Van de Graaff generator safety question (fwd)

[Steven Roys <sroys@xxxxxxxx>]

---------- Forwarded message ----------
Date: Thu, 25 Dec 2003 10:56:09 -0700
From: Terrell Fritz <terrellfone@xxxxxxxxxxxxxxxxxxxxxxx>
To: High Voltage list <hvlist@xxxxxxxxxx>
Subject: Van de Graaff generator safety question

Hi All,

I was thinking of making a Van de Graaff generator out of a 50cm diameter
(20 inch) stainless steel sphere I have laying around.  I got it from
Waterscapes:

http://hot-streamer.com/temp/Old2001Files/2001-04/Orb.jpg

http://www.waterscapesweb.com/orbs.html

If I calculate the top voltage assuming 3MV/m radius:

3MV / m x 25cm = 750VDC

If I calculate the top capacitance I get:

C = 4 x pi x Eo x R  =  4 x 3.1415... x 8.85E-12 x 0.25  ==  27.8031pF

Where I got worried was the stored energy:

E = 1/2 x C x V^2  =  1/2 x 27.8pF x 750000^2  ==  7.81 Joules!!

Assuming one is grounded and the thing arcs to one's forehead, assuming the
human is the biggest resistance in the path, then all the 7.8 joules is
dissipated in the human....

My question is how dangerous is it??  Making this thing would not be fun if
it was capable of killing anybody (well...  "easily").  If it would be
really dangerous, I would not want to build it since it would not be fun to
demonstrate if I had to really worry about it.

It is pretty much like a 30pF 750kV cap I would think.  But I am not a
great expert on this stuff so I thought I would ask.

Cheers,

	Terry