Walker, Halliday & Resnick — Electric Fields: Problem 7

7 Two large parallel copper plates are 8.0 cm apart and have a uniform electric field between them as depicted in Fig. 22-23. An electron is released from the negative plate at the same time that a proton is released from the positive plate. Neglect the force of the particles on each other and find their distance from the positive plate when they pass each other. (Does it surprise you that you need not know the electric field to solve this problem?)