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Conductor with shell problem getting zero as answer?

  1. Sep 27, 2014 #1
    1. Problem Statement:

    The figure shows a cross-section view of a very long cylindrical cable. There is an outer tube
    made of copper, inner radius 2R, outer radius 4R. The inner copper wire has radius R and is
    concentric with the tube. The inner wire has charge density –2λ (per unit length), while the tube
    carries total charge +λ. [You need to show solution method to get full credit on this problem!]
    (a) How does the charge arrange itself on the outer tube? On outside of course!
    b) What is the surface charge density (σ) at the outer surface of the tube, at 4R? Getting zero for this one?

    2. Relevant equations
    sigma=q/A

    3. Work.
    So I used the above formula and got zero for some odd reason. I calculated the sigma inner and thats what giving me zero. Work is attached in jpeg.
     

    Attached Files:

  2. jcsd
  3. Sep 27, 2014 #2
    Hi. Check... [itex]\frac{\lambda - 4\lambda \pi R}{8 \pi R} \neq \frac{\lambda-\lambda}{2}[/itex].
     
  4. Oct 6, 2014 #3
    Ahhh makes sense now. Thanks!
     
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