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Flux Integral Problem

  1. May 6, 2015 #1
    1. The problem statement, all variables and given/known data
    Hello,
    So I've got to evaluate a flux integral, but I get 0. Could someone please, check my workings and give me advice.
    Please find my workings and the problem attached here as an image.
    Thank you very much!

    flux%20integral_zpsq1b9chvp.jpg
     
  2. jcsd
  3. May 6, 2015 #2

    LCKurtz

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    I will give you that your handwriting is pretty nice. Still it would be better to type your work. Is that third component of your flux vector a ##z## or a ##2##? Some mathematical issues are: Is your normal directed upward like it is supposed to be? In fact, is it even a vector? That's another reason to type your question, so we can edit it. Also you can't say ##x^2+y^2=1## for the normal. That is only true on the boundary of the surface but not on the surface itself. And check your partial derivatives.
     
  4. May 6, 2015 #3

    Zondrina

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    You can parametrize the surface if you want, let:

    $$\vec r(x, y) = x \hat i + y \hat j + z \hat k = x \hat i + y \hat j + (1 - x^2 - y^2) \hat k$$

    Find ##\vec r_x(x, y)## and ##\vec r_y(x, y)##. Use those to obtain ##\vec r_x \times \vec r_y##. Now you have a theorem which states:

    $$\iint_S \vec F(x, y, z) \cdot d \vec S = \iint_D \vec F(\vec r(x,y)) \cdot (\vec r_x \times \vec r_y) \space dA$$

    Where the region ##D## is given by the plane ##z = 1 - x^2 - y^2## for ##0 \leq z \leq 1##.

    Plotting the plane for the points ##(x, 0, 0), (0, y, 0)##, and ##(0, 0, z)## will give you the limits easily, and a nice graph to look at.
     
  5. May 6, 2015 #4

    LCKurtz

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    The ##\pm## sign is necessary because one has to decide whether ##\vec r_x \times \vec r_y## points in the required direction to agree with the given orientation. The OP didn't check that.
     
  6. May 7, 2015 #5
    Got confused..

    g = x2 + y2 + z -1 = 0
    n = [itex]\frac{ \nabla g}{|\nabla g|}[/itex]
    n =[itex] \frac{ 2x+2y+1}{\sqrt{2x^2+2y^2+1^2}}[/itex]
    and dS =[itex]\sqrt{2x^2+2y^2+1^2}[/itex] dxdy
    ?
     
    Last edited: May 7, 2015
  7. May 7, 2015 #6

    HallsofIvy

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    This last is wrong. n is a vector not a scalar. What is [itex]\nabla g[/itex]?
     
  8. May 7, 2015 #7
    g(x,y,z) = z - f(x,y) = 0 according to my lecture notes.
    I tried out the latex output, since the preview didn't work out. I've made other edits.
    I am quite confused, I solved other questions using the method with g, but this one, don't know if it should be solved with rdrdθ or just dxdy . The confusing thing is that here I have z = 1-x2-y2
     
    Last edited: May 7, 2015
  9. May 10, 2015 #8
    Hello, I've got exam on the same things, so please give some feedback on my workings

    This surface is the graph of the function [itex]f(x,y) = 1-x^2-y^2 \mbox{ for } x^2+y^2 <= 1[/itex], so use the graph parametrization
    [itex] X(x,y) = (y,x, 1 - x^2 - y^2).[/itex]
    We shall use the following (very important!) general formula for the upward normal of a surface defined by an equation of the form [itex] z = f(x,y):[/itex]
    [itex]N(x,y)=(-f1, -f2,1)[/itex]

    For the surface of interest here, it follows that:
    [itex]N(x,y) = (-(-2x),-(-2y),1) [/itex]

    And we have [itex]F(X(x,y)) = (y,x,1-x^2-y^2)[/itex]

    So [itex]F \cdot N = 2xy+2xy+1-x^2 - y^2 [/itex]

    So we have [itex]\ \int_0^{2\pi} \int_0^1 (4cos( \theta )sin( \theta ) + 1 - r^2) rdrd \theta [/itex]
     
  10. May 10, 2015 #9

    Zondrina

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    Sorry for that a bit earlier where I mentioned ##z = 1 - x^2 - y^2## is a plane, it's a paraboloid.

    As for the actual question, I worked it out to be:


    IMG_0576.jpg

    Sorry for the squeeze near the bottom I was writing a bit too quickly and it got cut off.
     
  11. May 10, 2015 #10
    Yeah, I got the same, I just played it smart, but I think your solution is correct. I substituted r = x + y in the integral ## 1 - x^2 - y^2## with ## 1 - ( x^2 + y^2)## to get ## 1 - r^2## and also for x and y since r = 1 I think I saw somewhere to replace ##rcos( \theta ) ## with ## 1cos( \theta )##
    Otherwise your should be the correct one.
     
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