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Re: VanderGraaff safety (fwd)



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

William Beaty wrote:

On Wed, 6 Jan 1999, Raymond A. Rogoway wrote:

> circuit with a wire from each terminal of a standard 1.5V cell with a
> switch in one lead. He then soaked his hands in salt water and then
> inserted one wire into the palm on one hand and the other wire into the
> palm of the other hand.

If he poked the wires INTO his skin, he would bypass the large skin
resistance, and probably could kill himself.

Here's something I did once, while looking for new types of
demonstrations: placed a metal spoon in my mouth, touched one terminal of
a 9v battery with a wet finger, then touched the other terminal to the
spoon handle.  I hoped to produce a sudden bad taste in my mouth.
Instead, I saw a flash of "darkness" each time the battery touched the
spoon!  The tiny current was affecting my eyes and/or brain!  Yeeeesh, not
an experiment I care to repeat anytime soon.

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