Van de Graaff Generator

[John Steaver]

Hi, with a van de graaff generator, does the voltage increase with a higher RPM motor or a different material?

 

[Anon_LG]

Both. The Van De Graff generator is based on the triboelectric effect, different materials posses different charges. Rubber is the material of choice due to its placement on the triboelectric series, it posses considerable negative charge, and its flexibility. If you were to use say, steel, as the belt (would not be particularly flexible), no effect would be generated due to its lack of electrical bias. Additionally the extent of the energy (time*PD(voltage)*current) produced by the triboelectric effect is also affected by friction, the opposition of movement. See here for more information on the responsible phenomenon itself.

I hope this helps,

 

[JimW]

The last post is not entirely correct. There are three materials that are important in a standard V.D.G. generator: the top roller, the bottom roller, and the belt. Rubber is the belt of choice because it is an insulator (most important) and it is in the middle of the triboelectric scale. The top and bottom rollers must be of opposite sides of the triboelectric scale from the belt. One more positive, and the other more negative.

You will get more charge generation from your generator by selecting good material rollers. But a faster motor will increase the rate of charge as well. Friction between the belts and the rollers are a far lower factor to consider.

JimW

 

[hevans1944]

Hi, with a van de graaff generator, does the voltage increase with a higher RPM motor or a different material?

It depends on where your losses are occurring, and whether you have a "serious" Van de Graaf generator or just a "toy" good mainly for making hair stand on end.

In theory, as long as the charge collecting sphere is accumulating charge, and the charge does not "leak off," there is no limit to how much voltage the sphere will charge to. In practice, charge will reach a level where corona discharge (ionization of air) occurs, draining the charge from the sphere. Motor speed affects only the rate of charge accumulation, not the final voltlage. Materials are important if you are using the triboelectric effect to separate charges for deposition on the insulating belt, but again that only affects the rate of charge accumulation, not the final voltage on the sphere.

The basic principle of a modern Van de Graaf generator applies an electrical charge to a non-conductive belt or chain by means of either corona discharge or electrostatic induction. It then moves the charge, by mechanically moving the belt or chain, to the inside of a conductive sphere insulated from ground. The charge is extracted, again by induction, and deposited on the sphere where it accumulates since there is no intentionally conductive path to discharge the sphere.

Note that in a Van de Graaf generator there is a continuous transfer of charge from the bottom of the generator (at Earth potential) to the conducting sphere at the top. Assuming the transported charges do not "leak off" by conduction down the insulating support column, the sphere will continue to accumulate charge as long as charge is being transported to it.

An isolated sphere has a certain capacitance, determined by its radius and the dielectric constant of the air around it. The voltage that is developed on the sphere is calculated from V = Q/C, where Q is the charge in coulombs deposited on the sphere and C is the capacitance in farads. Nowhere is it stated that this voltage has anything to do with the speed of the belt or the materials used because it doesn't. The maximum voltage of a Van de Graaf generator mainly depends on how much voltage can be developed on the sphere before it causes the surrounding air to break down and corona discharges to occur. If you want higher voltage from your Van de Graaf generator, fit it with a larger sphere and a taller, better insulated, support column.

The triboelectric effect has no part in a modern commercial Van de Graaf generator design. Also, rubberized belts are becoming scarce and are being used less frequently. The belts also wear more quickly and are less stable than a pellet-chain design. In fact, it is not unusual to find a Van de Graaf accelerator that started life with a belt to now be fitted with a Pelletron charging system manufactured by the National Electrostatics Corporation (NEC). The NEC website has a nice animation showing how charge is transported using their Pelletron system.

You could also make a Van de Graaf generator by charging up ping-pong balls and using vacuum suction to deliver the charged balls up into a conducting sphere. By using a coaxial tube arrangement, you could shoot the charged balls up a center tube slightly larger in inside diameter than the diameter of the ping-pong balls and recover them down a much larger outer tube for recirculation at the bottom. Examine the design of the machine used to select lottery numbers for ideas on how to recirculate the balls.

 

[John Steaver]

Thanks for all your help. Now I understand why the sphere and columns are so big when the voltage is higher.
thanks again- John

 

[hevans1944]

Thanks for all your help. Now I understand why the sphere and columns are so big when the voltage is higher.
thanks again- John

Yeah, it's really hard to hold off corona discharges in air. Making the sphere larger helps "smooth out" the electrical field. The larger columns are needed to support the weight of the moving belt or chain. When these things were first invented, they sometimes built two of them with opposite charges on the spheres to effectively double the voltage capability.

Van de Graaf generators, and later Cockroft-Walton solid-state voltage multipliers, played an early role in developing the practical aspects of nuclear physics. A whole generation spent their time measuring nuclear reaction cross-sections for interactions between the elements in the periodic table. It was discovered that some of these collision reactions exhibit resonant absorption peaks at characteristic collision energies, greatly increasing their nuclear cross section. The resonant particle velocities occur over a very narrow range of energy. When I operated and maintained a small tandem particle accelerator, these resonant nuclear reactions were used to calibrate the accelerator beam energy.

Trying to build a high voltage machine much above about 200,000 volts to operate in air is extremely difficult. That is why modern particle accelerators are insulated with pressurized sulfur hexafluroide gas, which can hold off several million volts per inch. The tandem accelerator at the Oak Ridge Holifield Radioactive Ion Facility holds off 25 million volts with SF6.

Are you building, or plan to build, an educational Van de Graaf generator? It's a great way to get kids interested in physics. Arcs and sparks and hair standing straight out from the head always attracts attention.

 

[John Steaver]

just one more question, what is the capacitance of the sphere? Why cant normal capacitors discharge a continuous flow of energy?

 

[hevans1944]

just one more question, what is the capacitance of the sphere? ...

Google is your friend here. The answer is:

You might also want to visit this page.

Why cant normal capacitors discharge a continuous flow of energy?

Define "normal capacitors" please. The continuous discharge from a Van de Graaf occurs because a continuous charge is being delivered to its "spherical capacitor" by the moving belt (or chain in the case of a Pelletron).

 

[John Steaver]

ok thanks or all the help

 

[John Steaver]

wait, what if voltage was delivered to the spherical capacitor without any belts, combs etc

 

[hevans1944]

wait, what if voltage was delivered to the spherical capacitor without any belts, combs etc

You never "deliver" voltage to a capacitor, spherical or otherwise. Voltage is the result of charge being moved from one plate to the other plate of a capacitor: V = Q/C, where V is the voltage in volts, Q is the charge in coulombs, and C is the capacitance in farads. In the case of the spherical capacitor on a Van de Graaf generator, the Earth is one plate and the sphere is the other.

A Cockroft-Walton generator uses no "belts, combs etc" but it cannot continuously move charge to a sphere, and thereby accumulate arbitrarily high charge and the corresponding high voltage produced by a Van de Graaf generator. It has an upper voltage limit, determined by the excitation voltage and the number of multiplier stages.

 

[John Steaver]

thanks for replying so quickly, I understand now. thanks for help