van de graaff generator experiment

Van De Graaff Generator Wonders

Activity length, 20-40 mins., electricity forces and motion, activity type, discrepant event (demonstration only).

Most people have seen a Van de Graaff generator before at a science centre or on TV. You know that it makes peoples' hair stand on end, but do you actually know how it works?

Van de Graaff experiments are all based on the fact that like charges repel.

A Van de Graaff generator pulls electrons from the Earth, moves them along a belt and stores them on the large sphere. These electrons repel each other and try to get as far away from each other as possible, spreading out on the surface of the sphere. The Earth has lots of room for electrons to spread out upon, so electrons will take any available path back to the ground.

The grounding rod is a smaller sphere, attached by a wire to the Earth. It provides a convenient path for electrons to move to the ground. If we bring the grounding rod close enough to the large sphere, the electrons rip through the air molecules in order to jump onto the grounding rod, creating a spark and crackling noise.

When a fluorescent light tube approaches the negatively charged generator, the electrons on the generator flow through the tube and the person holding it. Flowing electrons result in an electrical current, lighting up the light tube. It doesn't take very much current to light a fluorescent bulb!

Putting Styrofoam peanuts or confetti on top of the Van de Graaff generator can create a cool trick. The electrons that collect on the sphere spread out into the Styrofoam peanuts and confetti, making the little, light objects negatively charged. When the negative charges on the peanuts repel the negative charges on the generator, the peanuts push off the sphere.

When a student puts a hand on the sphere, the electrons will spread out onto that person as they repel from the other electrons. They are most obvious in a person's hair because the like charges of the electrons repel each other and cause the hairs to stand up and spread away from each other. As long as the person is standing on an insulated platform, the electrons will not be able to travel down to the ground and their hair will remain standing up.

Explain how static charge causes materials to attract or repel each other.

Per Class or Group: A Van de Graaff Generator (available at Arbor Scientific ) a plastic stool Styrofoam peanuts (or confetti) metal pie pan a mirror

Key Questions

• Where are the electrons?
• What is making your hair stand on end?
• Why doesn’t the hair come down after the machine has been turned off?
• What caused a shock when the volunteer touched a fellow student?
• Why did your teacher ground the generator before allowing the volunteer to step off the stool?
• What is the role of the plastic stool?

Safety note:  Make sure you ground the large sphere after each use by touching it with the ground wire or small sphere. Although the Van de Graaff generator produces a very low current, it may cause problems with people who have heart problems or a pacemaker. Warn students they may get small shocks which will scare them more than hurt them.

Part 1: Making Sparks

• Touch the small sphere (connected to the ground wire) to the dome.
• Turn the knob counter clockwise.
• Turn the generator on.
• Slowly turn the knob clockwise so the motor turns the belt.
• Take the small sphere away and let a charge accumulate on the dome. Ask a student to turn off the lights to make it easier to see the sparks.
• Move the small sphere around the sphere in different positions so that everyone can see the sparks.

Part 2: Sautéing Styrofoam 

• Ground the dome by touching the grounding rod to it.
• Without removing the grounding rod, place Styrofoam peanuts (or confetti) on top of the large sphere.
• Take the ground away, and the Styrofoam peanuts will fly off the generator.
• This can be repeated by placing a metal pie panplate (or three!) on top of the generator and repeating the steps above.

Part 3: Hair-Raising Experience

• Ask a student to step up onto the insulated stool.
• Without removing the grounding rod, ask the volunteer to put one hand on the dome, the other hand by their side and make sure they understand not to move their hands until you tell them to.
• Take the ground away, and their hair will start to stand up. Shaking their head will help too!
• Hold the mirror so that the volunteer can see their new hair-do!
• Ask the volunteer to move their hand from the ball to their side, and to keep it there. Immediately ground the Van de Graaffand then turn it off.
• The volunteer can simply step off the stool or touch elbows with a classmate to get rid of their extra electrons (note: touching elbows will result in a shock!).
• Place a piece of fake fur on the large sphere, the individual fur strands will stand.
• Tape streamers to your volunteer, like an extra-long moustache!
• Have someone hold onto the large sphere while blowing soap bubbles with a wand, the bubbles will become positively charged and will be attracted to anything that is grounded e.g. a person walking by.
• A fly stick is a miniature, battery powered Van de Graaff generator. It charges mylar objects, which are then repelled by the stick (and by each other). You can make small objects hop up and down between the stick and your hand or levitate the more visible ones. For fun ideas, check out the Educational Innovations' teacher blog.

