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Homework Help: Gauss's Law

  1. Feb 22, 2010 #1
    Two long charged, concentric cylinders have radii of 3.0 and 6.0 cm. The charge per unit length is 5.0 x 10-6 C/m on the inner cylinder and -7.0 x 10-6 on the outer cylinder. Find the electric field at (a) r = 3.0 cm and (b) r = 8.0 cm, where r is the radial distance from the common central axis.

    2. Relevant equations
    ε0 ∮EdA = qenc
    λ= Q/L

    3. The attempt at a solution

    EA = qenc0

    E = λL/ π r2 ε0

    From here, I don't know what to do with L. The height of the cylinder should be irrelevant, right?
    I have another similar question using ρ and volume, leaving me with a height in the formula for the volume that I don't know what to do with.

  2. jcsd
  3. Feb 22, 2010 #2


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    EA = qenc/ε0

    this is the right formula, just make it clear, what A is.

  4. Feb 22, 2010 #3
    I figured out what I was doing wrong. The area is the surface area of the cylinder, not the area of the base. So, L cancels when (2 pi r)h is in the denominator.
  5. Feb 22, 2010 #4


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    Well done!

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