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Charged insulator

  1. Jun 19, 2009 #1
    I feel so ashamed to ask a question this simple, but how does an insulator get charged? Let's say rubbing a glass rod with cat's fur. I thought that insulator means there is no free electron.

    Does rubbing means turning the neutral atoms at the surface of the glass rod into ions?

    or something else is happening?
  2. jcsd
  3. Jun 19, 2009 #2
    Yep, electrons are pretty much stripped off the surface of the insulator and transferred to the other object. Touching the two together forms a chemical bond, and if the surface chemistry is just right (e.g., one material has a higher surface electronegativity than the other one), then pulling them apart will result in an imbalance, leaving the surfaces charged with opposite polarities. When you rub the two together, you are essentially repeating this process many times over. It's called the triboelectric effect.
  4. Jun 25, 2009 #3
    aha ic, so its surface chemistry, there were chemical reaction

    these things were never taught in high or first year physics, no wonder i don't understand at all.

  5. Jun 26, 2009 #4
    Also, if you have a strong radioactive beta-decay source near an insulator, betas can inbed themselves in the insulator and charge it up. In addition, visible or UV light or X-rays, through photoelectric interactions, can eject photoelectrons from the surface. In all these cases, because there is no conduction band in the insulator, the charges cannot be neutralized.
  6. Jun 26, 2009 #5
    I thought photoelectric effect only happens in metals?
  7. Jun 26, 2009 #6
    Insulators have a higher work function than metals, meaning it takes a higher energy photon to knock off an electron, but even insulators have electrons. The photons probably have to be vacuum-ultra violet (100 nm or shorter wavelength).
    [Added edit] We also know that in the x-ray region, deep core photoejection (a form of photoelectric effect) can knock k-shell electrons out, which may escape the insulator. Outer electrons will then fill the k-shell, and leave a hole in the valence band. But since there are no electrons in the conduction band, the hole is not filled.
    Last edited: Jun 26, 2009
  8. Jun 28, 2009 #7
    oooh ic thx every1
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