From: Paul Doherty (pauld@exploratorium.edu)
Date: Tue Mar 11 2003 - 07:59:17 PST
Message-Id: <l0311076eba93b7f72f89@[192.168.111.161]> Date: Tue, 11 Mar 2003 07:59:17 -0800 From: Paul Doherty <pauld@exploratorium.edu> Subject: Re: pinhole Leyden jars and safety
My rule is: never use a leyden jar with a Van de graaf or a whimshurst machine.
As Ron shows the electric discharge can be large enough to be dangerous.
I do use them with the wool and styrofoam charged electrophorus.
Paul D
>I mentioned Leyden jars in a recent message posted on Pinhole regarding
>"generators and safety" and received an e-mail message asking:
>
>
>>Do you have any idea if it is safe to use a Leyden jar with students?
>>I used a small Leyden jar in conjunction with a classroom sized van
>>de Graff generator. I stopped doing this a few years ago because
>>I sensed that the jolt was too much. I have never gotten a
>>significant jolt from the van de Graff generator but when it is
>>used to charge up a Leyden jar, the charge really packs a punch.
>
>I thought some Pinhole subscribers might find the reply of interest:
>
>_______________________________________________
>
>
>Here's a "back of the envelope" type of answer to your question:
>
>A Leyden jar is basically a large capacitor. Capacitors typically have a
>capacitance measured in the order of 10^-12 F or 1 pF (micro-micro farads
>as they use to say in my day). If your jar was a "typical" Leyden jar
>(about the size of a 1 qt. mayonnaise jar), it's capacitance could be
>around a thousand times larger (it depends on what was used and how it was
>put together). This would make it around 10^-9 F (or a nanofarad - that is,
>1 nF).
>
>A good Wimshurst machine will put out 50 kV. If you were to use it to
>charge up this Leyden jar, the amount of energy stored can be computed
>using the formula:
>
>E = (1/2) C X V^2 = 0.5 X (1X10^-9) X (50X10^3)^2 = 1.25 J.
>
>This is equivalent to a 1 kg mass dropped on your head from a height of 13 cm.
>
>(E = m X g X h = 1.25 J = 1 kg X 9.8 m/s^2 X h => h = 12.8 cm or 13 cm)
>
>Now you can see why the shock produced by the discharge of a good-size
>Leyden jar can be so disconcerting.
>
>The fact is that with the jar you not only got a large voltage but a
>considerably larger amount of charge than that which can be supplied by
>your Van de Graaff generator alone.
>
>From the standpoint of serious danger, the issue is a little more difficult
>to nail down.
>
>The amount of current is what does the damage. For short pulses like that
>from a Wimshurst machine or a Van de Graaff generator, 10 J delivered over
>one second is considered enough to electrocute an individual (i.e. stop the
>heart from doing it's job). But this is based on someone with a body
>resistance of 2000 ohms. If an individual has been made nervous by all the
>high voltage fireworks going on, the increased level of moisture and salts
>on their body could make their resistance considerably lower than this and
>1.25 J could be enough to put them in real danger.
>
>Of course, your Van de Graaff generator probably put out more than 50 kV.
>100 kV is not uncommon and this puts the possible amount of energy in your
>Leyden jar at 5 J. At 150 kV, the amount of energy in your jar could be as
>high as 11.2 J. At that point, there's a good chance that you or you
>students have crossed the line into an area where electrocution is a good
>possibility.
>
>So, be very careful when storing electrical energy in a Leyden jar. If the
>jars are made from 35 mm film canisters with small amounts of aluminum foil
>for the inside and outside conductors, there probably won't be much need
>for concern. But the larger ones that are typically brought out for
>display/demonstration/use should be handled with great care if they have
>been charged.
>
>A classic demonstration of how energy is stored in a Leyden jar involved
>using a Leyden jar that was specifically designed so that it could be
>disassembled after being fully charged.
>
>The instructor brings out the jar, takes it apart to show how it is
>constructed, puts it back together, charges it up using a high voltage
>charging machine (VdeG, Wmhrst, whatever) and then shorts out the jar.
>
>If the jar is of any size, the discharge will be quite dramatic.
>
> In charging the jar, the electric field produced by
> the charges on the conductors polarized the charges in
> the insulator. Like a plastic rod rubbed with fur or a
> glass rod with silk, the charges on the conducting
> plates are transferred to the insulator. When you short
> circuit the jar, you establish a conducting path between
> the two conductors and the charges take the path of least
> resistance - producing the discharge.
>
>The jar is recharged and, in order to avoid the possibility of accidentally
>shorting it out to the outer conductor (by way of the instructor), a
>non-conducting rod is used to remove the inner conductor from the jar. The
>outer conductor is separated from the insulator and, with bated breath
>(i.e. much drama), the two conductors are brought in contact with each
>other. Nothing seems to happen. At this point, a student is found who is
>willing to reach in and touch the inside of the cup-shaped insulator with
>his/her finger.
>
> Actually, finding such a willing subject can be difficult
> if the original discharge was of some size.
>
>Despite their trepidation, they find that they can do so without any harm.
>
> The amount of charge at the point of contact is usually
> too small to produce anything more than a tingling sensation.
>
>Then you "casually" put the jar back together (while actuallly being very
>careful about it) and short out the two conductors (saying something like
>"Standard practice with large capacitors is to..." or "just to be safe") -
>bam! A very large discharge - just like before!
>
>I would never do this demonstration with my kids. Some of them were so
>inquisitive that they would try to do it by themselves - despite the safety
>concerns - just to get the "hands-on" experience. Besides, I can be such a
>klutz sometimes that I probably would have terminated my delightful career
>in a blaze of light before my horrified students ("Gawd, he's done it
>again!").
>
>Count yourself lucky.
>
>ron
>
>
>
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