About the sticker

Artist: Jeff Kulak

Jeff is a senior graphic designer at Science World. His illustration work has been published in the Walrus, The National Post, Reader’s Digest and Chickadee Magazine. He loves to make music, ride bikes, and spend time in the forest.

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T-Rex and Baby

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Michelle is a designer with a focus on creating joyful digital experiences! She enjoys exploring the potential forms that an idea can express itself in and helping then take shape.

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Artist: Ty Dale

From Canada, Ty was born in Vancouver, British Columbia in 1993. From his chaotic workspace he draws in several different illustrative styles with thick outlines, bold colours and quirky-child like drawings. Ty distils the world around him into its basic geometry, prompting us to look at the mundane in a different way.

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Van de Graaff generator

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The Van de Graaff generator is a classroom classic with a surprising heritage in cutting-edge particle physics. As well as making your hair stand on end, these machines were used to accelerate particles through millions of volts.

1 × Van de Graaff generator 1 × electrically-insulating stool Some confetti, or aluminium foil, or foil cake trays

The demonstration

This demonstration involves high voltages, and so it should never be done by anyone with a pacemaker or other internal electrical device, or who thinks they might be pregnant.

The first part of this demo requires a volunteer from the audience. It works best on someone with long, light-coloured hair free from ties and styling products: light hair is often thinner, which means it will stand up more easily, and is also easier to see. Don’t pick on an individual (they might be pregnant, or just shy!), but if you could encourage someone fitting that description that gives you the best chance of success.

• Give your volunteer a round of applause and find out their name. Check that they aren’t wearing any metal jewelry etc, and ask that they remove it if so. Put it safely to one side.
• Get the volunteer to stand on an electrically-insulating stool. Place one hand on top of the generator dome, and get them to hold out their other hand flat. In this, place some confetti, pieces of aluminium foil, or cake tins. If you have a suitable light, dim the main lights and illuminate their head from behind to emphasise the forthcoming hairdo.
• Check they’re feeling OK, turn on the generator, and stand back. The only thing they need to do is not to take their hand from the dome and, should they do so, not try to replace it (or they’ll get a shock!). Make sure the earthed globe is a long way from the main one to avoid shocking the volunteer too.
• The objects in their hand will leap out and, by the time that’s finished, their hair should be standing up pretty nicely. Get them to shake their head around a bit to encourage this.
• Get the volunteer to take their hand off the dome, and jump down with both feet, and give them a round of applause!

Having shown the amusing effect of high voltage on a person, we can now explore the limitations of these devices as particle accelerators. The problem is sparks, and we can use the sparks to work out the voltage to which we charged up our hapless volunteer!

• If you haven’t already, dim the lights.
• Take the grounded sphere and place it near the dome of the Van de Graaff generator. When you get within a few centimetres, a spark should leap across with a crack. Do this a few times from different angles to show the audience.

Vital statistics

breakdown voltage of air: 30,000 V/cm

highest-voltage Van de Graaff: 25.5 MV

length of spark from Van de Graaff LHC: 7 TV ÷ 30 kV/cm = 2300 km

How it works

The rubber belt inside the Van de Graaff generator runs between two rollers made of different materials, causing electrons to transfer from one roller to the rubber, and from the rubber onto the other roller, by the triboelectric effect. Brushes at the top and bottom provide a source and sink for these charges, and the top brush is electrically connected to the Van de Graaff’s dome and so the charge will spread out across the dome.

