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Help, My Van de Graaff has Fallen Ill and it won’t Charge UP!

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.

bernard-williams
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Help, My Van de Graaff has Fallen Ill and it won’t Charge UP!

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  1. Help, My Van de Graaff has Fallen Ill and it won’t Charge UP! NYSS-AAPT 9/29/2007

  2. NYSS-AAPT 9/29/2007

  3. NYSS-AAPT 9/29/2007

  4. NYSS-AAPT 9/29/2007

  5. Outline • Common misconceptions • Theory of operation • Troubleshooting • Giant VdG at Syracuse University NYSS-AAPT 9/29/2007

  6. Dome Upper combs Belt Rollers Lower combs (grounded) VdG Schematic Not shown: motor, Insulating support NYSS-AAPT 9/29/2007

  7. 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

  8. Operation - - - - • Roller becomes charged • Induces charge on combs • Ionize gas • Belt carries charge away • Reverse happens at top - e- - - - - - - - - - - - - - - - - - - • Implies VdG functions as a current source Belt rotates clockwise + + + + + + + + + - NYSS-AAPT 9/29/2007

  9. - - - - - e- - - - - - - - - - - - - - - - - - - Belt rotates clockwise + + + + + + + + + - 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

  10. - - - - - - - - - - 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 + + + + C ~ ra for large rb rb ra + + + + + + NYSS-AAPT 9/29/2007

  11. 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

  12. Troubleshooting and Maintenance • Interference with charging mechanism • Prevent charge build up Ammeter - - - - - - - - - - - - - • Plasma? • Current? • Cleanliness? • Proximity of combs? • Sharp objects near dome? • Damaged dome? • HUMIDITY!!!! + + + + + + + + + - NYSS-AAPT 9/29/2007

  13. Syracuse University 1 m Van de Graaff NYSS-AAPT 9/29/2007

  14. Syracuse University 1 m Van de Graaff Plasma around upper comb Lower comb assembly NYSS-AAPT 9/29/2007

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