A Myriad of Demonstrations Using a Van de Graaff Generato

A Myriad of Demonstrations Using a Van de Graaff Generator

by This email address is being protected from spambots. You need JavaScript enabled to view it. , The Science House

A talk presented at the American Association of Physics Teachers Meeting, College Park, MD, August, 1996

Sparks

1. A spark between the aluminum globe of the generator and a grounded aluminum globe. On a good day you might see sparks about a foot long or so, but on a hot, humid day you will probably have very small sparks. On a hot, humid day, try using a hair dryer to lower the relative humidity near the generator. You might only need to blow around the outside of the aluminum globe to get sparks several inches in length, but you might want to blow inside the base of the generator to dry things out in there (be careful not to heat up the belt too much-- most models of classroom Van de Graaff generators have latex belts that can be damaged by excessive heat). Sometimes I crack the top half of the globe off and just blow a little warm air straight down the support tube and get decent sparks.

2. A spark between the aluminum globe of the generator and a finger. This is usually not very comfortable, and often the sparks are fairly short and/or faint because of the small radius of curvature of the end of your finger. The back of your arm might give you better sparks, and the sparks won't cause you as much discomfort. On days when I have been sweating a bit, the back of my arm doesn't work very well (for attracting sparks, that is)-- I suppose this is due to the humidity around me! In addition to just making sparks, try the next demonstration.

3. A spark between the aluminum globe of the generator and the back of the forearm making the muscles of the forearm contract causing the hand to twitch. Don't try a spark to the top of your head with your tongue sticking out: not only will you have a small sore spot on your head, you might bite your tongue. Be careful when bending down to pick up objects you have dropped on the floor around the generator for this same reason.

Repulsion

The Van de Graaff electrostatic generator develops either a net positive charge or a net negative charge. It will repel objects with a like charge. To determine if your Van de Graaff generator develops a positive or negative charge, the easiest thing to do is to rub a balloon on your hair or shirt to develop a net negative charge on the balloon, and then throw the balloon at the generator while it is charged. With most classroom demonstration models of Van de Graaff generators, the charge developed will be negative and the balloon will be repelled. If by some odd chance your generator develops a positive charge, it should attract the balloon, at least until the generator charges the balloon positively. Most classroom Van de Graaff generators rely on charges coming from the separation of a rubber belt and a felt-covered pulley. If you look at a triboelectric chart, you will see that this results in the belt carrying away electrons from the felt, the same as the balloon pulls electrons away from your hair or shirt. The electrons are drawn from the rubber belt to the globe at the top of the generator (through corona discharge), and therefore the generator will be charged negatively, repelling the negatively-charged balloon.

1. The hair. Everyone's seen this, many have done this, and many have tried and been disappointed (at least occasionally). Every now and then even I have a hard time getting someone's hair to stand up from the charges building up and repelling each other. Usually my hair works great for this demo, but if I have been sweating (which is usually the case during my Physics on the Road presentations), my hair sometimes won't budge. I generally have a volunteer from the audience come up for the first try (or two), stand on a plastic stool (mine is a Rubbermaid step stool), place one hand on the globe of the generator, hold a mirror in the other hand, and then turn on the generator, and if the hair stays down, I try mine. If that doesn't work, I use some other method to demonstrate the repulsive forces between like charges. Don't avoid any one type of hair except for short or braided hair. Hair spray (previously applied) can sometimes make this demo more impressive because large sections of the hair can be made to stand up instead of individual strands. One thing to watch for during a failure are charges leaking off the person quickly (you can sometimes hear this; if you hear the charges leaking off of the student's shoes, have the student rearrange his or her feet so that they are more completely on the stool). If you can pinpoint the leaks, you might be able to have the student reposition a body part or take off a certain piece of jewelry to stop the leak. Sometimes a leak is due to a nearby object which can be moved or avoided (speaking of which, don't put the stool close to the table or any other object that might cause the student to get shocked). To discharge the student, you can touch the generator with a chicken stick, but I usually just have the student quickly pull the hand away from the generator and step off the stool to discharge. The student might get a small shock to the feet or toes, but this is rare. The hair is always the best "repulsion of like charges" demo, but there are others.

