Register to reply

Flux through a sphere

by glid02
Tags: flux, sphere
Share this thread:
glid02
#1
Dec3-06, 05:12 PM
P: 54
Here's the question:

A fluid has density 2 and velocity field .
Find the rate of flow outward through the sphere .

So far I've found n, which is
1/2x+1/2y+1/2z

and F dot n gives z^2.

I converted to spherical coordinates and z^2 is equal to 4cos^2[phi].

My integral is set up as:
4*(int[0-2pi] int[0-pi] (cos^2[phi]*sin[phi]dphi dtheta.

The first integral is -1/3cos^3[phi] from 0-pi which is 1/3 - - 1/3 = 2/3

The second integral gives 2/3*2*pi, so the entire thing is 4*2/3*2*pi.

I thought I was just supposed to multiply that by 2 (the density) but that's not the right answer. Can someone tell me what I did wrong or what I'm supposed to do with the density?

Thanks a lot.
Phys.Org News Partner Science news on Phys.org
Scientists develop 'electronic nose' for rapid detection of C. diff infection
Why plants in the office make us more productive
Tesla Motors dealing as states play factory poker
StatusX
#2
Dec3-06, 07:24 PM
HW Helper
P: 2,567
One way to verify the answer is by Gauss' law (aka the divergence theorem). The divergence of the velocity field is seen to be 2, so the flux out of the sphere is 2*(4/3*pi*2^3), which is off by a factor of 4 from what you got. It seems like you forgot the r^2 factor in the surface area element. What you did with the density is correct since it's constant here.


Register to reply

Related Discussions
Flux through a sphere? Calculus & Beyond Homework 8
Flux through a hole in a sphere? Introductory Physics Homework 1
New Flux in a Sphere of moderator of Radius R Nuclear Engineering 15
Why doesn't changing the Gausssian sphere change the net flux? Introductory Physics Homework 7
Magnetic Flux & Electric Flux Classical Physics 2