## Demo Notes:

### The van de Graaff Generator

• Background
• Before doing this demonstration you should do a lab in which students explore the attraction and repulsion effects that charged objects exhibit when brought near one another
• Important concepts include:
• Rubbing different types of substances together creates static charges
• Charge comes in two types: positive and negative
• Negative charge comes about because of an excess of electrons on the atoms in the charged object (aside: the word electron comes from the Greek word for amber because the ancient Greeks noticed electrical phenomena when they rubbed amber)
• Positive charge comes about because of a loss of electrons from the atoms in the charged object; in effect the protons left behind now outnumber the electrons
• Neutrally charged objects do not represent a third type of electric charge; neutral just means that all the positive and negative charges are balanced out
• The nuclei of the atoms are not affected because the protons (which have a positive charge) and the neutrons (which are neutral) are 1,836 times more massive than electrons: they can’t move but the electrons can
• Positive charges are attracted to negative charges. Positive charges repel each other. Negative charges repel each other, too.
• Conductors are substances that allow electrons to flow through them easily. Examples include metals, graphite, and salty water. Electrons are very mobile within conductors and will always spread out to be as far apart as possible.
• Insulators are substances that do not allow electrons to flow through them easily. Examples include glass, dry paper, air, rubber, and very pure water. Electrons can be added to or removed from the surfaces of such objects but electrons will not move from atom to atom within them.
• Electric current in a conductor is caused by the same force that causes charges to be attracted or repelled. The electrons move toward positive charges and away from negative charges. They will flow until all charges in contact with the conductor are in balance. That is, until every positive charge is matched up with a negative charge.
• Electric current will flow into or out of the ground until charges are balanced. The Earth is an infinite reservoir for electrons and can accept or provide as many as necessary. Electric charge is a conserved quantity and is constant at all times throughout the universe. Small local imbalances create the effects discussed here.
• See the links at the bottom of the web version of this document for simulations to take a look at before continuing.

