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Homework Help: [Gauss' Law] Hollow insulating sphere?

  1. Jan 29, 2012 #1
    1. The problem statement, all variables and given/known data

    A hollow insulating sphere with an inner radius of 6.58 cm and outer radius of 11.7 cm has a uniform charge density of 79.9 μC/m3 distributed throughout the volume between.

    If we want to use Gauss' Law to find the electric field at r = 17.2 cm, what "charge enclosed" should we use?

    2. Relevant equations

    RHO = Q/V

    a = .0658 m
    b = .1170 m
    r = .1720 m

    therefore r > b > a

    3. The attempt at a solution

    i kno RHO = Q/V

    i just don't kno what V to use

    i've used v = 4/3(pi)(.1720^3)

    i've used v = (4/3)(pi)(.1720^3 - .1170^3)

    think i've used every combination of r

    i kno i'm overlooking something incredibly simple
    blah!
     
  2. jcsd
  3. Jan 29, 2012 #2
    The charge is only in the middle region and the entire charge is enclosed in a sphere of radius r.

    ie,v = (4/3)(pi)(.1170^3 - .0658^3)
     
  4. Jan 29, 2012 #3
    yes that makes perfect sense
    and i could have swore i used those r's for v

    thanks humanist
     
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