Chapter 9: Vector Calculus
Section 9.8: Divergence Theorem
Example 9.8.4
Apply the Divergence theorem to the vector field F=x y3i+y z j+x2⁢z k and R, the region bounded above by the upper hemisphere of the unit sphere centered at the origin, and below by 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 in spherical coordinates:
∫02 π∫0π/2∫01ρ sinφsinθ3+ρ cosφ+ρ sinφcosθ2 ρ2sinφ dρ dφ dθ = 2360 π
To compute the flux through R, note that there are two boundaries, the upper hemisphere, and the disk that is its projection onto the plane z=0. To compute the flux through the upper hemisphere, note that on that surface
F·N dσ=x y3y zx2z·xyz dAz = x2y31−x2−y2+y2+x21−x2−y2 dA
If this be integrated over the unit disk in polar coordinates, the result is
∫01∫02 πr r5cos2θsin3θ1−r2+r2sin2θ+r2cos2θ1−r2 dθ dr = 2360 π
On the lower boundary (disk), the outward normal is N=−k, so F·N=−x2z, which becomes zero in the plane z=0. Hence, the flux through the disk 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, and represent it parametrically
Context Panel: Assign name
X=ρ sinφcosθ→assign
Y=ρ sinφsinθ→assign
Z=ρ cosφ→assign
Expression palette: Evaluation template
Common Symbols palette: Del and dot-product operators
Context Panel: Evaluate and Display Inline
Context Panel: Assign to a Name≻divF
∇·Fx=a|f(x)x=X,y=Y,z=Z = ρ3⁢sin⁡φ3⁢sin⁡θ3+ρ2⁢sin⁡φ2⁢cos⁡θ2+ρ⁢cos⁡φ→assign to a namedivF
Use spherical coordinates to obtain the volume integral of the divergence of F
Tools≻Load Package: Student Multivariate Calculus
Loading Student:-MultivariateCalculus
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
ρ3⁢sin⁡φ3⁢sin⁡θ3+ρ2⁢sin⁡φ2⁢cos⁡θ2+ρ⁢cos⁡φ
→MultiInt
∫02⁢π∫012⁢π∫01ρ3⁢sin⁡φ3⁢sin⁡θ3+ρ2⁢sin⁡φ2⁢cos⁡θ2+ρ⁢cos⁡φ⁢ρ2⁢sin⁡φⅆρⅆφⅆθ
=
2360⁢π
There are two parts to the boundary of R, the surface of the upper hemisphere, and, in the plane z=0, 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
Flux through a Surface Defined over a Disk
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:
Obtain ∇·F, the divergence of F
Invoke the Divergence command.
divF≔DivergenceF
y3+x2+z
Use the int command to integrate the divergence of F over R
intdivF,x,y,z=Region−1..1,−1−x2..1−x2,0..1−x2−y2,output=integral
∫−11∫−−x2+1−x2+1∫0−x2−y2+1y3+x2+zⅆzⅆyⅆx
intdivF,x,y,z=Region−1..1,−1−x2..1−x2,0..1−x2−y2 = 2360⁢π
Use the Flux command to obtain the flux of F through the hemispheric surface
FluxF,Surfacex,y,1−x2−y2,x=−1..1,y=−1−x2..1−x2,output=integral
∫−11∫−−x2+1−x2+1x2⁢y3−x2−y2+1+y2+x2⁢−x2−y2+1ⅆyⅆx
FluxF,Surfacex,y,1−x2−y2,x=−1..1,y=−1−x2..1−x2 = 2360⁢π
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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