5A20.20-Pith Balls

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Instructor’s video

Description: Shows attraction and repulsion of charges, Polarization and Coulomb’s law.

Equipment:

  • Pith ball on insulated stand
  • Rods and Fur
  • (Optional) Hairdryer to reduce humidity

Demonstration Procedure:

  1. Show that a neutral rod placed near the neutral pith ball does nothing.
  2. Put a charge on the rod with the fur and bring it close to the pith ball and note the attraction.
  3. Touch the pith ball with the charged rod and note the repulsion.

Tips: This demo works best in low humidity.

Concept: When you bring a charged rod near the neutral pith ball, the charges in the ball become polarized. The pith ball becomes attracted to the rod because opposite charges move to the side of the ball closest to the rod and like charges move to the side of the ball farthest from the rod. Since the opposite charges are closer together than the like charges, there is a larger force of attraction that repulsion (Coulomb’s law). When you touch the pith ball with the charged rod, it will have the same net charge as the rod and will repel.

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