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Help, My Van de Graaff has Fallen Ill and it won’t Charge UP!. Outline. Common misconceptions Theory of operation Troubleshooting Giant VdG at Syracuse University. Dome. Upper combs. Belt. Rollers. Lower combs (grounded). VdG Schematic. Not shown: motor, Insulating support.

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Presentation Transcript
outline
Outline
  • Common misconceptions
  • Theory of operation
  • Troubleshooting
  • Giant VdG at Syracuse University

NYSS-AAPT 9/29/2007

vdg schematic

Dome

Upper combs

Belt

Rollers

Lower combs

(grounded)

VdG Schematic

Not shown: motor,

Insulating support

NYSS-AAPT 9/29/2007

common misconceptions
Common Misconceptions
  • The VdG charges up because e- jump off the combs onto the belt
  • Combs must contact the belt in order to charge
  • The VdG won’t kill because the charging current is small

NYSS-AAPT 9/29/2007

operation
Operation

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  • Roller becomes charged
  • Induces charge on combs
  • Ionize gas
  • Belt carries charge away
  • Reverse happens at top

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  • Implies VdG functions

as a current source

Belt rotates clockwise

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NYSS-AAPT 9/29/2007

mythbuster

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Belt rotates clockwise

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Mythbuster
  • e- are not ripped off needles
  • No need for belt to contact brushes
  • Does not charge in a vacuum

NYSS-AAPT 9/29/2007

spherical capacitor

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For a toy VdG,

C = 2 x 10-12 F

For my 1 m diameter dome,

C = 5.5 x 10-11 F

E = 1/2CV2

= .25 J for 500,000 V

E = 1/2CV2

= 6.9 J for 500,000 V

Spherical Capacitor

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C ~ ra for large rb

rb

ra

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NYSS-AAPT 9/29/2007

parallel plate capacitor
Parallel Plate Capacitor

C = 100 μF

V = 10,000

E = 5,000 J

Charging time ~11 h

for I = 25 μA

E = 7J

V = 375 V

NYSS-AAPT 9/29/2007

troubleshooting and maintenance
Troubleshooting and Maintenance
  • Interference with charging mechanism
  • Prevent charge build up

Ammeter

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  • Plasma?
  • Current?
  • Cleanliness?
  • Proximity of combs?
  • Sharp objects near dome?
  • Damaged dome?
  • HUMIDITY!!!!

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NYSS-AAPT 9/29/2007

syracuse university 1 m van de graaff1
Syracuse University 1 m Van de Graaff

Plasma around upper comb

Lower comb assembly

NYSS-AAPT 9/29/2007