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Lecture Demonstrations Laboratory

Physics Rm. 202

(434) 924-6800


The Demonstrations Manual


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Electrostatics
Kinds of Charge and Charging
Attraction and Repulsion Between Charged Objects
Charging by Induction
Lord Kelvin's water-drop electrostatic generator
Wimshurst Machine
Fun Fly Stick
Xerox
Electric Fields And Potential
Capacitance
Current and Resistance
Electromotive Force And Current
DC Circuits
Magnetic Materials
Magnetism
Inductance
Induction
Complex Circuits
Semiconductors And Tubes
Transmission Lines and Antennas

Kinds of Charge and Charging

Purpose:

To demonstrate the two signs of charge, attraction and repulsion, charging of electroscopes by contact and by induction.

Procedure:

The most effective way to charge a rod by friction is to hold the cloth vertically in one hand and "whip" it vigorously with the rod held in the other hand. Note that when rubbing a glass rod with cat's fur (silk), the rod becomes positive; the fur (silk) negative. Use the electroscope to show that the rod and cloth carry charges of opposite sign. This can be done after the electroscope has been charged with a known sign, either by contact or induction. Then investigate the sign of charge on rods of other materials which have been rubbed with all the available cloths. Show repulsion and attraction between hanging balls charged similarly and oppositely. Show effect of charged rods on small bits of paper or an inflated balloon.

Hints:

Pith Balls: using them is tricky (they bounce around alot). Instead, you can use two balloons hung on fishing line - rub both in your hair, show like charges repel. Then, without touching balloon, bring your hand close to either one and show induced charge causes attraction.

Triboelectric Effect: when rubbing the following pairs of materials, one obtains a charge via friction.

Teflon Rod (neg) with Silk (pos).
Glass Rod (pos) with Cat Fur (neg).
Acrylic, Lucite or Glass Rod (pos) with Teflon Film (neg).

Some have suggested to use tinsel and helium filled conducting balloon. See http://www.youtube.com/watch?v=eCLu4t12LdE

Equipment:

  • Teflon Tod
  • Silk
  • Rubber
  • Pith Balls
  • Lucite
  • Glass
  • Bits of paper
  • Electroscope
  • Balloon