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?