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Homework Help: Problem related to two conductors in contact

  1. Oct 24, 2015 #1
    1. The problem statement, all variables and given/known data
    An isolated conducting spherical shell carries a negative charge. What will happen if a positively charged metal object is placed in contact with the shell interior? Discuss the three cases in which the positive charge is a) less than , b) equal to, and c) greater than the negative charge in magnitude

    2. Relevant equations
    Important point:
    The net electric charge of a conductor resides entirely on its surface.
    3. The attempt at a solution
    I want to know
    charged metal object is placed in contact with
    what does it mean?
    Does it mean charged metal object is placed inside the shell interior?
    in contct.png
    Is it correct?
  2. jcsd
  3. Oct 24, 2015 #2

    Simon Bridge

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    When something is placed in contact with something else it means the two things are touching each other.
  4. Oct 24, 2015 #3
    You mean conductor touches another conductor's interior?by?How would the setup look like?
  5. Oct 24, 2015 #4


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    Staff: Mentor

    It would look like a pair of 3 dimensional bodies as represented in your 2D sketch. Imagine a small metal egg held inside a larger hollow metal shell so that there is air all around the smaller one, insulating it. Then allow the middle one to fall onto the interior surface of the larger one when you want them to contact.
  6. Oct 24, 2015 #5
    Did not understand this .
  7. Oct 24, 2015 #6


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    Object in contact with the shell interior:

  8. Oct 28, 2015 #7
    @ehild diagram you drew greatly explained my doubt.
    Now ,could you please do one more favor?
    Can't we include charges in that particular figure?
  9. Oct 28, 2015 #8
    Is it right ?
    I am not showing that Q>q or any such thing.I am just considering the general case.
    Just want to know/confirm location of charges whatever the magnitude may be .
  10. Oct 28, 2015 #9
    After the conductors have been put in contact in contact with one another, those will no longer be the charges on them.
  11. Oct 28, 2015 #10
    You meant those will no longer bear the charges on them,right?
  12. Oct 28, 2015 #11
    Can you please tell the reason?
  13. Oct 28, 2015 #12
    What are your thoughts on this?
  14. Oct 28, 2015 #13
    I am unable to comprehend this.
    As I wrote
    How can we cancel them?
  15. Oct 28, 2015 #14


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    The charges will be redistributed. What facts do you know about the distribution of charge on a conductor?
  16. Oct 28, 2015 #15
    Gracy, here's a multiple choice quiz for you:
    1. In a conductor, positive and negative charges are regarded as (a) mobile or (b) not mobile?
    2. Positive and negative charges (a) attract or (b) repel?
    3. If two conductors are brought into contact with one another, charges can (a) freely pass between them or (b) not freely pass between them?
    4. If a positive and a negative charge are allowed to come into contact, they (a) neutralize each other or (b) don't neutralize each other?
  17. Oct 28, 2015 #16
    In a conductor
    positive charges - not mobile
    Negative charges-mobile
    Positive and negative charges attract each other.
    Before answering this I want to know one thing,are these conductors neutral?
  18. Oct 28, 2015 #17
    neutralize each other
  19. Oct 28, 2015 #18
    In my question, I was careful to use the words "are regarded as." Positive charges are just a deficit of electrons, so, if electrons can move in one direction, isn't that the same as positive charges moving in the opposite direction. If positive charges are not mobile, how come, when a conductor is charged positively, all the positive charges migrate to the outer surface of the conductor?
    If the conductors were neutral, there could not be a net migration of charge between them. So, what if they were not neutral to begin with?
  20. Oct 28, 2015 #19
    I thought by positive charge you meant proton.That's why I said not mobile.
  21. Oct 28, 2015 #20
  22. Oct 28, 2015 #21
    Previous post went all wrong.sorry
    They neutralize each other
    Last edited: Oct 28, 2015
  23. Oct 28, 2015 #22
    Even if a conductor has atoms within it that are positively charged because they had lost electrons, electrons can migrate to them from adjacent neutral atoms to neutralize the charge, while the adjacent atoms then become positively charged. Certainly this constitutes mobility of positive charge.
  24. Oct 28, 2015 #23
    The question was "If two conductors are brought into contact with one another, charges can (a) freely pass between them or (b) not freely pass between them?" The question was not "what will ultimately happen." And, if the charges on the two conductors were not equal and identical in sign to begin with, they certainly will not ultimately fully neutralize.
  25. Oct 28, 2015 #24
    Charges can freely pass between them.
  26. Oct 28, 2015 #25
    So now, based on the answers to these four questions, how would you analyze the original problem at hand?

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