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5B20.10-Surface Charge

5b2010surface_charge

Description: This demo shows that charge on a solid conductor resides entirely on the outer surface.
This is commonly referred to as Faraday’s Ice pail Experiment and is also the principle of the Van de Graff generator.

Equipment:

  • Pasco Electrometer
  • Insulated metal conductor surface
  • Insulated metal probe
  • Overhead Voltmeter
  • Open top double Faraday cage
  • Rods and fur
  • BNC to 2 alligator clip cable
  • Two long banana plug cables

Setup Procedure:

  1. Connect the electrometer to the Faraday cage using the BNC cable. Make sure the black (grounded) cable is attached to the outer cage, and the red cable is attached to the inside cage.
  2. Connect the overhead voltmeter to the electrometer output. Set the voltmeter to DC, 1V, centered.
  3. Turn the electrometer on and set it to locked zero mode (top position). Set the range to either 1,3, or 10. Set the mode to 10V DC.
  4. Make fine adjustment to the zero spot on the electrometer. Calibrate the voltmeter to read 0 (zero being centered).
  5. Place the voltmeter on the overhead projector and focus it.

Demonstration Procedure:

  1. Make sure the electrometer is set to locked zero mode.
  2. Use the rod and fur to deposite charge on the metal surface.
  3. Turn the electrometer to open zero mode (center position).
  4. Turn on the overhead projector.
  5. Using the insulted probe, touch the circular metal part to the inside of the metal surface. To test if there is any charge on the inside, place the probe in the middle part of the faraday cage. Note that the voltmeter does not move much, if at all.
  6. Discharge the probe.
  7. Touch the probe to the outer surface of the metal object. Bring it inside the middle Faraday cage and note the voltmeter moves.
  8. Explain that charge is not present on the inner surface of a conductor, but it is on the outside surface.
  9. Discharge the probe.
  10. This time touch the probe to the sharp point. Bring it inside the middle Faraday charge and note that more charge is present.
  11. This is because charge tends to concentrate at the sharp point.
  12. When finished, put the electrometer back to the locked zero mode. Turn off the electrometer.