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An electrostatics demo scenario.

One of my favorite electrostatics demos with a Van de Graaff is the
dissectable capacitor. Welch has a version for $33. It's two aluminum cups
which nest together with an insulating glass or plastic cup between. A
metal stem comes up from the inner cup, terminating in the usual solid
metal ball, so when assembled it is a Leyden Jar. But it can be taken
apart. This version, and the older version, has a hook at the top of the
center stem, like a coat hanger hook. The reason is obvious below.

Assemble it. I generally have a long ground wire running from the terminal
on the base of my Van de Graaff. I hold the ground wire (with my
hand--firmly; don't let it slip) against the outer cup and bring the
entire capacitor toward the dome of the Van de Graaff, so sparks jump to
the center conductor of the inner cup, charging the capacitor. If you
leave it too long, it will discharge by sparking through the plastic, so
you'll have to do it again. Experience. Practice.

Then set the capacitor on an insulating surface (wooden table ok). Don't
grab inner and outer conductors while assembled! It can produce a *hot*
thick spark. Show the class this spark, using one of those discharge
devices with two metal adjustable arms and an insulating handle, bridging
the outer and inner cans, but doing it slowly and dramatically. (Touch one
end to the outer can, then move the other end slowly toward the center
conductor rod. When students see the hot spark and hear the sharp *bang*
they are suitably impressed that this charged capacitor is something to
treat with respect. With our Van de Graaff the spark can often be 2 cm
long, or more, and quite thick and bright.

If you don't have one of the adjustable discharge (shorting) devices, you
can make one from heavy wire (coat hanger?) and an insulating (wood or
plastic) handle.

Now recharge the capacitor, as before. Set it down. Turn off the VDG if
you wish, so the room is quiet for dramatic effect. Casually use an
insulating rod (follow instructions to the letter here--I said
*INSULATING*) to lift the hook on the capacitor, lifting out the center
conducting can.  Set it aside, on an insulating surface. Now, boldly (!)
grasp the outer can and the inner insulator and take them apart. Some
students wonder at your daring. Set the insulator down. Handle the can
freely. Pick up the other can and handle it freely. Place the small can
inside the large one.  Nothing happens. Discharge (!) both cans on a metal
pipe if you wish, for effect. If you wish, say, "*Now* these shouldn't
have any excess charge on them."

Now reassemble the capacitor. Pay attention here. Place the plastic can in
the outer can. Now *use the insulating handle* to lift the hook of the
inner metal can, and lower the metal can into the plastic can. The
capacitor is now reassembled. You can handle it by the outer can, but
don't touch the inner can. Freely hold the capacitor for effect as you say
"Let's see if there's any charge left on this capacitor", but don't comment
on or explain your actions--yet.

Now use the discharging tool again, slowly bringing it toward the center
ball until that, huge hot spark happens again, just about as strongly as
before. You feign surprise. "There was charge there all along!"

Still seeming surprised, say "We took the thing apart, handled the parts,
and even discharged them on a grounded water pipe in the usual way, yet
when reassembled, the capacitor still had as much charge as before!" Let
them think about this.  Notice that you have deceived (lied) to them. Note
the careful wording of the sentence above. You gave the impression you had
handled and discharged *all* the parts, when, in fact, you did not handle
or discharge the plastic cup. Will any student notice this deception? Will
any volunteer comment? Most students wrongly think that the charge must be
on the metal cans, so will not consider it important to examine the
plastic cup.

==> At the end of the demo, recap this point, restating your deceptive
sentence, and pointing out how *some* people can make the wrong inference,
and how people who desire to mislead or deceive can have a field day with
folks who don't examing assertions critically, just by clever choice of
words, by selective omissions and selective emphasis.  Take every
opportunity to encourage skeptical and critical thinking in students.
Also, in the recap, emphasize the importance of *observing* details of
what happened, for example, the fact that no attempt was made to discharge
the insulating cup, and that an insulating tool was used to lift the inner
metal cup out of the charged capacitor, and to replace it later, but no
such care was required when handling the outer metal cup. Ask them why.
<==

Now ask them to think about where the charge was, while you fire up the
VDG and charge the capacitor again. By then they have finally figured out
that it resides on the inner and outer surfaces of the insulating glass
or plastic can. Disassemble the capacitor (carefully) as before. Pick up
the insulating can by its bottom, and offer the can to the nearest
student suggesting that they put their fingers inside to see if there's
any charge in there. Assure them it is safe, but don't force it on
someone who is adamantly unwilling. There's always some gullible fool in
the class who will take the teacher's word that something is safe.

Ask the class to be 'very, very quiet' as the student (probably with
great caution) inserts fingers into the insulating can. Student will feel
the charge, and usually others can hear the 'crackling' sound, but student
feels nothing even slightly painful, just a pleasant coulomb tickling.
Point out that there's charge on the outside of the insulating can also.

Even after the student has done that, significant charge remains. The
capacitor can be re-assembled and a healthy spark drawn from it.

Assemble and charge the capacitor again, while you discuss what has been
seen. Discuss *why* the charge went to the inner and outer surfaces of the
insulating cup, and why there actually was no charge on the metal cups after
you took the capacitor apart. Show this by disassembling the capacitor
and bringing the metal parts near a charged electroscope.

Recap as indicated above between ==> and <==

I've hit the important points. Embellish as your flair for the dramatic
dictates.

We had a nice old brass dissectable capacitor with a perfectly fitting
large glass insulator between. It was big as one of those soda-fountain
glasses of my childhood. One of my colleagues foolishly handed it to a
student, who was startled by the crackle of charge, dropped it and broke
it. Moral: hang onto the bottom of it when you hand it to the student.
Fortunately we found a plastic soft-drink cup from a fast-food place
which nearly fit, and the thing works quite well with it. Moral: You can
probably build your own dissectable capacitor if your budget is skimpy.
Just fashion inner and outer cups of metal around a suitable large
plastic glass. (Heavy aluminum foil, perhaps? Self-stick aluminum sheet
[comes in rolls]? A coat hanger for the inner rod?)

Lots of good electrical demos can be built from scratch. I recall as a
child building an electrophorus in the kitchen (with my mother's
indulgence) with a pie plate and a phonograph record, then charging a
capacitor made of aluminum foil and a sheet of window glass. Later I used
layers of foil and waxed paper all rolled up. I was a relatively good
child, and resisted any opportunity to zap a barnyard cat with it (I liked
cats). The neighbor boy who stopped by to visit wasn't so lucky when he
foolishly asked "What are you doing?"

Many good things we do in class, we just do, and never 'script' them. We
encourage students to take notes, write out lab strategy in advance,
write clear accounts of lab procedure. I guess we ought to take our own
good advice more often.

        -- Donald

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Dr. Donald E. Simanek                            Office: 717-893-2079
Prof. of Physics                    Internet: dsimanek@eagle.lhup.edu
Lock Haven University, Lock Haven, PA. 17745          CIS: 73147,2166
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