Working of van de graaff generator

Hey friends, In this blog you will understand full working process of van de graaff generator. 

Van de graaff generator

To understand van de graff generator we should understand these simple points :

(a)  if a small charged sphere suspended inside a spherical sphere   and both have same centre of origin
Small charged sphere contain +q charge and have radius r
Outside sphere contain +Q charge and have radius R

It means,
       
     Total potential on the outer sphere = potential due it's own charge Q + potential due to the charge q on the inner sphere

     = 1/4πEo[Q/R+q/R]

                                         

                      AND

Total potential on inner sphere = 1/4πEo[q/r+Q/R]

Hence, 

       The potential difference =Vr-VR  = q/4πEo[1/r-1/R]

  Where, 

           Vr = total potential on inner sphere

           VR = total potential on outer sphere

So,  if q is positive, the potential of the inner sphere will always be higher than that of the outer sphere. Now if the two spheres are connected by a conducting wire, the charge q will flow entirely to the outer sphere, irrespective of the charge Q already present on the outer sphere. This is true for conductors of any shape. A vande graff generator works on this fact. 

(b) charge density is more at sharp points 

                               Or 

      Charge density is inversly proportional to 
      Radius of sphere.

    Working of van de graaff generator:

(i)  Discharging action of sharp points (corona discharge) i.e., electric discharge takes place in air Or gases readily at the pointed ends of conductors. 

(ii)  if a charged conductor is brought into internal contact with a hollow conductor, all of its charge transfers to the hollow conductor, however high the potential of the latter may be. 

Construction: A large spherical conducting shell (of few metres radius) is supported at a height several metres above the ground on an insulating column. A long narrow belt of insulating material, like  runner Or silk, is wound around two pulleys, P1 at ground level and P2 at the centre of the shell. 

    This belt is kept continuously moving by an electric motor attached to the lower pulley P1. 

Near the bottom and the top of it's run, the belt passes close to two sharply pointed brass combs B1 and B2, pointing towards the belt. The comb B1, called spray comb is given a positive potential of 10 kv with respect to the earth by means of a battery ; while the comb B2, called collecting comb, is connected to the spherical shell S. 

                  

Working.   Due to the high electric field at the pointed ends of comb B1, the air of the neighborhood gets ionised and it's positive charge is repelled Or sprayed on to the belt, which moves up into the shell S. As it passes close to comb B2, it induces a negative charge at the pointed ends of comb B2 and a positive charge spreads uniformly on the outer surface of the shell S. The high electric field at the pointed ends of comb B2 ionises the air there and repels the negative charges on to the belt which neutralise it's positive charge. This process continues

 As more and more positive charge is given to the shell, it's potential continues to rise. In this way, a high potential of 6 to 8 million volts can be built upon the sphere. 

                               Notes

1. The high potential difference set up in a van de graaff generator is used to accelerate charged particles like protons, deutrons, alpha-particles, etc. To high energies of about 10Mev, needed for experiments to probe the small scale structure of matter

2.it was invented by American physicist Robert j. Van de graaff  in 1929.
3. Small  van de graaff generator is used in science study.