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B Why is current in Van De Graaff generator so low?

  1. Oct 11, 2016 #1
    I know that current depends on the impedance and potential difference b/w two points so why does a van de graaff generator pose no serious shock hazard i.e. why the extremely low current between the generator and ground? As seen in this video demonstration the instructor safely touches the generator and ground:
     
  2. jcsd
  3. Oct 11, 2016 #2

    sophiecentaur

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    It's simply because all the charges are being taken to the ball on the moving insulated belt. Those charges are formed on the belt at the bottom by electrostatic induction and the rate they are produced is very small. Current is rate of flow of charge. Few charges per second equals low current.
     
  4. Oct 11, 2016 #3

    anorlunda

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    @sophiecentaur gave you the right answer. A belt picking up one electron at a time is no competition for a wire for conducting currents.

    As a college student, I met Doctor Van De Graff. He was very proud of his invention. A company was making large quantities of large Van De Graff generators for use in making heat-shrinkable plastic. They may still use the same method of making heat-shrink today, but I don't know.
     
    Last edited: Oct 11, 2016
  5. Oct 11, 2016 #4

    davenn

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    for me, and maybe the OP, that doesn't even begin to answer the question, as it is not about the charge transfer to the dome from the belt rather it is about the transfer from a dome that is now fully charged to a person/other object
    The dome sitting at say 250kV and discharging into some object is a higher rate of current flow.
    I have just finished reading a dozen links on google and none of them gave any sort of answer regarding current flow from a charged sphere to an object is so low.
    One description read that it max's at around 200mA and others commented that the sphere can be considered a constant current source

    It does appear as tho it is all about impedance ...
    the impedance between the sphere and a discharge want is much lower than that between the sphere and a hand on the sphere ... hence the larger flow of current via the spark discharge ????


    Dave
     
    Last edited: Oct 11, 2016
  6. Oct 11, 2016 #5
    From what I read a current of 6mA is enough to shock you on dry skin. While it's true charge build up is slow on a Van De Graff generator it still says that to achieve a potential that big a huge amount of charge is needed and if you touch the ground while touching a generator it should shock you appreciably. Given it's DC and human skin resistance is around 1Mega Ohm
     
  7. Oct 11, 2016 #6
    Is it because of the short duration of shock?
     
  8. Oct 11, 2016 #7

    sophiecentaur

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    You can look at it as a high resistance (charges carried by the belt at a slow rate) and a small capacitance to Earth. At 250kV (on a good day) and with not many pF, the charge available is very small.
     
  9. Oct 11, 2016 #8
    But isn't a lot of charge required to reach the potential in the first place?
     
  10. Oct 11, 2016 #9

    sophiecentaur

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    Q=CV
    If you have a small capacitance, you only need a small charge to reach a high voltage. That's why Van deGraaff generators are approved for Schools; you really can't harm anyone with one of those. Wimshurst Machines, on the other hand, are not allowed anywhere near a School because the Leydon Jar capacitors ('Condensers') have a significant capacitance.
     
  11. Oct 11, 2016 #10

    anorlunda

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    If it makes a flash every 3 seconds, then discha

    The deceiving part is the impressive lightning flashes. Unlike natural lightning, these flashes have very low currents. That is why the humans don't get injured.
     
  12. Oct 12, 2016 #11

    sophiecentaur

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    . . . . and charge. In the end, it's total charge passed that governs the damage done. Initial current from a 250kV source through a human will be very high ( a substantial fraction of an Amp, even). That current is only sustained for a very short while (fraction of a millisecond) so it's harmless.
     
  13. Oct 12, 2016 #12

    sophiecentaur

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    Capacitance is very relevant here. If you get a chain of half a dozen kids to hold hands, standing on plastic buckets, with one end attached to the Van de Graaf (switched 0ff). Then start the motor and wait a minute or so. The total capacitance of kids plus ball is much greater and will give a seventh kid much more of a belt when they touch one of the charged kids.
     
  14. Oct 12, 2016 #13
    Yes I agree it's the short duration which makes the current spark sting from a V.D generator but nothing lethal
     
  15. Oct 12, 2016 #14
    So the potential should go down right? With C increased so much?
     
  16. Oct 12, 2016 #15

    sophiecentaur

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    Leakage may get worse and limit the final volts a bit but with a higher C, it just takes longer to charge up. The input current is, as always, limited by belt speed and rate of deposited charges on the best.
     
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