Chapter 9: Vector Calculus
Section 9.3: Differential Operators
Example 9.3.10
Derive the expression for the curl in cylindrical coordinates.
Solution
Mathematical Solution
An extension of the results in Example 9.2.9 gives
i=cosθ er−sinθ eθ
j=sinθ er+cosθ eθ
k=ez
and
er=cosθ i+sinθ j
eθ=−sinθ i+cosθ j
ez=k
so that the Cartesian form of F=fr,θ,z er+gr,θ,z eθ+hr,θ,z ex becomes
G=(x f−y g)/r(y f+x g)/rh≡ABC
The curl of the Cartesian vector G is then
∇×G=Cy−BzAz−CxBx−Ay≡uvw
where, by the chain rule,
Cy=hr yr+hθ xr2=hr sinθ+hθcosθr
Bz=fz yr+gz xr=fz sinθ+gz cosθr
Cy−Bz=hr sinθ+hθcosθr−fz sinθ−gz cosθr≡u
Az=fz xr−gz yr=fz cosθ−gz sinθ
Cx=hr xr−hθ yr2=hr cosθ−hθ sinθr
Az−Cx=fz cosθ−gz sinθ−hr cosθ+hθ sinθr≡v
Bx=r y f+x gx−y f+x g xrr2=y fr xr−fθ yr2+g+x gr xr−gθ yr2r−x y f+x2gr3
Ay=r x f−y gy−x f−y g yrr2=x fr yr+fθ xr2−g−y gr yr+gθ xr2r−x y f−y2gr3
Bx−Ay
=−1rfθ x2+y2r2+2 gr+gr x2+y2r2−g x2+y2r3
=−fθr+2 rr+gr−gr
=−fθr+gr+g≡w
The Cartesian vector u i+v j+w k becomes the cylindrical vector
u cos(θ)−sin(θ)0+v sin(θ)cos(θ)0+w 001 = u cos(θ)+v sin(θ)−u sin(θ)+v cos(θ)w
where
u cosθ+v sinθ
=hr sinθ+hθcosθr−fz sinθ−gz cosθr cosθ +fz cosθ−gz sinθ−hr cosθ+hθ sinθrsinθ
=hθr−gz
−u sinθ+v cosθ
=fz cosθ−gz sinθ−hr cosθ+hθ sinθr cosθ −hr sinθ+hθcosθr−fz sinθ−gz cosθr sinθ
=fz−hr
so ∇×F=hθr−gzfz−hr−fθr+gr+gr.
Maple Solution - Interactive
The Context Panel can only be invoked on the displayed form of objects. The displays in this interactive derivation are very large; a few small adjustments to notation help alleviate this problem. But the computation can seem overwhelming because of the visual clutter.
Define a sequence of the cylindrical coordinate variables.
P≔r,θ,z:
Suppress the appearance of the arguments r,θ,z.
Typesetting:-Suppressfp,gP,hP:
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 F, a vector field in cylindrical coordinates
Write the free vector. Context Panel: Evaluate and Display Inline
Context Panel: Student Vector Calculus≻ Conversions≻Apply Co-ordinate System≻ Conversions≻To Vector Field≻Assign to a Name≻F
f,g,h = f⁡pg⁡r,θ,zh⁡r,θ,z→apply coordinatesf⁡pg⁡r,θ,zh⁡r,θ,z→to Vector Fieldf⁡pg⁡r,θ,zh⁡r,θ,z→assign to a nameF
Convert the cylindrical vector field F to a Cartesian vector field G
Write F. Context Panel: Evaluate and Display Inline
Context Panel: Student Vector Calculus≻ Conversions≻Change Co-ordinate System
Context Panel: Assign to a Name≻G
F = f⁡pg⁡r,θ,zh⁡r,θ,z→change coordinatesf⁡p⁢xx2+y2−g⁡x2+y2,arctan⁡y,x,z⁢yx2+y2f⁡p⁢yx2+y2+g⁡x2+y2,arctan⁡y,x,z⁢xx2+y2h⁡x2+y2,arctan⁡y,x,z→assign to a nameG
Compute the curl of G, which possible since the form of the curl in Cartesian coordinates is known.
Convert this curl from Cartesian to cylindrical coordinates.
Common Symbols palette: Apply ∇× to G and press the Enter key.
Context Panel: Student Vector Calculus≻Conversions≻Change Co-ordinate System (See Figure 9.3.10(a).)
Context Panel: Simplify≻Assuming Positive
Context Panel: Simplify≻With Side Relations (See Figure 9.3.10(b).)
Context Panel: Apply a Command≻convert, diff (See Figure 9.3.10(c).)
Context Panel: Expand
Figure 9.3.10(a) Change coordinate system
Figure 9.3.10(b) Simplify with side relations
Figure 9.3.10(c) Apply a command
∇×G =
D1⁡h⁡x2+y2,arctan⁡y,x,z⁢yx2+y2+D2⁡h⁡x2+y2,arctan⁡y,x,zx⁢1+y2x2−D3⁡g⁡x2+y2,arctan⁡y,x,z⁢xx2+y2−D3⁡g⁡x2+y2,arctan⁡y,x,z⁢yx2+y2−D1⁡h⁡x2+y2,arctan⁡y,x,z⁢xx2+y2+D2⁡h⁡x2+y2,arctan⁡y,x,z⁢yx2⁢1+y2x2D1⁡g⁡x2+y2,arctan⁡y,x,z⁢xx2+y2−D2⁡g⁡x2+y2,arctan⁡y,x,z⁢yx2⁢1+y2x2⁢xx2+y2+2⁢g⁡x2+y2,arctan⁡y,x,zx2+y2−g⁡x2+y2,arctan⁡y,x,z⁢x2x2+y23/2+D1⁡g⁡x2+y2,arctan⁡y,x,z⁢yx2+y2+D2⁡g⁡x2+y2,arctan⁡y,x,zx⁢1+y2x2⁢yx2+y2−g⁡x2+y2,arctan⁡y,x,z⁢y2x2+y23/2
→change coordinates
D1⁡h⁡r2⁢cos⁡θ2+r2⁢sin⁡θ2,arctan⁡r⁢sin⁡θ,r⁢cos⁡θ,z⁢r⁢sin⁡θr2⁢cos⁡θ2+r2⁢sin⁡θ2+D2