Van de Graaff Generator

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Van de Graaff Generator

Q. With a neat sketch, explain the working of a Van-De-graff generator. What are the factors that limit the maximum output voltage obtained ?

  • A Van de Graaff generator is a device that creates high electric potential.
  • The Van de Graff generator was invented by Robert J. Van de Graaff, and American physicist (1901 – 1967).
  • Van de Graaff generators can produce electric potentials up to many 10s of millions of volts. 
  • Van de Graaff generators can be used to produce particle accelerators.
  • We have been using a Van de Graaff generator in lecture demonstrations and we will continue to use it.

Working Principle of Van de Graaff Generator :

Van de Graaff Generator
Figure A
  • The Van de Graaff generator work by applying a positive charge to a non conducting moving belt using a corona discharge.
  • The moving belt driven by an electric motor carries the charge up into a hollow metal sphere where the charge is taken from the belt by a pointed contact connected to the metal sphere.
  • The charge that builds up on the metal sphere distributes itself uniformly around the outside of the sphere.
  • For this particular Van de Graaff generator, a voltage limiter is used to keep the Van de Graaff generator from producing sparks larger than desired.
  • Friction between acrylic roller and rubber belt.
  • Lower comb sprays on negative charge by induction.
  • Upper comb sprays positive charge to leave negative charge on dome.
  • Millions of volts achievable in dry conditions.

Working of Van de Graaff Generator
Figure B


Van de Graaff Generator
Figure C
Bigger Van de Graff Generator
Figure D

The Giant Van De Graaff High Voltage Generator in its Airship-Dock Laboratory near New Bedford, Massachusetts.(Courtesy of Professor Van de Graaff)
Figure E

The biggest -25 Million Volts, Oak Ridge National Lab in Tennessee. Working of Van De Graaff Generator
Figure F
The biggest -25 Million Volts, Oak Ridge National Lab in Tennessee. Working of Van De Graaff Generator.
Figure G

Step by Step Working of Van De Graff Generator
Figure H


Step by Step Working of Van De Graff Generator.
Figure I

Van de Graaf Generator :

  • Rubber band steals electrons from glass.
  • Glass becomes positively charged.
  • Rubber band carries electrons downward.
  • Positively charged glass continues to rotate.
  • Wire “brush” steals electrons from rubber band.
  • Positively charged glass steals electrons from upper brush.
  • Sphere (or soda can) becomes positively charged to 20,000 volts.
Prototype Model of Van De Graff Generator
Figure J

Power Required to move the Belt

  • An insulated belt is moving with uniform velocity v in an electric field of strength E (x). Suppose the width of the belt is b and the charge density consider a length dx of the belt, the charge dq = σbdx
  • The force experienced by this charge (or the force experienced by the belt) is given by,

dF = Edq = E  σbdx or F =  σb ∫Edx

Since the electric field is uniform, therefore,

F = σ bv

The power required to move the belt –

= Force x Velocity

= F v = abVv

Now, I = dq/dt = ob (dx/dt)

Therefore 1 = σ bv

Hence Power required to move the belt is

P = F v = σ bVv = VI

Advantages of Van de Graaff Generator :

  • Very high voltages can be easily generated.
  • Ripple free output.
  • Precision and flexibility of control.

Disadvantages of Van de Graaff Generator :

  • Low current output.
  • Limitations on belt velocity due to its tendency for vibration. The vibrations may make it difficult to have an accurate grading of electric fields.

Applications of Van de Graaff Generator :

  • 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.

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