This accumulated charge would like to distribute itself over as large a volume as possible, and so it will also spread out across anything you connect to the metal dome, including your volunteer. The reason it’s important to stand them on something electrically-insulating is that the charge would like even more to spread out over the whole Earth, and connecting them to that will both massively reduce the effect, and also cause an electric shock as the current flows from the Van de Graaff to earth through its unfortunate human intermediary.

When insulated, the build-up of charge on the volunteer causes forces light objects to spread out as far as possible too, causing the confetti or foil to leap from their hand and then causing the individual hairs on their head to stand up. When they jump off the stool, the charge immediately flows to earth and their hair will immediately return to normal.

To work out the voltage of the Van de Graaff generator, and thus the voltage on our volunteer, we can use the length of the sparks in combination with the breakdown voltage of air—the voltage required to cause air to dissociate into ions and become conductive. This voltage is about 30,000 V/cm for dry air (hot, humid or lower-pressure air will tend to spark more easily). Sparks from the Van de Graaff are typically a few centimetres long, giving a voltage between 50,000 and 150,000 V.

Their propensity to generate sparks is the fundamental limitation of Van de Graaff accelerators, or indeed any accelerator design based on a large, static voltage. Those used for research managed to get up to over 20 MV by clever use of insulating materials, right down to careful choice of the gas in which the generator sits to minimise the chance of sparks. A Van de Graaff can thus be used to accelerate particles up to reasonably high energies: moving an electron through 1 V gains it an energy of 1 eV, so energies of over 20 MeV are achievable by this method (and more if accelerating nuclei with greater than a single electron charge).

However, modern particle physics has gone some way beyond this: the Large Hadron Collider will ultimately use beams which have 7 TeV of energy each: equivalent to accelerating a proton through 7,000,000,000,000 V. If we divide that by the breakdown voltage of air, we can work out the length of a spark we might get from an LHC employing a single, giant Van de Graaff generator to accelerate its particles. We get 2,300 km: easily enough to stretch, for example, from Switzerland to anywhere in the UK.

• HowStuffWorks: How Van de Graaff generators work
• Google Answers: high voltage arcing distances
• Wikipedia: Van de Graaff generator

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5A50.30 Van de Graaff Generator

Electrostatics, triboelectricity, dielectric breakdown

Static charge builds up on the Van de Graaff generator dome. Many different individual demos can be done with it. The Van de Graaff can be used to charge up the hair of a Barbie doll or person. When tart pans or rice krispies are charged up, they rain down on the ground one by one. Fluorescent lights will flash when sparked by the generator. A grounded dome or discharge wand can demonstrate dielectric breakdown in air.

As with all electrostatic experiments, unless conditions are ideal (dry, cold), the demonstration may perform worse than expected.

• [1] Van de Graaff generator
• [1] Discharge wand
• [1] Barbie doll
• [1] Plastic step stool
• [10] Tart pan
• [1] Grounded tower
• [1] Ornament on a stick
• [1] Fluorescent light with attached banana cable and clip
• [1] Leiden jar and wand
• [1] Semiconductor and pin
• [1] Flashlight
• [1] Extension cord

Classroom Assembly

• Put the Van de Graaff somewhere away from computers and plug it in.

Important Notes

• People with pacemakers, insulin pumps, cochlear implants, or other critical devices should not come near the Van de Graaff.
• Ensure the Van de Graaff is not near any computers or other electronics, such as phones.
• People being charged on the Van de Graaff should not touch the dome again after letting go.
• For the demo of charging someone's hair, try to pick someone with shoulder-length hair.
• When transporting the Van de Graaff on a bumpy path, the plexiglass casing and attached belt mount can rotate, twisting the belt. Be sure to straighten that out before use.
• Test the Van de Graaff just before show time to avoid untimely failures.

