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Electric Flux question.

  1. Aug 31, 2008 #1
    A charge of 12 x 10^-6 C is at the geometric center of a cube. Find the Electric Flux through one of the faces?

    My class would have rather left early on friday than go over this example so now I have no clue how to find this.
     
  2. jcsd
  3. Aug 31, 2008 #2

    Doc Al

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    What does Gauss's law tell you about the total flux?
     
  4. Aug 31, 2008 #3
    From what I've read, if the charge distribution is spread uniformly over a plane, we can determine the direction of E from it's symmetry. And if we know charge, we can find magnitude of E and vice versa.
     
  5. Aug 31, 2008 #4
    how much flux passes through all the faces?
     
  6. Aug 31, 2008 #5

    Doc Al

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    You're not asked to find E, just the flux through each face. What's the total flux?
     
  7. Aug 31, 2008 #6
    For my specific problem, the total flux through a cube will be zero.
     
  8. Aug 31, 2008 #7

    Doc Al

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    :bugeye:

    What does Gauss's law tell you about the total flux from a point charge?
     
  9. Aug 31, 2008 #8
    are you trying to visualize it? the whole point of flux lines is so you can easily visualize the field.
     
  10. Aug 31, 2008 #9
    That the total flux through any closed surface is proportional to the net electric charge inside the surface. And at each point on the surface, E is perpendicular to the surface, and its magnitude is the same at every point.

    And yes, Ive tried visualizing it but like I said before the teacher didn't get into details with it and my book doesn't give me any reassurance on what I've visualized.
     
  11. Aug 31, 2008 #10

    Doc Al

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    That's what you need to know. Now how is that flux distributed over the six faces of the cube? Which face gets the most flux?
    That's only true in special cases. It doesn't apply here--luckily you are not asked to find E, only the flux.
     
  12. Aug 31, 2008 #11
    that part you got right

    dont have any idea where you got this. the surface is arbitrary. why would you think then that E would be perpendicular to it?
     
  13. Aug 31, 2008 #12
    Well, with a cube the Area is S2 and out of the 6 sides I only calculated two that did not have a Flux of 0 and I got -ES2 and +ES2
     
  14. Aug 31, 2008 #13
    would it help you to think 2 dimensionally? draw a box with a point charge in the center then draw lines of flux. then generalize the result to 3 dimensions.

    hint:its much much easier than you are making it.
     
  15. Aug 31, 2008 #14
    nslinker, I think an issue you're having is that you're making the problem harder than it needs to be. Think about what flux is and what Gauss's law tells us. One thing is tells us is that the flux is the same through any closed surface with a point charge inside of it. The charge is located at the center so the flux is the same through the cube as it would be through a sphere around the point charge.

    One hint is that you do not need E at all to solve this problem. There's an easier way which involves the symmetry of a cube.
     
    Last edited: Aug 31, 2008
  16. Aug 31, 2008 #15

    Doc Al

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    Nope. Forget about E or the area of the sides. You don't know E, so that will get you nowhere. (That's the hard way to solve this probem.) This problem is much simpler than all that.

    For the nth time, Gauss's law will tell you how to find the total flux from that charge. What is it? (I want the actual value of the total flux.) Hint: The total flux has nothing whatsoever to do with the shape of the container--it will be the same for a cube, a sphere, or any closed surface that contains the charge.
     
  17. Aug 31, 2008 #16
    Okay, I have done a little more reading into the next chapter of my book and and saw that Flux = E x A, and after I put in terms I come out with Flux = q / Epsilon. But I think that's for spheres.


    I tend to have problems with making questions tougher than they need to be and I buckle during tests because of that.
     
  18. Aug 31, 2008 #17
    Your statement about Flux is correct. But Gauss's law tells us the flux through a surface is independent of the shape of that surface. So Gauss's law is for spheres, cubes, pyramids, etc.
     
  19. Aug 31, 2008 #18
    you're missing the whole point of flux lines. the number of lines coming out of each change is proportional to the amount of charge. each line continues outward forever. you're simply counting the number of lines that exit the box. draw the box I told you about and count the number of lines that cross it. it should be perfectly obvious.
     
  20. Aug 31, 2008 #19

    Doc Al

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    Total flux = q/epsilon is for any closed surface, not just spheres. That's what Gauss's law says. You might want to review Gauss's law: Gauss's Law
     
  21. Aug 31, 2008 #20
    Okay, so if Flux = q / Epsilon. Then it's (12x10-6) / (8.854x10-12), correct?
     
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