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 operating principle of the Van de Graff generator.
Equipment:
- Keithley 614 Electrometer
- Insulated metal conductor surface
- Insulated metal probe
- DC Voltmeter
- Open top double Faraday cage
- Fur and Hard Rubber/PVC
- BNC to 2 alligator clip cable
- Two long banana plug cables
Setup Procedure:
- 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.
- Connect the voltmeter to the electrometer output Red to pre-amp out and Black to COM. Set the voltmeter to 15V and +
- Turn the electrometer on and set it to nC, .2, supress off.
- Zero the electrometer by pushing the zero button in and then out. Calibrate the voltmeter to read 0 (zero being centered).
- Place the voltmeter on the doc cam or camera and focus it.
Demonstration Procedure:
- Make sure the Zero button is out.
- Use the rod and fur to deposit charge on the metal surface.
- Turn on the doc cam
- 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.
- Discharge the probe.
- Touch the probe to the outer surface of the metal object. Bring it inside the middle Faraday cage and note the voltmeter moves.
- Explain that charge is not present on the inner surface of a conductor, but it is on the outside surface.
- Discharge the probe.
- This time touch the probe to the sharp point. Bring it inside the middle Faraday charge and note that more charge is present.
- This is because charge tends to concentrate at the sharp point.
- Turn off the electrometer.
28-E3
