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How to uniformly charge an insulating sphere?

  1. May 11, 2013 #1
    In my Physics book there was this problem of finding electric field produced by the sphere, such that electric charge is distributed uniformly throughout the volume of an insulating sphere.

    I know that excess charge tends to distribute itself on the surfaces, but since this sphere is made from insulating materials excess charge cannot leave individual molecules to do so?

    Anyway, how it is possible to charge an insulating sphere throughout the volume?
     
  2. jcsd
  3. May 11, 2013 #2

    mfb

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    Those spheres are just hypothetical objects - there are no perfect insulators, if you wait long enough the charge will be at the surface only.
    You can place electrons at specific regions with electron beams, but I doubt that the result will give a uniform charge distribution.
     
  4. May 11, 2013 #3
    is it not possible to uniformly heat the "insulating sphere" as to release conduction electrons?
     
  5. May 11, 2013 #4

    sophiecentaur

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    You could take a charged 'wand' and touch the surface of the sphere at all points over its surface. Alternatively, you could put the sphere in a vacuum and bombard it with electrons whilst tumbling it at a steady rate. Electrons would then be spread (painted) over the surface. Of course, you would need to adjust the energy and focus of the electron beam because it would be deflected by electrons already on the surface. Woops - just read the previous post which says more or less the same thing.

    What about covering the sphere with a plastic coating and ripping the coating off. You would then get a charge all over the surface as you do with a roll of cling film. That. I think, is a method that could actually deliver some sort of a practical result.
     
  6. May 12, 2013 #5

    mfb

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    As far as I know, those induced charges are quite unpredicable, and not constant.

    In addition, it charges the surface only, not the full volume.
     
  7. May 12, 2013 #6

    sophiecentaur

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    You're a hard man to please.
    Could your big sphere be made of many smaller spheres?
     
  8. May 12, 2013 #7

    mfb

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    I would use something more space-filling, like cubes (with special parts for the surface?), but that is possible, sure.
     
  9. May 12, 2013 #8

    Vanadium 50

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    You're unhappy with perfectly insulating, but are fine with perfect spheres?

    Like a lot of problems in physics, these are idealizations or approximations. Just like frictionless planes, massless and stretchless ropes, etc.
     
  10. May 12, 2013 #9

    sophiecentaur

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    Perfect 'enough'?
     
  11. May 12, 2013 #10
    What physics book?
     
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