Chapter 9: Vector Calculus
Section 9.8: Divergence Theorem
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Example 9.8.4
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Apply the Divergence theorem to the vector field and , the region bounded above by the upper hemisphere of the unit sphere centered at the origin, and below by the plane .
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Solution
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Mathematical Solution
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The divergence of F:
Implement the integral of over the interior of in spherical coordinates:
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To compute the flux through , note that there are two boundaries, the upper hemisphere, and the disk that is its projection onto the plane . To compute the flux through the upper hemisphere, note that on that surface
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If this be integrated over the unit disk in polar coordinates, the result is
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On the lower boundary (disk), the outward normal is , so , which becomes zero in the plane . Hence, the flux through the disk at the bottom of vanishes, and the flux through the upper surface matches the volume integral of the divergence.
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Maple Solution - Interactive
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The Student VectorCalculus package is needed for calculating the divergence, but it then conflicts with any multidimensional integral set from the Calculus palette. Hence, the Student MultivariateCalculus package is installed to gain Context Panel access to the MultiInt command.
Initialize
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Tools≻Load Package: Student Vector Calculus
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Loading Student:-VectorCalculus
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Tools≻Tasks≻Browse: Calculus - Vector≻
Vector Algebra and Settings≻
Display Format for Vectors
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Press the Access Settings button and select
"Display as Column Vector"
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Display Format for Vectors
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Define the vector field F
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Enter the components of F in a free vector.
Context Panel: Evaluate and Display Inline
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Context Panel: Student Vector Calculus≻Conversions≻To Vector Field
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Context Panel: Assign to a Name≻F
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Obtain , the divergence of F, and represent it parametrically
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Context Panel: Assign name
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Expression palette: Evaluation template
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Common Symbols palette: Del and dot-product operators
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Context Panel: Evaluate and Display Inline
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Context Panel: Assign to a Name≻
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Use spherical coordinates to obtain the volume integral of the divergence of F
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Tools≻Load Package:
Student Multivariate Calculus
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Loading Student:-MultivariateCalculus
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Write the name given to the divergence and press the Enter key.
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Context Panel: Student Multivariate Calculus≻Integrate≻Iterated
Complete the dialogs as per the figures below.
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Context Panel: Evaluate Integral
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There are two parts to the boundary of , the surface of the upper hemisphere, and, in the plane , the unit disk that is the projection of the upper hemisphere. For the flux through the upper surface, use a task template.
Tools≻Tasks≻Browse:
Calculus - Vector≻Integration≻Flux≻3-D≻Through a Surface Defined over a Disk
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Flux through a Surface Defined over a Disk
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For the Vector Field:
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On the lower boundary (disk), the outward normal is , so , which becomes zero in the plane . Hence, the flux through the disk at the bottom of vanishes, and the flux through the upper surface matches the volume integral of the divergence.
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Maple Solution - Coded
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Initialize
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Install the Student VectorCalculus package.
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Obtain , the divergence of F
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Use the int command to integrate the divergence of F over
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Use the Flux command to obtain the flux of F through the hemispheric surface
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On the lower boundary (disk), the outward normal is , so , which becomes zero in the plane . Hence, the flux through the disk at the bottom of vanishes, and the flux through the upper surface matches the volume integral of the divergence.
Maple takes a noticeable amount of time to execute the integrations in Cartesian coordinates. In the Interactive section, these integrations are performed in spherical coordinates, and the time taken is insignificant.
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