Date: Sat, 30 Jun 2001 11:44:47 -0700

Author: Bernard Cleyet

Subject: Re: VDG

Post:

more:

I found some info on the L of various conductor configurations -- this led me to think of predicting the discharge of a sphere thusly: one could assume the dome discharges in slices -- the first one defined by the point of discharge and the succeeding ones behind it with their respective L (and C) somewhat like a delay or transmission line --
interesting one can treat a sphere as if it has a Z!

bc

Bernard Cleyet wrote:

> p. 539 Hdb. C. & P. ninth ed. table of sparking potential up to 74.3 k V (1 cm balls 10 cm. spark)
>
> the 35th ed (p.2322) goes up to 300 kV. (25 cm balls) and 13.3 cm spark)
>
> the latter source is AIEE. since 10" ~= 25 cm, diff. ~ 15%
>
> using a diff. method for energy: Stray cap of a 25 cm ball is ~12 pF [4Pi ( kappa zero) radius], therefore, U = ~0.7 J {Dick's method = ~ four micro J.} I expect the charging current to be on the order of a few micro A. and the discharge multi A.
>
> An isolated cap with v. little inductance should discharge in nano S's. My impression from measuring the discharge of a 5 nF cap charged to 2kV discharging through an ~ one mm gap. The resistance is on the order of an ohm. The discharge had a rise time of ~ 20nS ('scope limit) and then rang for ~ 5 cycles at ~ 200nS period (implies ~ 5 E-6 H
> ?). I think this behavior is due to the high neg. resistance of a one atm. air discharge. With critical damping the dominant term is exp(-Rt/2l) This gives a time constant of ~ either ten microS if I use the ringing period or a tenth (or less) if one uses the initial rise time. (The initial peak was > 10X the beginning of the ringing (off scale
> -- 50 V/cm (with 10X probe) 8 cm scale across a one ohm resistor Using a heavy wire resulted in little change suggesting that the inductance was as important as the R.)
>
> Anyway, 300 kV on a globe of 12 puff is ~ 4 micro Coulombs or in one micro S four amps.
>
> Dick Heckathorn wrote:
>
> > Greetings,
> >
> > Yes, over 100,000 volt is possible.
> >
> > One figure that I use is that for each 1 inch of spark, the electric potential is 40,000 volts. Source ???
> >
> > Information that came with a Van de Graaff that I had at my former school indicated with a 10 inch diameter dome, the maximum voltage under ideal conditions is of 350,000 volts, with a current of 10 to 15 microamps with the spark lasting 1 to 15 microseconds. Energy = Voltage times current times time. I'll let you estimate the amount of energy per
> > spark.
> >
> > Another figure I have is that the electric field is 3 million volts per meter radius of the dome. Source??? With a 10 inch diameter, or 0.254 m, which is 0.127 m radius, one gets 381,000 volts.
> >
> > Results are similar. I'll stick by them until I hear otherwise. With evidence of course.
> >
> > Dick
> >
> > > Helping teachers who teach, motivating students who learn.
> > > "Science is nothing more than learning how to communicate with nature in such a manner that it will talk back."
> > >
> > > Dick Heckathorn 146
> > 65 Pawnee Trail Middleburg Hts, OH 44130-6635 440-826-0834
> > > Physics Teacher Cuyahoga Valley Christian Academy 4687 Wyoga Lake Road Cuyahoga Falls, OH 44224 330-929-0575 VM 120 > gnper090@wviz.org
> >
> > -----Original Message-----
> > From: Jim Krider [SMTP:James.Krider@asu.edu]
> > Sent: Friday, June 29, 2001 4:33 PM
> > To: tap-l@listproc.appstate.edu
> > Subject: VDG
> >
> > A professor asked me what the electric field strength of a Van de Graaff generator was.
> > We have the Winsco N100-V electrosta
> > tic generator.
> > I have heard that 100,000 volts could build up on the typical classroom VDG.
> > Does this seem reasonable?
> > Jim Krider, Laboratory Coordinator Senior
> > Arizona State University
> > Physics and Astronomy Instructional Resource Team
> > PO Box 871504 Tempe, Arizona 85287-1504
> > (480)965-8086 Fax: (480)965-7954 email:jkrider@asu.edu
> > web:http://www.public.asu.edu/~jkrider/
> >
> > ------------------------------------------------------------------------
> >
> > Part 1.2 Type: application/ms-tnef
> > Encoding: BASE64

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