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Capacitance with Earth?

  1. Apr 22, 2016 #1
    earth capacitance.png
    1. In above image an insulated metal plate has been placed inside earth (soil), and an voltage is applied between plate and earth groung what will be the capacitance here??

    2. one plate is the metal and the other plate is entire earth, so it is a big assymetrical Capacitor or What?

    all replies welcomed friends.
     
  2. jcsd
  3. Apr 22, 2016 #2
    I think it will fail to work as capacitor almost all supply voltage will occur across the insulation of the buried plate.
     
  4. Apr 22, 2016 #3

    CWatters

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    I think it can be modelled as a capacitor. The insulation will act like the dielectric between the plates...

    Plate.jpg
     
  5. Apr 22, 2016 #4

    CWatters

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    The capacitance will be roughly twice that of a two plate capacitor eg..

    C = 2*εRε0* A/d
    where
    ε0 is the permittivity of free space
    εR is the relative permittivity of the insulation
    A is the area of the plate
    d is the thickness of the insulation.

    You might need to include some "equivalent series resistance" due to the properties of the earth.
     
  6. Apr 22, 2016 #5
    I do not think resistance of the earth's crust will appear in the formula for capacitance. It will only affect the time for charging. The simplest equivalent circuit appears to be consisting of three plates separated by two insulating sheets: the outer plates will be connected to the negative terminal of battery and the central plate to the.positive terminal of the battery.
     
  7. Apr 22, 2016 #6
    if all votage appear across insulator, won't it try yo polarize it and store charge there by making capacitance?
    ....by the by i'll put another circuit in the thread with modifications
     
  8. Apr 22, 2016 #7

    CWatters

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    It won't appear in the formula for capacitance, that's not what I meant. However you may need to include it in the model depending on what this is being used for.

    See the drawing I posted. That's why there is a 2 in the equation.
     
  9. Apr 22, 2016 #8
    earth cap2.png
    Friends now apply my same question here, what will you think the capacitance will be ?.

    ( both the images are same except the distances between plates inside the ground )
     
  10. Apr 22, 2016 #9
    that's what i'm thinking Sir....please do see my second diagram also
     
  11. Apr 22, 2016 #10
    Modelling wise you seem to be correct. But can we consider the whole soil of earth as one conducting connection between two plates of a parallel capacitors.Just because we think the whole earth is at same potential.
     
  12. Apr 22, 2016 #11

    jim hardy

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    Sounds to me like the right approach.
    But why 2X ?
    Seems to me we have something between a single plate capacitor and a two plate capacitor.
    What's relative permittivity of topsoil where it's buried?
    Moisture ought to affect that pretty strongly with its 80-ish dielectric constant
    and conductivity will make it a lossy capacitor .

    If the soil is highly conductive,
    the model first posted is just like an electrolytic capacitor where you get a lot of capacitance
    because the insulation is so thin -
    it's just a few molecules of aluminum oxide on the surface of the aluminum plate.
    Conductive electrolyte is the other "plate". Wet salty mud should behave similarly....
    So i'll guess upper limit is half CWatters' answer.

    If on the other hand the topsoil is not highly conductive, it becomes an insulating layer between the single plate capacitor and the rest of the universe . It's hard for me to imagine something with less permittivity than free space so i'll guess the lower limit is same capacitance as a single plate of same area surrounded by free space.

    old jim
     
  13. Apr 22, 2016 #12
    I agree with Hardy. As first step it is ok. Finally we will have to arrange in this manner and study what equivalent capacitance we get experimentally. Better to model after experimentation.
     
  14. Apr 22, 2016 #13
    Because teh whole earth can be considered to have the same potential hence that is equivalent to a single conductor, which is also equi-potential.
     
  15. Apr 22, 2016 #14

    jim hardy

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    If your initial assumptions include that earth is highly conductive medium , then yes it's equipotential.
    Since that wasn't stated
    and substantial potential difference can exist between points on earth because it's only a mediocre conductor........

    In your though experiment , tie down these loose ends before starting.
     
  16. Apr 22, 2016 #15
    thank you mr. jim and let's think, i think i got some clearence

    I actually got this idea on attempt to invent a overunity device.

