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Solid sphere inside a hollow sphere.

  1. Feb 4, 2008 #1
    Question Details:
    The figure shows a solid metal sphere at the center of a hollow metal sphere.


    1- What is the total charge on the exterior of the inner sphere?

    2- What is the total charge on the inside surface of the hollow sphere?

    3- What is the total charge on the exterior surface of the hollow sphere?

    These problems seem easy, but I just can't figure them out. No similar examples are found in my book. Any help is much appreciated!


    This is what I've done:
    Create an imaginary Gaussian sphere of radius 8cm so that it passes through the point where E is 15000N/C.
    The flux should equal to (-15000)(4pi0.08^2) = q/(8.85x10^-12) the answer i get is wrong!!
     

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    Last edited: Feb 4, 2008
  2. jcsd
  3. Feb 4, 2008 #2

    Doc Al

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    Looks good to me!
     
  4. Feb 4, 2008 #3
    If this is a web-based problem, you may have used the incorrect significant figures. Since the radius is only 1 significant figure of accuracy, your answer should be only 1 sig. fig. when typed in. Give that a try So... -1x10^-8C I think is what you get.

    Steve
     
  5. Feb 25, 2008 #4
    Maybe you DEVIDED instead of MULTIPLYING.. make sure to multiply the FLUX on the left side of the equation X Epsilon constant...
     
  6. Mar 18, 2009 #5
    I have the exact same problem and I am having trouble with parts b and c. Wouldn't the inside surface of the hollow sphere just be the same charge as the solid inner sphere but positive?

    Thanks,
    KEØM
     
  7. Mar 18, 2009 #6

    Doc Al

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    That's right.
     
  8. Mar 18, 2009 #7
    Thank you for your reply. For part c, I can't say now that the charge on the outer surface of the hollow sphere is exactly negative of the charge on the inside surface of the hollow sphere can I? I think I have to use Gauss's Law and choose my gaussian surface to be a sphere with a radius 17cm long measured from the center of the solid inner sphere. Would this be right?

    Thanks in advance,
    KEØM
     
  9. Mar 19, 2009 #8

    Doc Al

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    No, that's not true in general. (That would be true if the total charge on the hollow sphere--counting both inner and outer surfaces--happened to be zero.)
    Exactly right. Gauss's law will give you the total charge within the gaussian surface. That, along with the previous results, will tell you the charge on the outer surface.
     
  10. Mar 19, 2009 #9
    Thanks again for all your help Doc Al.

    KEØM
     
  11. Apr 24, 2009 #10
    Question, I am doing the same problem, but i do not understand why a (-) sign is put for part A) but not B) & C).
     
  12. Apr 24, 2009 #11

    Doc Al

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    How do the answers for parts A) and B) relate to each other? How did you solve part B)?
     
  13. Apr 29, 2010 #12
    Hey I'm just wondering where did you get this quesion from because this was on my midterm and if theres a database of these questions I'd like to study from them
     
  14. May 3, 2010 #13
    Although I am not the original poster, I got mine from the textbook "Physics for Scientists and Engineers" by Randall D. Knight.

    Sorry if that is not much help.
     
  15. Oct 23, 2011 #14
    Why the inside surface of the hollow sphere has the same charge as the solid inner sphere but positive?
     
  16. Oct 23, 2011 #15

    Doc Al

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    Apply Gauss's law, realizing that the field anywhere within the conducting material must be zero.
     
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