Re: Dissectible Capacitor

[Robert Morse <Robert_Morse@CATHEDRAL.ORG>]

> Leigh Palmer wrote:

> > I will suggest that the cookie tin experiment I described not be
> > attempted by anyone. Today I managed to generate a potentially
> > lethal charge using only a hand cranked Wimshurst machine.

> Ludwig Kowalski wrote:
> The danger become real when the energy stored exceeds a certain
> number of Joules (does anybody remember how many?). I would
> not hesitate experimenting with C=5 pF but would avoid much
> higher C. (U=0.5*C*V^2. With V= 100,000 V U=25 mJ).

For a quick take on classroom electrostatic safety, I found the following
information when researching the subject about 10 years ago- I would
appreciate references to other information -

A. D. Moore  (Electrostatics,1968)  notes that a 10 Joule discharge is
dangerous- 250 mJ gives a heavy shock.  Cross ( Electrostatics: Principles,
Problems and Applications, 1987 p 353 ) notes people experience different
degrees of discomfort with shocks of above 10 mJ. Winburn (Practical
Electrical Safety, (cant find date) , p15) cites possible ventricular
fibrillation from impulse shocks and surge discharges with energies between
13.5 and 27 Joules.  A Sargeant-Welch dissectilbe Leyden jar (#1989) had a
capacitance of 100 pF and a glass pint Leyden jar had 350 p. From
experience, both of these can give all the shock I want to take when charged
with a classroom Van de Graf generator. I would not experiment with a Leyden
jar bigger than a pint or a flate plate capacitor with plate area bigger
than about 250 square cm surface area without being rather careful.

See Teaching about Electrostatics, R. A. Morse, AAPT,(1992) , p 327.

Bob Morse, St. Albans School, Washington, DC