1. Explain Van de Graaff Generator.

In 1929 Robert .J. Van de Graaff designed and built an electrostatic generator which is used in Nuclear Physics Research. The generator is called Vand de Graaff generator. It is an arrangement for producing a very high potential difference so as to accelerate charged particles like deuterons and protons. Such charged particles is used to trigger nuclear reactions and to break atoms for various experiments in physics.
Principle:
Van de Graaff generator is based on the following principles :
1.    Action of points: Sharp pointed surfaces have larger charge densities so these can be used to setup charging and discharging action.
2.    Uniform distribution of charges on the surface of a spherical conducting shell. If a hollow conductor is in contact with another charged conductor, then the hollow conductor continues accepting the charge irrespective of its potential.

 Construction and working

Van de Graaff generator consists of a highly polished metal doom mounted over insulating stands. An endless belt made up of good insulating material allowed to run between the pulleys P₁ and P₂ controlled by a motor. C₁ and C₂ represents two metal combs, C₁ is the sparyer comb which sprays charges into the belt and C₂ is a collector comb which collects the charges from the belt and transfers to the metal doom. A very high voltage is applied to C₁. D represents the discharge tube which provides the path for the accelerated particle.
Let the spray comb be charged to a high +ve potential which sparys +ve charges to the belt. Since the belt is moving up, so it carries this positive charge upward. The collector comb collects these charges from the belt and transfers it to the metal doom. The process continues till the doom becomes charged to the saturated value. When the charge on the doom becomes charged to a high potential, leakage due to ionization of the surrounding air also increases. To prevent the leakage, the generator is enclosed in an earthed steel tank filled with air or methane under very high pressure.
When the potential inside the doom becomes very high, the particle which is to be accelerated is released inside the tube. The particle gets accelerated downwards due to high potential inside the doom and hit the target after emerging from the tube.
The energy acquired by a charge q is qV, when V is the potential of the spherical doom.

Disadvantages:
1.    It is very dangerous due to the high potential of the doom.
2.    The efficiency of the generator is very small with respect to other type of particle accelerators.
3.    Electrons cannot be accelerated.
4.    The size of the generator is extremely large.
The only advantage of the accelerator is that the same instrument can be used to accelerate both positive and negative charges with out any change in the design of the instrument.

2. A paper strip after folding into two lightly ironed and placed. What do you observe. Why?

Two halves of the folded strip repels away. This is because, ironing of the strip results into frictional electrification of the strip. Here both the strips are charged in the same way and so they repel each other.

3. What is a point charge?

Point charge is that charge whose dimensions are negligible in comparison to its distance from the point where its effects are desired. Coulomb’s law, ideally holds good for point charges.

4. Define an ideal dipole.

An ideal dipole is just a point dipole, having no size. The amount of charge q is very large in an ideal dipole while its separation a is very small such that their product p = aq is a constant.

5. Although ordinary rubber is an insulator, the special rubber tyres of aircraft are made slightly conducting. Why is this necessary?

Collision with high-speed wind develops charge on the tyres. In case the tyres are non-conducting,  charges will accumulate on the tyres in large amounts, which is quite dangerous. On the other hand, the conducting tyres lose their charge through metallic bodies hence the danger is eliminated.

6. Vehicles carrying inflammable material usually have metallic ropes touching the ground during motion. Why?

Moving vehicles get charged due to friction with air. The vapour which escapes from the inflammable materials carried by the vehicle may catch fire due to spark produced by the charged vehicle. Usually vehicles carrying inflammable materials have metallic ropes touching the ground during motion. Through these ropes, the charge accumulated in the vehicle gets transferred to the earth.

7. Why repulsion is the surest test for electrification?

Two bodies can attract under two conditions.
1.    One is charged and the other is oppositely charged.
2.    One is charged and the other is uncharged.
But two bodies will repel only when both are charged identically. Hence repulsion is the surest test for electrification.

8. Electrostatic experiments do not work well on humid days. Why?

At ordinary conditions air is an insulator. This insulating property is greatly affected by humidity. As humidity increases air becomes conducting. So electrostatic experiments do not work well on humid days.

9. Can a metal sphere of radius 1 cm hold a charge of one coulomb?

When a charge of one coulomb is given to a sphere of radius 1 cm, the potential at a point on the surface of the sphere becomes very high. At high potential the insulating property of air gets converted to zero and the air becomes a good conductor. So the insulation of air breaks down causing discharge of charges form the sphere of radius 1 cm. Thus a sphere of 1 cm radius is unable to hold one coulomb of charge.

10. Why does printing paper get charged when it is passed through high speed printing press? What should be done to avoid this?

The paper gets charged because of friction. It can be prevented by ionizing the surrounding air with the help of a gas flame or by using an ionizer. The charges in air help in neutralizing the charges on paper.

11.Give any two characteristics of the force between a pair of point charges.

       (a) The force on each charge will be equal and opposite.
       (b) The force acts along the line joining the charges.

12. Two identical conducting spheres carry charges +Q and –Q respectively. If distance between their centres is r, then why correction is need in coulomb’s law?

According to coulombs law F = k Q²/r². But the charges on the metallic spheres will not remain uniformly distributed on their surfaces because of mutual attraction. The redistribution of charges will not permit the use of distance of separation of charges as r. Hence correction is needed.

13. Why is the metallic body of an electroscope earthed during experiments?

The charge on the leaves of an electroscope induces opposite charge on the inner surface and similar charge on outer surface of the body of the electroscope. The free charge on the body of electroscope is sent to earth by grounding it. In this way, only opposite charge remains on the body of the electroscope, which helps in the increase in divergence of the leaves of the electroscope. Thus earthing of the body of electroscope increases the sensitivity of electroscope.

14. What are the important properties of electric charge?

1.    Electric charge is scalar.
2.    There exists two kinds of electric charges.
3.    Like charges repel but unlike charges attract.
4.    Electric charge is quantized.