Van De Graaff Generator

What is Van De Graaff Generator?

A Van de Graaff generator is a device used for building up extremely high potential differences in the order of a few million (as high as 20 million volts). Currently, Van de Graaff generator produces a potential difference of 5 Mega volts.

The Van de Graaff generator was designed by Robert Jemison Van de Graaff in1929 A.D. 

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While standing in an insulated platform, if we touch the spherical part of this generator, the charge of the sphere would pass along our body and would transmit to our hairs. 

Since the same charge would produce on our hairs, and these charges would repel each other. This is what we could see in the picture below:

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Van de Graaff generators are delineated as the "constant current" electrostatic devices. When we put a load on a Van de Graaff generator, the current remains constant., but the voltage alters with the load.

On this page, we will learn about the following:

  • What is Van De Graaff Generator?

  • Working, principle, and construction of Van De Graaff generator

  • Uses of Van De Graaff generator

Principle

Van de Graaff generators are “Constant Currentelectrostatic devices that work mainly on the two principles: 

  • Corona discharge.

  • Accumulation of charge on the outer sphere.

 Construction

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Van De Graaff Generator Consists of the Following:

  • An outer terminal -  An aluminum or steel sphere

  • Upper brush -  A piece of fine metal wire 

  • Upper pulley (P1) - A piece of nylon  

  • A long narrow conveyor belt of insulating material like Silk, rayon or rubber wound around the pulleys P1 and P2

  • Motor

  • Lower brush

  • Lower pulley (P2) - A piece of nylon covered with silicon tape.

  • B1 - Sharply pointed spray comb

  • B2 - Sharply pointed collecting comb

 Working

When the spray comb is given a positive potential (= 10 ^ 4 volts) w.r.t. the earth via the high tension source H.T. Due to corona discharge action of sharp points, a positively charged electric wind is set up, which sprays a positive charge on the belt as soon the motor is turned on, the lower pulley (P2) begins turning the positively charged belt upwards, and the lower pulley (P1) establishes a negative charge. Since the pulley P2 is capturing electrons from the belt which is passing over this pulley P2.

Here, we can see that the charge on the pulley P2 is more concentrated than the belt because a strong electric field is generated at the lower pulley.

As the belt reaches the sphere, a negative charge builds upon the collecting comb B2 and a positive charge on the farther side of the comb B2.

This positive charge shifts to the outer surface of S. The discharging action of sharp points of the comb B2, a negatively charged electric wind is set up. Which in turn would neutralize the coated positive charge on the belt, and the belt would turn down again.

The belt will collect the positive charge from comb B1, and then would be collected by the comb B2.

This process continues, the charge accumulates on the sphere S and the excess charge shows up on the outer surface of the sphere.

 Capacitance of electrical sphere

 C = 4 x π x 𝔪o x  R (R =radius of the shell)

 C = Q / V  =  V /  4 x π x 𝔪o x R 

Where  V is the potential difference

C = Capacitance

Q= The charge in the spherical shell

𝔪o =Permittivity of free space

Hence, the potential difference V increases with an increase in charge Q.



As soon as the potential of the shell exceeds the breakdown field value of air (3 x 10 ^ 6 Volt /meter), the air around  S gets ionized, and leakage occurs.

This leakage is minimized by housing the shell inside the steel chamber filled with nitrogen and methane at high-pressures.

Uses of Van de Graaff generator

Particle  Accelerators

These accelerators are also known as Particle Smashers. The high energy supply by the generator accelerates the subatomic particles in an evacuation tube, and these particles crash into atoms.

The ability of a generator to create these high-energy collisions is the cornerstone of Particle and used to accelerate ions, protons, and electrons needed for various experiments of Nuclear physics.

These generators are used as accelerators to generate X-Ray beams for nuclear research and nuclear medicine.

  • Science Education

In physics education to teach Electrostatics.

  • Sterilizing Food

Accelerates electrons to sterilize food and process materials

Summary

There are two kinds of Van de Graaff generators: The one employs high-voltage power supply for charging and one utilizes belts and rollers for charging.


Van de Graaff generator, as we set about the output terminal (sphere) with a grounded object, the voltage will minify, but the current will remain the same. 


The charges are produced due to friction by two insulating bodies rubbing against each other, and they can’t move on their own (at rest) which means they (charges) are static by nature are called Static charges.


SAQs

Q1:  A Spherical Shell of Radius k with Charge D is Spread out to Radius h. Find the Work done by the Electrical Force in this Procedure.

Ans: Work done =  Initial Stored energy - final stored energy

Since the formula is given by W = 1/2  x D ^ 2  / 𝔪  x r

= 1/ 2 x D ^ 2 / 4 x π x 𝔪  x k  -  1/ 2 x D ^ 2 / 4 x 𝔪 x π  x  h

Therefore, Work done = D ^ 2 / 8 x  π x 𝔪 x [ 1 / k - 1 / h]

Q2: Two Insulated Spheres of Radii 40 cm and 56 cm Holding a Similar Charge are Wired by a Conductor and then they are Set Apart. Which of the Two Spheres will Carry More Charge?

Ans: Bigger sphere will carry more charge as its capacity is larger (D = M  x G). The potential difference G turns the same on getting them attached with a wire.

FAQ (Frequently Asked Questions)

Q1: How can you Prevent Corona Discharge?

Ans: Corona discharge depends upon various factors:

Shape: A smooth surface (electric field uniformly spreads) would have less corona than the stranded surface (as electric field distributes over the surface at different locations). 

Size: Conductors having a large diameter have fewer chances of corona discharge.

Line Voltage: Lesser the line voltage, less will be the corona discharge.

Q2: How can the whole Charge of a Conductor be Transferred to another Isolated Conductor?

Ans: This can be done by placing a hollow insulated conductor inside the charged conductor and connecting the two conductors by a wire, the whole charge will shift to the isolated conductor.