
         
                             
      Pyroxene                
                             
      Amphibole                
                             
      Biotite
    (Black Mica)
      Plagioclase
    (Sodium rich)
      Relative
    Weathering
    potential
                             
               
          Orthoclase          
                           
          Muscovite
    (White Mica)
             
                           
          Quartz          
                      Low

    This indicates that the potassium rich minerals (orthoclase, muscovite, biotite) are relatively resistant in comparison to sodium rich minerals (sodic plagioclase, some amphiboles and clinopyroxenes). By logical extension, sodium is over-represented in sea water and evaporites, whereas potassium is over-represented in detrital sands and sandstones. 

  7. 1 hour ago, exchemist said:

    I have a feeling it may be to do with the stability of complex minerals containing K, e.g. aluminosilicates. Perhaps the greater ionic radius of K+ forms more energetically favourable lattices , or it is harder for the larger ion to migrate within minerals and eventually be leached out by water.

    Reckon so. I've seen somewhere that albite (sodium feldspar) weathers at 10 times the rate of orthoclase (potassium feldspar). 

  8. 5 hours ago, exchemist said:

    I am reminded of the "Brownian Ratchet": https://en.wikipedia.org/wiki/Brownian_ratchet

    Excellent link!

    Cherry-picking one particularly apt paragraph:

    Quote

    The Feynman ratchet model led to the similar concept of Brownian motors, nanomachines which can extract useful work not from thermal noise but from chemical potentials and other microscopic nonequilibrium sources, in compliance with the laws of thermodynamics.[3][4] Diodes are an electrical analog of the ratchet and pawl, and for the same reason cannot produce useful work by rectifying Johnson noise in a circuit at uniform temperature.

     

  9. 9 hours ago, Ken Fabian said:

    I don't know why Dr Novack thinks it but I think it because I think the photons interacting with an antenna don't add (all of) their energy as heat to the antenna, some energy making electrical potential. Some gets converted to electrical energy and carried away. It may initially be at thermal equilibrium but it isn't a closed system. Note, I don't think this as a certainty - I don't know enough.

    But if the antenna can't absorb IR because the temperature of the antenna makes it radiate IR - not the antenna transmitting IR because that takes electricity from somewhere else, but the materials radiating it in the normal way things that are warm radiate - then, yes, it is at thermal equilibrium and an IR rectenna cannot work.

    I had thought that interaction between EMR and an antenna was independent of antenna temperature but that was most likely ignorance - most working antenna aren't tuned to the band the antenna normally radiates in.

    We're pretty much aligned here, and I too am pushing the limits of my understanding of EM field behaviour.

    I was hoping that @swansont or someone else with expertise in this field would have picked up on my previous post and confirmed or otherwise the mental picture I have of this. But in the absence of such... 

    I think it reasonable to picture the thermal energy of a metallic antenna as being largely contained in the motion of a 'gas' of the unbound electrons. If you can persuade a significant excess of these to move in a coordinated way in a particular direction then that yields an electrical current distinct from the thermal motion. However, I suspect that this requires the incoming radiation to be both directional and/or phase-coordinated. Thermal radiation from the sun satisfies the directionality requirement and I presume the higher frequency part of the spectrum may have sufficient 'kick' to push a few electrons across the junction gap of a diode which might explain the measurable current reported in that scenario. But ambient thermal radiation is omnidirectional and the individual photons are much weaker. Trying to extract energy from this scenario just sounds a little too Maxwell's Demonish for comfort.  May be this picture is all wrong, but it at least tends towards consistency with the Kirchoff's Law/2nd Law objections I raised earlier.  

  10. 4 hours ago, swansont said:

    This is not owing to heat transfer - the radiation isn't thermal. It's doing work. 

    I'm getting a picture here of transmitted photons perturbing the EM field of an antenna aligned with the source somewhat analogously to a steady trade wind generating oceanic waves. And just as the energy of oceanic waves  can be harvested by an appropriate machine, the waves in the field of the antenna induce an alternating pd across the terminals of the antenna which can in turn be harvested.

    Is this analogy a useful one?

  11. 4 hours ago, Ken Fabian said:

    Simply, the energy that is diverted away isn't turned into heat in the receiver. In an otherwise closed system it has a leak; there will be loss of energy, ie cooling.

    You're effectively claiming the ability to extract useful work from a system at thermal equilibrium.

    Not sure there's anywhere to go from here.

