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A question about capacitor

Discussion in 'Electronic Basics' started by aman, Apr 23, 2005.

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  1. aman

    aman Guest

    When I think of capacitor I can see there are two conducting parallel
    plates seperated by an insulator dielectric. So as there are opposite
    charges on the inside of the plates they attract each other thus
    containing the charge.

    Here is my question. If there is a perfect insulator used as
    dielectric, does it mean that the charge is fully contained and if the
    dielectric material gets a little conducting the charge on the plates
    gets reduced(cannot be retained fully) and some charge flows in the
    dielectric. Am I correct to some extent ?

    I am asking this because I am constructing a kind of capacitor detector
    which is seperated by dielectric which is water(different kind of
    samples with different conductivity and am trying to nuetralise the
    harmful free ions in water).
  2. Two plates is the minimum, often exceeded.
    Yes, although what would be called the charge is fully
    contained whether the dielectric conducts or not.
    If you know your chemistry, you know that water
    becomes something like an insulator when you
    get the free ion concentration low enough. Are
    you using leakage to guage that concentration?
    The capacitance will stay just about the same
    unless you have very high impurity levels.
  3. aman

    aman Guest

    So doesnt capacitance depend on conductivity of the dielectric ? I mean
    wont a more conductive sample of water give a different capacitance
    than less conductive water ?
  4. No. The effects act in parallel. When you apply an
    electric field to a slightly conductive dielectric, some
    bound charges move a limited distance then stop,
    and some unbound charges drift continously. We
    call the first displacement (or capacitive) current.
    We call the second leakage. The movements of
    those different charges are largely independent.
    I don't think you will be able to measure a capacitance
    change as you alter the ion concentration until you have
    such a large concentration that the dielectric property
    of the solute itself is a factor. By that time, measuring
    the capacitance would be quite a trick.
  5. aman

    aman Guest

    In C= KA/d if K does not depend on conductivity, what makes a
    dielectric have different dielectric constant K. What are the
    parameters on which K depends on?
  6. Usually, 'K' refers to the relative dielectric "constant" and
    would be multiplied by the permittivity of free space in
    the above formula.
    It is affected by how easily bound charges can be
    displaced and the density of those charges. For
    example, in water, the bound charges are at
    sort of opposite ends of the H2O molecule and
    displacement occurs as they become polarized
    in an E field.

    Your question covers a lot of territory.
  7. aman

    aman Guest

    So doesnt this imply that if there are free ions in water there will
    be a different K from a nuetral sample of water with no free ions ? I
    am actually adding chemicals(ionic) to water which nuetralises harmful
    ions in water to form a nuetral particle floc. So I need to detect zero
    crossing. As i am adding ions into water i need to detect a zero
    crossing in going from postive charge to negetive.
  8. I don't think so. The free ions will drift under the
    influence of an E field, contributing in-phase current,
    not lagged current such as displacement produces.
    The only way free ions could change the K would be
    if there were enough of them to act as a dielectric
    and they had a K different than the H2O they displace.
    Sorry, but I cannot make sense of that. Maybe it
    would help to show your circuit or block diagram.
  9. Woops, that should be:
    not leading current such as displacement produces.
  10. aman

    aman Guest

    I am adding salt to water. I read something in one of the forums below
    which confuses me. It also says "adding virtually any contaminant to
    water will alter its conductivity. That also ruins the diaelectric
    constant . Only pure water will have a measurable diaelectric

    It says that pure water is insulator and adding salt changes
    conductivity. I am adding salts to contaminated water to which makes it
    an insulator. But if more salts are added water crosses the point at
    which it is an insulator(pure) and again starts conducting. So I need
    to detect that zero crossing.

    But according to the forum in the link above, conductors cant be
    dielectric. But what if I use conducting water as dielectric. What
    happens. I know dielectric is supposed to support electric field but
    oppose current. But if there is leakage current the capacitor can't
    hold charge or what ?

    Also what is meant by "Only pure water will have a measurable
    diaelectric constant."
  11. I don't know what a ruined dielectric constant is.
    That is gibberish as far as I am concerned.
    I don't understand the zero crossing. Pure water
    conducts. There are always H+ and OH- ions
    present, and they can carry current. What you
    will see, if you can reduce free ions by adding
    salts to react with already present ions so as to
    produce a non-ionic product (a precipitate, I
    presume), is a decrease in conductivity which
    will approach the conductivity of pure water,
    possibly followed by an increase if you add too
    much. There will be no zero crossing.

    To me, it looks like you should almost forget
    about capacitance and measure conductance.
    If you are using AC for that measurement, it
    may be a good idea to measure only in-phase
    current so as to ignore the capacitance.
    For good conductors, the dielectric properties do
    not matter and are difficult or impossible to measure.
    A counter-example to that statement can be found
    in any electrolytic capacitor. They all leak current,
    so, because the material between their plates is a
    conductor (albeit a poor one), and taking the above
    as true, they cannot also be capacitors. Countless
    devices say otherwise.
    The question should not be "Can it hold charge?" but
    "How long can it hold charge?" Leakage limits the
    time that a capacitor can hold charge.
    That overstates the case. If we are to believe that,
    then adding one ion molecule to a vat of pure water
    will turn it from being a dielectric to being a poor
  12. A more accurate statement might have been that adding things to water
    that increase its conductivity interferes with many dielectric
    measurement methods.

    Imagine measuring the value of a capacitor with a capacitance bridge.
    The null is nice and sharp and determining the value with high
    precision is easy. Now, connect various resistances in parallel with
    that capacitor. The bridge null gets shallower and wider as more and
    more of the instrument's bridge current detours through the resistance
    and less and less of it passes through the capacitance. But
    paralleling those resistors did not alter the dielectric constant of
    the insulating material in the capacitor.
    The key word, here, is "measurable". Using a higher AC excitation
    frequency lowers the capacitive impedance while the Resistive
    component is essentially unchanged. So measuring conductive samples
    is more practical with a higher excitation frequency. The caution is
    that the actual value of high dielectric materials, especially liquids
    can vary dramatically as the excitation frequency passes through
    motional resonances of the polar molecules. So you will need
    calibration standards that you can use at the frequency you choose.
  13. Gareth

    Gareth Guest

    There will be some ions, but not many. Only about 1 in 500 million
    water molecules in pure water break up into H+ and OH- ions.

    Pure water is actually a good insulator (conductivity < 0.06 uS/cm)

  14. Rich Grise

    Rich Grise Guest

    What you're actually looking for is a pH meter.

    This isn't really related to dielectric constant at all, unless, as
    Larry said, the solute has a dramatically different K than water, and
    if it were in enough concentration to affect a capacitive readout,
    you wouldn't have a capacitor but an electroplating tank.

    Good Luck!
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