GETTING A CHARGE
OUT OF ELECTROSTATICS
An electrostatics demo scenario by Donald Simanek.
For this demonstration you will need either a Van de Graaff electrostatic generator or a Wimshurst machine and a dissectable capacitor.
One of my favorite electrostatics demos with a Van de Graaff is the
dissectable capacitor. Scientific supply houses sell them.
This device has two
aluminum cups that 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 that when assembled it is
a Leyden Jar. But its three component pieces can be easily 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 the Van de Graaff. I hold the ground wire
(with my hand--firmly; don't let it slip) against the outer metal
cup and bring the entire capacitor toward the dome of the Van de
Graaff, so sparks jump to the hook attached to the center conductor
of the inner cup, charging the capacitor. If you leave it too long,
it discharges by sparking through the plastic, so you'll have to
do it again. Experience. Practice.
Then set the capacitor on an insulating surface (a wooden table is
ok). Don't grab the inner and outer conductors while it is
assembled! It can produce a hot, thick spark. Show the class this
spark, using one of those discharge devices with two metal
adjustable semicircular arms with a ball at each end and an
insulating handle. Touch one end to the outer can, then bring the
other ball end of the discharger slowly and dramatically toward the
ball on the hook of the Leyden Jar. 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 this spark can often be 2 cm long, or more, and quite
thick and bright.
If you don't have one of those 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 on the table.
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.
Now, slowly and carefully offer the can hanging on the insulating rod
to a nearby student, saying (casually) "Would you hold this for me?"
Most students recoil from it and refuse. "Oh, be that way," you
say. "I'll do it myself." Casually take the metal inner can in your
hand and set it on the table, or other insulating surface.
Now, boldly (!) grasp the outer can and the insulating cup and take
them apart. Some students wonder at your daring. Set the insulator
down. Handle the can freely. Pick up the inner can and handle it
freely. Place the small can inside the large one. Nothing happens.
Discharge (!) both cans by touching them to a metal pipe if you wish,
for effect. If you wish, say, before you do it, "Now these shouldn't have any excess charge on them."
Offer the nested metal cans to another student to hold. (I've had
students refuse, even after seeing me handle them freely. What ever
happened to trust?) Don't press this point, just set the cans on the
Now reassemble the capacitor. Pay attention here. Place the plastic cup
inside 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 cup.
The capacitor is now reassembled. You can handle it by the outer can, but
don't touch the inner can at the same time. Freely hold the capacitor for
effect as you say "Let's see if there's any charge left on this
capacitor." Don't comment on your actions or explain themyet.
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
Still seeming surprised, say "We took the cans 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 anyone
volunteer comment on it? At this point most students wrongly think
that the charge must be on the metal cans, so they 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 other folks who don't examine 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 the charge resides on the inner and outer
surfaces of the insulating glass or plastic cup. Disassemble the
capacitor (carefully) as before. Pick up the insulating cup by its
bottom, and offer it to the nearest student, telling him or her to
put a hand inside to see if there's any charge in there. Assure
the student that it is safe, but don't force the cup 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.]
I usually choose a girl for this part, looking her straight in the
eye with a sincere 'trust me' look, accompanied by non-threatening
body language. I've never had anyone refuse to put a hand inside
the cup. Psychology?
Ask the class to be 'very, very quiet' as the student (probably
with great caution) inserts her fingers into the insulating cup. The
student feels the charge, and others can hear the 'crackling'
sound, but the student feels nothing even slightly painful, just
a pleasant Coulomb tickling. Point out that there's charge on the
outside of the insulating cup also.
Even after the student has done that, significant charge remains.
The capacitor can be re-assembled and a healthy spark drawn from
it. The hand and fingers made contact only with a small fraction
of the cup's inner surface, and the parts not touched kept their
Assemble and charge the dissectable 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 (or very little)
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.
Footnote: Some discussions of this experiment use the term
"dielectric" to describe the insulator. I have not used this word, because
it is unnecessary, and potentially misleading, in this context. While the
dielectric properties of the insulator are certainly there, they play
little role in this experiment. To see why, consider the details of the
process of charging a capacitor.
First look at the case of a capacitor without anything (vacuum) between
the plates). How does it get charged? Some potential source (perhaps a
battery) moves electrons from one plate, through the external circuit to
the other plate. The external source doesn't add charge to the capacitor
(it had zero net charge at the start, and zero at the end). Therefore the
usual term "charging" is misleading. The external source simply did the
work necessary to transport free charges from one plate to the other via
the external circuit.
