2. Van de Graff generator
• INTRODUCTION
• INVENTION
• PRINCIPLE
• CONSTRUCTION
• WORKING
• APPLICATIONS
• LIMITATIONS
3. 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.
• Invented in 1929 by American physicist
Robert J.
Van de Graff at Princeton University, with
help from colleague Nicholas Burke.
4. PRINCIPLE This Van de Graff generator works on
the principle of electrostatic induction and action at
points.
Uses two principles of Electrostatics
• Corona Discharge :- Discharging action
of sharp points, i.e., electric discharge
takes place in air or gases readily, at
pointed conductors.
•Receiving action-- 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
5. Construction
• Hollow metal sphere --There is a large spherical
conductor named S having a few meters of radius.
• Insulating pillars-- It is placed on two pillars at a
certain height from the ground.
One is p1 and other is p2.
• An endless belt --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.
• 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.
6. • A motor M-- A motor M is used whose main
function is to create a spin in the belt.
• Two sharp headed combs-- Two sharp
headed combs, spray comb B1 and collecting
comb B2 are used.
• A discharge tube D-- 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.
• Nitrogen or Methane gas --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.
7. Working
The initial start given to generator is by giving the first comb B1 a positive potential by a strong
source of near about 104 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.
8. Charge carrying capacity of the sphere will be shown with the formula.
= 4 R
In the above equation, R is the radii of the spherical shell.
As we know that V= Q/C
Therefore V=Q/ 4 R
From the equation it is clear that the potential and charge are directly proportional to each other.
As we will increase the charge the V i.e. potential will also increase.
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.
9. • As the belt continues to move, a constant charging current travels via the belt,
and the sphere continues to accumulate negative 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.
APPLICATIONS
• They are used where a very high voltage at very low current is needed.
• Used to accelerate projectiles like proton, deuteron etc. to carry out different
nuclear reactions.
• In medicine, such beams are used to treat cancer.
• Used to study collision experiments in physics.
10. LIMITATIONS
• It produces very low intensity of current as compared to voltage.
• Maintenance 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.
