Van de Graaff generator : Van de Graaff generator is an electrostatic generatr, that can produce high potential of the order of millions of volt.
Construction :
` P _ 1 ` = Pulley rotated with motor
` P _ 2 ` = Pulley
` B ` = An endless belt made up of insulating material.
` C _ 1 ` = Comb connected to high voltage supply and is used to spray positive charges.
` C _ 2 ` = Comb connected to dome
` S` = Metallic dome
D = Gas dischange tube
T = Target.
It consists of a hallow metal polished sphere S, called dome which is supported on an insulating column. An endless insulated fabric belt is made to run over two pulleys ` P _ 1 and P _ 2 `. Pulley ` P _ 1 ` is rotated using an electric motor. ` C _ 1 ` is metal comb with sharp metal spikes and is connected to high voltagee suppy. Comb ` C _ 2 ` is called as collecting comb which is connected to dome.A discharge, tube D, acts as a source of positive ions such as protons, deuterons. target nucleus is kept at other end of discharge tube. The generator is enclosed in a steel chamber filled with nitrogen or methane at high pressure.
Working : when an electric motor is switched ON, endless belt wound over pulley, ` P _ 1 and P _ 2 ` starts rotating. A spray comb ` C _ 1 ` is given a high voltage rectifier ( H.V.R.) supply. charge from metal comb is transferred to belt. The charges is continuously carried upto large hollow sphere. Negative charges are induced on the sharp ends of comb ` C _ 2 ` and an equal positive charges are induced on farther end of ` C _ 2 ` where comb ` C _ 2 ` collectes these charges, which are spread on the outer surface of dome. The potential of dome continuously increases. The charges are crowded on outer surface of dome which leaks to the surrouding in the form of spark. To avoid such a leakage, Van de Graaff generator is enclosed in earth connected steel tank filled with air under pressure.
The positive ions are introduced from ion sources in the upper part of the evacuated accelerator tube. These ions are accelerated in downward direction. Due to very high fall of potential the positive ion acquire high energy. These charged particles are directed towards target ( T)
Numerical :
Given : `R _ g = 25 Omega , I _ g = 10 mA = 10 xx 10 ^( - 3) A = 10 ^( - 2 ) A, V = 100 V`.
M.C.G is converted into voltmeter by connecting high resistance ` ( R _ s ) ` in series with it which is given by
` R _ s = ( V) / ( I _ g ) -R _ g `
` R _ s = ( ( 100)/ ( 10 ^( -2))) - 25 `
` R _ s = 10000 - 25 `
` R _ s = 9,975 Omega ` in series.
