The classic Van de Graaff generator, named "Van de Graaff" in honor of his inventor, is a machine to produce high voltage potentials using the physics principle
of friction: two different materials in close contact produces an electron exchange between then, originating a voltage potential. The Van de Graaff generator is such a machine that makes this
process practical, not only generating the potential by friction in a continuous way but also providing an efficient form of amplification and accumulation of the electrical charge generated,
to achieve this goal the Van de Graaff parts are the following:
- A charge conveyor belt
- An upper and lower roller
- An upper and lower comb
- A hollow metallic sphere
His function is to transport the electric charge produced with the lower roller to the upper part of the Van de Graaff electrostatic machine.
The Van de Graaff belt should be made of a non conductive material, because the electric charges are transported mechanically and must stay "bounded"
to the belt surface. A Van de Graaff generator don´t need to be vertical, in fact this position is chosen because
the extremities of the machine need to be kept apart, and with a horizontal structure can be more difficult to maintain that isolation.
The lower roller
On lower roller the primary charges are produced, the lower roller and the belt correspond to different positions
in the triboelelectric series, what means that the materials that composes these Van de Graaff part has different
ability to gain or loose electrons. Ideally the lower roller should be mad with a material that is in an extremity of the triboelectricity
series, the belt are in the middle and the upper roller are in the opposite extreme. To simplify we can say that the friction
between the roller and the belt produces an exchange of electric charge, in fact, is not the friction between the materials, but just
the close contact between them, the different ability to gain or loose electrons is that produces the electrostatic interaction providing
electrons exchange. As result the belt exhibit an opposite charge related to the roller, as the belt is rolling
it carries away the charge to the upper part of Van de Graaff machine, but on roller the charge stays concentrated.
The lower comb
The lower comb transfer charges from an charge source to the belt, the concentrated charge on lower roller
attracts opposite charge from the lower comb that has a sharp ends and is connected to the earth, an infinite source of charge. The charges forms a plasma that fills the gap between the
comb extremity and the belt, this enable that more charge are deposited over the belt, as they are attracted to the lower roller.
In a practical Van de Graaff generator the lower comb is connected to the machines structure,that provides the charges for machine
The upper roller
This roller is situated inside the metallic hollow sphere and also interacts with the belt for charge generation.
When the charged belt encounters the upper roller, due to the triboelectric difference the upper roller will be
charged with a polarity which has the same polarity that the belt initially have, also some opposed charge are created over the belt
what neutralizes few initial charge. The electric charge concentrated on the upper roller repel the initial charge on belt and attracts
opposite charge from the upper comb.
The upper comb
Made of a conductive material, this element is responsible to make possible the charge exchange between the belt and the metallic sphere,
it has a sharp end that forms a small gap with the belt and is connected with the inner part of the sphere. In the same way that the lower comb,
the sharp end creates a high potential difference to the roller producing an air ionization around it. Is the ionized air that makes possible the charges to move
from the metallic sphere, that is electrically connected with the upper comb, in direction to the roller. As the charges travels toward the upper roller it encounters
the belt midway and interacts with the charges already on it neutralizing the belt electric balance and enabling the process to start again. As result of this charge
exchange processes the metallic sphere on top of Van de Graaff generator retain some electric charge.
The metallic hollow sphere
Is the hollow metallic sphere that stores the charge that is generated on Van de Graaff electrostatic machine, these charges are transferred to it by means of
the upper comb. When the charge arrives at the sphere, it tends to migrate to his external surface, because of the electric repulsion among then. The charge are
injected continually while generator is running and grows until it reaches an electrical potential high enough to break the air isolation. When an electrical conductor
object is put near the charged sphere, if it is close enough, the electric barrier is broken and a bright and thunderous spark is produced. The distance from the sphere
that the spark begins is an indicating on how much charged was the sphere, also the sphere should not have any sharp edges because if they exist the charges will
not be accumulated properly as small sparks will appear around the edges discharging the sphere.
Van de Graaff generator of the first Hungarian linear particle accelerator showing the transport belt,
the sphere in this case is replaced by a hollow metallic structure with round edges.
By Zátonyi Sándor, (ifj.) (Own work) [GFDL
], via Wikimedia Commons
Build yourself a VandeGraaff generator with easy to find parts, that can be hand operated requiring no electricity or batteries, get the DIY plans
here at innovationview.com.