Theory: The American physicist, Dr.Robert Jemison Van de Graaff invented the Van de Graaff generator in 1931. The device has the ability to produce extremely high voltages - as high as 20 million volts. Van de Graaff invented the generator to supply the high energy needed for early particle accelerators. These accelerators are known as atom smashers because they accelerates the sub atomic particles to very high speeds and then “smash” them in to the target atoms. The resulting collision creates other sub atomic particles and high energy radiations such as X-rays. The ability to create these high energy energ y collisions is the foundation of particle and nuclear physics.
Working of the generator is based on two tw o principles: 1. Discharging action of sharp points, i.e., electric discharge takes place in air or gases readily, at pointed conductors. 2. If the charged conductor is brought in to internal contact with a hollow conductor, con ductor, all of its charge transfers to the surface of the hollow conductor no matter how high the potential of the latter may be.
Theory behind construction: If we have a large conducting condu cting spherical shell of radius „R‟ on which we pla ce a charge Q, it spreads itself uniformly all over the sphere. The field outside the sphere is just that of a point charge Q at the centre, while the field inside the sphere vanishes. So the potential outside is that of point charge and inside it is constant.
The potential inside the conducting sphere = Now suppose that we introduce a small sphere of radius „r‟, carrying a cha rge q, into the large one and place it at the centre. The potential due to this new charge has following values.
Potential due to small sphere of radius r carrying charge
at the surface of small
sphere at the large shell of radius R. Taking both charges q and Q in to account we have for the total potential V and the potential difference given by,
Now assume that q is positive. We see that, independent of the amount of charge Q that may have accumulated on the larger sphere, it is always at a higher potential: the difference V(r) V(R) is positive. The potential due to Q is constant upto radius R and so cancels out in the difference. This means that if we connect the smaller and larger sphere by a wire, the charge q on the former will immediately flow on to the matter, even though the charge Q may be quite large. The natural tendency is for positive charge to move higher to lower potential. Thus, provided we are somehow able to introduce the small charged sphere into the larger one, we can in this way pile up larger and larger amount of charge on the latter. The potential of the outer sphere would also keep rising, at least until it reaches the breakdown field of air.
Construction: It consists of a large metal sphere mounted on high insulating supports. An endless belt b, made of insulating material such as rubber, passes over the vertical pulleys P1 and P2. The pulley P2 is at the centre of the metal sphere and the pulley P1 is vertically below P2. The belt is run by an electric motor M. B1and B2 are two metal brushes called collecting combs. The positive terminal of a high tension source (HT) is connected to the comb B1. Due to the process called action of points, charges are accumulated at the pointed ends of the comb,
the field increases and ionizes the air near them. The positive charges in air are repelled and get deposited on the belt due to corona discharge. The charges are carried by the belt upwards as it moves. When the positively charged protion of the belt comes in front of the brush B2, by the same process of action of points and corona discharge occurs and the metal sphere acquires positive charges. The positive charges are uniformly distributed over the surface of the sphere. Due to the action of points by the negative charges carried by the gas in front of the comb B2, the positive charge of the belt is neutralized. The uncharged portion of the belt returns down collects the positive charge from B1 which in turn is collected by B2. The charge transfer process is repeated. As more and more positive charges are imparted to the sphere, its positive potential goes on rising until a surface maximum is reached. If the potential goes beyond this, insulation property of air breaks down and the sphere gets discharged. The breakdown of air takes place in an enclosed steel chamber filled with nitrogen at high pressure.
Explain Van De Graff generator? 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 nuclear physics 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 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 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 4 source of near about 10 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
Therefore 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 presen t 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.
