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Homework Help: Electric Flux/Gauss' Law

  1. Mar 18, 2013 #1
    1. The problem statement, all variables and given/known data
    A 10 nC point charge is at the center of a 2 m x 2 m x 2 m cube. What is the electric flux through the top surface of the cube?

    2. Relevant equations

    Electric Flux = E x A x cosθ
    Electric Flux = ∫E x dA = Q(inside)/ε
    E (point charge) = kq/r^2 where k = 1/(4pi ε)

    3. The attempt at a solution
    Aside from finding the area, volume, and anything else that might be considered obvious, I've no idea where to head from here. I'm not sure if the cube is completely closed or if the top surface isn't uniform. I'm hoping someone can help walk me through this problem.
    Last edited: Mar 19, 2013
  2. jcsd
  3. Mar 18, 2013 #2


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    It's an imaginary cube. So it's closed and doesn't have any properties that could be nonuniform. You can compute the total flux through the cube, right?? Any reason to believe that more passes through one face than any other?
  4. Mar 19, 2013 #3
    I'm... not sure? The point charge is in the center of the cube itself, so... no?
  5. Mar 19, 2013 #4


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    Ok, so 1/6 of the total flux must pass through each face. Total flux is easy to compute.
  6. Mar 19, 2013 #5
    I think I got it.

    Total Flux of Cube = Q (charge) / ε = (10x10^-9) / (8.85x10^-12)

    1129.94 / 6 = 188.32 N m^2 / C.

    This is the answer I was given. Thanks for the help!
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