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Gauss Law

  1. Feb 17, 2013 #1
    1. The problem statement, all variables and given/known data

    -38.0 nC of charge is uniformly distributed throughout a spherical volume of radius 34.0 cm.
    How much charge is contained in a region of radius 23.0 cm concentric with the charge distribution?


    2. Relevant equations

    Charge density = λ/area

    3. The attempt at a solution

    I don't know how to approach this problem. I dont know if it is right to use gaus's law.Please if anyone can give me directions or what topic in physics this problem comes from, I will gladly appreciate. Thanks
     
  2. jcsd
  3. Feb 17, 2013 #2

    tiny-tim

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    Science Advisor
    Homework Helper

    Hi Pruddy! :smile:
    You're told that the distribution is uniform, so this is just geometry

    "-38.0 kg of cheese is uniformly distributed throughout a spherical volume of radius 34.0 cm.
    How much cheese is contained in a region of radius 23.0 cm concentric with the cheese distribution?" :biggrin:
     
  4. Feb 17, 2013 #3
    :confused: Tiny-Tim,
    So am I to used the Electric flux formula to solve this problem? But the question is looking for how much charge?
     
  5. Feb 17, 2013 #4
    This question has nothing to do with the Gauss's Law or electric flux ( since it doesn't mention the electric field ).

    As pointed out in post #2, this is a geometry problem.

    Imagine starting with your sphere and removing a section with radius 23cm.
    What is the relation between the removed part and the whole sphere?
     
  6. Feb 17, 2013 #5

    Doc Al

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    Staff: Mentor

    What you need is charge density ρ = Q/volume.

    How does the total volume (radius 34 cm) compare to the volume of the region (radius 23 cm)?
     
  7. Feb 17, 2013 #6
    Set up a Ratio, [itex]\frac{-38nc}{V_{1}}[/itex]=[itex]\frac{Q_{2}}{V_{2}}[/itex]
     
  8. Feb 17, 2013 #7
    Hi, whynot314.
    I used the formular and this are my calculations:

    q2 = -38 x 10^(-9)/4*pi*r(0.34)^2 = q2/(4*pi*r(0.23)^2

    = -38 x 10^(-9)*4*pi*r(0.23)^2 /(4*pi*r(0.34)^2
    = -2.293 x 10^(-9)
    I dont know if this is right...
     
  9. Feb 17, 2013 #8

    Dick

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    You should be using volumes, not surface areas. This isn't a Gauss' law problem. It's a charge density problem. The volume of a sphere of radius r is (4/3)pi*r^3. But even using area the numbers still don't come out the way you say they do.
     
    Last edited: Feb 17, 2013
  10. Feb 17, 2013 #9
    Hi, whynot314,
    Thanks a lot! You are the best.
     
  11. Feb 17, 2013 #10
    Hi, whynot314,
    I got it now.
     
  12. Feb 18, 2013 #11
    I am studying this stuff right now to so this problem helped me as well.
     
  13. Feb 18, 2013 #12
    wow. That's awesome!
     
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