Education

Van de Graaff generators in the classroom: Theory, operation, and safety

20 April 1998

Theory

The generator uses a Teflon pulley at the lower end of the machine, attached to an electric motor. A rubber belt passes over the pulley. As the pulley turns, the pulley acquires a negative charge while the inside surface of the belt near the pulley acquires an equal amount of positive charge. The outside surface of the belt acquires an equal amount of negative charge by induction. An electrode in the form of a wire screen (called the lower brush) drains away these negative charges to the ground. A similar screen electrode (called the upper brush) at the top of the belt removes the positive charges and deposits them on the collector dome. The Teflon pulley retains the negative charges that it acquires.

Positive charges stay on the inside surface of the belt and travel upwards as the belt moves. At the top, it runs over an aluminum pulley that conducts the positive charges and retains them. Free electrons from the metallic pulley flow on to the belt and are carried down to the the lower plastic pulley. As the belt keeps running, more charges are deposited on both pulleys. The positive charges are transferred to the collector dome and the negative charges are drained into the ground. The belt plays an important role in transporting negative charges from the upper comb to the lower comb and positive charges from the lower comb to the upper comb. There is no transfer of charge from the electrical lines. This device would work exactly the same if it were powered by a hand crank.

On the metallic collector dome, the positive charges spread out due to electrostatic repulsion and become uniformly distributed due to the dome's spherical shape. The buildup of positive charges on the dome continues until ionization intensity is reached. At equilibrium, the potential difference between the collector dome and the generator housing can reach one-half million volts. The air between the collector experiences dielectric breakdown and the generator discharges the accumulated static charge in the form of a spark. This discharge causes the potential difference to drop below the ionization threshold but is brought up again in a matter of seconds. This process continues as long as the generator is running.

Source: Zafar A Ismail, Van de Graaf Generator: Instructions & Applications

Operation & safety

• Do not allow students to use the Van de Graaff generator unsupervised.
• People with cardiac pacemakers should never operate the generator or come in contact with it.
• Obviously, we are dealing with high voltage here. Stay about three feet away from the collector while it is charged. Full intensity, white-hot sparks can jump as far as 15 inches, less intense, red-purple sparks can jump 20-30 inches. While the current is too low to injure you, a surprise spark is no fun. Keep the generator at a safe distance from the outlet where you plan to plug it in. If you're too close, you won't be able to turn it off safely.
• Always discharge the collector dome between experiments and when you are finished. Use the discharge wand for this. Connect the alligator clip to a gas main or similar grounded object. Hold the discharge wand by the handle. Do not touch the grounding strap when discharging the generator. The voltage is so high that the current can pass through the insulation into your hand.
• The motor produces a lot of heat that could damage the belt or the motor itself. Do not run the generator continuously for long periods of time. Turn it off when not in use. This is an expensive device and we should try to keep it operating perfectly for a long time.
• Leave the upper and lower combs alone. They are not supposed to touch the belt. Do not bend them or mash them. They were working fine when last tested and do not require any adjustment.
• Keep the entire device clean and dry. Dust and moisture degrade the generator's performance.
• Handle the aluminum parts with care. The collector, housing, and discharge wand are easily dented. The collector already has a sizable ding in it from and accidental drop of only three inches!

