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Flux in a cone

  1. May 24, 2010 #1
    1. The problem statement, all variables and given/known data

    what is the flux entering a cone of radius r and height h located in a uniform electric field E parallel to its base?

    2. Relevant equations

    flux = E.ds

    3. The attempt at a solution
     
  2. jcsd
  3. May 24, 2010 #2

    ehild

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    What is ds?

    ehild
     
  4. May 24, 2010 #3

    gabbagabbahey

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    Better yet, what does the divergence theorem tell you?
     
  5. May 24, 2010 #4
    it is the small element of the effective area through which flux passes
     
  6. May 24, 2010 #5

    gabbagabbahey

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    He was hinting that your first step should be to find an expression for ds (if you evaluate the flux integral directly).
     
  7. May 24, 2010 #6

    ehild

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    And what is the effective area of a curved surface like that of a cone?

    ehild
     
  8. May 24, 2010 #7
    dat is what i am confused of
     
  9. May 24, 2010 #8
    i see 2 possibilities one the curved surface area of the cone and the other the triangular plane perpendicular to base but which one?
     
  10. May 24, 2010 #9

    ehild

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    In your first formula dΦ= E˙ds is the product of E with the area normal to it. As the field lines are parallel to the base, these surface elements are parts of that triangular cross section of the cone which is perpendicular to the electric field. As E is constant simply multiply E with the area of the triangle.
     
  11. May 24, 2010 #10
    thanx but one more doubt cant we take curved surface area resolved into the field direction in this case?
     
  12. May 24, 2010 #11

    ehild

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    I do not quite follow you, but this is what we do. By definition, the flux is E ds cos (alpha) where alpha is the angle between E and the normal of the surface element. ds cos(alpha) is the projection of the surface area onto a plane normal to E.

    ehild
     
    Last edited: Jun 29, 2010
  13. May 24, 2010 #12
    thanku so much got the point now
     
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