A van de Graff generator is a device used for building up high potential differences of the order of a few million volts such high potential differences are used to accelerate charged particles like electrons, protons ions etc. needed for various experiments of nuclear physics.
It was designed by van de Graff in the year 1933.
This generator is based on:
The action of sharp points the phenol-men on of corona discharge.
The property that charge given to a hollow conductor is transferred to outer surface and is distributed uniformly over it.
The essential parts of van de Graff generator are shown in a large spherical conducting shell of radius equal to a few meters. This is supported at a suitable height (of several meters above the ground) over the insulating pillars P1 P2, A long narrow belt of insulating material like, silk rubber or rayon is wound around two pulleys P1, and P2, P1, is at the ground level and P2 is at the centre of S. the belt is kept moving continuously over the pulleys with the help of a motor, B1 and B2 are two sharply pointed metal combo fixed as shown B1 is called the spray comb and B1, is called the collecting comb.
The positive ions to be accelerated are produced in a discharge tube D. they on source lies at the head of the tube inside the spherical shell. The other end of the tube carrying the target nucleus is earthed.
The generator is enclosed in a steel chamber C filled with nitrogen or methane at high pressure.
The spray comb is given a positive potential (≈104 volt) the earth of high tension sources H.T, due to discharging action of sharp points a positively charged electric wind is set up, which discharge) as the belt moves, and reaches the sphere a negative charge is induced on the sharp ends of collecting comb B2 and an equal positive charge is induced on the farther end of B2. This positive charge shifts immediately to the outer surface of S. due to discharging action of sharp points of B2. A negatively charged electric wind is set up. This neutralizes the positive charge on the belt. The uncharged belt returns down collects the positive charge form B1, Which in turn is collected by B2. This is repeated. Thus the positive charge on S goes on accumulating.
Now the capacity of spherical shell
C =4π ∈0R, where R is radius of the shell,
As V = Q/C
∴ V = Q 4π∈0R
Hence the potential V of the spherical shell goes on increasing with increase in Q.
The breakdown field of air is about 3 x 106 v/m . the moment the to entail of spherical shell excesses this value, air around S is ionized and leak – age of charge starts. The leakage is minimised by housing the generator assembly inside a steel chamber filed with mirage or methane at high pressures.
If q is the charge on the ion to be accelerated and V is the potential difference developed across the ends of the discharge tube, then energy acquired by the ions = qV. The ions hit the target with this energy and carry out the artificial transmutation etc.
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