Why Don't We Use the Jacobian in Surface Integrals?

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Discussion Overview

The discussion centers around the use of the Jacobian in the context of evaluating surface integrals, specifically questioning why it is not applied when transforming coordinates for a surface integral defined in three-dimensional space.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions the absence of the Jacobian in the evaluation of a surface integral defined by a parameterization of a surface in three-dimensional space.
  • Another participant argues that the Jacobian is not applicable because the situation does not involve changing between coordinate systems of the same dimension, as the surface is two-dimensional within three-dimensional space.
  • A participant seeks clarification on scenarios where the Jacobian would be relevant for surface integrals.
  • There is a suggestion that the Jacobian would be used if the parameterization involved three variables, indicating a transition from one three-dimensional coordinate system to another.
  • It is noted that using the Jacobian would be appropriate when changing parameterizations of a surface, which involves two-dimensional coordinate systems.

Areas of Agreement / Disagreement

Participants express differing views on the applicability of the Jacobian in surface integrals, with no consensus reached on the initial question regarding its use.

Contextual Notes

The discussion highlights the distinction between changing coordinate systems of the same dimension versus working with a surface in a higher-dimensional space, which remains unresolved in terms of specific applications of the Jacobian.

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Why is it that when we evaluate a surface integral of:

f(x, y ,z) over dS, where

x = x(u, v)
y = y(u, v)
z = z(u, v)

dS is equal to ||ru X rv|| dA

Why don't we use the jacobian here when we change coordinate systems?
 
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Because you are NOT "changing coordinate systems"- not in the sense of replacing one 3 dimensional coordinate system with another or replacing one 2 dimensional coordinate system with another. The Jacobian is the determinant of an n by n matrix and so requires that you have the same dimension on both sides. That is not the situation when you have a two dimensional surface in a three dimensional space.
 
What would be a case then where the jacobian matrix would be used in evaluating a surface integral?

Thanks for the response.
 
Would the Jacobian be used if:

x = x(u, v, w)
y = y(u, v, w)
z = z(u, v, w)

?
 
Yes, but of course that's not a "surface integral"- that's changing from one three-dimensional coordinate system to another. You might, after forming the integral over a surface, decide that the integral would be simpler if you chose different coordinates, that is a different parameterization, for the surface. Then you would use the Jacobian to change from one two-dimensional coordinate system to another.
 

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