CBSE Class 12-science Answered

It is used to generate high potential differences of near about million volts. Then the generated potential differences are used to speed up the particles like ions etc.

It is a device most commonly used in nuclearphysics experiments.
In 1931 Van De Graff generator was designed.

Two main principles of Van De Graff Generator are given below:
• It is designed on the basis of the property that the charge given to a conductor which is hollow inside, is transferred to the outer surface and scatters uniformly over it.
• It is founded on the phenomenon of Corona Discharge.

Construction of VanDeGraff generator: The diagram of Van DE Graff generator is shown below.
There is a large spherical conductor named S having a few meters of radius. It is placed on twopillars at a certain height from the ground. One is P1 and other is P2. A belt made up ofinsulating material is moving over two pulleys P1 and P2. The pulley P2 is present at the center of the spherical conductor S and the pulley P1 is present near the ground. A motor is used whose main function is to create a spin in the belt. Two sharp headed combs, B1 and B2 are used. Spray Comb and collecting comb are the names of B1 and B2 respectively. 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 preassure inside the chamber is maintained very high.

What is the working of a Van De Graff generator?
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.