Divergence theorem with inequality

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Homework Help Overview

The discussion revolves around the application of the divergence theorem in the context of calculating the flux of a vector field, specifically F(x,y,z)=4x i - 2y^2 j +z^2 k, through various surfaces of a cylinder defined by the inequalities x^2+y^2<=4 and 0<=z<=3. Participants are exploring the implications of using inequalities versus equalities in defining the surfaces for flux calculation.

Discussion Character

  • Exploratory, Assumption checking, Problem interpretation

Approaches and Questions Raised

  • Participants are questioning the differences in outcomes when using inequalities compared to equalities in the surface definitions. They are also discussing how to express the flux through different surfaces of the cylinder, including the top, bottom, and side surfaces, and the implications of these expressions.

Discussion Status

There is an ongoing exploration of how to express the surfaces in terms of inequalities and what that means for the flux calculations. Some participants have provided specific parameterizations for the surfaces, while others are seeking clarification on the concept of the "tangent surface." The discussion is active, with modifications to questions being made as participants refine their understanding.

Contextual Notes

Participants have noted previous mistakes in their questions and are working to clarify their understanding of the problem setup. There is a focus on ensuring that the definitions used align with the requirements of the divergence theorem.

kelvin56484984
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Homework Statement



F(x,y,z)=4x i - 2y^2 j +z^2 k

S is the cylinder x^2+y^2<=4, The plane 0<=z<=6-x-y

Find the flux of F

Homework Equations



The Attempt at a Solution



What is the difference after if I change the equation to inequality?

For example :
x^2+y^2<=4, z=0

x^2+y^2<=4 , z=6-x-y

x^2+y^2=4, z=6-x-y

thanks!
 
Last edited:
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kelvin56484984 said:

Homework Statement



F(x,y,z)=4x i - 2y^2 j +z^2 k

S is the cylinder x^2+y^2=4, 0<=z<=3

Homework Equations



The Attempt at a Solution


[/B]
I find that the answer is 84 pi

What is the difference after if I change the equation to inequality?

For example :
x^2+y^2<=4, z=3

x^2+y^2<=4 , z=0

X^2+y^2=4, z=3

thanks!

Make a sketch and everything will become clear.
 
kelvin56484984 said:

Homework Statement



F(x,y,z)=4x i - 2y^2 j +z^2 k

S is the cylinder x^2+y^2=4, 0<=z<=3

Homework Equations



The Attempt at a Solution


[/B]
I find that the answer is 84 pi

Answer to what??

What is the difference after if I change the equation to inequality?

For example :
x^2+y^2<=4, z=3

x^2+y^2<=4 , z=0

X^2+y^2=4, z=3

thanks!

One inequality gives the surface of the cylinder and the other gives the solid cylinder.
 
Sorry, I made some mistake previously and I modified the question.
If I want to find the flux of the bottom face, top face, side surface of cylinder and the tangent surface of the cylinder,
How can I express it in inequality?
 
kelvin56484984 said:
Sorry, I made some mistake previously and I modified the question.
If I want to find the flux of the bottom face, top face, side surface of cylinder and the tangent surface of the cylinder,
How can I express it in inequality?
Bottom: ##\vec R(x,y) = \langle x,y,0\rangle,~0\le x^2+y^2\le 4##
Top: ##\vec R(x,y) = \langle x,y,3\rangle,~0\le x^2+y^2\le 4##
Side: ##\vec R(\theta,z) = \langle 2\cos\theta,2\sin\theta,z\rangle,~0\le \theta\le 2\pi,~0\le z\le3##
I have no idea what you mean by the "tangent surface".
You could, and likely should, use polar coordinates for the first two instead of ##x## and ##y## parameters.
 

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