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Electric field in tube

  1. Oct 22, 2007 #1
    A long tube charge with charged with uniform spatial density [tex]\rho[/tex].
    The inner radius of the tube is: a
    The outer radius of the tube is: b

    Need to find the electric field in: a<r<b

    My approach is Gauss:

    E*S=4 [tex]\pi[/tex] kq

    The S is the Gaussean Surface it should be 2 [tex]\pi[/tex] r^2 ??

    and q should be [tex]\rho[/tex]*(r^2-a^2)??
  2. jcsd
  3. Oct 22, 2007 #2
    The answer should be:
    E=2 [tex]\pi[/tex] k[tex]\rho[/tex][tex]\frac{r^2-a^2}{r}[/tex][tex]\hat{r}[/tex]

    So I did something wrong.....
  4. Oct 22, 2007 #3


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    Gold Member

    Is the tube closed? because if it isn't then Gauss law can't be used.
  5. Oct 22, 2007 #4
    what do you mean closed?
    The tube is hollow.....

    If I can't use gauss, what should i use?
  6. Oct 22, 2007 #5
    I still don't understand why I can't use gauss here.
  7. Oct 23, 2007 #6
    Anyone? :)
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