Barbie or tart pans

• Place Barbie or tart pans on top of the dome. Tart pans should have the open part facing down.
• Turn on the Van de Graaff generator. Barbie's hair should flare out or the tart pans should fly off.
• While grounding the Van de Graaff, turn it off.
• Get the volunteer to stand on the step stool, putting one hand on the Van de Graaff dome.
• Tell the volunteer to not touch the dome again if she or he lets go.
• Start the generator.
• Tell the volunteer to shake one's head.
• Show the volunteer what he or she looks like in the mirror.
• Use just the grounded banana cable ( not the grounding rod) to make the volunteer's hair dance by bringing the end of the cable close to the dome, then away, then close, and so on.
• Tell the volunteer to let go of the Van de Graaff and turn it off.
• Note how the volunteer's hair is still standing up.
• Tell her or him to jump off the stool, and most of the charge should drain away. Alternatively, tell the volunteer discharge by elbowing the elbow of another volunteer standing on the ground, then jumping off the stool.

Chain of pain (zap several people)

• Get a few volunteers.
• Everyone except the one standing on the stool and touching the Van de Graaff should link hands or pinkies. The last one in the chain should be touching a grounded object, like a faucet, if possible.
• Start the Van de Graaff generator.
• The person touching the Van de Graaff should make elbow-to-elbow contact with the next person on the chain.
• Stop the Van de Graaff generator.

Fluorescent light

• Ground the light by plugging the banana cable into the grounding port on the Van de Graaff generator.
• Bring the fluorescent light closer to the generator. Further from the dome, the light will periodically illuminate from relatively large sparks. Close to the dome, the light will almost continuously illuminate from many small sparks.
• Ground the generator and turn it off.

Dielectric breakdown

• Set up the grounded tower next to the Van de Graaff generator, with grounding wire plugged in to the generator's grounding port.
• Start the Van de Graaff.
• Vary the distance between the Van de Graaff and the grounded tower as required.
• Ground the Van de Graaff and turn it off.

Oscillating ornament

• Set up the grounded tower 20–30 cm from the Van de Graaff generator, with grounding wire plugged in to the generator's grounding port.
• Place the ornament on a stick such that the ornament is suspended between the Van de Graaff and grounded tower.
• Adjust the ornament's string length or position until the ornament oscillates.

Additional Resources

• PIRA 5A50.30
• Don't attempt this at home!

Last revised

• Original construction: purchased. Van de Graaff generator is Winsco N100-V. It takes Winsco RP-601 replacement belts.
• Replacement belts also available from Boreal (as of 2022) as item number 470005-528.

Related demos

If you have any questions about the demos or notes you would like to add to this page, contact Ricky Chu at ricky_chu AT sfu DOT ca.

The National MagLab is funded by the National Science Foundation and the State of Florida.

Van de Graaff Generator

The Van de Graaff generator is a popular tool for teaching the principles of electrostatics. You might remember it as the thing that made your hair stand on end. It’s now largely used for educational purposes, but it was invented by Robert J. Van de Graaff in 1930 to power early particle accelerators.

The Van de Graaf generator creates a buildup of static electricity around a metal sphere. Electric charge in the form of electrons builds until the voltage is so high that air molecules can be ionized and a spark discharge can take place to a nearby object. In this tutorial the object is a grounded rod.

Anything or anyone in direct contact with the Van der Graaf generator’s sphere, becomes charged. If a person holds their hand on the sphere, they become charged and their hair may stand on end since like charges repel and the hairs are pushed away from the like-charged metal sphere and each other.

Instructions

• Find the key elements of the generator labeled below.
• Notice the direction the electrons are traveling. See how they move up from the bottom roller and collect around the sphere.
• Watch the generator send a spark to the grounded rod when the sphere can’t hold any more electrons.
• Turn the generator off. Observe how the number of electrons remains fixed.

Inside the generator are two rollers connected by a belt. The motor turns the bottom roller, thus rotating the belt and upper roller. A metal brush drags across the bottom roller, and static electricity begins to build.

The rollers are always made from different materials. The bottom roller is constructed from material that holds a negative charge, such as silicon. The belt is made of a neutral material like rubber. The top roller is made from a material that holds a positive charge, like aluminum, and is surrounded by a hollow metal sphere. The difference in charge draws the electrons up the belt. When the electrons reach the upper belt, they meet another metal brush which transports them to the sphere.

