This is a project on van de graaff generator for class 12 physics investigatory project

1. INTRODUCTION
Van De Graff generator is used to generate high
potential differences of near about million volts. The high
voltages generated by the generator can be used for
accelerating subatomic particles to high speeds, making
the generator a useful tool for fundamental physics
research. It can be used to demonstrate the presence
and nature of electric fields and the functioning of the
Van de Graaff Accelerator.
A Van de Graaff generator is an electrostatic
generator which uses a moving belt to accumulate very
high amounts of electrical potential on a hollow metal
globe on the top of the stand. It discharges to a smaller
sphere and, in ideal conditions, it can produce a voltage
of 400,000 volts with a short circuit current of 20 micro-
amps.
It was invented by American physicist Robert J. Van de
Graaff in 1929. The potential difference achieved in
modern Van de Graaff generators can reach 5
megavolts. A tabletop version can produce on the order
of 100,000 volts and can store enough energy to
produce a visible spark.

2. HISTORY
The Van de Graaff generator was developed, starting in
1929, by physicist Robert J. Van de Graaff at Princeton
University on a fellowship, with help from colleague
Nicholas Burke. The first model was demonstrated in
October 1929. The first machine used an ordinary tin
can, a small motor, and a silk ribbon bought at a five-
and-dime store. Where upon he went to the head of the
physics department requesting a hundred dollars to
make an improved version. He did get the money, but
with some difficulty. By 1931 he could report achieving
1.5 million volts, saying "The machine is simple,
inexpensive, and portable. An ordinary lamp socket
furnishes the only power needed.
According to a patent application, it had two 60-cm-
diameter charge-accumulation spheres mounted
on borosilicate glass columns 180 cm high, the
apparatus cost only $90 in 1931. Van de Graaff applied
for a second patent in December 1931, which was
assigned to MIT in exchange for a share of net income.
The patent was later granted. In 1933, Van de Graaff
built a 40-foot (12-m) model at MIT's Round Hill facility,
the use of which was donated by Colonel Edward H.R.
Green.

3. PRINCIPLE
Corona Discharge :- Discharging action of sharp points,
i.e., electric discharge takes place in air or gases readily,
at pointed conductors.
If the charged conductor is brought in to internal contact
with a hollow conductor, all of its charge transfers to the
surface of the hollow conductor and scatters uniformly
over it no matter how high the potential of the latter may
be.

4. CONSTRUCTION
There is a large spherical conductor named S having a
few meters of radius.
It is placed on two pillars at a certain height from the
ground. One is p1 and other is p2.
An endless belt b made up of insulating material such as
rubber, silk or a similar flexible dielectric material, is
moving over two pulleys P1 and P2.
The pulley P2 is present at the centre of the spherical
conductor S and the pulley P1 is present vertically below
P2 near the ground.
A motor M is used whose main function is to create a
spin in the belt.
Two sharp headed combs, spray comb B1 and
collecting comb B2 are used.
A discharge tube D is used in which the acceleration of
ions is done. The point from where the ions originate is
present at the head end of the discharge tube. But the
other end of the tube is earthed.
The whole apparatus is placed in a steel compartment.
Compartment is filled with Nitrogen or Methane.
The pressure inside the chamber is maintained very
high.

5. WORKING
The initial start given to generator is by giving the first comb B1
a positive potential by a strong source of near about 10^4 Volt.
Electric wind having a positive charge will be produced.
Production of wind occurs due to the discharging of charge
from the sharp edges. Belt moves continuously and will reach
the sphere on moving. When the belt will reach the sphere an
induced negative charge will be produced on the sharp edges
of the comb B2. At once a induced positive charge will be
produced on the other side of the comb B2. After this shifting of
the charge,it will be transferred to sphere S. Similarly, due to
the discharging action of the B2, wind will be produced but this
will be negatively charged. The negatively charged wind helps
to make the positively charged belt neutral. After this the belt
will be totally discharged. Again after rotation, the belt will come
down. It will take the positive charge from the comb B1. Then
again this charge will be taken by the comb B2.
The above process repeats again and again. Due to the
repetition the charge will start collecting on the sphere S. The
air present around the sphere S will start ionizing when the
potential of the spherical belt will cross the value of 3 X 10^6.
3 X 10^6 is the value of the air’s breakdown field. When the
ionization of air starts then side by side leakage of charge will
also take place. As we have discussed earlier that the
generator is packed into a steel compartment filled with gas
such as methane or nitrogen. So, leakage is minimized by this
steel chamber. The larger the sphere and the farther it is from
ground, the higher will be its final potential.

6. APPLICATIONS
AND USES
In modern times, the application of Van De Graff
generators is largely limited to academic purposes to
demonstrate the practical aspects and concepts of
electrostatic behaviour of particles.
Primarily designed as a particle accelerator, the Van De
Graaff generators are used in laboratories for
demonstration purposes only.
Van de Graaff generators were one of the first methods
used to study nuclear physics before the advent of
better methods to accelerate particles.
The beam of these charged accelerated particles is
used to trigger the nuclear reaction.
Accelerated particle beams are used to break atoms for
various experiments in physics.
In medicine, such beams are used to treat cancer.

7. LIMITATIONS
It produces very low intensity of current as compared to
volatge.
Maintainance and construction is costly and bulky.
For air, a maximum of 3 million volt can be generated as
after that air will ionize.
It cannot accelerate neutral particles.

8. REFERENCE
Webliography
Van de Graaff Generator – Wikipedia
Bibliography
VK Physics Satya Prakash Class XII
NCERT PHYSICS PART 1 for Cass XII
The Britannica guide to Particle physics Edited by
Erick Gregersen
HOME MADE LIGHTNING- Creative Experiments
in Electricity- R.A Ford