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 table.

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 them—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 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 charge.

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 plate,

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 dramatic dictates.

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 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. 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 passerby.

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 making measurements.

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