Gradient and Divergence in spherical coordinates

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  • #1
physicss
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Homework Statement
Hello, are my answers correct?

I had to calculate the gradient of f=z and f=xy in spherical coordinates.
My solution for f= z is: cos(θ) er+ (-sin(θ)) e(θ)

f=xy

rsin^2(θ)sin(2φ) er+ rcos(θ)cos(φ)sin(θ)sin(φ) e(θ)+ rcos(2φ)sin(θ) e(φ)

I also had to calculate the divergence of the following vectorfield (Image)

My result is: (scalarfield)

Sin^2(θ)cos^2(φ) er + rcos(2θ)cos^2(φ) e(θ)-rsin(θ)cos(2φ) e(φ)


Thanks in advance
Relevant Equations
er, e(θ) and e(φ) are the spherical base vectors
9527BE5E-8449-4D24-8FA2-A3BE2FC41DD0.jpeg

Vectorfield for the divergence
 
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  • #2
physicss said:
rsin^2(θ)sin(2φ) er+ rcos(θ)cos(φ)sin(θ)sin(φ) e(θ)+ rcos(2φ)sin(θ) e(φ)
I get a slightly different ##\vec e_\theta##.
Please learn to use LaTeX.
 
  • #3
physicss said:
My result is: (scalarfield)

Sin^2(θ)cos^2(φ) er + rcos(2θ)cos^2(φ) e(θ)-rsin(θ)cos(2φ) e(φ)
If it is a scalar, how come it has ##\vec e_r## etc?
I also note a dimensional inconsistency.
 
  • #4
You can get the gradient in spherical (or any other) coordinates by direct substitution, using basic linear algebra. From there you can get div, curl, and the Laplacian just by vector operations:
 

FAQ: Gradient and Divergence in spherical coordinates

What is the gradient in spherical coordinates?

The gradient in spherical coordinates (r, θ, φ) is given by the expression:

∇f = (∂f/∂r) e_r + (1/r) (∂f/∂θ) e_θ + (1/(r sinθ)) (∂f/∂φ) e_φ

where f is a scalar field, e_r, e_θ, and e_φ are the unit vectors in the directions of increasing r, θ, and φ respectively.

How do you compute the divergence in spherical coordinates?

The divergence of a vector field A = A_r e_r + A_θ e_θ + A_φ e_φ in spherical coordinates is given by:

∇·A = (1/r^2) ∂(r^2 A_r)/∂r + (1/(r sinθ)) ∂(A_θ sinθ)/∂θ + (1/(r sinθ)) ∂A_φ/∂φ

Why are spherical coordinates used in gradient and divergence calculations?

Spherical coordinates are often used in gradient and divergence calculations because they are well-suited for problems with spherical symmetry. This includes many physical problems in fields like electromagnetism, fluid dynamics, and gravitational fields, where the geometry of the problem naturally suggests the use of spherical coordinates.

What are the unit vectors in spherical coordinates?

The unit vectors in spherical coordinates are:

e_r: Points radially outward from the origin.

e_θ: Points in the direction of increasing polar angle θ, orthogonal to e_r.

e_φ: Points in the direction of increasing azimuthal angle φ, orthogonal to both e_r and e_θ.

How do you convert Cartesian coordinates to spherical coordinates?

To convert Cartesian coordinates (x, y, z) to spherical coordinates (r, θ, φ), use the following transformations:

r = sqrt(x^2 + y^2 + z^2)

θ = arccos(z / r)

φ = arctan(y / x)

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