Demonstrations

• Sparks
  When the grounded discharge wand is brought near the collector dome, lightning discharges will occur, accompanied by a crackling sound. Try varying the distance between the wand and the collector to see the different types of sparks the generator can produce. Some are white-hot and quite intense, others are a red-purple and less intense. Try to see how long you can get the white-hot sparks to jump. Under the best circumstances, sparks can leap up to fifteen inches. I believe the rule is three inches for every 100,000 volts.
• Current Kills, not Voltage
  If you feel adventurous you can try discharging the generator without the wand. Bring the side of your forearm near the collector. Do not allow the generator to run long before doing this. If you come up on it pretty fast it won't have time to accumulate a lot of charge and you can zap yourself with relatively little pain. You could also try holding a fluorescent tube. The length of the tube offers some protection from really violent shocks and the tube will light up. The voltage is still high enough to hurt slightly but the current is so low it really can't harm you
• Hair Raising
  Pick a student volunteer with long blonde hair for this demo. Darker hair is too heavy and short hair isn't very dramatic. Your volunteer should also have clean, unprocessed, un-moussed hair. Thoroughly discharge the generator before beginning. Have your volunteer stand on the insulating footstool in the accessories box and place one hand on the generator. Your volunteer should not be touching the table or anything else. Your volunteer should also not be wearing a jacket, hat, or layers of loose clothing. These can serve a discharge points. Turn the generator on and wait. Your volunteer's hair will begin to levitate by electrostatic repulsion. Show you volunteer the results in a mirror, but stay a safe distance away. When your volunteer looks thoroughly charged have her hold her other hand out horizontally with spread fingers. Under the right conditions, the charge will safely leak off her fingers making a crackling noise. When you are done turn off the generator. Tell your volunteer to remove her hand. Discharge the generator with the discharge wand. Tell your volunteer to "shake off" the excess charges before stepping of the footstool. This will reduce or eliminate the shock.
• Lightning Rod
  In the accessories kit you will find a coil of heavy gauge wire bent so that the inside of the coil stands up like an antenna. Place it on top of the uncharged generator and turn the generator on. Place your hand nearer to the generator than normal. You will find you can get quite close before there is a spark and that the spark is mild in comparison to those generated earlier. This happens as the charge concentrates at the tip of the wire and discharges continuously, reducing the charge of the collector sphere. You can also touch the wire with similar results. This illustrates the operating principle behind the lightning rod. Contrary to popular belief, lightning rods reduce the likelihood of a lightning strike by discharging the area around them.
• More repulsion demos (and one repulsion-attraction demo)
  • Lay a piece of animal fur on the generator and turn it on. The hairs will stand up. Bring your hand near the fur. Corona discharge between your fingers and the fur will discharge parts of the fur. If you do it right you can make the fur look as if it were being animated by a mad scientist.
  • Lay the stack of pie plates on the generator and turn it on. The plates will rise off one at a time by electrostatic repulsion as if they were an armada of UFOs.
  • Drop confetti on the generator and turn it on. The confetti will fly off.
  • Tear long narrow strips of newspaper and tape them on to the collector with scotch tape. Turn on the generator. The strips will try to align themselves with the electrostatic field. Do the same with the smaller Van de Graaff generator and put the two generators near each other. You will get a crude approximation of the field around two positive charges. Thoroughly remove the tape when you are done to keep the performance of the generator high.
  • Bring a lit candle near the collector after the generator is turned on. The flame will deflect away from the collector. This shows the flow of ions away from the collector and mimics the solar wind to a certain extent. If you bring the flame very close, a portion of the flame will be attracted toward the collector. The ions in the flame are separating by charge.
• Dissectible Leyden jar
  Use extreme caution when performing this demonstration. Do not touch the Leyden jar when it is charged and discharge it completely when you are finished. You cannot tell the difference between a charged and uncharged Leyden jar until it's too late.
  • Assemble the Leyden jar. Holding the jar by the outer aluminum cup, touch the terminal of the jar to the generator's collector dome. Turn the generator on. You may prefer to use the smaller generator. The jar can also be charged from the static on a TV screen or a charged balloon Turn the generator off. Place the jar on the table. Stay clear of the terminal. Use the wooden handled chemical scoop to discharge the jar by placing it in contact with both the inner and outer aluminum cups. You will get a vicious sounding spark.
  • Recharge the Leyden jar as before. Using the wooden handled chemical scoop, lift the jar by the terminal and remove the middle plastic cup and the inner aluminum cup together. Let the plastic cup slide off the inner cup and place all three parts on the table in a row. Show the students how you can now safely handle all of the parts. Using the scoop reassemble the jar by sliding the inner cup into the middle cup until they stick. Quickly lift them and then slide them into the outer cup. Discharge the fully assembled jar. You will get another vicious sounding spark (the intensity may be a bit less). It looks like a magic trick. The charge resides on the plastic cup. Being an insulator, the large quantity of charge cannot leave easily. When the unit is reassembled, the charges on the cup can exit violently though the metal surfaces.
• Untested demos
  The Van de Graaff generator comes with a newspaper-like set of instructions and applications. There are a few demonstrations listed there that we were not able to make work. Maybe you would like to try them. We also have a Faraday Cage. We do not know how to use it effectively. If you come up with any useful demonstrations please share them.