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Homework Help: Flux problem

  1. Jun 17, 2007 #1
    1. The problem statement, all variables and given/known data
    Calculate the flux:
    line from (4,6) to (2,6)

    using formular: integral v.n dS where v = -yi+xj

    n = j
    x = t:
    y = 6
    Therefore r(t) = ti+6j
    r'(t) = i
    dS = |r't|dt =>dS = dt

    v.n = (-6i+tj) . (j) => t

    (As line is going from right to left (4,6) to (2,6))
    Lower bound of integration = 4:
    Upper bound of integration = 2:


    integral of t dt = t^2/2

    therefore Answer is [2^2/2] - [4^2/2] = 2-8 = -6.

    The correct answer i'm given is 6 but i don't know where i've gone wrong. could someone please tell me :biggrin:

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Jun 17, 2007 #2
    Depends on the direction of flux the question is interested in-- by taking n =j, you assumed they were interested in the flux in the +y direction-- maybe the question wanted the flux in the -y direction? (Which would give you the negative of the n you obtained)
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