When a polythene rod is rubbed with a dry cloth, it becomes negatively charged due to the accumulation of electrons from the cloth. This process is a form of triboelectric charging, where electrons are transferred from one material to another through friction.
When the charged polythene rod is held above small pieces of paper, there are a few physics concepts at play:
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Electrostatic Attraction: The negatively charged polythene rod will induce a positive charge on the surface of the small pieces of paper. This happens because the electrons in the paper are repelled by the negatively charged rod, causing the near side of the paper to become positively charged (due to a slight imbalance of charge).
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Attractive Force: The paper is now attracted to the rod due to the electrostatic force between the negatively charged rod and the positively induced charge on the paper. This attraction causes the paper to jump up towards the rod.
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Contact and Release: When the paper makes contact with the rod, it can become slightly charged (often becoming negatively charged as it may gain some electrons from the rod). Once it becomes negatively charged, it experiences a repulsive force due to the like charges (negative repelling negative), causing the paper to be pushed away from the rod.
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Gravity and Repetition: As the paper is pushed away from the rod, gravity pulls it back down to the table. Once it is back on the table, it loses its charge (either dissipating or neutralizing) and the cycle can repeat when the polythene rod is brought back above it again.
This continuous cycle of attraction and repulsion results in the small pieces of paper jumping up and down between the table and the rod.
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