    " Assymetrical Capacitor is the Key to overunity"- Prof Donald Lee Smith.
     
  17. Apr 22, 2016 #16

    jim hardy

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    be careful where you take the thread - perpetual motion is a disallowed subject at PF.
    If there's a quirk of math that leads one into over-unity engines - i'd say you should examine the derivation carefully for a divide by zero someplace.

    See also Asimov's "The Gods Themselves"
    https://en.wikipedia.org/wiki/The_Gods_Themselves

    Have fun , and be rigorous with your thinking
    Well, i suppose one could argue we(at least some of us) think in pictures which we laboriously translate into words...
    old jim
     
  18. Apr 22, 2016 #17
    agreed Mr Jim. i won't do it again.
     
  19. Apr 22, 2016 #18

    berkeman

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    Yes, as Jim said, discussions about PMMs and over-unity devices are not allowed at the PF. From the Forbidden Topics portion of the PF Rules (see INFO at the top of the page):
    The discussion about your buried capacitor and the conductance of soil is fine. :smile:
     
  20. Apr 22, 2016 #19

    berkeman

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    Thank you :smile:
     
  21. Apr 22, 2016 #20

    CWatters

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    I treated it like a three plate capacitor with the two outer plates connected together. That's equivalent to two capacitors in parallel hence x2.
     
  22. Apr 22, 2016 #21

    CWatters

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    Looks like you have an AC voltage source driving two capacitors in series.
     
  23. Apr 22, 2016 #22

    jim hardy

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    Thanks CW....

    now that i see your picture it's crystal clear....
     
  24. Apr 22, 2016 #23

    Baluncore

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    Is this an AC or a low frequency DC circuit? The connection wires have significant capacitance due to their field. If it is a significant frequency AC, then it becomes a transmission line problem.

    The good or bad conductivity of the soil is not really an issue as it is an equipotential. Resistance is in quadrature with capacitive reactance so it does not effect capacitance.

    Re: top diagram of post #8. The two buried plates are in series, but the two sides of each are in parallel. That cancels to C = one surface plus wires.
     
  25. Apr 22, 2016 #24

    jim hardy

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    I'm not sure i can buy that.
    It's only equipotential if it conducts, else there's an electric field across it which isn't possible if it's a conductor.

    Let me change the shape from a flat plate to a sphere.
    I need to do that because the capacitance of a single sphere is straightforward, see
    http://hyperphysics.phy-astr.gsu.edu/hbase/electric/capsph.html
    but the capacitance of a single plate is fearsome ,
    and i know my limits way too well to tangle with Poincaire operators and eignenfunctions .

    So here goes:
    oneplatecap.jpg

    If the soil is nonconductive and has permittivity not much different from that of free space
    then an electric field exists between the surface of the sphere and the 'ground' wire that's also buried in the soil
    and that volume holds whatever energy is stored in the capacitor.
    If the distance to that ground wire is infinite then it's an isolated sphere with capacitance 4πε0R , per that hyperphysics link

    If the soil is conductive, then
    free charges in it migrate to the surface of the insulator and the electric field is constrained to the volume of the insulator
    and capacitance will be εA/d = ε4πR2/d

    so I hypothesize that conductive soil probably increases capacitance by ratio (ε4πR2/d ) / ( 4πεR ) = R/d

    For OP's flat plate offering the difference between conducting and nonconducting soil might simplify to some reasonable function resembling √area / d
    and that's why electrolytic capacitors work so well with one plate of metal, one plate of conductive semi-liguid paste, and insulation just a microscopically thin layer of oxide on the metal plate's surface. Gives a very small denominator.

    That's what i think
    and i'll welcome corrections. (See recent General Discussion thread "Have you ever failed"..)

    old jim
     
  26. Apr 22, 2016 #25

    NascentOxygen

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    I'm not sure that this shows the complete picture. Assuming attention has been given to ensuring soil conductivity, then in addition to the capacitance of your plate and cable combo, I think we'll also have a parallel capacitance of the entire Earth's sphere hanging off your buried cathode over on the right. If correct, this seems to place an extra 710uF in parallel to what you would calculate.

    https://en.wikipedia.org/wiki/Capacitance
     
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