Calculate Outward Flux Through a Sphere

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The discussion revolves around calculating the outward flux through a sphere, with initial confusion regarding the direction of the unit normal vector due to contradictory problem wording. The user attempted the integration using polar coordinates but arrived at an incorrect answer after extensive calculations. They later realized that applying the divergence theorem simplifies the problem significantly, leading to the correct answer of π/6. The conversation highlights the importance of recognizing when to use the divergence theorem to streamline complex integrals. Ultimately, the correct approach revealed that the solution was much simpler than initially perceived.
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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.
\hat{n}=x\vec{i}+y\vec{j}+z\vec{k}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\sigmad\sigma=\frac{1}{\sqrt{1-x^2-y^2}}\,.dxdy
Converting to polar coordinates:
\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
Let's just say i spent a certain amount of time doing the integration.
\int^1_0 \frac{r^3 {\cos}^2 \theta}{\sqrt{1-r^2}}\,.dr=\frac{2{\cos}^2 \theta}{3}
\int^1_0 2r^2\sin\theta\,.dr=\frac{2\sin\theta}{3}
\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}
\int^{\frac{\pi}{2}}_0 \frac{2{\cos}^2 \theta}{3}+ \frac{2\sin\theta}{3}= \frac{\pi}{6}+ \frac{2}{3}
Somehow i got it wrong, as the correct answer is \frac{\pi}{6}
 
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
dV=\frac{1}{8}\times \frac{4\pi r^2}{3}
where r=1, so dV=\frac{\pi}{6}
div F=1, so the answer is \frac{\pi}{6}
I can't believe it was so easy...
 
Question: A clock's minute hand has length 4 and its hour hand has length 3. What is the distance between the tips at the moment when it is increasing most rapidly?(Putnam Exam Question) Answer: Making assumption that both the hands moves at constant angular velocities, the answer is ## \sqrt{7} .## But don't you think this assumption is somewhat doubtful and wrong?

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