How Can I Use Stokes' Theorem to Show Integral of fgrad(g)*dr=0?

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Homework Help Overview

The discussion revolves around applying Stokes' Theorem to a problem involving two continuously differentiable scalar fields, f and g, defined on a surface S with boundary C. The original poster seeks to demonstrate that the integral of f grad(g) * dr equals zero under the condition that grad(f) is perpendicular to grad(g) x n, where n is a unit normal to the surface.

Discussion Character

  • Conceptual clarification, Mathematical reasoning, Assumption checking

Approaches and Questions Raised

  • The original poster expresses confusion regarding the implications of the perpendicularity condition and its relevance to the problem. They question how this condition aids in demonstrating the integral's value.
  • Another participant clarifies the meaning of perpendicularity in the context of vectors and emphasizes the significance of the dot product being zero.
  • Further discussion includes the introduction of vector identities relevant to the problem, with one participant suggesting that these identities could aid in the solution.
  • The original poster attempts to manipulate the expressions involving the gradients and cross products but seeks clarification on the relationship between grad(f) x grad(g) and f grad(g).
  • Another participant points out that grad(f) x grad(g) and f grad(g) are not equal and suggests using vector identities to compute curl(f grad(g)).

Discussion Status

Contextual Notes

Participants are navigating the complexities of vector calculus and Stokes' Theorem, with some expressing uncertainty about the definitions and relationships between the involved mathematical entities. The original poster's understanding of the problem setup and the implications of the given conditions is still developing.

JaysFan31

Homework Statement


Let S be a simple parametrically defined surface with boundary C as in Stokes' Theorem. Let f and g be two continuously differentiable scalar fields defined on S. Let n be a choice of unit normal on S. If grad(f) is perpendicular to grad(g) x n everywhere on S, show that integral of fgrad(g)*dr=0.
Note: x is cross product and * is dot product


Homework Equations


Stokes' Theorem:
integral of F*dr=integral of curlF*ndS


The Attempt at a Solution


I'm pretty confused on this one.
First off, what does the perpendicular part mean? What does it tell me? Doesn't it just tell me that the cross product exists everywhere? How does this information help me solve the problem and show that the integration equals zero?
 
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You do not know what "perpendicular" means? It means here exactly what it means in elementary geometry- that the two vectors are at right angles to one another. What you are really asking is "What does it tell me". No, it does not tell you that the cross product exists everywhere! As long as the two vectors exists, their cross product exists. What it tells you is much simpler than that: that the dot product of the two vectors is 0.
 
To solve this (and the last post) all you need are some vector identities. Here they are at no extra charge. f is a scalar, a,b,c are vectors. (. is dot, * is scalar multiply).

curl(grad(f))=0
a.(bxc)=b.(cxa)=c.(axb)
curl(f*a)=grad(f)xa+f*curl(a)

I'm not charging for this service because I'm sure you already had them.
 
Ok. So I get that
grad(f) . (grad(g) x n)=0 where x is cross product and . is dot product since the vectors are perpendicular.

Using the second identity shown,

n . (grad(f) x grad(g))=0.
This equals (grad(f) x grad(g)) . n=0.

How do I show however that (grad(f) x grad(g))=fgrad(g) to make it work?
 
They AREN'T equal. Use one of your free identities to compute curl(f grad(g)). The point to Stokes is integrating curl(F) over the interior and integrating F over the boundary. curl(F) and F are TWO DIFFERENT FIELDS.
 

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