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Homework Help: Very quick insulating sphere electric field question

  1. Apr 4, 2012 #1
    Given the standard formulas of an insulating sphere, inside and outside respectively:

    [1] Q/4πΣσr^2
    [2] Qr/4πΣσR^3

    - where R referes to the radius of the solid insulating sphere.

    Charged paint is spread in a very thin uniform layer over the surface of a plastic sphere o diameter 12.0cm, gicing it a charge of -35*10^-6C, Find E:
    a) just outside the paint layer?
    b) just inside the paint layer?
    c) 5.00cm outside the surface of the paint layer?

    My Attempt:

    a) just outside: Using [1] with r=6.0cm and Q = -35*10^-6 , I obtain 8.74*10^7N/C , is the correct answer
    b) just inside: E = 0 (From Gauss's Law)
    c) just outside: Using [1] with r = 5.0cm and R = 6.0cm, I obtain 1.26*10^8N/C, however the correct answer is 2.60*10^7N/C

    If anyone could point my in the right direction for c, greatly appreciated, ta =]
  2. jcsd
  3. Apr 4, 2012 #2


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    Homework Helper

    Um, that should be outside and inside respectively because the first equation is for outside and the second equation is for inside.

    Yes, you should be using equation 1, but since you've mentioned R, I'm guessing you actually used equation 2?
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