Van de Graaff generator

an de Graaff generator. The large sphere acts as a capacitor to store the charge transferred up its supporting column. The small sphere (connected to the ground potential) will draw an arc from the larger when the air gap breakdown voltage is exceeded.

The Van de Graaff generator is an impressive electrostatic generator that is capable of producing enormously large static electric potentials. In fact, giant Van de Graaff generators can produce miliions of volts leading to awesome displays of corona and lightning. More modest “class room” sized Van de Graaff generators typically produce 100,000 V to 500,000 V.

Originally the generator was built using a pure silk ribbon driven by a small motor. The charges on the ribbon were collected on a terminal that actually was a tin can.

Van de Graaff generators have been instrumental in a number of fields, including particle acceleration, electrostatics, ESD, as well as providing an awesome display of the power of electricity.

Review of literature

1.    Robert Jemison Van de Graaff
b. December 20, 1901, Tuscaloosa, Alabama, U.S.A.
d. January 16, 1967, Boston, U.S.A.

American physicist and inventor of the Van de Graaff generator, a type of high-voltage electrostatic generator that serves as a type of particle accelerator. This device has found widespread use not only in atomic research but also in medicine and industry.

2. Nikola Tesla b. January 12, 1981, Scientific American march.

3. Charrier Jacques b.March 1934, French scientist, worked on Van de graaff in the year 1934.

4. Paolo Brenni in the year 1999 worked on van de graaff.

Working of V.d.G generator

A Van de Graaff generator is an electrostatic machine which uses a moving belt to accumulate very high electrostatically stable voltages on a hollow metal globe.The potential differences achieved in modern Van de Graaff generators can reach 5 megavolts.

Description of Fig.
1) hollow metal sphere
2) upper electrode
3) upper roller (for example in acrylic glass)
4) side of the belt with positive charges
5) opposite side of the belt with negative charges
6) lower roller (metal)
7) lower electrode (ground)
spherical device with negative charges, used to discharge the main sphere
9) spark produced by the difference of potentials

A simple Van de Graaff generator consists of a belt of silk, or a similar flexible dielectric material, running over two pulleys, one of which is surrounded by a hollow metal sphere.Two electrodes and in the form of comb-shaped rows of sharp metal points, are positioned respectively near to the bottom of the pulley and inside the sphere  is connected to the sphere, and a high DC potential (with respect to earth) is applied to a positive potential in this example.

The high voltage ionizes the air at the tip of  repelling (spraying) positive charges onto the belt, which then carries them up and inside the sphere. This positive charge induces a negative charge to the electrode  and a positive charge to the sphere (to which  is connected). As the belt continues to move, a constant charging current travels via the belt, and the sphere continues to accumulate positive charge until the rate that charge is being lost (through leakage and corona discharges) equals the charging current. The larger the sphere and the farther it is from ground, the higher will be its final potential.

Since a Van de Graaff generator can supply the same small current at almost any level of electrical potential, it is an example of a nearly ideal current source. The maximum achievable potential is approximately equal to the sphere’s radius multiplied by the e-field where corona discharges begin to form within the surrounding gas. For example, a polished spherical electrode 30 cm in diameter immersed in air at STP (which has a breakdown voltage of about 30 kV/cm) could be expected to develop a maximum voltage of about 450 kV. Van de Graaf generators often have ground wires.

Applications

• These high voltage generators include driving X-ray tubes.
• Accelerating electrons to sterilize food and process materials.
• Accelerating protons for nuclear physics experiments.
• Van de Graaff generators can produce miliions of volts leading to awesome displays of corona and lightning.

The Van de Graaff generator can be thought of as a constant-current source connected in parallel with a capacitor and a very large electrical resistance.

Van de Graaff generators have been instrumental in a number of fields, including particle acceleration, electrostatics, ESD, as well as providing an awesome display of the power of electricity.

Future Scope

• DEMONSTRATION OF A WORKING MODEL OF A THUNDERSTORM

  Drops of water falling from an artificial cloud about ten feet in length would charge it to a high voltage, producing sparking (a miniature artificial lightning) to earth. This would be followed by a colored slide of a pictorial diagram showing how the electrical operation of the large electrostatic generator is closely similar in principle.

• FUTURE POSSIBILITIES OF AN UNLIMITED SOURCE OF ATOMIC POWER

  Possibility of the use of accelerators for the production of atomic power using hydrogen reactions.
  Some possible uses, including propulsion of airplanes and space rockets.

• DEMONSTRATIONS OF PRESENT-DAY LIGHTNING PROTECTION

  Automobile.
  Power line.
  House wiring.
  Fences.

Bibliography

Web sites surfed :

• www.google.com
• www.wikipedia.com
• www.ccmr.cornell.edu
• www.physicsweb.com

Books referred :

• Pradeeps Fundamentals
• Halliday/Resnik/Walker