  12. 20 minutes ago, CharonY said:

    Here, you have to go back to the hierarchies that I a referred to earlier. On the mechanistic levels a lot of thing manipulate genetic material. Either by making errors or allowing errors and so on. Their impact on evolution is not zero, but rather it affects the overall genetic landscape in terms of e.g. expected mutation rate, potential role of horizontal gene transfer and so on. However, when it comes to the mechanisms of evolution, e.g. selection, the same rules apply as for any other traits that a cell or organism might have.

    Just to emphasise this crucial point, consider the major introgression of 'alien' DNA the ancestors of some of us experienced in the not too distant past. Yet the vast majority of our genome has been swept clean of neanderthal alleles (the so-called neanderthal deserts). This example is far from the picture of small random mutations slowly accumulating over time. And yet the end result is just what Darwin would have predicted. We retained useful alleles, particularly those helpful to climate adaptation and immune responses, and lost those those that didn't sit well with the core sapiens 'team' of genes. It undoubtedly influenced the speed of evolution, but the fundamental principle of evolution by natural selection remained unchallenged.

     

    As we're in the Speculations section, I might add that I'm inclined to the view that evolution by natural selection is an inevitable process driven by differential efficiencies in the utilisation of energy flow through a system. The detailed mechanics of genetics then becomes more of a consequential effect than a prime.cause.

  13. 1 hour ago, MigL said:

    I don't see how an observer would be able to see anything ahead of himself upon passing through the Event Horizon, as there are no geodesics for light to follow in the outwards direction.
    The only available geodesics are forward in time to the center.

    Does the light need to be travelling in the outward direction?

    If the infalling observer is falling faster than the wavefronts are receding, he catches up with them doesn't he?

  14. The detailed mechanics of aerial design are way beyond my pay grade, but you seem to be making two distinctions here that I would be wary of:

    5 hours ago, Ken Fabian said:

    ... it not being a thermal phenomena

    Until it is. Can you say this when the received phonons are indistinguishable from the sea of thermal phonons flooding the lattice of the receiving aerial? 

    5 hours ago, Ken Fabian said:

    ... ie cooling

    Cooling is thermodynamically difficult. I'd be reluctant to use the word in place of "not getting quite as warm as you might otherwise expect".

    Revisiting Kirchoff's Law briefly, there are some useful bits of information to be had from Electromagnetic Reciprocity.

    This caught my eye:

    Quote

    Forms of the reciprocity theorems are used in many electromagnetic applications, such as analyzing electrical networks and antenna systems.[1] For example, reciprocity implies that antennas work equally well as transmitters or receivers, and specifically that an antenna's radiation and receiving patterns are identical.

     

  15. The bottom line here is Kirchoff's Law of Thermal Radiation which is often expressed as:

    Quote

    For an arbitrary body emitting and absorbing thermal radiation in thermodynamic equilibrium, the emissivity is equal to the absorptivity.

    If this were not true then there could be spontaneous nett heat flow from a cool body to a hotter one which simply doesn't happen.

    However, the proviso 'in thermal equilibrium' is key since 'arbitrary bodies' can have widely different emissivities at different wavelengths corresponding to different equilibrium temperatures. This is where the misunderstandings arise.

    Taking an arbitrary example quote from https://en.wikipedia.org/wiki/Emissivity

    Quote

    Solar water heating system based on evacuated glass tube collectors. Sunlight is absorbed inside each tube by a selective surface. The surface absorbs sunlight nearly completely, but has a low thermal emissivity so that it loses very little heat. Ordinary black surfaces also absorb sunlight efficiently, but they emit thermal radiation copiously.

    Here the absorptivity is with reference to thermal equilibrium radiation at the surface temperature of the sun, whereas the emissivity is with reference to local thermal equilibrium in the vicinity of the collectors. The collectors are NOT in thermal equilibrium with the surface of the sun. Without the temperature difference, there would be no loophole to exploit.

    10 hours ago, Ken Fabian said:

    Won't the receiver in this case only gain heat from what is not converted to electricity and will radiate back less than reaches it? Seems to me the rate of absorption by the antennae is independent of temperature of the antennae.

    Okay so far. But note that emission by the antennae is NOT independent of the temperature of the antennae,

    10 hours ago, Ken Fabian said:

    A cooling emitter - radiating away some of it's energy, yes - but a cooling receiver too (?), diverting what reaches it to electricity instead of raising it's temperature (and radiating it back). Again, adding to efficiency??

    The receiver does not reradiate incident solar radiation because it is not at the same equilibrium temperature. Therefore there is an asymmetry between absorption and emission that can be exploited. 

    10 hours ago, Ken Fabian said:

    I recall one of the suggested possible uses for Optical Rectenna is surface coatings on walls for cooling rooms, an alternative to A/C - "waste" heat turned to electricity as a bonus.

