Evaluate Surface Integral f.n ds for Sphere x^2+y^2+z^2=a^2

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Discussion Overview

The discussion revolves around evaluating the surface integral of the vector field \( f = xi + yj - 2zk \) over the surface of a sphere defined by \( x^2 + y^2 + z^2 = a^2 \) that lies above the x-y plane. Participants explore different approaches to tackle the integration and the implications of the symmetry of the problem.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant expresses difficulty with the integration process and seeks assistance, noting the need to consider the sphere's orthogonal projection onto the x-y plane.
  • Another participant suggests that using the right theorem could simplify the calculation of flux integrals, implying that there may be easier methods available.
  • A third participant reiterates the problem and emphasizes the importance of finding the normal vector to the surface, recommending a change to spherical coordinates due to the problem's symmetry.
  • Another participant points out the anti-symmetry of the vector function \( f \) about the origin and questions what this symmetry implies given the symmetric nature of the sphere, suggesting the use of the Divergence theorem to evaluate the integral over the sphere's interior.

Areas of Agreement / Disagreement

Participants present multiple approaches and insights, but there is no consensus on a single method to evaluate the integral. The discussion includes both supportive suggestions and challenges regarding the integration techniques.

Contextual Notes

Some participants highlight the symmetry of the vector field and the sphere, which may influence the evaluation of the integral, but the implications of this symmetry remain unresolved. The discussion also touches on the potential use of the Divergence theorem without reaching a definitive conclusion on its application.

Who May Find This Useful

Individuals interested in vector calculus, particularly those studying surface integrals and the application of the Divergence theorem in symmetric regions, may find this discussion relevant.

anand
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Problem : Evaluate [double integral]f.n ds where f=xi+yj-2zk and S is the surface of the sphere x^2+y^2+z^2=a^2 above x-y plane.

My effort:: I know that the sphere's orthogonal projection has to be taken on the x-y plane,but I'm having trouble with the integration.Please help!
 
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Calculating flux integrals can be a bit tedious. Although some are very easy when you invoke the right theorem. Like this one.
 
anand said:
Problem : Evaluate [double integral]f.n ds where f=xi+yj-2zk and S is the surface of the sphere x^2+y^2+z^2=a^2 above x-y plane.

My effort:: I know that the sphere's orthogonal projection has to be taken on the x-y plane,but I'm having trouble with the integration.Please help!

you want to find f.n where n is obviously the normal to the surface .. find that first... the easiest way to do this is probably change to spherical coordinates...given the symmetry of the problem
 
Didn't we just have this question? Or was it also posted on a different board?

The vector function, f(x,y,z)= xi+ yj- 2zk, is obviously "anti-symmetric" about the origin: f(-x,-y,-z)= -(f(x,y,z)), while the region of integration, a sphere centered at the origin, is symmetric. What does that tell you?

Or you can use the "Divergence theorem" and integrate \nabla \cdot f over the interior of the sphere, as Galileo suggested. Here \nabla \cdot f is particularly simple.
 

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