When a charged rod is moved towards an uncharged paper cylinder suspended by a string, the cylinder gets attracted to the rod initially but then moves away with a jerk. Why?

Initially, the electrically neutral cylinder gets attracted to the charged (suppose negatively charged )rod. The cylinder, like any other object, has millions of atoms with an equal number of negative and positive charges. When the rod is moved towards the cylinder, the charges in the cylinder move away from the rod, leaving positive charges near the rod. This is why the cylinder gets attracted to the rod. Also, remember that negative charges are free to move about, while positive charges are not.
When, however, the rod touches the cylinder, some of the excess negative charges on the rod get transferred to the cylinder. The cylinder then acquires a net negative charge and is repelled by the rod. Thus, it moves away with a jerk.

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