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I Electrostatic polarization and paper bits attraction

  1. Jan 26, 2017 #1
    Hello,
    I have been reflecting over this for the past few days. We can charge two insulators by rubbing them against each other. The two materials end up having an equal amount of opposite charge. For example, a glass rod rubbed with silk will become positively charged and the silk negatively charged.

    Small bits of paper can be attracted and moved around when the positively charged glass rod is brought close (without touching) to the paper bits. The neutral paper bits gets polarized (separation between the centers of positive and negative charged) and Coulomb's attraction take place: the induced negative charge on the paper bit faces the positively charged rod. But we can also touch the paper bits with the rod and the bits will stick to the rod. Wouldn't the positive charge on the rod neutralize the negative induced charge on the paper bit leaving the paper bit with a positive charge? If that was true, the paper bit would immediately get repelled by the positive charged rod. But that does not happen. Why? Is it because, both materials are insulators and even when the rod touches the neutral paper the positive charge on the rod will remain on the rod and the induced negative charge on the paper bit will remain on the paper?

    However, that seems to go against the fact that simple contact between two insulator can allow charge to transfer between them (rubbing simply increases the amount of contact). So the positively charged rod should neutralize the negative induced charge on the paper bit...

    Thanks,
    Fog37
     
  2. jcsd
  3. Jan 27, 2017 #2

    goodphy

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    Gold Member

    Hello.

    I'm trying to do my best to explain your question. I can be wrong:)
    By rubbing a glass rob with a silk, some electrons in the rob transfer to the silk due to different electronegativities of both materials, I guess. As a result, the rod becomes positively charge as it has a lack of electrons in its volume.

    Now, you get your charged rod closer to the paper (not touched yet). Then the electrostatic field from the rod makes the paper polarized, but the paper itself is not in a deficit of electrons; it is just polarized, not ionized. In this case, you can move the paper by moving the rod due to the electrostatic force between them. When the rod touches the paper, electrons in the paper may or may not transfers to the rod. But in either case, one of them, the rod and paper, is neutral and polarized and the other is ionized. So there must be an electrostatic force between them and they're attracting one another, like an ionic bond, literally:).

    I'm sure that they're pushing each other when they're touching and the paper is grounded as they can be neutralized simultaneously by receiving enough number of electrons from earth (the earth is a source of an infinite number of electrons).
     
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