Chapter 9: Vector Calculus
Section 9.8: Divergence Theorem
Example 9.8.8
Apply the Divergence theorem to the vector field F=x y3i+y z j+x2⁢z k and R, the region bounded by the elliptic paraboloid z=5−3 x2−2 y2 and the plane z=0.
Solution
Mathematical Solution
The divergence of F:
∇·F=∂xx y3+∂yy z+∂zx2z=y3+z+x2
Implement the integral of ∇·F over the interior of R:
∫−5/35/3∫−5−3 x2/25−3 x2/2∫05−3 x2−2 y2divF dz dy dx = 875216⁢6⁢π
To compute the flux through R, note that there are two boundaries, the upper elliptic paraboloid, and the ellipse and its interior that is the intersection of the paraboloid and the plane z=0. To compute the flux through the upper surface, note that on that surface
F·N dσ
=x y3y zx2z·6 x4 y1 11+36 x2+16 y2 1+36 x2+16 y2 dA
=6 x2y3+4 y2z+x2z
=6 x2y3+4 y2+x2 5−3 x2−2 y2
If this be integrated over the ellipse 3 x2+2 y2=5, the result is
∫−5/35/3∫−5−3 x2/25−3 x2/26 x2y3+4 y2+x2 5−3 x2−2 y2 dy dx = 875216⁢6⁢π
On the lower boundary (ellipse), the outward normal is N=−k, so F·N=−x2z, which becomes zero in the plane z=0. Hence, the flux through the interior of the ellipse at the bottom of R vanishes, and the flux through the upper surface matches the volume integral of the divergence.
Maple Solution - Interactive
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
Tools≻Load Package: Student Vector Calculus
Loading Student:-VectorCalculus
Tools≻Tasks≻Browse: Calculus - Vector≻ Vector Algebra and Settings≻ Display Format for Vectors
Press the Access Settings button and select "Display as Column Vector"
Display Format for Vectors
Define the vector field F
Enter the components of F in a free vector. Context Panel: Evaluate and Display Inline
Context Panel: Student Vector Calculus≻Conversions≻To Vector Field
Context Panel: Assign to a Name≻F
x y3,y z,x2z = →to Vector Field →assign to a nameF
Obtain ∇·F, the divergence of F
Common Symbols palette: Del and dot-product operators
Context Panel: Evaluate and Display Inline
Context Panel: Assign to a Name≻divF
∇·F = y3+x2+z→assign to a namedivF
Obtain the volume integral of the divergence of F
Tools≻Load Package: Student Multivariate Calculus
Loading Student:-MultivariateCalculus
Context Panel: Assign to a Name≻Y
5−3 x2/2→assign to a nameY
Write the name given to the divergence and press the Enter key.
Context Panel: Student Multivariate Calculus≻Integrate≻Iterated Complete the dialogs as per the figures below.
Context Panel: Evaluate Integral
divF
y3+x2+z
→MultiInt
∫−13⁢1513⁢15∫−12⁢−6⁢x2+1012⁢−6⁢x2+10∫0−3⁢x2−2⁢y2+5y3+x2+zⅆzⅆyⅆx
=
875216⁢6⁢π
Calculus palette: Iterated triple-integral template
There are two parts to the boundary of R, the surface of the upper hemisphere, and, in the plane z=0, the interior and boundary of the ellipse 3 x2+2 y2=5. 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 an Ellipse
Flux through a Surface Defined over Interior of an Ellipse
For the Vector Field:
Select Coordinate SystemCartesian [x,y,z]Cartesian - othercylindricalsphericalbipolarcylindricalbisphericalcardioidalcardioidcylindricalcasscylindricalconicalellcylindricalhypercylindricalinvcasscylindricallogcylindricallogcoshcylindricaloblatespheroidalparaboloidalparacylindricalprolatespheroidalrosecylindricalsixspheretangentcylindricaltangentspheretoroidal
Maple Solution - Coded
Install the Student VectorCalculus package.
withStudent:-VectorCalculus:
Set display of vectors via BasisFormat.
BasisFormatfalse:
Define F via the VectorField command.
F≔VectorFieldx y3,y z,x2z:
Invoke the Divergence command.
divF≔DivergenceF
Use the int command to integrate the divergence of F over R
intdivF,x,y,z=Region−5/3..5/3,−5−3 x2/2..5−3 x2/2,0..5−3 x2−2 y2,output=integral
intdivF,x,y,z=Region−5/3..5/3,−5−3 x2/2..5−3 x2/2,0..5−3 x2−2 y2
Use the Flux command to obtain the flux of F through the hemispheric surface
FluxF,Surfacex,y,5−3 x2−2 y2,x=−5/3..5/3,y=−5−3 x2/2..5−3 x2/2,output=integral
∫−13⁢1513⁢15∫−12⁢−6⁢x2+1012⁢−6⁢x2+106⁢x2⁢y3+4⁢y2⁢−3⁢x2−2⁢y2+5+x2⁢−3⁢x2−2⁢y2+5ⅆyⅆx
FluxF,Surfacex,y,5−3 x2−2 y2,x=−5/3..5/3,y=−5−3 x2/2..5−3 x2/2
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