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Charging by induction

  1. Jan 25, 2010 #1
    1. The problem statement, all variables and given/known data
    I have a question that has been puzzling me: I know that when we bring a negatively charged rod close to a neutral metallic sphere, the negative charges are distributed to the opposite side of the sphere because of the negatively charged rod and the positive charges remain stationary (they did not move). When we attach it to the ground, some of the electron charges leave through the ground wire, leaving a positive charge on the sphere and the electrons readily redistribute themselves.

    Now here is my problem: When we bring a positively charged rod towards a neutral metallic sphere, there is a charge separation (the electrons are brought towards the positive rod and the positive charges in the sphere are repelled to the other side. When we attach the sphere to the ground, the positive charges are then neutralized, leaving only electrons that distribute themselves.

    I thought that only electrons are free to move and NOT protons? I don't understand why the second case underwent a charge separation and the first case did not...maybe I'm misunderstanding but maybe both cases actually undergo a charged separation?


    2. Relevant equations



    3. The attempt at a solution
     
    Last edited: Jan 25, 2010
  2. jcsd
  3. Jan 25, 2010 #2

    kuruman

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    In which case there is a deficit of negative charges in the conductor when you break the contact to the Earth with the rod still in place. This makes the originally neutral conductor have a net positive charge.

    No. The only positive charges in the conductor are the nuclei and they are not free to move.
    When you attach the sphere to the ground, more electrons from the Earth are free to move closer to the positively charged rod which brings an excess of negative charges on the sphere. When you remove the ground (with the rod still in place) the negative charges are trapped so when you remove the rod, the sphere has a net negative charge.
     
  4. Jan 25, 2010 #3
    But do the positive charge redistribute themselves when the negative rod is removed because I thought that positive charges do not move. In one textbook, it says that they do not move but in my other textbook, it physically shows the positive charges redistributing themselves when the negative rod is removed...that is confusing me...
     
  5. Jan 25, 2010 #4

    kuruman

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    Positive charges do not move. Consider a conductor that is initially neutrally charged. If a volume element within the conductor loses negative charges, it acquires a net positive charge and some other volume element in the conductor acquires a net negative charge; if the volume element gains net negative charge, some other part of the conductor acquires a net positive charge. The motion of negative charges within the conductor creates "absences" of negative charge (i.e. a positive charge) in other parts of the conductor. What moves inside a conductor is not the positive charges, but the absence of negative charges, if you can imagine that. That's probably what your other textbook is (should be) trying to illustrate.

    I hope I did not confuse you, but the matter is a bit confusing.
     
    Last edited: Jan 25, 2010
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