Calculate Outward Flux Through a Sphere

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In summary, the conversation discusses finding the outward flux through the spherical part of an octant, with some confusion over whether the unit normal vector should be outward or inward. The conversation goes on to discuss converting to polar coordinates, doing the integration, and eventually realizing that the answer is simply \frac{\pi}{6}. The conversation also mentions the possibility of using the divergence theorem instead.
  • #1
DryRun
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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]
 
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  • #2
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.
 
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  • #3
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...
 

Related to Calculate Outward Flux Through a Sphere

1. What is the concept of outward flux in physics?

Outward flux is a measure of the flow of a vector field through a surface. It is used to determine the total amount of a quantity, such as electric field or fluid flow, that passes through a given surface in a specific direction.

2. How is outward flux calculated?

The outward flux is calculated by taking the dot product of the vector field and the unit normal vector to the surface, and then integrating this product over the surface. This gives us the total flux through the surface.

3. What is the significance of outward flux in fluid dynamics?

In fluid dynamics, outward flux is used to determine the rate of fluid flow through a given surface. This is important in understanding fluid behavior and predicting how it will move in a certain direction.

4. Can outward flux have negative values?

Yes, outward flux can have negative values. This occurs when the vector field and the normal vector to the surface are in opposite directions, resulting in a negative dot product. This indicates that the flow is entering the surface rather than leaving it.

5. How does outward flux relate to Gauss's law?

Gauss's law states that the net flux through a closed surface is equal to the enclosed charge divided by the permittivity of the medium. Outward flux is used to calculate the total flux through a surface, making it a crucial concept in understanding and applying Gauss's law.

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