Van de Graaff Generator: Principle, Working, and Construction

A Van de Graaff generator is a device designed to accumulate electric charge on a hollow metal sphere on the top of an insulated column creating very high electric potentials which further create static electricity and make it available for experimentation.

• Van de Graff Generator was invented by Dr. Robert Jemison Van de Graaff, an American physicist, in 1931.
• Van de Graaff generator used for building up high potential differences of the order of a few million volts.
• The Van de Graff generator system is highly electrostatic in nature and is capable of generating extremely high voltages, up to 20 million volts.
• The generator was invented by Van de Graaff to provide the high energy needed by early particle accelerators.
• Atom smashers are named for the fact that they accelerate subatomic particles to extremely high speeds before smashing them into the target atoms.
• Other subatomic particles and high-energy radiations, such as X-rays, are produced as a result of the collision.
• Particle and nuclear physics are built on the ability to produce these high-energy collisions.

Key Terms: Van de Graff, Generator, Electric Field, Charges, Electrostatic Potential, Accelerator, Voltage, Electric Current, Capacitance, Collision, Atoms, X-Rays, Radiation, Speed

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Van de Graaff Generator- Operational Principle

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The Van de Graff generator uses mobile belts which accumulate charges on a hollow spherical metallic structure. This is then placed on top of an insulating column such that a very high electric potential ranging to a couple of volts is created. This produces a large electric field that is capable of accelerating charged particles.

The basic principle behind the working of the Van de Graff generator is as follows:

• Sharp point discharge, i.e., electric discharge, occurs readily in air or gases at pointed conductors.
• When a charged conductor makes internal contact with a hollow conductor, all of its charges pass to the hollow conductor's surface, regardless of how strong the latter's potential is.

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Van de Graaff Generator- Working

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If we put a charge Q on a large conducting spherical shell of radius R, it spreads uniformly across the sphere. Outside the sphere, the field is identical to that of a point charge Q in the middle, while the field within the sphere vanishes. As a result, the potential outside is that of a point charge, while the potential inside is constant.

Within the conducting domain, there is an opportunity

Assume we insert a small sphere with a radius of ‘r' and a charge of ‘q' into the larger one and put it in the middle. This new charge's potential has the following values.

\(\frac{1}{4 \pi \varepsilon_0} \frac{Q}{R}\)

• Potential due to charge-carrying small sphere of radius ‘r’

​\(q = \frac{1}{4 \pi \varepsilon_0} \frac{q}{R}\)

• Potential on the surface of a massive R-radius shell

\(= \frac{1}{4 \pi \varepsilon_0} \frac{q}{R}\)

• The total potential V and the potential difference is given by when both charges q and Q are taken into account

\(V(R) = \frac{1}{4 \pi \varepsilon_0}( \frac{Q}{R} + \frac{q}{R})\)

\(V(r) = \frac{1}{4 \pi \varepsilon_0}( \frac{Q}{R} + \frac{q}{R})\)

\(V(r) - V(R) = \frac{q}{4 \pi \varepsilon_0} (\frac{1}{r} - \frac{1}{R})\)

• Assume that q is now a positive number. We can see that the larger sphere is still at a higher potential, regardless of how much charge Q has accrued on it.
• The difference V(r) - V(R) is positive. Up to radius R, the potential due to Q is constant, so it cancels out in the gap.

Conclusion

This implies that if we bind the smaller and larger spheres with a wire, the charge q on the smaller sphere will flow into the matter immediately, even if the charge Q is very high. The positive charge has a normal tendency to shift from higher to lower potential. As a result, if we can somehow insert the small charged sphere into the larger one, we can pile up an increasing amount of charge on the latter. The outer sphere's capacity would rise as well, at least until it reached the air breakdown zone.