The electrons gather around the surface of the sphere until it reaches a critical potential. The metal sphere must release some of the electrons. With a quick zap, the Van de Graaff generator releases a spark of electrons towards the metal of the discharge rod. As long as the generator is switched on the cycle will repeat. When the generator is turned off, and the stream of new electrons stops, the electrons around the sphere can stay put.

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UCSC Physics Demonstration Room

Demonstration Resources for UCSC

Van de Graaff Generator

Van de Graaff (With “Hair”)

Van de Graaff generator (without hair)

The Van de Graff Generator produces high static voltage of up to 200,000 V. To demonstrate the static charge developed on the outer conducting shell of the generator, five variations of this demo are offered.

The five demos offered include:

Flying Ball (Figure 1): Lightweight metallic pith ball, thread, insulating or plastic wand

Silver Snake (Figure 2): Metallic ribbon, Insulating or plastic wand

Puffed Rice Fireworks (Figure 3): Puffed rice, pie tin, broom and dustpan recommended for cleanup

Hairs (Figure 4): Pom-pom

Plane for Charge Transfer (Not Shown) Circular proof plane with plastic handle Electroscope

To operate the Van de Graff generator, turn the knob on the base to adjust the belt speed. Set the belt speed to low before turning on the generator, then adjust to the desired speed to create static charge on the outer shell of the conducting dome.

• Flying Ball Charge the Van de Graff generator and turn off after charging for about 5 seconds. Use the plastic wand with the pitch ball hanging by a thread to touch the pitch ball to the dome. The pitch ball will collect some charge and repel from the charged dome. Discharge the charged dome using the grounded discharge wand.
• Silver Snake Charge the Van de Graff generator again. Use the metallic ribbon wand around the dome or touching the dome; it will attract or violently repel away from the dome. Compare its geometry with the geometry of the ball.
• Puffed Rice Fireworks Make sure that the Van de Graff generator is discharged and off. Place a pie tin face down on the dome, and add a small amount of puffed rice on the pie tin. Charge the Van de Graff generator. The cereal will fly off the top of the dome as they collect negative charges and are repelled by the negatively charged dome.
• “Hairs” Make sure that the Van de Graff generator is discharged and off. After placing the plastic pom-pom on the dome, charge the Van de Graff generator. The”Pom-pom” plastic strands spread away from the dome (“hairs standing on end”) when charge is built up.
• Proof Plane for Charge Transfer (not pictured) A circular proof plane with a plastic handle can be used to transfer charges from the Van de Graaff to an electroscope.

Explanation:

Figure 5 – The Van de Graff Generator

As the rubber belt moves, it rubs against a conducting brush, building up static charges. The conducting brush is connected to the conducting metal dome, and the charges travel to the the most energetically favorable position.

The lowest energy position for the electrons will be to be spread as far apart from one another as possible. The surface area of the outer shell is greater than the surface area of the inner shell, so the charges will build up on the outer shell.

Troubleshooting 

Pom-pom “hair”: Occasionally the strands of the pom-pom are attracted to the conducting shell of the Van de Graaff generator instead of being repelled by it. This is directly contrary to what is supposed to happen in the demonstration. In theory, the charge that is deposited on the conducting shell should spread out and move to the strands of the pom-pom, so they repel  each other and the outer shell as everything becomes similarly charged. When the pom-pom sticks to the conducting shell rather than being repelled, it suggests that the charge of the strands are different from the charge of the conducting shell. The general consensus of the demonstration room staff is that on especially hot or humid days, additional water in the air acts to suck charge off of the strands faster than can be deposited by the rubber belt. This effect would be seen on hot days because hot air can hold more moisture, even if it doesn’t feel humid. This would mean that while the conducting shell remains charged, the strands become neutrally charged and the two attract. Please note this may not be a correct or complete answer to the issue, so if you have any insights please email us!

