where Ris the radius of the disk and σis the surface charge density on the disk. See Eq. 22-26. The magnitude of the field at the center of the disk (z= 0) is Ec= /2ε0. We want to solve for the value of zsuch that E/Ec= 1/2. This means 22111.zzzR−=⇒=++Squaring both sides, then multiplying them by z2+ R2, we obtain
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