Explain why, when a polythene rod which has been rubbed with any cloth is held high above small pieces of paper on a table, the pieces of paper jump up and down repeatedly between the table and rod.

Answer 1

could it be the net charge on the rod is not zero? Those excess charges attract the paper pieces, as the charges in the paper (an insulator), move, twist, or however, move toward the rod, this creating an attraction with the rod. The paper pieces are being charged by "induction". Read about it here.
http://stao.ca/res2/demo/9AP-P2.pdf