2. Packing peanuts. You know what I mean: put some packing peanuts in a plastic cup and tape it to the top of the generator. Turn on the generator and away they go! I use the packing peanuts made of corn starch when I do this since they are biodegradable (and if they get stepped on and squashed, I can wash them down the drain). On a good day, the cup of foam pieces can be held in one hand while touching the generator with the other hand (and, of course, standing on a plastic stool), and when the generator is turned on, the pieces will fly out. This can also be done with other small, lightweight things, such as some cereals or confetti. If you want a real mess, put some black pepper in a small dish on top of the generator, but be prepared to sneeze! Grits (not cooked!) work well also, but I might be getting too far north for that. 3. Pie pans. Place a pie pan or several stacked pie pans upside-down on the top of the generator. When the generator is turned on, the pie pans lift off one at a time and fall to the sides. This can make quite a racket, and if the generator is not charging the pans well enough, they might just fall off from the vibrations of the generator before they charge up enough to lift off. If you have a chance, try it with some of the small aluminum ashtrays like you find in fast food restaurants. These are cheaper (unless you get caught!), and are easier to store, but then again they are easier to pack away and be lost because of their smaller size.

4. A small balloon attached to the generator by a string taped to the globe will be charged to the same sign as the globe of the generator. The balloon will rise up straight above the generator if the string is not too long ("too long" is determined by the size of the generator and how well the generator is charging; I usually only use a string about six inches in length). Try this with a piece of tinsel in place of the string.

5. A polystyrene cup works the same way as the balloon, but variations on the demos are welcomed by students. Maybe your artistic talents can be demonstrated by drawing on a foam cup.

6. Speaking of your artistic talents, you might want a Van de Graaff designer hat. Take a polystyrene bowl, attach several balloons and/or foam cups with strings and tape, tape a string to hold the hat on your head like a cowboy hat, and let the charging begin! If you don't like making a fool of yourself, just tape the contraption onto the generator (I was going to say, if you don't like making a fool of yourself, you're probably in the wrong profession, but I decided to be nice to those fuddy-duddies).

7. Make a string wig and attach it to the generator. It does about the same thing as hair, but it is extremely more reliable.

Attraction

The generator will attract objects with the opposite charge and many objects with no net charge (through induction).

1. Hold a paper towel near but not touching the generator. It will be attracted to the generator through inductive charging. Don't let go of the paper towel unless you want to do a demo from the next section! Also don't get too close to the generator: paper towels can be conductive and give you a shock. This works with almost any small lightweight object. Sometimes you can even see one of the grounding globe accessories wobbling back and forth from the attraction before the spark jumps across.

2. With someone's hair standing up or with the string wig attached to the generator, move your hand or the chicken stick near the hair or strings. The hair or strings will be attracted to your hand or the chicken stick because your hand or the chicken stick are charged through induction to the opposite sign as the generator charges the hair or strings. Do not get close to the person's head while doing this demo: both you and the student might get a pretty good shock, and shocks to the head are not soothing! This same effect occurs when you move your hand near the polystyrene cup or balloon attached to the generator by a string.

3. Being careful not to pour water on the base of the generator, pour some water from one beaker to another near the generator. The water stream will be bent towards the generator. Sure, this can be done with a piece of Styrofoam rubbed in your hair instead of the Van de Graaff generator, but while you have it out performing other demonstrations, why not give it a try. Sometimes the water is pulled farther to the side than you expect, and small droplets get pulled onto the globe of the generator, so have some paper towels ready.

Attraction followed by repulsion

When an object is charged by induction by holding it near the generator, it will be attracted towards the generator. If it is allowed to get close enough to the generator (sometimes even touching), it can be charged to the same sign as the generator through conduction and will be repelled.

1. Hold a paper towel or coffee filter by the edge near the generator. It should be attracted to the generator. Let go of the paper towel or coffee filter, and it will fly to the generator, get charged, and fly back toward your hand. The same thing works with many small lightweight objects.

2. Blow some small soap bubbles near the generator. I usually use a soda straw as the bubble blower for this demo so that I get small bubbles. This one can make a soapy mess, so don't do it prior to other demos!