• Performing the Demonstrations
For all demonstrations always describe the experiment and ask students what they think will happen. Ask them why they think it will happen that way. Finally, after the demonstration discuss whether they were right and whether their reasoning was correct.
• Opposite Plastic Find two kinds of plastic which can be rubbed with paper to obtain opposite electric charges. Demonstrate that like kinds of plastic repel and opposite kinds of plastic attract. This is best done by suspending one of each kind of plastic, charging one by rubbing and bringing another non-suspended piece, which has also been rubbed, close to the suspended piece. This demonstration is best carried out by students.
• The paper either adds electrons to the plastic to give it a negative charge or removes electrons to give it a positive charge. The motion of attraction and repulsion is subtle and requires a good observer to make it visible. Students learn that there are two kinds of charge, that opposites attract and likes repel.
• Bits of Paper Tear paper into a large number of very small uneven pieces. Charge a piece of plastic by rubbing it with paper. Bring the plastic close to a pile of the pieces of paper. The bits of paper will be attracted to the plastic, regardless of the charge.
• If the charged plastic has a net negative charge it causes the electrons to move away from it within each atom of the paper. Similarly, the positively charged atomic nuclei are attracted toward the negatively charged plastic. This motion of particles within an atom is called polarization. It is an extremely tiny change but for a light object like a tiny scrap of paper it is enough for the attractive force to be observed. If the charged plastic has a positive charge then the electrons and nuclei move in the opposite direction…but the effect ist the same.
• This demonstration shows attraction of opposite charges and induction of charge in non-conductive objects (polarization).
• Bouncing Ball Suspend a small foam ball covered in aluminum foil between two pieces of metal. The distances between them should not be great. Move the ball away from the pieces of metal. Then use pieces of plastic that gain opposite charges when rubbed with paper to give the two pieces of metal opposite charges. Now bring the ball between the two pieces of metal again. It will bounce back and forth between them.
• The aluminum foil on the ball experiences a separation of charge due to the proximity of the charged metal. Electrons move through it toward the positive charge and away from the negative charge. Due to the fact that the pieces of metal are undoubtedly charged to different extents the ball will move toward one but not the other. When it comes in contact it either transfers electrons to a positively charged piece of metal or accepts them from a negatively charged piece. Once this occurs the two objects have the same charge and the ball is repelled toward the other piece of metal. There the same process occurs and the ball bounces back and forth until the charge in the two pieces of metal has become equalized.
• This demonstration shows how opposite charges attract, like charges repel, and how electrons move through conductors.
• Van de Graaff Disassembly Unplug the van de Graaff generator. Take the top dome off of the generator. Show students the band which can move and the comb which accepts the charges from the moving band. Explain that the band is charged by rubbing within the mechanism and that the comb electrodes at either end serve to spray electrons/accept electrons. The result is that the metal sphere at the top gains a large net electric charge.
• Flying Pie Pans Place an aluminum pie pan on the top of the van de Graaff and turn it on. Once it has obtained the same charge as the generator it will fly off. Try it with two pans. Then try it with as many as you have: say 10. All the pans will fly off one by one in all directions.
• Like charges repel. The pans are held down by gravity due to the weight of the pans on top of them. Once the weight is reduced sufficiently they fly away.
• Foam Hailstorm Fill an aluminum pie pan with foam packing pellets. Place it on top of the generator and encourage it to stay in place with a small piece of clay. Turn on the generator. The pellets all go flying in all directions!
• Like charges repel. The pellets obtain the same charge as the sphere and each other and the result is a force which makes them all fly apart.
• Dancing Bubbles Use commercial bubbles solution to blow bubbles toward the van de Graaff while it is running. At first the bubbles will be attracted due to the polarization of charge in the atoms of the bubbles. Once they approach more closely they will be repelled because they will be charged by the ionized air flowing away from the generator. This sometimes is so violent as the pop the bubbles. If it is not it may be possible, with practice, to levitate a charged bubble by attraction to your hand. This works because the charged bubble induces polarization in the atoms of your hand so that the opposite charge is closest to the bubble.
• Charge induction by polarization. Like charges repel.
• Hair Raising Have a volunteer with long hair (preferrably that is not too thick or wavy) stand on an insulator. A pile of 3 or 4 textbooks works well. Ask them to place their hands on the generator before starting it. Tell them that to avoid shocks they must by no means let go of the generator! If they do they will quickly regret it due to the shocking sparks that result. Turn the generator on and their hair should rise up and stand on end. This demonstration works very poorly if the air is too humid or if the hair has been treated to make it stick together (water, hairspray, etc.)
• Like charges repel. The individual hairs all obtain the same charge and repel each other. The books are necessary to prevent the volunteer from being grounded. If they are then all the charge leaks away into the ground without charging up the hair. The volunteer shouldn’t let go because this lowers their electrical potential and sparks can fly, shocking the person.
• Spark on Knuckle Turn on the generator and once it has been running for a while bring a knuckle close to the sphere with a quick, sharp motion. A large luminous spark will fly from the sphere to your hand. This is mildly uncomfortable but bearable. The grounding sphere should also be used to demonstrate the generation of sparks.
• This demonstration looks like magic. Students should be asked whether the sparks are flying from or to the demonstrator’s hand. The demonstrator is grounded and electric charge always flows to the ground since it is an infinite sink. Energy is used to create a separation of charge so that the sphere at the top of the van de Graaff becomes strongly charged. This energy can be released in the form of a spark because the tendency of all electric charges is to be attracted to the opposite charge and to move until all charges are equalized.
• Spark Chain Have students hold hands while standing on books. Have the one closest to the generator place a hand on it and promise not to let go. Turn on the generator and though hair might rise, nothing else happens. When the demonstrator grounds the last person in the chain with a touch all participants feel a small spark.
• The voltage created by the generator is so high that people are conductors. All of the participants obtain the same charge as the generator and the chain can be grounded when anyone lets go, steps onto the floor, touches someone who is not charged, or lets go of the generator. Upon grounding the sparks fly in the small spaces between hands.
• Glowing Discharge Tube Prepare a small fluorescent tube (not the spiral bulb style) or an atomic emission spectroscopy tube by attaching a length of wire to one end. Turn off the room lights for better effect. Turn on the van de Graaff and point the other end of the tube at it while holding on to the wire. Because you are grounded the ion wind will energize the tube and make it glow.
• The van de Graaff ionizes air molecules close to its surface. These particles then fly away due to electrical repulsion. These moving ions can carry enough energy to cause an emission tube to light up as long as the other end of the tube is grounded, allowing the charge to flow through it.

• Safety
• The sparks can hurt and some people will be very afraid of them. The danger is more from sudden motion due to overreaction than from the electric discharges from the van de Graaff. The voltage created is very high but electrical current will be very low. This is a pretty safe situation.
• Do not do the demonstration near computers or other electronic equipment. Students may wish to keep personal music players and cell phones far away.
• Persons with pacemakers should use all due caution! Don’t get near the van de Graaff just in case a discharge causes a malfunction.

• Cleanup
• Pick up and store or dispose of all the flying bits of paper, foam, pie plates and what have you.
I owe a lot of this material to Van de Graaff Generator Hints, Demos, & Activities ©1994 William J. Beaty http://amasci.com/emotor/vdgdemo.html
Some of the background material was informed by Lessons In Electric Circuits -- Volume I Chapter 1 by Tony R. Kuphaldt ©2000-2008 http://openbookproject.net/electricCircuits/DC/DC_1.html

Last updated: Nov 30, 2008       Home