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Homework Help: Flux Through a Cube

  1. Oct 2, 2011 #1
    1. The electric field in the region of space shown is given by E=(8i + 2yj) N/C where y is in m. A) What is the magnitude of the electric flux through each face of the cube. (3.0 M sides, 2.0m from yz axis)? B) Is there any charge in the cube?

    2. E * da

    3. To be honest I have absolutely no idea how to even begin. We did half of a kinda similar problem on the board on Friday. My professor is notorious for pulling this kind of stuff. There are a number of similar problems like this around but I can't seem to follow the logic. I am more concerned with understanding how to do the problem than what I get for a grade on this quiz. Thanks in advance for helping me with this
  2. jcsd
  3. Oct 2, 2011 #2


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    Staff: Mentor

    Part of your problem is no doubt that the question is not clear. What's a "yz axis"? Where is the cube actually located? What is its orientation?
  4. Oct 2, 2011 #3
    The flux can be found using [tex] \Phi = \int E \cdot dA [/tex]
    The dot product inside the integral means you are looking only at the electric field passing perpendicularly though the surface. If the perpendicular field is constant over the surface the equation reduces to [tex] \Phi = \int E \cdot dA = E\int dA = EA[/tex] I would look at each of the six surfaces on the cube individually and find expression for the perpendicular field through it. If it is constant over that surface you can just use EA, but if it varies you will have to do the integral. The total flux will be the sum of all six surfaces. For part b look up Gauss's law.
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