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Homework Help: Electric Field Between Two Disks

  1. Sep 12, 2014 #1
    1. The problem statement, all variables and given/known data

    Two 10-cm-diameter charged disks face each other, 29cm apart. The left disk is charged to -50 nC and the right disk is charged to +50 nC. What is the magnitude of the electric field E⃗ at the midpoint between the two disks?

    The answer is 3.9 x 104 N/C

    2. Relevant equations

    Edisk = (KQ/R2)[1 - z/√(z2 + R2)]
    Q = Charge of the disk in Coulombs = 5 x 10-8 C
    R = Radius of the disk in meters = 0.05m
    Z = Distance from the disk in meters = 0.145m

    3. The attempt at a solution

    This seems like a simple plug-and-chug to me. I've tried using the Electric Field of a Disk formula above and I've tried doubling my answer to accommodate for the two plates. Can anyone else hit that magic number, and if so, how did you do it? I'd like to know for an exam coming up.

  2. jcsd
  3. Sep 12, 2014 #2


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    Don't forget that you have two disks that are separated (by 29 cm)!
  4. Sep 12, 2014 #3


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    Are you sure about your formula for Edisk?
  5. Sep 12, 2014 #4
    You know, I'm beginning to think that my problem lies there. As I look around the internet, it seems like everyone has their own personal formula for Edisk, such as this website which essentially multiplies my formula by 2.

    Now that's not to say I'm not peeved that my professor picked a homework website that didn't jive with his in-class notes, but that's a separate issue.
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