Van de Graaff Generator - physics notes for class 12
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What is Van de Graaff generator -
It is a device which is used for building up high potential difference of the order of 10(4) volt. Such charged particles like deutron, proton, ions etc. can be accelerated by using this generator. It was designed by Robert J. Van de Graaff in the year 1931.
Principle -
It is based upon the following principles :
- Discharging action of sharp points i.e. Corona discharge.
- Charge is uniformly distributed on the outer surface of the spherical shell.
Construction -
It consists of a large and highly polished hollow metal sphere called dome. It is insulated from ground with the help of the insulating supporters. Two combs, like metallic needles, are provided at point C1 and C2 as shown in the figure. The comb at point C1 is called as spraying comb and the comb at point C2 is called as collecting comb. There is a provision of two pulleys, around which a belt of insulating material, eg. silk, rayon, reinforced rubber moves. The whole setup is fixed inside a steel chamber so as to minimize the leakage.
Working -
Due to high density of positive (+ve) charge at the sharp points of the teeth of the comb C1, positive (+ve) charge is sprayed on the belt ( Corona Discharge ). As the belt is rotated by the electric motor, these positive (+ve) ions move upward along the belt. A negative (-ve) charge is induced on the sharp points of the teeth of the collecting comb C2 and an equal positive (+ve) charge is induced on the farther end of the comb C2. This positive (+ve) charge shifts to the outer surface of the conducting shell S. Due to corona discharge at the sharp points of the teeth of comb C2, a negatively (-vely) charged electric wind is set up. This neutralises the positive (+ve) charge on the belt. The uncharged belt return down, collect the positive (+ve) charge from C1, which in return is collected by C2. This process continues and sphere goes on getting more of the more positive (+ve) charge.
The leakage is minimised by housing the whole generator inside a steel chamber filled with N(2) or methane sulphur hexafluoride or some other inert gas at high pressure.
Uses -
It is used for accelerating charged particles which are used for -
- triggering nuclear reaction
- to break up atoms in physics
- in medicines to treat cancer.
Let's have some more knowledge -
Who was Robert J. Van de Graaff ?
Ans - This American engineer, physicist was born on December 20, 1901, in Tuscaloosa, Ala., United States. Robert Jemison Van de Graaff was popularly known for his invention of Van de Graaff generator. He died on January 16, 1967, in Boston. He used to teach at Princeton University and Massachusetts Institute of Technology.
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