Lightweight pith balls can show electrostatic attraction and repulsion.
Other Demos of Interest:
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Franklin's
Bells
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Bending Water with Static Electricity
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Styrofoam Peanuts on a Van de Graaff
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Teachable Topics:
- Positive and Negative Charge
- Electrons and Electrostatics
- Attraction and Repulsion
Theory:
'pith\ n 1a: a usu. continuous central strand of spongy tissue in the stems of most vascular plants that prob. functions cheifly in storage.
A pith ball is a very small, lightweight object that picks up electric charge quite well. A charged pith ball works well to show the Coulomb force between two charged objects. If a glass rod is rubbed with silk, the silk rubs some electrons off the rod. The rod thus becomes positively charged.
Touching the positively-charged rod to two pith ball gives the balls each a small positive charge. Since like charges repel, the pith balls repel one another.
If an ebonite rod is rubbed with fur, the rod rubs some electrons off the fur. The rod thus becomes negatively charged and attracts the positively-charged pith balls.
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Figure 1: Charging a glass rod
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 Figure 2: Charging the pith balls
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Figure 3: Charging a ebonite rod
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Apparatus:
- Two pith balls hung by string
- A glass rod
- An ebonite rod
- A piece of fur and a piece of silk cloth
Procedure:
- Charge up the glass rod with the silk rod first. Then touch both pith balls with the rod. The two balls should now repel one another.
- Next, charge the ebonite rod with the fur. Bring the rod close to the balls. You will see the positively-charged balls are attracted to the rod.