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Cylindrical Capacitor Computation

  1. Mar 24, 2009 #1


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  2. jcsd
  3. Mar 24, 2009 #2
    The first equation for a cylindrical capacitor is correct, and is written for an air-filled capacitor (no dielectric).

    the equation for a dielectric-filled cylindrical capacitor is

    C = 2 pi ek e0 L / Ln(b/a) Farads

    where ek == dielectric constant of membrane (unitless)
    e0 == 8.85 x 10-12 Farads per meter
    L == length of capacitor in meters
    Ln(x) == natural log of x
    b, a == outer and inner radii in meters
    Check consistency of all units in above equation
    Look up the dielectric constant of mylar (DuPont)
    Compare to dielectric constant of a myelinated nerve
    Look up the description of multiple sclerosis
    Look up the definition and description of demyelination
    Last edited: Mar 24, 2009
  4. Mar 24, 2009 #3


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    The two last homework lines aren't necessary at all since I'm a health professional. :wink:
    I understand the first one and may compare the two dielectric constants but myelinated/unmyelinated dielectric vary with authors.

    What happens when b grows toward the value of L?

    Edit: I found a dielectric value for myelin around 6 to 10.
    Last edited: Mar 24, 2009
  5. Mar 24, 2009 #4
    RE What happens when b grows toward L? This is a difficult 3-D problem, so I looked in my favorite E&M book (Smythe) but could not find a good answer. My suggestion is if L< b then replace L with b.

    I will make a few comments about signal transmission lines and capacitance, and I will try to make them applicable to nerve signals. There are two basic types of analog transmission lines (delay lines) used by engineers: The L - C transmission line (usually dispersionless) and the R - C delay line (usually dispersive). In both types, the signals travel through the series components; L (inductance), or R (resistance), or axons or schwann cells. In all types the capacitance C is between the center conductor (axon or schwann cells) and the outer ground conductor, separated by a space between a and b (insulating dielectric or myalin sheath).

    The signal velocity along the center conductor (along axons) is proportional to 1/sqrt(LC) or 1/(RC), and the signal amplitude (voltage or spikes) is proportional to sqrt(L/C) or (1/C). For high amplitude signals and for faster signals, a low capacitance is better. The capacitance for a given geometry (given a, b, L) is proportional to the dielectric constant: Air = 1. mylar (duPont sheets) = 3.2, myelin sheath around axon = 6 to 10, and water = 80. So to preserve both fast nerve signals and high amplitude signals (for given a, b, and L), air is best, followed by mylar, myelin sheath, and water (worst). For a given L and a, larger b is better (but scales only as Ln (a/b))
    I hope this helps.
  6. Mar 24, 2009 #5


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    The electric field was calculated by applying Gauss' law to an infinite cylinder. Which is why L must be >> than b.
    http://hyperphysics.phy-astr.gsu.edu/hbase/electric/capcyl.html" [Broken]
    Last edited by a moderator: May 4, 2017
  7. Mar 25, 2009 #6


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    Thanks Bob but I simulated the lines with Micro Cap 9 (http://www.spectrum-soft.com/index.shtm" [Broken]) and I found effectively the same values of a signal dampening. There is a low pass filtering and, of course, a lower capacitance helps to enhance the cut-off frequency of the filter. But enhancing the limit of the filter doesn't give any mean to enhance the velocity of a signal, I think?
    Secondly, the cut-off frequencies are around 10/50 Hz and the the mean sinusoidal frequency (first harmonic) of a firing neuron is often > http://en.wikipedia.org/wiki/Action_potential#Taxonomic_distribution_and_evolutionary_advantages" but it is very well transmitted. It's a kind of huge contradiction.

    That's why I asked
    Last edited by a moderator: May 4, 2017
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