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## Homework Statement

A capacitor consists of two large metal disks of radius 2.4 meters placed parallel to each other, a distance of 0.7 millimeters apart. The capacitor is charged up to have an increasing amount of charge +Q on one disk and -Q on the other. At about what value of Q does a spark appear between the disks?

## Homework Equations

E = [Q/(A/2(epsilon))](1- Z / (R^2+Z^2)^1/2)

R>>Z

E = [Q / (pi(r^2) / 2 * 8.85E-12)]

## The Attempt at a Solution

I know that since one disk is positive and one disk is negative, the field points in one direction, so it should be E1 + E2. I tried plugging it into the equation, but then I'd still have 2 variables, E and Q. How do you find the value of Q when a spark appears? Is there a value for this that they did not provide?

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