# explain the working of VAN DE GRAFF GENERATOR?

Vande Graff generator is a machine which can produce large electrostatic potential difference. ​It is founded on the phenomenon of Corona Discharge.

Initially the generator is started by giving the first comb a positive potential by a strong source of near about ${10}^{4}$ Volt. Then an electric wind will be produced having positive charge. It occurs due to the discharging of charge from the sharp edges. The belt will continuously move and when it will reach the sphere an induced negative charge will be produced on the sharp edges of the comb B2. Then immediately an induced positive charge will be produced on the other side of the comb B2. So, the charge will shift and it will be transferred to sphere S. Again, due to the discharging action of the B2, wind will be produced but this time it will be negatively charged. This negatively charged wind will help to make the positively charged belt neutral. After this action the belt will be totally discharged. Again after rotation when the belt will come down it will take the positive charge from the comb 1 and again this charge will be taken by the comb B2.
This process repeats again and again and charge will start collecting on the sphere S.

• 15

Van De Graff generator 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 experiment
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 two pillars at a certain height from the ground. One is P1 and other is P2. A belt made up of insulating 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 dischargetube 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.

Charge carrying capacity of the sphere will be shown with the formula.
= 4  R

In the above eqation, R is the radii of the spherical shell.
As we know that V= Q/C

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.
Definition in brief: Let V be the potential difference present along the discharge tube and q be the charge present on the ion which is to be accelerated then the energy obtained by the ions will be given by: qV. This is the energy with which the ions strike the aim.

Things to Remember
• When capacitors are connected in parallel, and both the plates of capacitor are joined with the common battery. Then the potential difference possed by each each capacitor wil be same.
• Charge on a capacitor may or may not be same. It depends upon the capacity of the capacitor.
• In a series combination there is only one route for the movement of charge.
• Due to higher capacity earth can take unlimited of charge.

• 4

Van De Graff generator 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.

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:

There is a large spherical conductor having a few meters of radius. An insulating belt similar to conveyor belt moves on the two pulleys. Two combs , 1 is the spray comb which is responsible for spraying of charge and the other is collecting comb for collecting charges are named B1 and B2 respectively. These combs work on the principle of corona discharge or action of sharp points. Acceleration of ions take place in the discharge tube D.

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.

Working:

The system is started by giving positive potential to B1 by a high tension source of 104V .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.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.

Charge carrying capacity of the sphere will be shown with the formula.= 4π ε0 R

• 7

The Van de Graaff generator is working according to the principle of uniform distribution of charges over a spherical body and corona discharge. When an isolated spherical body is charged, the charge distributes uniformly over the surface of the body. When the potential at the surface of the sphere reaches the breakdown potential of air, it ionizes the air surrounding it and gives off corona discharges.

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@ishan goyal: well written. your answers are always useless to meritnation. keep posting !

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^Useless? AHAHHAHAHH

• 1

explain briefly corona discharge and how wind can be ionised

• 1

where we use it

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