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Homework Help: Why does the E-field inside a cylinder =0?

  1. Jan 28, 2013 #1
    1. The problem statement, all variables and given/known data

    Figure 24-32 shows a section of a long, thin-walled metal tube of radius R, carrying a charge per unit length  on its surface. Derive expressions for E in terms of distance R from the tube axis, considering both (a) r > R and (b) r < R. Plot your results for the r = 0 to r = 5cm, assuming that  λ= 2*10^-8
    C=m//and R = 3cm

    [i cant put the figure since the question is in the book]

    2. Relevant equations

    q = ε0 ∫ E dA

    3. The attempt at a solution

    i got the answer to (a) E=λ/2∏rε0

    i dont understand why (b) is 0
  2. jcsd
  3. Jan 28, 2013 #2
    urgent help due tomorrow
  4. Jan 28, 2013 #3


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    Science Advisor
    Homework Helper
    Gold Member

    You know that field inside a closed conductor, with no internal charges, is 0. You can think of this as being because field lines cannot pass through it. This tube is 'long'; as long as you are not near the ends, any field reaching in from outside will be small.
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