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Flux in 3 dimensions

  1. Nov 25, 2009 #1

    zcd

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    1. The problem statement, all variables and given/known data
    Find the flux of field [tex]\mathbf{F}=x\mathbf{i}+y\mathbf{j}+z\mathbf{k}[/tex] across the portion of the sphere [tex]x^{2}+y^{2}+z^{2}=4[/tex] in the first octant in the direction away from the origin in three ways:

    a. using formula for flux when sphere is a level surface
    b. using formula for flux when sphere is a parametric surface
    c. using Stoke's theorem

    2. Relevant equations
    [tex]S=\iint \limits_{R}\frac{|\nabla f|}{|\nabla f\cdot\hat{p}|}dA[/tex]

    [tex]Flux=\iint \limits_{S} \mathbf{F}\cdot\mathbf{n} d\sigma=\iiint \limits_{V}\nabla\cdot\mathbf{F}dV[/tex]

    Stoke's theorem (I thought Stoke's theorem involved circulation and curl)?
    [tex]\oint \limits_{C} \mathbf{F}\cdot d\mathbf{r}=\iint \limits_{S}\nabla\times\mathbf{F}\cdot\mathbf{n}d\sigma[/tex]

    Divergence theorem (we haven't learned this yet but I think it will apply better than stoke's theorem for part c)
    [tex]\iint \limits_{S}\mathbf{F}\cdot\mathbf{n}d\sigma=\iiint \limits_{D} \nabla\cdot\mathbf{F}dV[/tex]

    3. The attempt at a solution
    for part a:
    [tex]Flux=\iint \limits_{S} \mathbf{F}\cdot\mathbf{n} d\sigma=\iiint \limits_{V}\nabla\cdot\mathbf{F}dV[/tex]
    the gradient vector is always perpendicular to a surface f(x,y,z)=c, so [tex]\mathbf{n}=\frac{\nabla f}{|\nabla f|}[/tex]
    this should lead to [tex]\iint \limits_{S} \frac{(\mathbf{F}\cdot\nabla f)|\nabla f|}{|\nabla f||\nabla f\cdot\hat{p}|}dA[/tex]
    after some simplifying and conversion to polar coordinates...
    [tex]\int_{0}^{2\pi}\int_{0}^{2} r^{3}+r\sqrt{4-r^{2}}dr d\theta=...=\frac{40\pi}{3}[/tex]

    I temporarily skipped part b cause of the work involved for finding [tex]d\sigma=|\frac{\partial{\mathbf{r}}}{\partial{\phi}}\times\frac{\partial{\mathbf{r}}}{\partial{\theta}}|d\phi d\theta[/tex]

    for part c:
    using divergence theorem to find flux-
    [tex]\nabla\cdot\mathbf{F}=\frac{\partial{M}}{\partial{x}}+\frac{\partial{N}}{\partial{y}}+\frac{\partial{P}}{\partial{z}}=3[/tex]
    [tex]\iint \limits_{S}\mathbf{F}\cdot\mathbf{n}d\sigma=\iiint \limits_{D} \nabla\cdot\mathbf{F}dV[/tex]
    [tex]=\int_{0}^{2\pi}\int_{0}^{\pi}\int_{0}^{2}3\rho^{2}\sin\phi}d\rho d\phi d\theta=...=32\pi[/tex]

    and here's where I have the problem. The same flux from two different methods should not be different. Where in my work did I go wrong?
     
    Last edited: Nov 25, 2009
  2. jcsd
  3. Nov 25, 2009 #2

    LCKurtz

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    You are given just a portion of a sphere which is in the first octant. There is no volume enclosed by it so the divergence theorem is irrelevant. The sphere is cut off by the 3 coordinate planes.
     
  4. Nov 25, 2009 #3

    zcd

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    I overlooked the part about being in first octant, but how would I apply Stoke's theorem to find flux of the field across a surface? I thought Stoke's theorem was flux of the curl of the field?

    Reworking part a:
    [tex]\int_{0}^{\frac{\pi}{2}}d\theta\int_{0}^{2}\frac{4r}{\sqrt{4-r^{2}}}dr=4\pi[/tex]
     
    Last edited: Nov 25, 2009
  5. Nov 25, 2009 #4

    LCKurtz

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    I agree Stokes' theorem doesn't seem appropriate since your [itex]\vec F[/itex] is not the curl of a field. Your answer or [itex]4\pi[/itex] is correct.
     
  6. Nov 25, 2009 #5

    zcd

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    Thanks for the verification. Is there another theorem I can use to finish the problem? Or should I just solve for circulation and give 0 as an answer?
     
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