Where does the excess free charge reside after the process finishes? On
the very inside of the negative plate, as close as it can get to the
positive plate (which has an electron deficiency).
Now in the capacitor with an insulator-dielectric between the plates, what
happens? The same thing. Except now, since the insulator is in close
proximity to the plates, many free electrons from the negative plate easily
jump the small gap from that plate to the surface of the insulator. Since
it is an insulator, that's as far as they can go toward the positive
During the capacitor disassembly, there's still a strong field between the
plates, in such a direction as to keep the excess charges in place on the
surface of the insulator until the plates are too far away for the charges
to go anywhere else. Therefore these free charges stay put on the surface
of the insulator.
Recap the demo as indicated above.
I've hit the important points. Embellish as your flair for the
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. A plastic soft-drink cup is good.
Heavy aluminum foil may be formed around it for the outer and inner cans.
A coat hanger may serve 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) from 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
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. We ought to take
our own good advice more often.
CARE OF THE VAN DE GRAAFF
The Van de Graaff needs cleaning every so often. Its dome and
center column collect pollution and finger prints. Its belt
attracts dust and pollution.
We disassemble the machine, and swish the belt around in water with
mild detergent, then rinse it thoroughly in clean water and let it
air-dry. The plastic center column and metal dome is cleaned in
the same way, inside and out.
The metal parts often acquire an oily film from hand prints and
atmospheric pollution. The detergent treatment usually suffices,
but can be preceded by wiping the metal with alcohol.
FAILURE TO ATTAIN A CHARGE
The Van de Graaff just doesn't work well on humid days. If a
charged electroscope discharges in a few seconds, don't expect the
Van de Graaff, or any other classic electrostatic machine, to work.
Wait for a dryer day.
Be sure the motor is running up to speed (you can tell by the
sound). Our Tel-Atomic machine uses a sewing machine motor with
carbon brushes. Your local sewing-machine repair shop can probably
clean, lubricate and put in new brushes, though a mechanically
clever student can do the job.
If there's a DC power supply to put potential onto the comb at the
bottom of the belt, check to make sure it is functioning.
Make sure that the wire and spring contact that carries charge
from the upper comb to the dome has continuity. This is often phosphor
bronze. It may acquire an oxide film, which can be removed with any
commercial metal cleaner, followed by removing the cleaner with
distilled water. We often use the liquid metal cleaner sold for
cleaning silverware and metal pots and pans. It contains dilute
thiourea and hydrochloric acid.
Don't use anything on the rubber belt that would be harmful to rubber.
SHOWMANSHIP WITH THE VAN DE GRAAFF
On a dry day our Van de Graaff produces sparks five or six inches
long. These can sting a fingertip. Keep a grounding wire nearby to
discharge the dome when not using it, for it can surprise a
If you wish to show your hair standing on end, stand on an insulating
stool or platform. A low wooden stool is good. Place your hand on
the VDG dome before charging it. Likewise if you have a student do
it. A person with fine, dry hair is best.
Since the fingertips are very sensitive, use the back of your hand
to draw sparks from the dome, or use your knuckles. A neat trick
is to 'throw sparks' with the hand. After you've determined the
range of the sparks (the maximum length of them), lunge the back
of your hand toward the dome, stopping suddenly as your knuckles
are just in range of the spark. The spark jumps between knuckle and
dome. To the audience, it seems as if you have 'thrown' a spark at
the dome. This is a magician's ploy, taking advantage of the
psychology of vision. The eye follows large motions, ignoring
smaller ones. The magician makes a sweeping gesture, while doing
the dirty work with small motion of the other hand. Here, the eye
follows the lunge of your hand. When your hand stops, the observers
think they see the motion continue in the spark. Many students, if
asked, swear that the spark started on your knuckle and jumped to
the dome. Do this in a darkened room, for best effect.
I usually explain such deceptions after the fact, to remind
students how easily the mind is fooled into thinking we see things
that aren't there, or that aren't exactly the way we think we see
them. This is why scientists are cautious about trusting their
unaided senses, but prefer to design unbiased instruments for
You may wish to include in these demos some one-liners like:
"I really get a charge out of this demonstration!"
"This machine has the potential to give you a nasty surprise."
For more about electrostatics equipment you can build, see
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