A conducting ball hangs from the ceiling with a flexible insulating. Initially,

Question

A conducting ball hangs from the ceiling with a flexible insulating. Initially, the ball is fixed, and a positively charged small conducting sphere is placed near it as shown in the figure What happens if the ball is released (allowed to move) while still hanging from the ceiling, but only after the positively charged small conducting sphere is allowed to make contact with the ball? If the ball were made of an insulating material (say, a plastic ball), what would you expect to happen in the similar situation as 1)?

Explanation / Answer

(1) If ball is released while still hanging from ceiling, then it will be attracted towards positively charged sphere. When conducting ball was fixed, then the electrons which were free to move inside conducting ball got attracted towards charged sphere. There was a charge distribution with negative charges closer to charged sphere and positive charges farther from charged sphere. Attractive force between negative charge on conducting ball and positive charge on conducting sphere was larger than repulsive force between positive charge on conducting ball and positive charge on conducting sphere. Hence ball got attracted towards conducting sphere after being released.

(2) Now the ball will repel conducting sphere after release. While they were in contact, there was charge distribution on them due to conduction. Both acquired positive charge due to conduction. Since like charges repel each other, ball will repel conducting sphere after being released.

(3) If the ball were made of insulating material, then in case 1, the ball may have got attracted towards conducting sphere. Due to positive charges on sphere, there would be polarization on insulating ball with electrons slightly attracted towards sphere. There would not be charge distribution as electrons cannot move freely inside insulator. If the force of attraction between slightly displaced electrons and positive charge on sphere was sufficient enough then insulating ball may get attracted towards conducting sphere.

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