Transformation matrix derivation problem

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Homework Help Overview

The discussion revolves around deriving a transformation matrix to convert vectors from Cartesian coordinates to spherical coordinates. The original poster outlines an equation involving transformation coefficients and expresses uncertainty about how to derive the transformation matrix T.

Discussion Character

  • Exploratory, Assumption checking, Conceptual clarification

Approaches and Questions Raised

  • Participants discuss the relationship between Cartesian and spherical coordinates, with some suggesting starting with a two-dimensional case for simplicity. There is a focus on finding the transformation coefficients and understanding the angles involved in the transformation.

Discussion Status

Participants are actively engaging with the problem, with some providing guidance on approaching the derivation of the transformation matrix. There is recognition of the complexity involved in converting between coordinate systems, particularly regarding the linearity of the transformation.

Contextual Notes

Some participants note that the transformation matrix is intended for local vectors at a specific point rather than global coordinates, which raises questions about the assumptions underlying the problem setup.

Fryderyk
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Homework Statement



As part of an assignment, I need to derive a transformation matrix to convert a vector in cartesian coordinates to spherical coordinates.

Homework Equations



What I've got so far is:

For an arbitrary vector V,

<br /> \textbf{V}=\left[\begin {array}{ccc}V_{x}&amp;V_{y}&amp;V_{z}\end{array}\right]\left[\begin{array}{c}\textbf{i}\\\textbf{j}\\\textbf{k}\end{array}\right] = \left[\begin{array}{ccc}V_{R}&amp;V_{\theta}&amp;V_{\phi}\end{array}\right]\left[\begin{array}{c}\textbf{e}_{R}\\\textbf{e}_{\theta}\\\textbf{e}_{\phi}\end{array}\right]<br />

Knowing that VR is the component of V in the eR direction, then:

<br /> V_{R}=aV_{x}+bV_{y}+cV_{z}<br />

where a, b and c are some transformation coefficients to be derived from geometry.

Similarly,

<br /> V_{\theta}=dV_{x}+eV_{y}+fV_{z}<br />
and
<br /> V_{\phi}=gV_{x}+hV_{y}+iV_{z}<br />

From this, a transformation matrix T exists such that:

<br /> \left[\begin{array}{ccc}V_{x}&amp;V_{y}&amp;V_{z}\end{array}\right]\textit{T}=\left[\begin{array}{ccc}aV_{x}+bV_{y}+cV_{z}&amp;dV_{x}+eV_{y}+fV_{z}&amp;gV_{x}+hV_{y}+iV_{z}\end{array}\right]=\left[\begin{array}{ccc}V_{R}&amp;V_{\theta}&amp;V_{\phi}\end{array}\right]<br />

The Attempt at a Solution



My problem is finding this matrix T.
It's obvious from inspection that:

<br /> T=\left[\begin{array}{ccc}a&amp;d&amp;g\\b&amp;e&amp;h\\c&amp;f&amp;i\end{array}\right]<br />

But, I'm not sure that just stating 'it's obvious that' will be sufficient.
I'm wondering if and how the below equation can be rearranged and solved to give T.

<br /> \left[\begin{array}{ccc}V_{x}&amp;V_{y}&amp;V_{z}\end{array}\right]\textit{T}=\left[\begin{array}{ccc}V_{R}&amp;V_{\theta}&amp;V_{\phi}\end{array}\right]<br />

After this point, finding the coefficients shouldn't be a problem.

Thank you
 
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Welcome to PF!

Hi Fryderyk! Welcome to PF! :smile:

(have a theta: θ and a phi: φ :wink:)
Fryderyk said:
My problem is finding this matrix T.
It's obvious from inspection that:

<br /> T=\left[\begin{array}{ccc}a&amp;d&amp;g\\b&amp;e&amp;h\\c&amp;f&amp;i\end{array}\right]<br />

But, I'm not sure that just stating 'it's obvious that' will be sufficient.
I'm wondering if and how the below equation can be rearranged and solved to give T.

<br /> \left[\begin{array}{ccc}V_{x}&amp;V_{y}&amp;V_{z}\end{array}\right]\textit{T}=\left[\begin{array}{ccc}V_{R}&amp;V_{\theta}&amp;V_{\phi}\end{array}\right]<br />

Yes, you're really just stating the obvious …

Finding T and finding a b c etc are the same thing.

Try it this way … you have two sets of axes, with angles between them, and you first need to find the angles …

do the two-dimensional case first (it's easier! :wink:) …

er and eθ are at angle θ to i and j, so the matrix is … ? :smile:
After this point, finding the coefficients shouldn't be a problem.

:smile: :smile:
 
hi, I am new to PF and i have a very similer problem to this. I am also having trouble finding the T matrix. so far i have found:
R= (x^2+y^2+z^2)^1/2
theta= sin-1(y/(x^2+y^2)^1/2)
phi =??

hope some one can help...
thanks
 
Welcome to PF!

Hi demha! Welcome to PF! :smile:

(have a square-root: √ and try using the X2 tag just above the Reply box :wink:)
demha said:
hi, I am new to PF and i have a very similer problem to this. I am also having trouble finding the T matrix. so far i have found:
R= (x^2+y^2+z^2)^1/2
theta= sin-1(y/(x^2+y^2)^1/2)
phi =??

You seem to be trying to find a matrix that transforms "global" coordinates.

A matrix is linear, and only gives linear transformations, but Cartesian to polar vectors isn't linear.

The matrix T isn't intended to convert vectors which start at the origin … it's only for converting "local" vectors at a particular point (x,y,z) = (r,θ,φ) … so you're converting from i j and k at that point to er eθ and eφ at that point.

For that, you don't need √ or sin-1, you just use the angles. :wink:

As I said to Fryderyk, try the two-dimensional case first. :smile:
 

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