Also Check out : NCERT Solutions for Class 12 Physics Chapter 2 

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Van de Graaff Generator - Construction

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The following are the vital parts of a Van de Graff generator:

1. Hollow metal sphere
2. Upper electrode
3. Upper roller (for example an acrylic glass)
4. Side of the belt with positive charges
5. The opposite side of the belt, with negative charges
6. Lower roller (metal)
7. The lower electrode (ground)
8. An aspherical device with negative charges
9. The spark produced by the difference in potential

The following are some points to consider about the Van de Graff Generator working:

• It is made up of a huge metal sphere that is supported by high-insulating supports.
• Over the vertical pulleys P₁ and P₂, an infinite belt b made of an insulating material such as rubber passes.
• The pulley P₂ is located in the metal sphere's middle, while pulley P₁ is located vertically below P₂. An electric motor M drives the belt. Collecting combs B₁ and B₂ are two metal brushes.
• The comb B₁ is attached to the positive terminal of a high-tension source (HT).
• Charges accumulate at the pointed ends of the comb due to a phenomenon known as the action of points, which causes the field to increase and ionize the air surrounding them.
• Due to corona discharge, positive charges in the air are repelled and deposited on the belt.
• When the belt passes, the charges are borne upwards by it.

• As the positively charged portion of the belt passes in front of brush B₂, the metal sphere acquires positive charges by the same phase of point and corona discharge.
• Positive charges are evenly distributed around the sphere's surface.
• The positive charge of the belt is neutralized by the action of points caused by the negative charges carried by the gas in front of comb B₂.
• The belt's uncharged part returns down and absorbs B₁'s positive fee, which is then obtained by B₂. The method of charge transfer is repeated.
• When more positive charges are applied to the sphere, its positive potential rises until it reaches a surface limit.
• If the potential is exceeded, the air's insulation property fails, and the sphere is discharged. In an enclosed steel chamber filled with nitrogen at high pressure, the air is broken down.

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Previous Year Questions

1. A bar magnet is10 cm  long is kept with its north….[BITSAT 2019]
2. A conducting sphere of radius R=20cm is given a charge Q….[JIPMER 2015]
3. A metallic sphere is placed in a uniform electric field. The line of force follow the path (s) shown in  ….[VITEEE 2018]
4. As shown in the figure, charges are placed at the vertices…. [VITEEE 2011]
5. On moving a charges , the potential difference between the points is… [VITEEE 2011]
6. Voltage rating of a parallel plate capacitor is….[JEE Main 2018]
7. In the figure shown below, the charge on the left plate of the 10μF capacitor is −30μC….[JEE Main 2019]
8. In The Figure Shown After The Switch S Is Turned from postion a to b….[JEE Main 2019]
9. the effective capacitance of the whole circuit is to…...[JEE Main 2019]
10. In the circuit shown, find C if the effective capacitance of the whole circuit is….[JEE Main 2019]
11. A solid conducting sphere, having a charge Q, is surrounded by an uncharged conducting hollow  ….​.[JEE Main 2019]
12. A Plane Electromagnetic Wave Of Frequency 50 MHz travels in….[JEE Main 2019]
13. A Parallel Plate Capacitor With Square Plates Is F….[JEE Main 2019]
14. The dielectric constant of a material which when fully inserted in above capacitor, gives same capacitance….[JEE Main 2019]
15. Watt per ampere is unit of…
16. The electron is accelerated through a potential difference of 10 V. The additional energy acquired by the electron is…
17. What is the final potential difference across each capacitor? [KEAM]
18. The effective capacitance between two points is….
19. While a capacitor remains connected to a battery, a dielectric slab is slipped between the plates…..[KCET 2001]
20. The energy stored in a capacitor of capacity C and potential V is given by...[NEET 1996]

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Things to Remember

• When a charged conductor makes internal contact with a hollow conductor, all of its charges pass to the hollow conductor's surface, regardless of how strong the latter's potential is
• The Van de Graff generator was invented by Dr. Robert J Van de Graaff in 1931.
• The system is capable of generating extremely high voltages. 
• Sharp point discharge, i.e., electric discharge, occurs readily in air or gases at pointed conductors. 
• When a charged conductor makes internal contact with a hollow conductor, all of its charges pass to the hollow conductor's surface, regardless of how strong the latter's potential is.
• The positive charge has a normal tendency to shift from higher to lower potential.