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

[Allenlbq]

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?

 

[lightgrav]

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?)