capacitance or inductance?

Discussion in 'General Electronics Chat' started by strantor, Jun 7, 2011.

  1. strantor

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    1000v is passed through a 5000ft piece of wire on a spool. When the voltage is removed, a charge is left in the spool; if you touch it, it will shock you. Is this capacitance or inductance? I thought the spool was an inductor, but someone who I consider to be more knowledgeable than I, has told me that it is the effect of capacitance.
     
  2. #12

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    Inductive effects stop the instant all current stops. Inductance is inertia of current. The only answer left to you is capacitance.
     
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  3. Barnaby Walters

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    Hi there,

    Strantor — If you are referring to charge stored in the coil (I.E. once removed from a power source, the thing can still shock you after a few minutes) then that would have to be capacitance. Are you getting it confused with the backing EMF produced when a magnetic field generated by current flowing through a coil collapses and releases energy?

    Thanks,
    Barnaby
     
  4. strantor

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    Yes, that's what I had in mind. is it not the same thing? Is the shock felt, not back EMF?

    Does the coil configuration have anything to do with the capacitance? If I were to stretch that 5000ft wire out in a straight line, would it still shock me?
     
  5. shortbus

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    In an older thread this was also discussed. The effect of the insulation makes it a capacitance when in a coil. This is why magnet wire is used for transformers.

    Laid out straight it probably wouldn't Unless it was close to a broadcast tower/antenna.
     
  6. kubeek

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    Well, there is a chance you are talking about a coaxial cable. If you charged the coax to some voltage (between the inner and outer conductor, both ends unterminated) then the cable can hold quite large charge, depending on the voltage even enough to shock you.

    Plain wire in a spool will "self-reset" in a very short time, leaving no voltage between the two ends.
     
  7. steveb

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    What kind of wire is it? Is it coaxial wire, or just insulated single strand wire, or maybe even uninsulated single strand wire?

    Coaxial line is a very good capacitor actually, and it would be effective either coiled or straight.

    A plane old single insulated wire would have capacitance much more variable with geometry of how it is spooled (or not).

    An uninsulated wire would behave a lot like a metalic can once spooled because it would all be shorted out into one conductive sheet, and a conductive can is a crude single conductor capacitor not too different than the sphere on a Van de Graaff generator.
     
  8. strantor

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    The spool in question that generated this specific question is actually a 7 insulated conductor cable, with pairs wire-nutted together to form a single conductor running back and forth through the length of the cable. the spool is about 5ft wide X 6ft diameter. The cable has 2 outer layers of heavy duty armor wire. the shock came between the conductor and the armor wire.

    I have however had a similar experience with a 35,000ft spool of single conductor wire on a spool about 2ft wide X 2ft diameter. for that incident, the spool had been megger'd the night before and still had significant charge between the conductor and the reel to shock the p*ss out of me.
     
  9. steveb

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    So, I suspect the first example (the one that generated the question) is much more like the coax cable because you have an outer armor wire that acts similarly to the shield, and an inner conductor running back and forth inside. My guess is that this would be just as effective when unspooled.

    The second example, may be much different when unspooled, but that's just a guess. Was the reel conductive?
     
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  10. kubeek

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    Exactly what constitues a kind of coax cable. You can try measuring its capacitance, it will be quite high. Next time discharge it before touching ;)

    The other example probably creates a capacitance between the wire and the spool, which hold charge in a similar way.
     
  11. strantor

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    yes, steel reel.
     
  12. Ron H

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    If you charge up any large conductor (coax or not) to 1000V relative to earth, and then leave it floating, it will take a long time for that charge to bleed off. If you touch it, you will get shocked. It's similar to shuffling your feet on the carpet and then touching a grounded (or large ungrounded) conductor: You will get shocked.
    You could charge your big mechanic's toolbox (with insulating wheels) to 1000V, and then get shocked when you touch it.
     
  13. strantor

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    So,
    1. The capacitance is between the inner conductor and a conductive body outside; and the capacitance is due more to this fact, than the fact that it is in a spool?
    2. I can think of the wire's insulation as a dielectric?
    3. Where does back EMF come into all of this? Is back EMF what you were referring to when you mentioned the charge that dissipates very quickly?
     
  14. kubeek

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    If the wire were laying flat on a metal surface, the result would be the same. Maybe a bit more since the outer layers of the spool add very little to the total capacitance, unlike the inner ones.

    Yes, inuslation=dielectric up to the rated voltage.

    back EMF comes from current flowing through the wire being suddenly interrupted, the effect you felt was because of different voltage between two insulated conductors.
     
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  15. #12

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    Back emf has nothing to do with this. Back emf is about inductance. You have only capacitance working here, and yes, insulation = dielectric.
     
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  16. strantor

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    Ok, theoretical question,
    If I were to charge up a co-ax cable to 1000V, then that charge exists between the inner and outer conductors, seperated by the insulation, correct? so, then if I were able to remove either the inner or outer conductor, without touching the two together, would the charge cease to exist? where would it go? If I seperated the two and placed one on each side of my house, surely there would not be a spark if I rejoined them sometime later, right?
     
  17. ErnieM

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    As capacitance in inversely proportional to distance a thinner insulation leads to more capacitance. Magnet wire is used in transformers because the insulation is so very thin you can stuff more wire into the core.
     
  18. kubeek

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    When you have a capacitor charged with some charge, the relation is Q=C x V, charge stored is equal to capacitance times voltage. When you take the two conductors away this happens: the charge remains the same (a kind of energy conservation law) and the capacitance decrases rapidly, so the voltage has to incre ase to keep the relation.
    So as you move the conductors apart, the voltage increases until you get a spark or the insulation breaks down and the charge discharges.
     
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  19. #12

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    Thanks kubeek. It made my head hurt to try and figure that out.
     
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