VIP's Electrosatics Demonstrations

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The demonstration were submitted to the e-mail list system called PhysicShare.

Date: Fri, 3 Jan 1997 08:02:59 MDT
From: "Merrell, Duane
Emery High School

Van de graf,

Two long wire conductors, thin is good. Two mylar helium balloons, big is great. Attach one conductor to each balloon with tape, make sure ends are not varnished and will conduct. Tape the other end of each conductor to the top of the van de graf. You should have two mylar balloons floating above the vandegraff attached to the vandegraff. Turn the van de graf on and since both balloons gain the same charge they will repel. Should be big seperation. Seems to always work here in Castle Dale, Utah.

Duane Merrell

One of the best Oh-Ah van de Graff demos is to place a group of about 10 small aluminum pie pans (tart size not the full size one) stacked upside down on top of the van de Graff. Then turn it on. They sequentially float off of the top of the dome. It's even better if you paint a happy face on the dome.

Ray Rogoway

Van de Graaff ideas: IN ALL OF THESE WATCH FOR ANY STUDENT WITH HEART PROBLEMS

• fl. tube lighting
• large clear bulb lighting (a la plasma sphere)
• large ring of students holding hands, etc.
• place styro bowl of puffed rice cereal atop it, crank it up, and look out foil covered (then popped) balloon suspended near it - big pith ball (use mylar balloons, too)
• Leyden jars (try homemade ones, too) - BUT BE CAREFUL!
• hair on end (student must be insulated from floor - build stand or use rubber mats)
• charge on a hollow conductor
• Faraday cage

Sean P. Lally - Science Chairman
High School Physics Teachers Assn.
Sewickley Academy

More Van de Graaff ideas:

Get a standard fluorescent light bulb (the long and skinny variety), stand next to the Van de Graaff generator, and hold the bulb near one end. Let the generator arc to the middle of the tube, and you'll get a nice flash of light in the bulb. You'll feel a small jolt in your arm, but it's not painful. (Although after doing this demo for three periods in one day I did have a buzz in the right half of my body for about 12h.)

Stand on a wooden ladder, and hold the electrodes of the fluorescent bulb on the generator sphere. The bulb will glow, and you can move your other hand up and down the bulb to cause the length of the "glow" to increase or decrease.

Another thing I've tried with success is to hang two Al cans from the ceiling (each on a different string) so that they are touching one another as they hang. Stand on a wooden ladder next to the generator,place one hand on the sphere, and hold a metal rod in the other. When you fire up the generator, you can point the rod at the cans (from a distance of a few feet or so) and put a charge on them that will cause them to repel like the leaves of an electroscope.

I never realized how difficult it would be to describe these in words only! If only I was an accomplished ascii artist. <g> I also do the other popular van de Graaff demos, but my students really love these. They especially like to try them themselves.

Good luck. I hope they work for you.

-Tim

I find the Van De Graff generator particularly useful when I am introducing electrostatic fields. What I do is to take a metalized pith ball attached to the end of a silk thread which is in turn taped to the end of a wooden meter stick. I mount the meter stick on a ring stand. Next I charge the pith ball positive using a polyethylene dry cleaning bag rubbed against a plastic ruler. Then I turn on the generator keeping the pithball far enough away from the generator so that it doesn't touch but close enough to be strongly attracted by the sphere. Used carefully it can be used to demonstrate the direction of the electric field - particularly noting its radial nature - by moving the pith ball to various locations around the negatively charged Van De Graph sphere.

Here's some ideas from my home page, in the section on teaching scenarios. These were posted about a year ago here, but the text has been edited now, and a GIF supplied (uuencoded, at the end).

GETTING A CHARGE OUT OF ELECTROSTATICS

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 has 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 that when assembled it is a Leyden Jar. But its three 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, "*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 push 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 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 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 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 Leyden jar 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.

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. 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 gets cleaned in the same way, inside and out.

The metal parts often acquire an oily film from handprints 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 which 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 cleaning the cleaner off with distilled water. We often use the liquid metal cleaner sold for cleaning silverware and metal pots and pans. It contains dilue thiourea and hydrochloric acid.

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 stook 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 the magician's ploy, which takes advantage of our 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 which 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."

-- Donald

Build a full size Faraday cage. I made one from 12-14 feet of 36 inch wide aluminum screening (hardware stores don't understand meters). Fold the screen in half to make a 6 foot envelope. Staple the sides shut (I later used wire to sew them shut). Put a piece of duct tape over the bottom edges to prevent snagging of clothing. Slip the screen over a person. Conduct charges from the Van De Graff (or a small Tesla coil). The idea of shielding becomes very real. The person inside feels nothing. And it gets their attention!

Lowell Sylwester

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