Problem 12 - A Complete Resource Book in Physics for JEE Main

Question

A table tennis ball which has been covered with a conducting paint is suspended by a silk thread so that it hangs between two metal plates. One plate is earthed. When the other plate is connected to a high voltage generator, the ball (A) is attracted to the high voltage plate and stays there. (B) hangs without moving. (C) swings backward and forward hitting each plate in turn. (D) is repelled to the earthed plate and stays there.

Step-by-Step Solution

Answer

The correct answer is (C) the ball swings backward and forward hitting each plate in turn.

Step 1: Analyze the initial situation

Initially, the table tennis ball is neutral and is hanging between two plates. There is no motion because there is neither charge on the ball nor on the plates.

Step 2: Explain the charge accumulation

The high voltage generator gives positive charge to one plate, the plate then polarizes the conducting paint on the ball. The side of the ball facing the charged plate becomes negatively charged (as the electrons in the conducting paint are attracted to the positive plate) and the far side of the ball becomes positively charged (as positive charges are left behind).

Step 3: Explain the motion of the ball

Due to the negative charge on the side of the ball facing the plate, the ball is attracted to the positively charged plate. The ball hits the plate and gets a positive charge from it (as it is a conductor and can share charges).

Step 4: Illustrate grounding and repelling

The positively charged ball is then repelled by the positively charged plate and attracted by the earthed plate. Upon hitting the earthed plate, the ball's positive charge is neutralized because the grounding provides an easy path for the excess charge to return to the Earth.

Step 5: Illustrate the continuous movement

The ball is again neutrally charged so it gets charged by the high voltage generator plate and the process repeats, causing the ball to swing back and forth between the plates.