An uncharged, metallic ball is suspended in the region between two vertical metal plates. If the two plates are charged, one positively and one negatively, then describe the motion of the ball after it is brought into contact with one of the plates.
The two charged plates create a region with a uniform electric field between them, directed from the positive towards the negative plate.
Once the ball is disturbed so as to touch one plate (say, the negative one), some negative charge will be transferred to the ball and an electric force will act on the ball, that will accelerate it to the positive plate.
Once the ball touches the positive plate, it will re’ewe its negative charge, acquire a positive charge and at accelerate back to the negative plate. The metallic ball will continue to move back and forth between the plates until it has transferred all their net charge, thereby making both the plates neutral.