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[Fwd: VanderGraaff safety (fwd)]



Here's another good idea.

poj
--- Begin Message --- Paul,
Does your subject have to experience an ac signal or could it be dc? If
dc is okay, then you can hook the small 9v batteries together. Their true
voltage is between 9 and 10-v when they are new. With this ~10v step you
can get to the threshold in a much safer manner. If this is a good thread,
feel free to post.

Tom McCarthy
Saint Edward's School
1895 St. Edward's Drive
Vero Beach, FL 32963
561-231-4136
Physics and Astronomy
-----Original Message-----
From: paul o johnson <pojhome@FLASH.NET>
To: PHYS-L@LISTS.NAU.EDU <PHYS-L@LISTS.NAU.EDU>
Date: Sunday, January 10, 1999 3:59 PM
Subject: Re: VanderGraaff safety (fwd)


Boy, you chaps gave me some great ideas. Tomorrow I will:
(1) Insert a 100-Kohm resistor in each lead.
(2) Insert a 1:1 isolation Xformer ahead of the variac.
(3) Put in a work order to get GFI outlets installed in all our labs.

Any more good ideas?

poj

L. R. Cartwright (Larry) wrote:

I definitely agree that additional precautions like a 1:1 isolation
transformer and redundant resistances in each output line would be a
great
idea. "An old variac" connect directly to AC service is a catastrophe
waiting to occur, no matter how many times the device has performed
safely
up to now.

The most important thing is: ONLY use your device in a GFI protected
outlet. Anyone whose lab doesn't have ground fault interrupt protection
needs to talk to your building administrator(s) right away. It's code
everywhere in the US that gfi's be installed in any room with plumbing.
Gfi's aren't very expensive, and pretty effectively eliminate the
direct-path-to-ground concerns.

Best wishes,

Larry

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Larry Cartwright
Physics, Physical Science, Internet Teacher
Charlotte High School, 378 State Street, Charlotte MI 48813
<physics@scnc.cps.k12.mi.us> or <science@scnc.cps.k12.mi.us>

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

On Sat, 9 Jan 1999, paul o johnson wrote:

This van de Graff thread has been extremely interesting. It has moved
me to
describe a demo I set up for my physics 2 class (2nd half of Serway).

Most inquisitive kids have briefly gotten across 120 VAC by the time
they're in
college. They know what it feels like. But most of my students have no
idea
what it feels like to get an electrical shock. First we talk about it.
Very few
of us can feel anything if we put our dry fingers across a 1.5 V
flashlight
battery or even a 6 V lantern battery. A few more can feel a slight
tingle by
putting our dry fingers across a 12 V car battery. But 120 V knocks us
on our
butt. Where is the threashold?

I rigged up an old variac with its output connected to two brass screws
about
two inches apart and also to an AC voltmeter. I put a 100-kohm
current-limiting
resistor in series with one lead to the screws. I ask a volunteer to
place two
fingers of the same hand on the screw heads (to avoid sending any
current
through the chest) and use the other hand to slowly turn the variac up
from
zero while watching the voltmeter. So the individual student is in
complete
control of the voltage between the finger contacts.

With dry skin, my most sensitive students can start to feel a faint
tingle at
about 20 V while I, myself, can feel nothing at all until about 65 V.
Then we
re-try it after dipping our fingers in a beaker a pure tap water, We
can then
feel that initial tingle at perhaps 50% of the dry-finger voltage.

I've done this in about four classes, now, and my students say it is an
interesting and memorable experience. Many for the first time in their
lives
feel electric charge going through their hand. I figure the current
when we
first feel the tingle is between 10 and 100 microamps.

OK, colleagues, what are your thoughts on this demo? Is it too
dangerous? Is
there a better way to let students actually feel an electric shock?

poj



--- End Message ---