From: SFPhysics@aol.com
Date: Thu Jan 10 2002 - 02:44:45 PST
From: SFPhysics@aol.com Message-ID: <bd.1a122dca.296eca9d@aol.com> Date: Thu, 10 Jan 2002 05:44:45 EST Subject: Van de Graaff for students? CD effects?
<<
> Has anyone ever tried discharging a Van de Graaf
> generator to a CD? Will it conduct and etch little
> fractal lightning bolts on it like it does in the
> microwave? Just curious. Do you think the small
> generator safe for elementary school students?
> With firm handshake,
> Raleigh
>>
Hello Raleigh:
Please forgive me if I repeat some of my previous posting to answer your
questions.
The small 75 kV 3" generator is completely safe for students to do everything
from lighting NE-2 neon lamps to drawing sparks to their bare knuckles. It
has very low storage capacity and will deliver only small currents during
discharge. This is the unit with the 3" collector sphere and was first sold
in the 1950s as the ATOMOTRON science toy complete with several experiments
and an experiment book. I do not know if they are still made this small but
I prize the one I made from a kit in 1956 and have kept it running well for
all the intervening years. I even managed to put together a small class set
from finding these things at flea markets then repairing them. A lot of good
Physics equipment appears at flea markets and now on eBay.
The 200 kV generator is borderline student safe and can deliver a solid jolt
of moderate current thanks to the extra storage area of the 7" sphere.
Always discharge into a metal object held in the hand and not onto bare skin
which will show a small painful burn point. Students may use this size
generator but with strict teacher supervision and a solid prelab preparation.
Students with any heart irregularities should go nowhere near the unit
because it can put a fair amount of current through a body to the floor
(cement floors with steel rebar are the worst).
The 10" sphere 400 kV and larger units are really not safe for student use
unless the students are exceptional and can remember all of the safety rules
and procedures. In top shape these units can deliver enough current to make
you think you have been hit by a stun gun. These really are more for
teachers to give demonstrations.
As to CDs reacting with Van de Graaff generators, they do not. The metalized
area sandwiched between the plastic laminations is not effected in any way by
the low current static charge. A CD with a metalized coating in a microwave
develops large currents that do change the metallic layer. This is due to
high voltage with high currents resulting from eddy currents with the passing
high intensity wave fronts of the microwaves. The radio frequency energy is
converted to heat energy because the metalized film acts as a short circuit
to the eddy currents. The tracks you see are really burn marks. Not all CDs
will do this. Some have a reflective coating that only looks metalized but
is not.
Hope that answers your question and helps a bit,
Al Sefl
The Dr. Frankenstein of the Physics Lab....
Who once fried the solid state phone system in Mission High School (SF) when
he answered the phone while doing a demo and let the cord get too near the
Van de Graaff. It was only a measly 6" spark but required two weeks and a
handful of new cards to go into the telephone system card cage for the
repair. That'll teach 'em to interrupt my demonstrations!
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