Chapter 9: Vector Calculus
Section 9.4: Differential Identities
Example 9.4.5
If F is a sufficiently well-behaved vector field in spherical coordinates, show that ∇×F is solenoidal.
Solution
Mathematical Solution
Let F=u(ρ,φ,θ)v(ρ,φ,θ)w(ρ,φ,θ) be a vector field in cylindrical coordinates.
From Table 9.3.3, the curl of F is given by
∇×F=eρρ2sinφeφρ sinφeθρ∂ρ∂φ∂θuρ vρ w sinθ=w cos(φ)ρ sin(φ)+wφρ−vθρ sin(φ)uθρ sin(φ)−wρ−wρvρ+vρ−uφρ ≡ UVW
From Table 9.3.2, the divergence of ∇×F is given by ρ2Uρρ2+V sinφφρ sinφ+Wθρ sinφ. Table 9.4.5(a) takes each term separately.
ρ2Uρρ2
=∂ρρ2w cosφρ sinφ+wφρ−vθρ sinφρ2
=∂ρρ w cosφsinφ+ρ wφ−ρ vθsinφρ2
=wcosφsinφ+ρ wρcosφsinφ+wφ+ρ wφ ρ−vθsinφ−ρ vθ ρsinφ ρ2
=wρ2cosφsinφ+wρρcosφsinφ+wφρ2+wφ ρρ−vφρ2sinφ−vφ ρρ sinφ
V sinφφρ sinφ
=∂φuθρ sinφ−wρ−wρsinφρ sinφc
=∂φuθρ−w sinφρ−wρsinφρ sinφ
=uθ φρ−wφ sinφ+w cosφρ−wρcosφ−wρ φsinφρ sinφ
=uθ φρ2sinφ−wφρ2−wρ2cosφsinφ−wρρcosφsinφ−wρ φρ
Wθρ sinφ
=∂θvρ+vρ−uφρ ρ sinφ
=vθρ+vρ θ−uφ θρρ sinφ
=vθρ2sinφ+vρ θρ sinφ−uφ θρ2sinφ
Table 9.4.5(a) Divergence of the curl in spherical coordinates
Table 9.4.5(b) gathers up the three sets of terms that are to be summed.
wρ2cosφsinφ+wρρcosφsinφ+wφρ2+wφ ρρ−vθρ2sinφ−vθ ρρ sinφ
uθ φρ2sinφ−wφρ2−wρ2cosφsinφ−wρρcosφsinφ−wρ φρ
vθρ2sinφ+vρ θρ sinφ−uφ θρ2sinφ
Table 9.4.5(b) The terms in ∇·∇×F that are to be combined
Careful matching of terms and the equality of mixed partials leads to the vanishing of the sum of terms in Table 9.4.5(b).
Maple Solution - Interactive
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 spherical vector field F
Write the vector field as a free vector. Context Panel: Evaluate and Display Inline
Context Panel: Student Vector Calculus≻Conversions≻Apply Co-ordinate System (Complete dialog as per figure on right.)
Context Panel: Student Vector Calculus≻Conversions≻To Vector Field
Context Panel: Assign to a Name≻F
uρ,φ,θ,vρ,φ,θ,wρ,φ,θ = u⁡ρ,φ,θv⁡ρ,φ,θw⁡ρ,φ,θ→apply coordinatesu⁡ρ,φ,θv⁡ρ,φ,θw⁡ρ,φ,θ→to Vector Fieldu⁡ρ,φ,θv⁡ρ,φ,θw⁡ρ,φ,θ→assign to a nameF
Compute the divergence of the curl of F
Common Symbols palette: Del, dot product,and cross product operators
Context Panel: Evaluate and Display Inline
∇·∇×F = 0
Maple Solution - Coded
Load the Student VectorCalculus package and execute the BasisFormat command.
withStudent:-VectorCalculus:BasisFormatfalse:
Implement notational simplifications with the declare command in the PDEtools package
PDEtools:-declareuρ,φ,θ,vρ,φ,θ,wρ,φ,θ,quiet
Use the VectorField command in the Student VectorCalculus package to define F
F≔VectorFielduρ,φ,θ,vρ,φ,θ,wρ,φ,θ,sphericalρ,φ,θ =
Verify ∇·∇×F=0
Apply the Curl and Divergence commands from the Student VectorCalculus package.
DivergenceCurlF = 0
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