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Length of the vector (electrostatic cylinder)

  1. Nov 19, 2012 #1
    http://img842.imageshack.us/img842/2816/unavngivettz.png [Broken]

    My problem is that I'm confused about a hint I was given in this problem. I usually use the law of cosine to find the length of [itex]\vec{r}-\vec{r'}[/itex]. But the hint here says that I should make it [itex][r^2 + (z - z_0)^2]^{1/2}[/itex]

    Where does this come from? I can't quite get my head around the geometrical idea of this hint. Can't the law of cosine be used here?
    Last edited by a moderator: May 6, 2017
  2. jcsd
  3. Nov 19, 2012 #2
    It's really hard to answer these questions when you don't specify what any of the terms mean, so I can only guess at what r-r' even is. It looks like a Pythagoras approach to give you the hypotenuse of the triangle with sides r and z-z0.

    How would you use the law of cosine, and what problem would using it solve?
  4. Nov 19, 2012 #3
    I don't know how the hint is specifically formulated but I think the best way here is to use cylindrical coordinates.
    It may be that r' is the cylindrical radius of the charge element.
    The point P has r=0 and z=zo.
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