Explain the principle of a device that can build up high voltage of the order of a few million volts.
Draw a schematic diagram and explain the working of this device.
Is there any restriction on the upper limit of the high voltage set up in this machine? Explain.
Draw a schematic diagram and explain the working of this device.
Is there any restriction on the upper limit of the high voltage set up in this machine? Explain.
Van de Graff generator is the device used for building up
high potential differences of the order of a few million volts.These high
potential differences are used to accelerate charged particles such as
electrons, protons, ions.
It is based on the principle that charge given to a hollow
conductor is transferred to outer surface and is distributed uniformly over it.
Construction:
It consists of a large spherical conducting
shell (S) supported over the insulating pillars. A long narrow belt of
insulating material is wound around two pulleys and two sharply pointed
metal combs. These are called the spray comb and collecting comb.
Working -The spray comb
is given a positive potential by high tension source. The positive charge gets
sprayed on the belt.
As the belt moves and reaches the sphere, a
negative charge is induced on the sharp ends of collecting comb and an
equal positive charge is induced on the farther end of
This positive charge shifts immediately to the
outer surface. Due to discharging action of sharp points, the positive charge
on the belt is neutralized. The uncharged belt returns down and collects the
positive charge, which in turn is collected. This is repeated. Thus, the
positive charge goes on accumulating.
The main limiting factor on the value of high potential is
the radii of sphere.
If the electric field just outside the sphere is sufficient
for dielectric breakdown of air, no more charge can be transferred to it.
For a conducting sphere,
Electric field just outside sphere
Thus, E = VR
Van de Graff generator is the device used for building up high potential differences of the order of a few million volts.These high potential differences are used to accelerate charged particles such as electrons, protons, ions.
It is based on the principle that charge given to a hollow conductor is transferred to outer surface and is distributed uniformly over it.
Construction:
It consists of a large spherical conducting shell (S) supported over the insulating pillars. A long narrow belt of insulating material is wound around two pulleys and two sharply pointed metal combs. These are called the spray comb and collecting comb.
Working -The spray comb is given a positive potential by high tension source. The positive charge gets sprayed on the belt.
As the belt moves and reaches the sphere, a negative charge is induced on the sharp ends of collecting comb and an equal positive charge is induced on the farther end of
This positive charge shifts immediately to the outer surface. Due to discharging action of sharp points, the positive charge on the belt is neutralized. The uncharged belt returns down and collects the positive charge, which in turn is collected. This is repeated. Thus, the positive charge goes on accumulating.
The main limiting factor on the value of high potential is the radii of sphere.
If the electric field just outside the sphere is sufficient for dielectric breakdown of air, no more charge can be transferred to it.
For a conducting sphere,
Electric field just outside sphere
Thus, E = VR
