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Homework Help: Question on Gauss's Law

  1. Feb 13, 2010 #1
    1. The problem statement, all variables and given/known data
    A charge of 9 pC is uniformly distributed
    throughout the volume between concentric
    spherical surfaces having radii of 1.8 cm and
    3.8 cm.

    Let: Ke = 8.98755 × 109 N · m2/C2 .
    What is the magnitude of the electric field
    2.9 cm from the center of the surfaces?
    Answer in units of N/C

    2. Relevant equations
    Electric Flux: (Ie) = E*A = q/epsilon
    Electric Field: E = Ke*q/r^2

    where A = Surface Area of gaussian sphere, and epsilon is a constant = 8.8542e-12

    3. The attempt at a solution
    My approach to this problem was assuming their was an inner charge at the center, which I labeled q. To start I used the formula for net flux Ie (electric flux) = E*A = q/epsilon. I neglected the outer sphere completely and used the principle that E (the electric field) is the electric field just outside the conductor and let that equal the charge given in the problem, and epsilon is a constant equal to 8.8542e-12, and A is the surface area of the inner sphere of radius .018m.

    E = 9 pC (9e-12)
    A = 4pi*r^2 = .004071504079
    q = ?
    epsilon = 8.8542e-12

    Solved for q (my theoretical inner charge)

    I used this equation to find my theoretical inner charge of the inner sphere (q) and then applied the generic formula for an electric field at a point P created by a charge q, E = Ke*q/r^2 to solve for the Electric field at P(a distance r from q).

    Ke = 8.98755e+9
    q = 3.244492027e-25 (answer from first part)
    r = .029m

    Using this approach I came out with E=3.467304913e-12, which needless to say, was incorrect. I feel I may have overcomplicated this problem and am approaching it incorrectly. Any help would be greatly appreciated, thanks.
  2. jcsd
  3. Feb 13, 2010 #2


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    Can you formulate Gauss' Law?

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