Question

A very small uncharged metal-coated Styrofoam ball is suspended in the region between two parallel oppositely charged metallic plates. A uniform electric field exists between the two plates. 

 

Describe the motion of the ball when it is brought into contact with one of the plates.

Answer 1

• Once the ball is brought in contact with one of the charged plates, say the negatively charged plate, some negative charge gets transferred to the ball. Soon after it gets repelled by the negatively charged plate and attracted to the positive plate at the other end. 
• The ball swings to strike the positive plate. When in contact, the ball loses its negative charge, neutralizes some of the positive charges on the plate, and gains some positive charge on its surface. 

The ball is repelled by the plate in contact and attracted to the opposite plate. 

So the ball now swings towards the negative plate.

• Subsequently, the ball keeps swinging back and forth between the two plates.

The charge keeps getting transferred from one plate to another till both the plates get completely neutralized. The ball stops swinging thereafter.