The document describes the Van de Graff generator, which uses a moving belt to generate very high voltages. It was invented in 1929 and can now produce voltages up to 5 megavolts. It works by using combs to impart negative charges to a moving belt from a grounded lower electrode and transfer those charges to an upper electrode and metal sphere at high voltage. Applications include accelerating subatomic particles for nuclear reactions and medical cancer treatments. However, it produces very low current, is costly to maintain due to its large size, and has limitations in the maximum voltage it can generate due to air ionization.

2. Introduction
History
Principal
Construction
Working
Applicati
ons
Limitatio
ns

3. 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

4. It was invented in 1929 by American
physicist Robert J. Van de Graff. The potential
differences achieved in modern Van de Graff
generators can reach 5 megavolts.
 The Van de Graff generator can be thought
of as a constant-current source connected in
parallel with a capacitor and a very large
electrical resistance

5. 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.

6. Van De Graff Generator consist of :-
1) Hollow metal sphere
2) Upper electrode
3) Upper roller (metal)
4) Side of the belt with
positive charges
5) Opposite side of the belt
with negative charges
6) Lower roller
7) Lower electrode
(ground)
8) Spherical device with
negative charges, used to
discharge the main sphere
9) Spark produced by the
difference of potentials

7. As the belt passes in front of the lower comb, it receives negative charge that
escapes from its points due to the influence of the electric field around the lower
pulley, that ionizes the air at the points.
As the belt touches the upper roller (3), it transfers some electrons, leaving the
roller with a negative charge (if it is insulated from the terminal), which added to
the negative charge in the belt generates enough electric field to ionize the air at
the points of the upper comb.
 Electrons then leak from the belt to the upper comb and to the terminal, leaving
the belt positively charged as it returns down and the terminal negatively charged.
The sphere shields the upper roller and comb from the electric field generated by
charges that accumulate at the outer surface of it, causing the discharge and
change of polarity of the belt at the upper roller to occur practically as if the
terminal were grounded.
 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

8. They are used where a very high
voltage at very low current is
needed.
The beam of these charged
accelerated particles is used to
trigger the nuclear reaction .
In medicine, such beams are used
to treat cancer.
Accelerated particle beams are
used to break atoms for various
experiments in physics .

9. •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.

10. PPT BY :-
KINJAL JAIN
XII-B
ROLL NO. – 10