• Avoid any contact with the large dome before discharging it with the small grounded sphere .
• There are two Van de Graafs – one has a blue base and an attached grounding sphere (use that one). The other is larger and has a black base and needs a separate grounding sphere to be attached (harder to use, kept in Thimann 130)

Note: All electrostatic demos work better on cold, dry days.

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7 Van De Graaff Generator Activities

A set of activities to show how the generator works and the principles behind it.

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• Electrostatics
• Van De Graaff Generator

Van de Graaff Generator - Working Principle

A Van de Graaff generator is an electrostatic generator, invented by an American physicist Robert J. Van de Graaff. It uses a moving belt that accumulates charge on a hollow metal structure designed like a globe, placed on the top of a column that is insulating in nature and thus, creating a very high electric potential in the order of a few million volts.  This results in a very large electric field that is used to accelerate charged particles.

Working principle of Van de Graaff Generator

Let us consider a large spherical shell of radius R. If we place a charge of magnitude Q on such a sphere, the charge will spread uniformly over the surface of the sphere and the electric field inside the sphere will be equal to zero, and that outside the sphere will be due to the charge Q placed at the center of the sphere.

So the potential outside is that of a point charge; and inside it is constant, namely the value at the radius R. We thus have:

Potential inside conducting spherical shell of radius R carrying charge Q = constant and is given by,

Let a small sphere be placed at the center of the large one such that the radius of the smaller sphere is r and the charge over its surface is q. The potential energy thus generated due to the smaller surface at different points in the system can be given as the following values,

At the surface of the small sphere:

\(\begin{array}{l} V _ { r 1 } = \frac{ 1 }{ 4 \pi \varepsilon_ { 0 } } \frac{ q }{ r } \end{array} \)

At the large spherical shell of radius R:

\(\begin{array}{l} V _ { r2} = \frac{ 1 }{ 4 \pi \varepsilon_{ 0 } }\frac{ q }{ R } \end{array} \)

If we consider the total charges in the system, that is, q and Q, then the total potential energy due to the system of charges can be given as,

\(\begin{array}{l} V _ { R } = \frac{ 1 }{ 4 \pi \varepsilon _ { 0 } } (\frac{q}{R} + \frac{Q}{R} ) \end{array} \)

\(\begin{array}{l} V _ { r } = \frac{1}{4 \pi \varepsilon_{0} }(\frac{q}{r} + \frac{Q}{R}) \end{array} \)

\(\begin{array}{l} V _ { R } \, – \, V _ { r } = \frac{ q }{ 4 \pi \varepsilon_{0}} (\frac{1}{R} – \frac{1}{r}) \end{array} \)

Assuming that q is positive, the inner sphere is always at a higher potential and is independent of the charge Q that is accumulated on the larger surface. The difference in potential given by the value V(r)-V(R) is positive. The potential due to Q is constant up to radius R and thus, the difference gets canceled out. If we connect the smaller and the larger sphere with a conducting wire, the charge q, however small, on the smaller sphere gets transferred to the bigger sphere. Here, if we introduce a small charged sphere into a larger spherical shell, as shown in the system, the charge on the larger sphere keeps increasing. Similarly, the potential at the larger sphere would also keep rising as the charge increases, until the breakdown field of air is reached.  The Van de Graaff generator works on the same principle.

Following is the table of link related to the difference between motor and a generator:

In the figure given above, we can see a Van de Graaff generator. Here, a large spherical shell is held at a height of several meters above the ground supported by an insulating column. Two pulleys are wound with a belt-like insulating material, with one being at ground level and the other one at the center of the shell. This belt undertakes a continuous motion, thus carrying a positive charge continuously from the ground to the top.  This belt is kept moving continuously by a motor driving the lower pulley. The positive charge is transferred to the larger shell by a carbon brush, thus rendering the outer shell with a very high potential over time.

Frequently Asked Questions – FAQs

Who invented van de graaff generator, energy is a scalar or vector quantity, how does the van de graaff generator work, what is an electrostatic generator, state true or false: van de graaff generator produces very high voltage direct current at low current levels..

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