Help with Vector Analysis in spherical velocity-space coordinates

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Discussion Overview

The discussion revolves around integrating a solid angle represented as a vector over the surface of a unit sphere. Participants explore the implications of the integration bounds and the nature of the integrand, questioning whether the operation resembles a dot product and how it relates to the concept of solid angles in spherical coordinates.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested, Homework-related

Main Points Raised

  • One participant expresses confusion about integrating a solid angle multiplied by a vector over a sphere, noting that their current approach yields zero.
  • Another participant questions the nature of the integral being performed and asks for clarification on the integrand.
  • A participant suggests that integrating a vector quantity over the sphere results in zero due to symmetry, as inflow and outflow cancel each other.
  • Some participants propose that if the integrand were simply one, the integral would yield the expected solid angle of 4π.
  • There is a discussion about whether the integrand should be the solid angle itself or a vector quantity related to it, with some suggesting that the integral might be misinterpreted as a dot product.
  • One participant raises a concern about the interpretation of the solid angle in relation to points on the sphere, questioning how it can be defined at a point.
  • A later reply introduces the idea of visualizing the integral in the context of a neutron's travel direction, complicating the understanding of the solid angle's representation.
  • Another participant suggests using Green's theorem, though the relevance of this suggestion is not elaborated upon.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the correct interpretation of the integral or the nature of the integrand. Multiple competing views remain regarding the integration process and the meaning of the solid angle in this context.

Contextual Notes

There are unresolved questions about the assumptions underlying the integration, particularly concerning the definition of the solid angle and its representation as a vector. The discussion reflects uncertainty about the mathematical steps involved and the physical interpretation of the problem.

phrozenfearz
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I'm looking at integrating a solid angle multiplied by the vector representing that solid angle over an entire sphere (4pi).

I've split the terms into sin(\theta)cos(\phi)i, sin(\theta)sin(\phi)j, cos(\theta)k, but I am still integrating phi over [0,2pi] and theta over [0,pi]. This doesn't seem right and yields a value of zero for each term. I'm unsure of how to change my bounds of integration here.

Or is...
\int{\overline{\Omega}d\overline{\Omega}}
where \Omega=\{\theta,\phi\}
...similar to a dot-product operation and therefore would equal zero?

Thanks!

edit: I just noticed this forum isn't for homework questions... I seem unable to move it myself, so if any admins see this please move it to the homework forum.
 
Last edited:
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Welcome to PF!

Hi phrozenfearz! Welcome to PF! :smile:

(have a theta: θ and a phi: φ and a pi: π and an integral: ∫ :wink:)

Your limits are fine, but what are you integrating?
 


I have a solid angle being integrated over the surface area of a unit sphere. The only way I can interpret the answer I'm getting is that the integral of a vector w.r.t. a vector is equivalent to doing a dot-product. Therefore, the dot-product of omega with omega is zero.

Thanks for the symbols!
 
phrozenfearz said:
I have a solid angle being integrated over the surface area of a unit sphere.

Yes, but what is your integral …

how are you writing it (in θs and φs)?
 
Ah, sorry.. let me figure out how to export it from MathType...

\[<br /> \int {\Omega d\Omega } = \int {(\Omega _x e_x + \Omega _y e_y + \Omega _z e_z )d\Omega } = \int_0^{2\pi } {d\varphi } \int_0^\pi {\sin \theta d\theta (\Omega _x e_x + \Omega _y e_y + \Omega _z e_z )} <br /> \]<br /> \[<br /> \begin{array}{l}<br /> where \\ <br /> \Omega _x = \sin \theta \cos \varphi \\ <br /> \Omega _y = \sin \theta \sin \varphi \\ <br /> \Omega _z = \cos \theta \\ <br /> \end{array}<br /> \]<br />

Separating each component of Omega and integrating separately gives me zero for each term. In x and y, it's the ∫dφ over [0,2pi] reducing the term to zero, and ∫sinθcosθdθ over [0,pi] reducing the z term to zero. Thus, the integral of that vector/solid-angle is reducing to zero?
 
Of course, the integral over the entire sphere will be 0! Essentially what you are doing is calculating the flow of some vector quantity through the sphere, constant in time. Whatever goes into the sphere on one side, comes out on the other. There is no net "inflow" or "outflow" so the integral over the entire sphere is 0.
 
ah i see what you're doing …

no, if you're trying to integrate Ω over the whole sphere to get 4π, your integrand is just 1 …

∫02π∫0π sinθ dθdφ :smile:
 
tiny-tim said:
ah i see what you're doing …

no, if you're trying to integrate Ω over the whole sphere to get 4π, your integrand is just 1 …

∫02π∫0π sinθ dθdφ :smile:

Yeah, if the integrand were one, it would be simple. But the problem (as stated earlier, I posted this in the wrong forum) asks for the integrand to be Ω. Hence, my confusion. Is it just a dot-product, or am I doing something dreadfully wrong?
 
phrozenfearz said:
… the problem … asks for the integrand to be Ω.

Are you sure?

If the integrand is Ω times the outward unit vector, then the integral is obviously 0, since the vectors at opposite points on the sphere cancel each other!

What is the exact question (if it says "integrate the solid angle Ω over the whole sphere", it probably means ∫ dΩ, not something like ∫ Ω dΩ)?
 
  • #10
Can you help me to visualize this? It doesn't seem obvious to me, but integrals have never been a strong point once they move beyond x-y plots.

The exact question reads:

Integrate ∫ Ω dΩ. Hint: Ω = (iΩx + jΩy + kΩz)
 
  • #11
phrozenfearz said:
Can you help me to visualize this? It doesn't seem obvious to me, but integrals have never been a strong point once they move beyond x-y plots.

The exact question reads:

Integrate ∫ Ω dΩ. Hint: Ω = (iΩx + jΩy + kΩz)

hmm … that doesn't make sense to me …

at each point P of the sphere, what is the integrand as a function of P? :redface:

(It can't be Ω, since you can't have Ω "of a point" …

Ω is a solid angle, and points don't have them! :rolleyes:)

If it's thinking of, say, the electric field from a charge inside a sphere, so that the flux through any small surface area is kΩ , then the whole flux is 4πΩ, but that's from ∫ k (er.dΩ) :confused:
 
  • #12
It's meant to represent the angle in which a neutron is travelling. Omega is made up of theta and phi and if represented as a vector has a length of one. I'm not sure about that last bit you mentioned, but I don't think flux is relevant since we're only talking about one neutron.
 
  • #13
phrozenfearz said:
It's meant to represent the angle in which a neutron is travelling.

So this has nothing to do with the whole sphere?
Omega is made up of theta and phi and if represented as a vector has a length of one.

If it has a length of one, then it has nothing to do with solid angle, it's just the unit vector in the normal direction, er.
 
  • #14
use greens theorem
 

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