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Charging by induction - Electrons the ONLY mobile charge carriers?

  1. Dec 6, 2008 #1

    This is my virgin post. This forum has been a great source of information for the past one year. I am a high school teacher from Singapore.

    In the study of electrostatics (more specifically induction), we often encounter the scenario below:

    Two insulated metal spheres touching each other. A negatively charged rod is brought near to the left hand side of the system of spheres. A common question will be to label the distribution of induced charges, which in this case the solution will be: the positive charges (attracted) will be on the left hand side of the system, while the negative charges (repelled) will be on the right hand side. Explanation: electrons are repelled to the right hand side of the system. We can assume that the "loose valence electrons" on the left sphere can cross over to the right sphere (am i right to assume this?:shy:).

    My main question:
    Let's say if i modify the question by saying that a positively charged rod is brought near to the left hand side of the system of spheres. Is it right to say that the repelled positive charges move from the left sphere to the right sphere, instead of always saying that the mobile charge carriers are electrons (in this case moving from right to left under the influence of attraction)?

    My current understanding is no. Atoms in a metal solid are tightly held together. We can extend this strong rigid of particles to charged atoms (aka positive charges) as well. Hence, for metallic conductors, the only mobile charge carriers are electrons.

    I would like to seek the opinons of fellow forummers here. Comments and feedback are much appreciated. :smile:
  2. jcsd
  3. Dec 6, 2008 #2

    Doc Al

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    Staff: Mentor

    Your understanding is correct.
  4. Dec 6, 2008 #3
    You said insulated spheres right?

    For the main question, yes electrons are the mobile carriers of charge in metals. When there is fewer number of electrons in a particular region or group of atoms, there is development of positive charge there. Similarly for negative charge, there has to be an excess of electrons. The ions themselves don't move. Again, motion of electrons is also what causes electricity, not the positive ions of the conductor.
  5. Dec 6, 2008 #4
    In an ordinary metal, it's fair to say that the only carriers of charge and current are negatively-charged conduction electrons.

    But other materials (e.g. p-type semiconductors, or ionic conductors) can have positive carriers.
  6. Dec 6, 2008 #5

    Doc Al

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    Staff: Mentor

    I missed that. Why did you (klng) specify insulated spheres? If they were insulated, then no charge would flow between them.
  7. Dec 6, 2008 #6
    Hi Doc Al,

    Thanks for your reply. Its reassuring to hear that coming from you in ur first reply.

    By insulated, i mean that the 2 spheres are being supported by insulating stands. The metallic surfaces are still exposed for contact. :P
  8. Jan 2, 2009 #7
    i think the transfer of charges are due to the ionization in the air...but i too cant understand as how two physically strong spheres could exchange its electrons.....
  9. Jan 2, 2009 #8

    Doc Al

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    Staff: Mentor

    Why do you think that? Charge will be transferred just fine in a vacuum.
    What does the sphere being "physically strong" have to do with it?

    Electrons easily move about in a conductor. Put two conductors together and electrons can move between them.
  10. Jan 2, 2009 #9
    Doc Al's posts are accurate. In an extreme case of a very highly charged rod (high voltage) maybe some ionization would occur, maybe some sparking if the metal spheres are close but not in contact, but absent such high voltage static electricity the above discussion seems entirely appropriate.

    The ease with which static electricity can be generated in dry air, and jump air gaps, is well known to my dog...my new Yorkie has long hair and when the fireplace drys out our home air her hair sometimes stands straight out!!!
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