Why is the unit normal of a sphere important in vector calculations?

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Discussion Overview

The discussion revolves around the significance of the unit normal vector of a sphere in vector calculations, particularly in the context of vector fields and their representations. Participants explore the differences between the position vector and the unit normal vector, as well as the implications of these distinctions in calculations related to flux.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • Some participants express confusion regarding the distinction between the unit normal vector (n hat) and the position vector (r) in vector field calculations.
  • It is noted that "n" is used to represent the unit normal vector because it is normal to the spherical surface, which is a standard notation.
  • One participant questions why the unit vector is not simply represented as r hat.
  • Another participant calculates the dot product of the force vector and the unit normal vector, arriving at a different result (1/r^4) than what was presented in the linked example (1/r^2), leading to further inquiry about potential errors in their calculations.
  • Clarifications are made regarding the definitions of the vectors involved, with emphasis on the relationship between the position vector and the normal vector for a sphere centered at the origin.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the calculations involving the force vector and the unit normal vector, with differing results presented and no resolution to the discrepancies noted.

Contextual Notes

There are unresolved questions regarding the calculations of vector fields, particularly the definitions and relationships of the vectors involved, which may depend on specific assumptions about the vectors' representations.

Hypersquare
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I was looking at this example:

http://keep2.sjfc.edu/faculty/kgreen/vector/block3/flux/node10.html

and was confused between the difference between \hat{}n and \vec{}r

Why is the original vector field not given in terms of a unit vector? And what difference does this make?

Thanks :)
 
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Sorry that's supposed to be n hat and the vector r, I am a latex noob.
 
I also don't quite get why the is unit vector is not just r hat
 
"r", with the arrow over it is the "position vector" at a given point on the sphere. n with a hat is the unit vector in that direction. I presume they are using "n" to represent the unit vector because it is "normal" to the spherical surface and "n" is the standard notation for a normal vector.

For a sphere with center at the origin, the normal vector at any point is in the direction of the position vector. For any other surface that would not be true.
 
Thank you Ivy. Very helpful.
 
I got \vec{f}.\hat{n} as 1/r^{4} not 1/r^{2} as they got. What have I done wrong?
 
There is no "f" so I assume you mean "F" at the site linked to. That is defined by
\vec{F}= \frac{\vec{r}}{r^3}
\frac{\vec{r}}{r} is the unit vector \vec{n} normal to the sphere so the length of \vec{F} is 1/r^2. I don't know how you would have gotten 1/r^4.
 
I got it by doing:

\vec{F} . \hat{n} = \frac{\vec{r}}{r^{3}} .\frac{\vec{r}}{r} = \frac{1}{r^{4}}

I don't see what is wrong with that.
 

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