    But refer to the relevant paragraph in https://en.wikipedia.org/wiki/Optical_rectenna

    Quote

    In an interview on National Public Radio's Talk of the Nation, Dr. Novack claimed that optical rectennas could one day be used to power cars, charge cell phones, and even cool homes. Novack claimed the last of these will work by both absorbing the infrared heat available in the room and producing electricity which could be used to further cool the room. (Other scientists have disputed this, saying it would violate the second law of thermodynamics.[16][17])

    The proposal neglects to consider the equivalence of absorptivity and emissivity at thermal equilibrium. So it falls foul of Kirchoff's Law, and by logical extension, the 2nd Law of Thermodynamics. 

  16. 1 hour ago, Ken Fabian said:

     

    Off the top of my head I don't think that is correct. I think IR intensity of the emitter is purely temperature dependent but is not dependent on a difference in temperature - ie is independent of the temperature of the receiver. Except maybe will be receiving IR or conducted heat back - which, if the receiver is heated by the process would counter intuitively increase the efficiency... ?? Doesn't sound correct, but...

    @sethoflagos Any thoughts on this?

    As I understand it, the theorical maximum efficiency from a 2nd Law point of view is 1 minus the temperature ratio of absorber over emitter. 

    This yields something like 85% for solar spectrum conversion.

    Trying to convert near ambient spectrum IR would yield zilch I suspect as the antenna would be emitting about as much as it absorbed.

  17. On 1/9/2024 at 10:57 PM, Ken Fabian said:

    Optical Rectenna can convert Infrared radiation - radiant heat - directly into electricity but in practice so far the yields are extremely low. Like an antenna does with longer (radio) wavelengths - like the old "crystal radio" that powered itself from the radio waves. Not sure how that works in entropy terms - heat loss in the conversion?

    Of all the out there possibilities this would be one I'd like to see get some serious attention because if they can be made to work we could not only make electricity from waste heat but from radiant heat of all kinds, including down-radiation from clouds and atmosphere by night as well as from sunshine by day.

    Imagine the current generated is used to charge a battery. Losses are generated by the required electrode overpotentials and internal Ohmic resistance of the cell resulting in radiation of waste heat to the environment.

    An overall picture of the thermodynamics can be found here.

    Closely related is Thermodynamic bounds on Work Extraction from Photocells and Photosynthesis which connected a lot of loose strings for me at least. Attached. Well worth a read imho.

    Photocells and Photosyntesis.pdf

  18. On 1/11/2024 at 7:22 AM, StringJunky said:

    Is that correct?

    Given that current generation PAC-2/3 interception missiles can top Mach 4, their speed deficit against say Avangard and  Starry Sky-2 hypersonic glide vehicles is perhaps less of a challenge than keeping track of these highly manoeuvrable devices as they actively evade defence systems.

    That they achieve their impressive success rates via direct kinetic impact (rather than nearby fragmentation) I find quite staggering.

    Perhaps the real test will come with the future deployment of the BrahMos II Russian/Indian scram jet powered hypersonic cruise missile which will be capable of Mach 8-ish.

  19. 2 hours ago, Paulsrocket said:

    Where is ground zero of the big bang?

    We call it 'the universe'

    2 hours ago, Paulsrocket said:

    Shouldn't there be a void 13 or more billion years wide void

    We call it 'space'. The pressure is <10-6 Pascals which is a near-perfect vacuum by earthly standards

    2 hours ago, Paulsrocket said:

    with an outward moving ring of mass?

    Mott-Gurney shrapnel distributions apply to this sort of 'big bang'

                                           images.png.8ec56f55cf7d349e9dbd97d0a8787bb0.png

    The uniform shrapnel distribution we see on the larger scale is more suggestive of free gas expansion. ie there wasn't any shell casing involved.

    3 hours ago, Paulsrocket said:

    and why would the CMB be coming from every direction

    Photons emitted from a uniform shrapnel distribution?

    3 hours ago, Paulsrocket said:

    and not all moving away from the source?

    Do you think photons emitted by the shrapnel can only travel in the same direction as the shrapnel?

  20. 7 hours ago, Photon Guy said:

    Anyway, I was thinking if there was some way to recycle heat back into usable energy. Usually heat is a nuisance but I was thinking if there was a way to make it into something useful. Maybe turn it back into potential energy somehow.

    You can recycle all of it back into 'useful energy' in principle if you have a handy heat sink at absolute zero and lots of time to realise near-reversible thermodynamic processes. 

    Trouble is the 'ifs'. They tend to cost a lot of capital.

     

     

     

     

     

     

     

     

     

    y

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