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Will it discharge?

  1. Aug 20, 2013 #1
    For example, if I negative charge a glass ball and put it into a rubber (or any insulator) case to shield it from being grounded, will it discharge on its own? I know that a capacitor will discharge with time( assuming that there is no input.)
     
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  3. Aug 20, 2013 #2

    berkeman

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    Welcome to the PF.

    That comes down to leakage current mechanisms. With capacitors, there is some leakage current spec associated with each type of capacitor (check their datasheets for the numbers).

    For other geometries, the leakage current mechanisms will be different. You can certainly come up with some situations where the leakage current is very low, like for an object in space, for example. Such an object will stay charged up for a very long time.
     
  4. Aug 20, 2013 #3

    Baluncore

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    The insulated container your negative charged sphere is placed in is a dielectric material.
    The charge on the surface of your sphere will attract positive charge to the inside surface of the insulator and so make the outside of the insulator less positive = negative. This is probably best considered as a problem of electrostatics, capacitance in series.
     
  5. Aug 22, 2013 #4
    Please clarify

    Berkeman, you mentioned "current mechanisms" which, in my mind, implies that the spare electrons that are gathered on the glass ball are moving more than the average electron which are already in the ball making the spare electrons electricity rather than just spare electrons, however, I was under the impression that in a charge object, the spare electrons don't move more than a normal electron without an outside force, am i wrong?
     
  6. Aug 22, 2013 #5

    davenn

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    You missed the important word when you quoted berkeman he said Leakage current mechanisms.
    I think you will find he is referring to the different ways a current/charge will leak from a charged surface. Think of say .. a capacitor .. .it has 2 parallel plates with a dielectric in between them. But no dielectric is a perfect insulator ... so the leakage mechanism there will be through the dielectric till the charge is balanced in both plates

    There will be a "slow" discharge of your glass sphere into the rubber or other material that its sitting in.

    on charged sphere ... the charge will be evenly distributed across the surface of the sphere

    cheers
    Dave
     
  7. Aug 22, 2013 #6
    davenn the reason I left out leakage is because that was not my point. My point was do extra electrons have a current? If they have a current, then I expect it to discharge, but if there is no current will it still discharge?
     
  8. Aug 23, 2013 #7

    davenn

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    it is the point :smile:

    electrons DONT have a current
    the movement of electrons is a current or moving charge

    as hinted at earlier, if the glass sphere is totally isolated from anything else then any charge on the sphere be it positive or negative will spread out evenly over the sphere and remain there

    And also as said earlier ... any contact with other material WILL cause leakage ( a small flow of current/charge) will, if left long enough, eventually find a balance of charge remaining on the sphere ( any other charged object)

    Dave
     
  9. Aug 23, 2013 #8
    Thank you

    That is all I wanted to know, thank you. :approve:
     
  10. Aug 23, 2013 #9
    If the distance between the ball and the insulating case is microscopically small all around the ball it may stay charged for years since such a thin layer of air is an incredibly good insulator in contrast to thick layers of air which are only moderately good insulators.
     
  11. Aug 23, 2013 #10

    berkeman

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    Interesting. Why is that?
     
  12. Aug 23, 2013 #11

    Baluncore

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    Maybe, if leakage is the result of ionising radiation reacting with the mass of air. Less air is less collisions, so less ions produced = lower leakage ? ? ?
     
  13. Aug 23, 2013 #12
    There are two effects here.
    For once the breakdown strenght of air increases as the gap width decreases. That is predicted by Paschen's law
    http://en.wikipedia.org/wiki/Paschen's_law
    At a few µm you need more than 40 MV/m while for a gap between 1cm and 1m it's around 3MV/m.
    The second effect was explained already by Baluncore. There are not many ions in a tiny air gap so you can't get much of a current.
    It's actually possible to "glue" two pieces of plastic foil together with static charge and under the right conditions (no moisture in between the foils, smooth surface so the gap is really small) the foil will stick together for many years.
    You probably know that a roll of plastic foil can become charged when it's unrolled. But most people don't realize that that charge was put on the foil in the factory. It's not produced when you unroll it. Equal amounts of positive and negative charge are trapped in the roll of foil causing very strong fields in the gaps between the layers, while the total net charge is zero. That roll can be sitting on a shelf for years without loosing it's charge.
     
  14. Aug 23, 2013 #13

    berkeman

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    Very interesting. Thanks! :smile
     
  15. Aug 24, 2013 #14
    Interesting DrZoidberg, but how would one know if there was the right level of air in between the material? or what the right level of air is?
     
  16. Aug 24, 2013 #15
    It doesn't need to be precise. You just need two smooth surfaces that touch each other, at least one of which should be a good insulator, the other one can either be insulating or conducting e.g. aluminum and polyethylen foil. Even though they appear to touch there really is a microscopically small gap over most of the area.
     
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