Van-de-graff generator

Van de graaff generator is used to generate high voltages of the order of a few million volts. This results in generation of large electric fields for experimental purposes.

Principle

• The inner sphere has a higher potential than outer if the charge q is positive.
• If the two spheres are connected through a wire, the charge will flow from high to low potential.
• Thus, providing smaller potentials at the inner sphere will keep building large amount of charge at the outer sphere, till the breakdown field of air (3 x 10⁶ V/m) is reached.
• This accumulates close to millions of volts.

Construction

• A large spherical conducting shell (of few metre radius) is supported at a height several meters above the ground on an insulating column.
• A long narrow endless belt insulating material, like rubber or silk, is wound around two pulleys – one at ground level, one at the center of the shell.
• This belt is kept continuously moving by a motor driving the lower pulley.
• It continuously carries positive charge, sprayed on to it by a brush at ground level, to the top.
• There it transfers its positive charge to another conducting brush connected to the large shell.
• Thus positive charge is transferred to the shell, where it spreads out uniformly on the outer surface and a voltage difference of as much as 6 or 8 million volts (with respect to ground) can be built up.