Force on a charge,F = QE, don't you think it looks similar to F= mg. Force is produced on a charge in the presence of electric field. Just as force is produced on a mass in the presence of gravitational field.In a constant electric field E, there will be constant acceleration of the charge , a = QE/m. In a constant g field(near the surface of Earth) , the constant acceleration a = F/m = g. If you can solve projectile questions well , you can do the same for a mass in g field or for a charge in E field. The difference being the acceleration of a charge is of a larger order of magnitude compared to a mass in g field.Typical values of g= 9.81ms-2 and a = 1.0 x 10(power 15 ) ms-2. Its acceleration and force are in the direction of E if a positive charge is placed within the plates where E is constant. If a negative charge is placed between the plates , E remain the same but the force is in the opposite direction and the acceleration is opposite to E .The equation of motion is applicable for both these cases of constant acceleration.
