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Makeshift zF capacitor

  1. May 29, 2017 #1
    Hi PF,

    I am in need of a capacitance in the zF (zeptofarad = E-21) range.

    Of course, they aren't sold this small, so I was wondering if maybe someone with more physics knowledge could help me create my own zF capacitor.

    Thanks in advance!
     
  2. jcsd
  3. May 29, 2017 #2

    Tom.G

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    The formula to calculate capacitance is:

    C = 0.224 * (n-1) * K * A / d

    C in pf (10[sup}-12[/sup]
    n = number of plates
    K = dielectric constant (1 for air or a vacuum)
    A = area of one side of one plate
    d = separation between plates

    That says the separation must be 0.224*109 as large as the plate area. For a plate 1 inch sq. the separation would be about 3,535 miles, a little more than the width of the North American continent. That's an awful long wire to the second plate, with its own capacitance to the environment. I suggest you try a different approach.
     
  4. May 29, 2017 #3

    Nidum

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    The capacitance of a plain wire can be much higher .

    The only situations that I know of where zeptoFarad levels of capacitance have any tangible meaning or significance are in the very specialised technologies of scanning capacitance microscopy and nano scale sensors .

    +1

    Is there any actual purpose to this question ?
     
    Last edited: Jun 1, 2017
  5. May 29, 2017 #4
    Thanks for your replies, guys.

    Actually, I meant to specify that this is a self-capacitance value I'm referring to. I wrote this really late last night, so I apologize.

    I would think that if I just wanted a zF capacitance in the conventional sense, I could just connect 5000000 0.05pF capacitors in series (since they add like resistors in parallel) to get about 10zF, right? (But you couldn't conclude that the self-capacitance of one plate of this zF capacitor would necessarily be in the zF range since it is independent of the distance of separation of the plates, etc.)

    I see on Wikipedia that the formula for self-capacitance of a circular disk is 8εa, which requires an area of about an angstrom to get 10zF. I'm hoping maybe there's a practical way to achieve this instead.
     
    Last edited: May 29, 2017
  6. May 29, 2017 #5
    Hi Nidum,

    Yes, a zF self-capacitance is necessary to satisfy multiple conditions in order to make it possible to prototype a certain apparatus.
     
  7. May 29, 2017 #6

    jim hardy

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    I'm stumped by the practical matter of how will will connect your leads to it .

    A Driven Guard can reduce apparent capacitance,
    but how would one even detect the charge on a 10-21 f capacitor to drive the guard? At a whole volt it's less charge than resides on a single electron.
    Best electrometer amplifier i know of has a picofarad or two in its Zin , which would swamp the zetafarad . A single electron at its input would produce only about a microvolt and get lost in the picoamp of input bias current .
    http://www.ti.com/lit/ds/symlink/opa128.pdf


    Interesting problem ... how does one detect Electron Millivolts ? and Atto-Amps?
    I'm out of my league here .

    http://www.tek.com/sites/tek.com/files/media/document/resources/2648 Counting Electrons1.pdf
    upload_2017-5-29_12-27-39.png

    Watching with interest.... old jim
     
  8. May 29, 2017 #7
    Actually, using the exact numbers from my calculations, the charge will be the exact charge of an electron. To be specific, 5V*32.04zF=1.602E-19C.

    I'll take a look into this "driven guard."

    Yes, it's interesting lol. I don't know, maybe surface mount?

    That's a very interesting article. Thanks, Jim.
     
    Last edited: May 29, 2017
  9. May 29, 2017 #8
    So I looked up the driven guard here:

    http://www.keysight.com/upload/cmc_upload/All/2-Parametric_Measurement_Basic.pdf?&cc=US&lc=eng

    and I see that it reduces effective parasitic capacitance in a triaxial cable by isolating the signal line from the shield.

    This would seem not very effective in my setup since what I need is actually an object whose self-capacitance is in the zF range. Perhaps this is only possible with nanotechnology; I was just hoping maybe there were some ingenious physics trick by which I could generate an "effective" zF self-capacitance without actually making the physical size so small, maybe by using an extremely low-density conductor like conductive foam or something (trying to find info on that now).
     
    Last edited: May 29, 2017
  10. May 29, 2017 #9

    jim hardy

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    Yes, point being it surrounds the signal cable with a shielding conductor(think Faraday cage) held at same potential
    so there's no incentive for current to flow through the capacitance between those two conductors.
    upload_2017-5-29_16-51-20.png
     
  11. Jun 1, 2017 #10

    Baluncore

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    A 10 zF capacitor could never be connected to a circuit so the concept of 10 zF is meaningless without context. You could make a negative capacitance, then adjust it to reduce it's own input capacitance to 10 zF. You would end up with something like a driven guard.

    A single atom might have about 10 zF capacitance relative to almost nothing nearby in a vacuum.

    You could simulate a 32 zF capacitor with an LED. When one electron flows the voltage drop would need to be 5 V. The LED would produce a photon with a wavelength of 1240 / 5V = 248. nm which is UVC, in the middle of the UV part of the spectrum. The voltage on the UVC LED would rise by 5V when it received a 248 nm photon.

    This thread should be closed if you cannot explain the context in which a 10 zF capacitor might be connected or interfaced.
     
  12. Jun 1, 2017 #11

    Nidum

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    +1
     
  13. Jun 1, 2017 #12

    berkeman

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    Agreed. Finger hovering over mouse button... :smile:
     
  14. Jun 1, 2017 #13

    jim hardy

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    Second the motion
     
  15. Jun 1, 2017 #14

    Baluncore

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  16. Jun 1, 2017 #15
    Interesting. Could you explain where the 1240 came from?

    I appreciate the many interesting facts and useful suggestions you've provided.
     
  17. Jun 1, 2017 #16

    Baluncore

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    The defined speed of light; c = 299792458. metre/second.
    The absolute permeability; Uo = 4×10-7 * Pi henry/metre.
    Absolute permittivity; Eo = 1 / ( Uo * c * c ) farad/metre.
    The Planck–Einstein relation gives photon energy E from wave frequency; E = h * f.
    Where h = Plank's constant = 6.626070040×10-34 J·s = 4.135667662×10-15 eV·s
    Converting also between frequency and wavelength in nanometres uses the constant = h * c * 109 = 1239.841973862093 = 1240.
    So for an LED that produces radiation of nm wavelength; voltage = eV = 1240 / nm.
     
  18. Jun 2, 2017 #17
    Ok, you can close it now. I got what I needed out of it.
     
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