Corona discharge

The electric field intensity around a conductor is inversely related to the radius of curvature of the object. Therefore the electric field is greatest near points and edges. Corona discharge occurs because the electric field is so great near points and edges that the air is easily ionized. This ionization can be seen as a blue glow around some objects. If you don't like getting shocked by the generator, keep a small (or large) piece of foil handy to drain charges away in small quantities. The foil actually works better as a chicken stick than a chicken stick, and it's a lot cheaper.

1. Tape a thumbtack to the top of the generator. Turn the lights off. See what happens (you might even be able to hear the corona discharge).

2. Crack the top half of the globe of the generator off. Turn the generator on. Look at the glow around the spray (the sharp piece of aluminum or screen wire or whatever your generator has near the belt to charge the globe) and around the top edge of the generator. You should be able to see the corona discharge, especially near the spray. The same thing happens at the spray in the base of the generator, but it is harder to see because the base is usually not transparent!

3. Instead of having a student place a hand on the generator to do the hair demo, have the student hold a small piece of aluminum foil near the generator to charge through corona discharge. Then to get rid of the charges, have the student point the foil away from the generator.

4. Hold a strip of thin aluminum foil near the generator by the end (try a piece about half-an-inch wide and the width of the roll long). The foil will strike at the globe like a snake, get charged through conduction or corona discharge, be repelled, discharge through corona discharge, and repeat. Try this with a piece of tinsel or string also. Also try connecting a ground wire to the end of the foil.

5. Take a small piece of foil about one square inch in area and hold it near the generator (you don't have to worry about getting shocked, at least not at this point). Let go of the foil and pull your hand away. The foil will be attracted to the globe, but the charges will be leaking off the edge away from the generator, and if the attraction reaches a steady state balancing out gravity, the foil will float around the bottom of the globe of the generator. The biggest problems with this are the shape of the foil (you might need to try several pieces of various shapes and sizes) and the foil sticking to the support column of the generator (I guess this could be overcome by holding the generator sideways or upside down). If it works, it is a very impressive demonstration that many have not seen. I cut a piece of foil the shape of a moth (at least I think it looked like a moth) and it fluttered around the generator for a while before resting on the plastic support tube. I have had pieces of foil flutter around the bottom of the generator for about ten minutes without them sticking to the tube, but I've also had some stick immediately.

Other

By now you are probably sick to death of these demos and have a few ideas of your own to try. If you come up with anything interesting, please email me at This email address is being protected from spambots. You need JavaScript enabled to view it. and let me know what you did. Playing with stuff like this is part of my job, so if you have something that doesn't quite work but you think it should, let me know and I might be able to play around with it till it works! Anyway, I thought I'd give you a few more demos to try, but with minimal discussion.

1. Light a candle and hold it near the generator, or fasten it to the top. As long as the flame is closer to the generator than your hand you don't need to worry about getting shocked.

2. Put some smoke near the generator. Maybe try some chalk dust too.

3. Pour an electrorheological liquid near the generator (you will probably get shocked with this one). You don't know what an electrorheological liquid is? An electrorheological liquid is one whose flow is interrupted by static electrical charges. An example of one is a colloid of corn starch and vegetable oil. The oil and corn starch (mixed up very well to form a liquid about the consistency of Elmer's glue) will flow normally until you bring it close to the generator. The charged generator makes the corn starch line up and hold its position and the oil is unable to flow around it. If you want to try this without getting shocked, do the same thing with a piece of Styrofoam insulation charged up by rubbing it with hair instead of the Van de Graaff. This is much easier, but my talk is about Van de Graaff generators, so I had to use one to work this into the talk.

4. Take the globe off of your generator (maybe you don't have the globe anyway-- it's amazing how many pieces of Van de Graaff generators there are sitting around). Take the upper spray off the generator (this might have come off with the globe). Stand on a plastic stool and point a small piece of aluminum foil at the upper pulley. Turn on the generator. You should develop a charge through corona discharge. Sometimes this works better than charging through conduction. If you're lucky, your hair is now standing on end. To discharge, point the piece of foil away from the generator, and there goes that corona discharge again. I guess it's demos like this that make physics teachers such pack rats. Being able to do stuff like this when you only have a somewhat insignificant part of the generator-- after all, the globe is the most important part, right?-- keeps you from throwing anything away!

5. Hold a fluorescent tube near the generator. Adjust the position of your hand to control how much of the tube lights! Also try this with spectrum tubes, especially neon.