Concepts

Semiconductor bands, corona discharge, dielectrics, photodoping

Overview

A sharp conductor on the Van de Graaff generator will emit electrons via corona discharge and prevent arcing to an adjacent grounded tower. A shard of GaAs on a Van de Graaff generator will also undergo corona discharge if photodoped with room lights or a flashlight. In total darkness, the GaAs does not conduct, so the Van de Graaff arcs to the grounded tower.

Details

Equipment

• [1] Van de Graaff generator
• [1] Grounding tower
• [1] Discharge wand
• [1] Mounted GaAs shard
• [1] Thumbtack
• [1] Flashlight
  
• [1] Extension cord

Classroom Assembly

1. Put the Van de Graaff somewhere away from computers and plug it in.
2. Place the grounding tower within arcing distance of the Van de Graaff.

Important Notes

• The room must be very dark for this demo.
  
• Ensure the Van de Graaff is not near any computers or other electronics.
• Users should not be have their phones or other electronics.

Script

1. Turn off the lights.
2. Perform the dielectric breakdown experiment, leaving the grounded tower close enough to spark.
3. Tape the pin on top of the Van de Graaff dome, pointy end up.
4. Start the Van de Graaff. No sparks should occur due to the corona discharge off the pin.
5. Ground the Van de Graaff and turn it off.
6. Replace the pin with a piece of semiconductor.
7. Turn off all the lights.
8. Turn on the Van de Graaff. Sparks should fly from the Van de Graaff to the grounding tower.
9. Shine the flashlight at the semiconductor. Sparks should stop.
10. Turn off the flashlight, ground the Van de Graaff, and turn off the generator.

Additional Resources