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Homework Help: Seperation Vector

  1. Aug 28, 2005 #1
    Separation Vector

    Let [itex]\vec{r}[/itex] be the seperation vector from a fixed point [itex](\acute{x},\acute{y},\acute{z})[/itex] to the source point [itex](x,y,z)[/itex].

    Show that:

    [tex]\nabla(\frac{1}{||\vec{r}||}) = \frac {-\hat{r}} {||\vec{r}||^2} [/tex]

    Now, I've attempted this comeing from the approach that [itex]||\vec{r}|| = (\vec{r} \cdot \vec{r})^\frac {1} {2} [/itex] but it dosent seem to get me anywhere, am I missing something blatently obvious?

    Last edited: Aug 28, 2005
  2. jcsd
  3. Aug 28, 2005 #2
    Go back to the definition of the gradient in spherical coordinates.
  4. Aug 28, 2005 #3
    Wouldnt that just complicate things further?

    In Spherical Coordinates:

    [tex]\displaystyle{ \nabla = \hat{r} \frac {\partial{}{}} {\partial{}{r}} + \frac {1}{r} \hat{\phi}\frac {\partial{}{}} {\partial{}{\phi}} + \frac {1}{r sin \phi} \hat{\theta}\frac {\partial{}{}} {\partial{}{\theta}} }[/tex]

    I just don't see how that could simplify things?
  5. Aug 28, 2005 #4
    Okay, nevermind on this...

    Went with a totally different appraoch and things worked out nicely without having to go into spherical coordinates.

    Thanks again.
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