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Length of the vector (electrostatic cylinder) 
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#1
Nov1912, 10:04 AM

P: 25

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? 


#2
Nov1912, 10:16 AM

P: 1,212

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 rr' even is. It looks like a Pythagoras approach to give you the hypotenuse of the triangle with sides r and zz0.
How would you use the law of cosine, and what problem would using it solve? 


#3
Nov1912, 01:26 PM

P: 1,970

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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