From: Ronald Wong (ronwong@inreach.com)
Date: Thu Feb 20 2003 - 02:03:10 PST
Message-Id: <l03102804ba7a496c319b@[209.209.19.77]> Date: Thu, 20 Feb 2003 02:03:10 -0800 From: Ronald Wong <ronwong@inreach.com> Subject: generators and safety
A while ago, eric mentioned a class demo using a Van de Graaff generator
and he ended with a question:
>...
>oh, i've been meaning to ask this for a while and now
>it's totally appropriate: i'm curious about how safe
>one of my van de graaff activities is. here's what i
>do: i have the kids make a chain around the classroom
>and hold hands. at the beginning of the chain i have
>one student with his/her hand ON the van de graaff and
>the other end of the chain has a student with his/her
>hand grasping the sink. i turn the generator on with
>the kids bracing for a shock. of course since the
>charge can flow freely, there is no shock. then i
>have the student at the end of the line let go off the
>sink. still no shock. THEN i have the last student
>grab the sink again ... WHAM!! the whole chain feels
>a jolt! it's interesting to ask them what they
>felt--most of them say they feel the shock in their
>wrists. that leads me to believe that the juice is
>running through the blood stream which is a super good
>conductor. so basically what i have is a pretty good
>jolt ruinning straight across the chest. seems like a
>BAD idea when i think about it.
>so what do ya'll think? is it dangerous?
It's not the voltage that you should be concerned about, it's the current.
While the student was "grasping the sink" the current flowing was very low
because the charge buildup on the Van de Graaff's dome was small. This was
due to the fact that the chain of students was basically grounding the
generator. Small charge on dome leads to a very small voltage difference
and thus a very small current flowing through the students. One which the
students didn't even notice.
Letting go of the sink created an open circuit. Now, while no current was
flowing - thus no shock, the charge could now build up on the dome. More
charge on the dome lead to a larger voltage so that when the student
grabbed the sink again a correspondingly larger current would flow - one
strong enough to get a reaction from the students.
The effects of electric currents coursing through the body become
significant when the current is in the milliampere range (60-80
milliamperes through the heart over a short period of time can kill you).
With the typical high school demonstration Van de Graaff generator, you
will be dealing with currents in the microampere range. This a matter of
design and, as a result, the current flowing is a thousand times smaller
than that which is considered dangerous. So enjoy the fun.
It's a different kettle of fish if you are using one of the larger Van de
Graaff generators where the charge is sprayed onto the belt from a source
of electrostatic charge.
ron
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