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An insulator can be charged by friction contact with a grounded conductor, such as wool. Insulators tend to lose electrons, and their electrons travel through the conductor to the ground.

Therefore, fewer electrons would mean it would be positively charged.

A non-grounded conductor can be charged by rubbing an insulator. Since conductors tend to conduct electrons, due to mechanical vibrations caused by friction the electrons will break from the insulator surface and travel into the conductor, and the conductor would have more electrons, meaning it would be negatively charged.

Reason: By frictional contact between an non-grounded conductor and an insulator.

By frictional contact between an insulator and a grounded conductor also charging happens.