Demonstration of Point Discharge Apparatus Van de Graaff generator, static spinner
Procedure Attach the static spinner to the dome of the Van de Graaff generator.
Turn on the Van de Graaff generator. Observation
The arms of the static spinner rotate. Conclusion
An electric wind is blown away from the points of the arms due to point discharge.
Fig 15.9
ROBERT VAN DE GRAAFF, 1901–67 (AMERICAN) He invented the high-voltage generator that bears his name. He also invented the insulating-core transformer and developed tandem generator technology.
Static spinner on dome of Van de Graff generator
SCIENCE,TECHNOLOGY & SOCIETY
Lightning and lightning conductors The American physicist Benjamin Franklin (1706–90) demonstrated that lightning is basically a very large electric spark. Buildings are often protected from lightning damage by use of lightning conductors.Typically these are rods of copper (because it is a good conductor) running along the outside of the building from the roof to the ground.They are pointed on top to allow point discharge so as to reduce the likelihood of a strike. The ions near the point accelerate away from it because they experience a very large force. These ions are moving charge carriers, thus the air becomes a conductor of electricity. The ions cause further ionisation by collision making the air a better conductor.The charged cloud will gradually discharge because current flows from the ground to it through the copper conductor, then up through the air. This gradual discharge is obviously far safer than the sudden discharge in a lightning bolt. However, if the cloud were to discharge by a lightning bolt, it would most likely strike the conductor because of the opposite charge at that point. It would then travel through the rod rather than any other path through the building, which would be likely to damage the building. It should be noted that any pointed conductor can act as a lightning conductor. It is for that reason that it is unadvisable to play golf, use an umbrella or to shelter under a tall tree during a lightning storm.
The Gold Leaf Electroscope The gold leaf electroscope is used to study electric charge. It is made up of:
• A metal cap that is attached to • A metal rod which passes through
