Charge rubber rod negatively or glass rod positively. Bring rod close to can. Can will roll toward rod.
||Water as Charge Detector
Charge a rubber rod with some fur and bring it near a faucet which has a very small stream of water running from it. The water will be obviously
diverted--always toward the rod. A glass rod rubbed with plastic bread wrap will have the same effect--showing that the opposite charge is
induced in the near surface of the water stream.
||Big Pith Balls
Epoxy the ends of about 1 m of strong thread to two ping pong balls. Coat the balls with a suspension of graphite in alcohol or with an aluminum-base paint to make the balls electrically conductive. When balls are hung from some type of
support, they can be charged with a rubber rod rubbed with fur to illustrate basic law of electrostatics. Glass rod rubbed with plastic bread wrap gives the opposite charge.
Do as a demonstration.
Qualitatively show F a q 2
and F a 1 / d 2
F a q 1 q 2 / d 2
Directions for demo.
Set-up: Hang two pith balls (styrofoam cups may be substituted) from the ceiling or from a tall ring stand. See Diagram.
1. Charge the pith balls, ping pong balls, or the cups by rubbing with fur.
2. Bring the charged objects 2-3 cm apart. Release the objects.
3. With a piece of tape, mark the position of greatest displacement.
4. Repeat steps 2 and 3 holding the charged objects 4-6 cm apart.
Discuss with the class the relationship between distance and electric force.