Van de Graaff generator - complete information


Introduction :

Van de Graaff  generator  is an electrostatic high voltage generator. It can generate and supply a few million volt. Robert J. van de Graaff  designed this device .

It is basically an arrangement for producing a very high potential difference of the order of several million volt say 20 million volt. 

Van de Graaff  generator
Van de Graaff  generator

Principle of Van de Graaff generator:

Let a small charged conducting shell of radius r be located inside a larger charged conducting shell of radius R.  If they are connected together with a conductor, then charge q from the small shell will move to the outer surface of the bigger shell irrespective of its own charge Q. 

Here,   potential difference    =   V(r) - V(R)

                                              =       q/4πε₀ ( 1/r − 1/R)

In this way the potential of the outer shell increases Considerably.

Thus,  large amount of charge can be collected on the outer shell. In nutshell,  the principal of Van de Graaff  generator is that large amount of charge can be placed on the outer surface of a spherical shell to increase its potential to a very high value. 


It should be remembered that the device is also known as high voltages generator or Van de Graaff accelerator also. 

 Construction and working of Van de Graaff generator :

It consists of a large conducting spherical shell of few metre radius. This spherical shell is supported on a insulated stand of height of many metre. A belt made of insulated fabric runs over two pulleys P1 and P2 . Pulley P1 and the belt are run by an electric motor. A spray comb C1 made of metallic needles is connected to a high voltage rectifier (H. V. R)  which is earthed.  A collecting comb C2 made of metallic needles facing the belt is connected to the spherical shell S. 

Van de Graaff  generator
Van de Graaff  generator

Now :

Let spray comb C1 be charged to a high positive potential which sparys positive charge to the belt. This in turn becomes positively charged. Sharp teeth of the comb has very high charge density which sets up corona discharging action. Thus  setting up a sort of wind of positive charges. Since belt is moving up, so it carries this positive charge upward. 

Opposite charge appears on the teeth of collecting comb C2 by induction from the belt. As a result of this, positive charge appears on the outer surface of shell S. As the belt is moving continuously, so the charge on the shell S increases continuously. Consequently, the potential of the shell (S)  rises to a very high value. when the charge on the spherical shell is very high, leakage due to ionization of surrounding air also increases.  To prevent the leakage, the generator is enclosed in an earthed steel tank filled with air (or methane or nitrogen)  under pressure. 

Now the charged particles at the top of the tube (T)  are at very high potential with respect to the lower end of the tube which is earthed.  Thus these particles are accelerated downward and hit the target after emerging from the tube. 

The energy acquired by a charge q is = qV, where V is the potential of  a spherical shell S. 

Note :

It should be remembered that the maximum potential occurs in the accelerator, when rate of charge carried by the belt is equal to the rate at which charge leaks from the shell due to ionization of air. 

Numerical : A Van de Graaff generator is capable of building up potential difference of 15 × 10⁶ v. The dielectric strength of the gas surrounding the electrode is 5×10⁷ v m ̄¹.  what is the minimum radius of the spherical shell required? 

solution : 

   Dielectric strength = V/r 

                               r  = V/dielectric strength 

                            =  15 × 10⁶ v / 5× 10⁷ v m ̄¹ 

                               = 0.3 m = 30 cm 

Uses of van de Graaff Generator :

1) In medicine beam of positively charged particles are used to treat cancer. 

2) Accelerated particle beams are used to break atoms for various experiments in physics. 

3) The Beam of positively charged accelerated particles are used to trigger the nuclear reaction

Conclusion : 

Van de Graaff  generator is one among the most important inventions of science. You may have seen a Van de Graaf generator in a science museum before. This article has discussed about the all the important things related to it The article has discussed about its principle, construction and working. It has also discussed its uses.

Van de Graaff  generator is an electrostatic generator, and creates static electricity ,  building up a very large voltages on its surface , moving a belt over a terminal and the electric charge accumulates on the surface of a hollow metal sphere.

I hope this article would have helped you a lot. please help us to reach out more better audience by sharing this on social media and other sources. For more information about different physics concepts visit physics guide


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