[8 points] Two square metal plates of edge length L are arranged parallel to each other a distance d apart (where L is much larger than d) and charged to equal and opposite magnitudes Q. An electron (mass m, charge −e) is fired through a small hole in the left plate, as shown. What minimum speed must the electron have as it passes through the left plate in order to reach the right plate? Express your answer in terms of Q, L, d, e, m, and any other constants.