Imagine a single, stationary charged atomic ion, say a Lithium anion or cation (Li+ or Li-). Now imagine another a single free, oppositely charged particle--perhaps an electron or Hydrogen ion (H+)--passing by the first stationary atomic ion at a "classical" non-relativistic speed. For simplicity, imagine they are both in a vacuum and both in zero gravity and free from outside electrical noise or other forces.
What is a good equation to use to calculate the trajectories and/or movement of both of these particles, knowing the velocities and masses of each?
While Ana V's answer below is very good, I'm really looking for an actual approximate answer of how close the two particles need to be to have measurable movement based on the forces. I'm not looking for a high level of accuracy. Just trying to get a sense of the scale of impact on the trajectory of the moving particle and the movement (if any) of the stationary particle as you vary the closest distance between them. Would they need to be very close to each other (say, less than a micron)? Or would the stationary ion still exert enough force at macro-scale distances (say, a meter?) to measurably change the moving particle's trajectory and/or push away the stationary ion? How close together do they need to be for the electrostatic forces to move one or both of them measurably? Thank you!