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Homework Help: The Permittivity of Insulators

  1. Sep 18, 2016 #1
    I just have a few conceptual questions:

    Is it as difficult to remove electrons from an insulator as it is to add to it?
    I understand insulators have a lower permittivity than conductors, and that they still allow charges to build upon the surface. However, I read that when you rub an insulator with a cloth, you are actually removing electrons. Can you add charges to an insulator by rubbing it with a cloth? So I'm curious which situation is more common.

    How can insulators of a uniform charge density exist? Additionally, are insulators with a uniform negative charge be as likely to occur as those of a positive charge?
    This one stumbles me because charges are supposed to build up on the surface. I do not understand how they can be dispersed evenly throughout a surface unless insulators in general allow electrons to move at a slower rate through its material. Can anyone please clarify?

    Thank you so much for anyone who helps. I really appreciate it.
  2. jcsd
  3. Sep 19, 2016 #2


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    Yes, you can add electrons to an insulator by rubbing it, it all depends on the type of material or more specifically the electronegativity of the two materials being rubbed together. If the cloth is more electronegative than the rod then it will remove electrons from it and leave the rod with a resulting positive charge. If the rod is more electronegative than the cloth then it will remove electrons from the cloth and become negatively charged. In my experience the last case is more common, that is most material charge up negatively when rubbed with a cloth. It also goes to reason that since the charging mechanism is rather random on a microscopic scale and that we are dealing with insulators, which do not allow charge to move freely about, that the resulting charge distribution on the solid material will not be uniform. Other ways of charging up solid insulators do exist, like spraying charge onto the surface, but still obtaining a uniform charge distribution is virtually impossible. The resistance of insulators do vary though and some movement do occur at higher voltages.
  4. Sep 19, 2016 #3

    rude man

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    This is incorrect. Conductors have relative permittivity of 1, typically. Insulators can have relative permittivities of several thousand.
    Actually, having a dielectric of ANY finite volume charge density is fiction. All materials have SOME conductivity so at least eventually all charge will always be on the surface. Volume charge densities exist only in textbooks etc.
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