Calculate Outward Flux Through a Sphere

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SUMMARY

The discussion focuses on calculating the outward flux through a sphere using vector calculus. The user initially grapples with the concept of the unit normal vector and ultimately confirms that the outward flux is calculated correctly as \(\frac{\pi}{6}\). The integration process involves converting to polar coordinates and applying the divergence theorem, leading to the conclusion that the divergence of the vector field is constant at 1. The final answer aligns with the expected result, demonstrating the effectiveness of the divergence theorem in simplifying the calculation.

PREREQUISITES
  • Understanding of vector calculus concepts, particularly flux and divergence.
  • Familiarity with polar coordinate transformations in double integrals.
  • Knowledge of the divergence theorem and its application in calculating flux.
  • Basic integration techniques, including handling trigonometric functions.
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  • Study the application of the divergence theorem in various geometrical contexts.
  • Learn advanced integration techniques in polar coordinates.
  • Explore examples of calculating flux in different vector fields.
  • Review the properties of unit normal vectors in vector calculus.
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Students and professionals in mathematics, physics, or engineering who are working on problems involving vector fields and flux calculations, particularly those studying advanced calculus or electromagnetism.

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Homework Statement
http://s1.ipicture.ru/uploads/20120121/YUmpPl4N.jpg

The attempt at a solution
First of all, taking a look at the wording in the problem itself. I am not sure if the unit normal vector is supposed to be outward or inward. There is the "outward flux" but the "inside the sphere" part is contradicting. Anyway, i went with "outward", although I'm not 100% sure.
[tex]\hat{n}=x\vec{i}+y\vec{j}+z\vec{k}[/tex][tex]Flux=\iint \vec{F}.\hat{n}\,.d \sigma=\iint x^2+2yz\,.d\sigma=\iint x^2+2y\sqrt{1-x^2-y^2}\,.d\sigma[/tex][tex]d\sigma=\frac{1}{\sqrt{1-x^2-y^2}}\,.dxdy[/tex]
Converting to polar coordinates:
[tex]\iint x^2+2y\sqrt{1-x^2-y^2}\,.d\sigma=\iint \frac{x^2}{\sqrt{1-r^2}}+2y\,.rdrd\theta=\iint \frac{r^3 {\cos}^2 \theta}{\sqrt{1-r^2}}+2r^2\sin\theta\,.drd\theta[/tex]
Let's just say i spent a certain amount of time doing the integration.
[tex]\int^1_0 \frac{r^3 {\cos}^2 \theta}{\sqrt{1-r^2}}\,.dr=\frac{2{\cos}^2 \theta}{3}[/tex]
[tex]\int^1_0 2r^2\sin\theta\,.dr=\frac{2\sin\theta}{3}[/tex]
[tex]\int^1_0 \frac{r^3 {\cos}^2 \theta}{\sqrt{1-r^2}}+2r^2\sin\theta\,.dr=\frac{2{\cos}^2 \theta}{3}+\frac{2\sin\theta}{3}[/tex]
[tex]\int^{\frac{\pi}{2}}_0 \frac{2{\cos}^2 \theta}{3}+ \frac{2\sin\theta}{3}= \frac{\pi}{6}+ \frac{2}{3}[/tex]
Somehow i got it wrong, as the correct answer is [itex]\frac{\pi}{6}[/itex]
 
Last edited:
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You can't use the divergence theorem? If I'm reading that right, you spent a lot of time finding the flux through the spherical part of the octant. The boundary of the octant also has planar parts. Integrating through all of the parts isn't the easiest way to do this.
 
Last edited:
After some more thinking, i realize that
[tex]dV=\frac{1}{8}\times \frac{4\pi r^2}{3}[/tex]
where r=1, so [tex]dV=\frac{\pi}{6}[/tex]
div F=1, so the answer is [tex]\frac{\pi}{6}[/tex]
I can't believe it was so easy...
 

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