Transcript Slide 1

Van de Graaff Generator
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A Van de Graaff generator is made by a belt of a
flexible dielectric material, running over two
rollers (wheels), one of which is surrounded by a
hollow metal ball.
1. Power Supply that pulls off electrons.
2, 4. Rollers.
3. Positively charged belt (lost some
electrons)
5. Metal comb that drops electrons from the
ball onto the belt.
6. Metal ball that gives up electrons to the
belt, then it is charged!
7. Negatively charged belt (gained some
electrons from the ball).
8. Grounding wand, it can give or take
electrons to make everything even again.
9. Discharge: A spark, a flow of electrons
through the air.
The bigger the ball, and farther from the ground
it is lets you build more charge up on it.
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Schematic view
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History
• The fundamental idea of friction
machine as high voltage supply
traces back to 17th Century.
• Generator invented to supply the
high energy needed for early
particle accelerators.
• The first model was demonstrated
in October 1929, using a silk
ribbon bought at a five and dime
store as the charge transport belt.
• Originally used as a research tool
in early atom-smashing and high
energy X-ray experiments.
1931, Robert J. Van de Graaff (left) poses with
his electrostatic generator and Karl T. Compton,
MIT president
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Recent Developments
• Tandem Van de Graaff
accelerator
– Contains one or more Van de
Graaff generators.
– Negatively charged ions are
accelerated through a
potential difference then
stripped of electrons and
accelerated again.
• Pelletron
– Belt is replaced by a chain of
short conductive rods
connected by insulating links.
– Air-ionizing electrodes are
replaced by a grounded roller
and inductive charging
electrode.
– Can be operated at much
higher velocity than a belt, so
both the voltage and currents
attainable are higher than a
conventional Van de Graaff
machine.
The largest VDG generator, on display at the
Boston Museum of Science
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