From: Ronald Wong (ronwong@inreach.com)
Date: Tue Mar 11 2003 - 01:27:20 PST
Message-Id: <l03102800ba84ca31b5d8@[209.209.19.142]> Date: Tue, 11 Mar 2003 01:27:20 -0800 From: Ronald Wong <ronwong@inreach.com> Subject: Leyden jars and safety
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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