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Homework Help: The force between charged parallel disks, when distance isn't given?

  1. Jan 27, 2014 #1
    Two circular disks, each of area 2.10×10-4 m2, are situated parallel to one another. The distance between them is small compared with their radii. Both disks are uniformly charged; their charges per unit area are σ = 5.10×10-5 C/m2 for one and -σ = -5.10×10-5 C/m2 for the other. Compute the force exerted by one on the other.

    Hint: You can assume the field from each plate to be that of an infinite sheet.

    Correct me if I'm wrong, but I believe the electric field is calculated using Gauss's Law : E=(charge density)/(2*permittivity of space). But, how in the world do you get the force exerted by one on the other if distance isn't given?
  2. jcsd
  3. Jan 27, 2014 #2


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    the E-field is _uniform_ from + plate to - plate. This means that the E-vectors do not spread, so Gauss is really easy to use. F = qE , like always. (how else would you EVER calculate electric Force?)
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