[Physics Class Notes] on Van De Graaff Generator Pdf for Exam

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. 

While standing on 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:

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.

 

Introduction:

Van de Graaff Generator is one of the devices that has an insulating effect and helps insulate various objects by creating a very high Potential. There is the acceleration of particles that are observed. There is also the presence of atom smashers where the subatomic particles are exposed to high-speed acceleration and are then smashed into the target atoms. Due to this smash collision happening there is the production of other subatomic particles along with other high energy radiation being produced. In order to understand the use of the Van de Graaf Generator and its uses here is an article on Van De Graaff Generator – Introduction, Working, Principle, Construction and Uses via that helps you get an idea about everything involved with the same.

Triboelectric Effect Seen in Van De Graaff Generator:

The triboelectric effect is a type of electrification in which certain materials tend to become charged electrically when they come into contact with one another and are then separated from each other. The polarity and strength of the charges that are being produced may differ in magnitude based on the materials that are used, surface roughness, temperature, strain, and other properties. To know more about the triboelectric effect students can also access the NCERT Solutions for Class 12 Physics which provides a detailed view on the same.

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 Current” Electrostatic devices that work mainly on the two principles: 

 

Construction

 

 

 

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 (= 10⁴ 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 times  π times ?o times R ] (R =radius of the shell)

C = [ frac {Q}{V} = frac {V}{4 times  π times ?o times R } ] 

Where  V is the Potential Difference

C = Capacitance

Q= The charge in the spherical shell

?o =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 times 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 Particles 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.

In physics education to teach Electrostatics.

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 wil
l decrease, 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.