Two $2\mathrm{.}5-$cm$-$diameter disks spaced $1\mathrm{.}5$ mm apart form a parallel$-$plate capacitor. The electric field between the disks is $4\mathrm{.}9105$ V$/$m$.$ What is the voltage across the capacitor? An electron is launched from the negative plate. It strikes the positive plate at a speed of $2\mathrm{.}5107$ m$/$s$.$ What was the electron's speed as it left the negative plate?