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Showing Divergence Theorem Equivalence

  1. Mar 26, 2014 #1
    1. The problem statement, all variables and given/known data
    The problem states that a cube encloses charge. This cube is given in three space by <0,0,0> and <a,a,a>. The electric field is given by:
    [itex]\hat{E}=\frac{4e}{a^{2}e_{0}}[\frac{xy}{a^{2}}\hat{i}+\frac{(y-x)}{a}\hat{j}+\frac{xyz}{a^{2}}\hat{k}][/itex]. I am to find the total charge enclosed using both methods or "sides" of the divergence theorem equivalence.

    2. Relevant equations
    [itex]\displaystyle \iiint \limits_U \left({\nabla \cdot \mathbf F} \right) d V = \iint \limits_{\partial U} \mathbf F \cdot \mathbf n \ d S[/itex]
    [itex]\nabla=\frac{\partial}{\partial x}\hat{i}+\frac{\partial}{\partial y}\hat{j}+\frac{\partial}{\partial z}\hat{k}[/itex]

    3. The attempt at a solution

    The first half worked out fine and makes sense, so I think this part is right. Do note; I do get a bit sloppy with notation; this is a rough draft:
    [itex]\int_{0}^{a}\int_{0}^{a}\int_{0}^{a}\nabla\bullet EdV[/itex]

    which makes sense, as there are 3 faces in which flux can go by?

    However, the surface integral is a mess:
    when z=a ; Sz(surface raised from xy by a)=a
    [itex]\int\int\hat{F}\bullet\hat{dS}=\int\int F\bullet-kdxdy
    =-\int_{0}^{a}\frac{xyz}{a^{2}}dxdy=-z\int_{0}^{a}\frac{xy}{a^{2}}dxdy=-\int\frac{x^{2}y}{2a^{2}}dy=\frac{-x^{2}z}{2a^{2}}\int ydy=\frac{-x^{2}zy^{2}}{4a^{2}}=-\frac{a^{3}}{4}
    [itex]\int\int\hat{F}\bullet\hat{dS}=\int\int F\bullet jdxdy
    =\frac{1}{a}\int_{0}^{a}(y-x)dxdz=\frac{1}{a}\int yx-\frac{x^{2}}{2}dz=yxz-\frac{x^{2}z}{2}=a^{2}-\frac{a^{2}}{2}
    [itex]\int\int\hat{F}\bullet\hat{dS}=\int\int F\bullet idydz
    \frac{xy}{a^{2}}dydz=\frac{x}{a^{2}}\int ydydz=\int\frac{xy^{2}}{2}dz=\frac{xy^{2}z}{a^{2}}=\frac{a^{2}}{2}
    And the total:

    which I know is wrong because the flux is zero at a=4.

    Where am I going wrong? I haven't done surface/volume integrals in about 4-5 years, so bear with me. I'm trying to relearn through Boas.
  2. jcsd
  3. Mar 27, 2014 #2


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    Your normal is pointing in the wrong direction. It should point outward from the volume, so on the z=a face, it should point in the ##+\hat{k}## direction.

    You need to calculate the flux through all six faces and sum them.

    Also, your expression for ##\vec{E}## looks wrong. Should the z-component be ##xyz/a^3## rather than ##xyz/a^2##? It doesn't work out dimensionally otherwise.

    Finally, I got a different result for the integral of the divergence, so you probably made an error integrating somewhere. Your setup looked fine.
  4. Mar 27, 2014 #3

    Thankx, it was a cubed!

    I actually made a mistake in the addition; it should be:
    I'm a bit lost with the surface integral.
    Last edited: Mar 27, 2014
  5. Mar 27, 2014 #4


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    I'm not sure what you mean by three flux possibilities, but the integral should evaluate to ##\frac{7 e}{\epsilon_0}##.

    There's no volume involved in doing the surface integrals. I'm not sure what you're getting at.
  6. Mar 27, 2014 #5
    Sorry, I edited it before seeing your response. I'm getting 7/4ths. I'll try it again. thank you.
  7. Mar 28, 2014 #6
    Right Answer

    Yes, same answer youu got, forgot the constant coeff. I should do this much more neatly in lyx; the whole issue was adding improperly there.

    Ok, so I finally got it done:

    And by the surface integral:
    [itex]First at x=0 (the yz axis)

    \int\int F\bullet-idydx=0

    \int\int F\bullet idydx=\frac{x}{a^{2}}\int\int ydydz=\frac{y^{2}zx}{2a^{2}}=\frac{a^{4}}{2a^{2}}=\frac{a^{2}}{2}
    [itex]at y=0

    \int\int F\bullet-jdxdz=\frac{1}{a}\int\int xdxdy=\frac{x^{2}z}{2a}=\frac{a^{2}}{2}
    This is where I went wrong; I automatically assumed this plane to be of zero when there was a y-x term not y-xy term or so.
    [itex]at y = a

    \int\int F\bullet jdxdz=\frac{1}{a}\int(y-x)dxdz=yxz-\frac{x^{2}z}{2}=\frac{a^{3}-\frac{a^{3}}{2}}{a}=a^{2}-\frac{a^{2}}{2}
    [itex]at z = 0

    \int\int F\bullet-kdxdz=0
    [itex]at z=a

    \int\int F\bullet kdxdz=\frac{z}{a^{3}}\int xydxdy=\frac{x^{2}y^{2}z}{4a^{3}}=\frac{a^{5}}{4a^{3}}=\frac{a^{2}}{4}


    Thank you!

    Aside: On a side note, I'm using lyx. I'm actually a MSCS student; I just want to work on scientific computation. Is there a real urgency in learning LaTex itself outside of WYSIG?
    Last edited: Mar 28, 2014
  8. Mar 28, 2014 #7


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    Good work!

    I don't know if there's an urgency to learn LaTeX for your field, but it's not very hard to get decent at it.
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