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Tuesday, 6 November 2012

Van de Graaff Generator


Van de Graff Generator



           In 1929 Robert J. Van de Graff designed an electrostatic machine which produces large 
           electrostatic potential difference of the order of the order of 107 V

                       The working of Van De Graff Generator is based on the principle of electrostatic 
           induction and action of points.
           
                      A hollow metallic sphere A is mounted on insulating pillars as shown in fig. A pulley  B 
          is mounted at the centre of the sphere and another pulley C is mounted near the bottom. A belt
          made of silk moves over the pulleys. The pulley C is driven continuously by an electric motor.
          Two comb-shaped conductors D and E having number of needles, are mounted near the pulleys. 
          The comb D is maintained at a positive of the order of 104  is connected to the inner side of the 
           hollow metal sphere.

                      Because of the high electric field near the comb D, the air gets ionised due to action of points,
           the negative charges in air move towards the needles and positive charges are repelled on towards the
          belt, moves up and reaches near the comb E.

                      As a result of electrostatic induction, the comb E acquires negative charge and the sphere 
           acquires positive charge. The acquired positive charge is distributed on the outer surface of the sphere.
           The high electric field at the comb E ionize the air. Hence, negative charges are repelled to the belt,  
          neutralises the positive charge on the belt passes over the pulley. Hence the descending belt will
          be left uncharged.
                    
                      Thus the machine , continuously transfer the positive charge to the sphere. As a result, the 
          potential of the sphere keeps increasing till it attains a limiting value(maximum). After this stage
          on more charge can be placed on the sphere, it stars leaking to the surrounding due to ionisation
          of the air

                      The leakage of charge from the sphere can be reduced by enclosing it in a gas filled steel
          chamber at very high pressure

                      The high voltage produced in this generator can be used to accelerate positive ions ( protons,
          deuterons) for the purpose of nuclear disintegration

                              

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