WBCHSE Class 12 Physics Notes For Elementary Phenomena Of Electrostatics Long Answer Questions

Unit 1 Electrostatics Chapter 1 Elementary Phenomena Of Electrostatics Long Questions and Answers

Question 1. When an insulated charged spherical conductor is brought near a light, a small spherical conductor suspended with a silk fiber,

  1. It quickly comes and sticks to the charged sphere.
  2. Instantly move away and remain stationary in a deflected position to explain the phenomena.


When the insulated charged spherical conductor is brought near the small spherical conductor suspended with a silk fiber, unlike charge is induced on the near end of the small sphere and like charge on the far end.

As the unlike induced charge is nearer to the inducing charge, the small conductor suspended by the silk fiber is attracted and comes in contact with the insulated charged sphere.

As soon as the small sphere comes in contact with the insulated charged sphere, the former gets the same charge on it. Due to mutual repulsion of like charges, the small sphere instantly moves away from the insulated charged sphere.

Up to a distance such that the tension in the fiber, weight of the suspended ball, and force given by the charged sphere keep the suspended ball in equilibrium

Question 2. State whether attraction can occur between two same kind of charges.

We know the same charges repel each other. But, if a highly charged object is brought near to a little charged object quickly, even though it has the same charge, attraction may happen. This happens due to electrostatic induction.

When a highly charged object is brought near to a little charged object, an opposite charge may be induced in the latter. If the quantity of induced charge exceeds the former amount of charge present, attraction instead of repulsion may happen between them

Question 3. If a charged ebonite rod is made to touch the disc of a gold-leaf electroscope, the leaves diverge. Then the rod is removed from the disc and it is found that the divergence of the leaves decreases a little—explain.

Suppose, the ebonite rod is charged positively. Ebonite is a non-conductor of electricity. So when the rod touches the disc of a gold-leaf electroscope, the charges of both of them existing at the place of contact spread on the disc and the leaves of the electroscope. So the leaves spread apart

Now the charges on the other parts of the ebonite rod do not move to the electroscope. So these charges induce a negative charge on the disc and a positive charge on the leaves. For this reason, the divergence of the leaves increases further.

Divergence of the leaves depends on both induction and conduction of charges. When the ebonite rod is removed, induction will be absent. So then the leaves will have positive charges due to conduction only. So, the divergence of the leaves decreases a little.

Question 4. Why are gold leaves used in a gold-leaf electroscope?

The leaves of the gold-leaf electroscope should be thin and light. They diverge due to the repulsion of similar charges on them. The divergence of the leaves is more if they are light. Large divergence makes observation easier.

Now gold is a ductile metal, that can be beaten to produce very thin leaves. Moreover, gold does not react with air. For these reasons, gold leaves are used in an electroscope.

Question 5. Where will the surface charge density be maximum in a charged cubical conductor?

The surface density of charge will be maximum at each of the cubical conductors.

Question 6. Why Is the metal box of the gold-leaf electroscope

During experiments with a gold-leaf electroscope, free charges exist on the outer surfaces of the tin (metallic) plates used in it. These free charges repel like charges on the leaves and decrease their divergence.

Because of the earthing of the electroscope, these free charges move to the ground. Hence the sensitivity of the device increases.

Question 7. Why is a drying agent kept inside a gold-leaf electroscope?

Though dry air is a good insulator, moist air conducts electricity moderately. If a charged body finds itself in moist air, charges will leak and ultimately get discharged.

Hence, due to the presence of moist air inside a gold-leaf electroscope, the divergence of the leaves will gradually decrease. To avoid this unwanted condition, drying agents (hygroscopic substances) are kept inside a gold-leaf electroscope.

Question 8. Why should not a strongly charged body be brought very close to a gold-leaf electroscope?

If a strongly charged body is brought very close to a gold-leaf electroscope, the amount of charge induced on the leaves of the electroscope will be very large. Then the divergence of the leaves may be large enough to detach them from the rod.

Question 9. Can an alternating static charge at one end of an isolated conductor, developed by an alternating current, be detected by a gold-leaf electroscope?

Yes, a gold-leaf electroscope can defect alternating static charges. The divergence of the gold leaves in this electroscope depends only on the magnitude of charge in a test body, not on the nature of the charge, i.e., not on whether the charge is positive or negative.

The average of this magnitude of charge, over a complete period of the alternating source, has a finite value—this average is never zero. So, when the test body is brought near the disc of a gold-leaf electroscope, this average non-zero value of charge will be the input on the electroscope. The leaves will diverge accordingly

Question 10. Why it is not possible to electrify a metal rod by rubbing it while holding it with a bare hand?

A metal rod as well as a human body is a good conductor of electricity. 5a by holding a metal rod with a bare hand if it is rubbed. the charge acquired due to friction flows through the body of the experimenter. That’s why the rod remains uncharged.

Question 11. How much is one safe while taking shelter in a vehicle for protection during lightning?

The steel frame but not die tires of the topped vehicle can protect one from lightning in the condition that the fellow is not touching metal parts another hide. The lightning charge flows around die outside of there, creating a partial electric screen and protecting the inside

Question 12. How can charge be fully transferred from one spherical conductor to another?

If a charged spherical conductor is placed inside a relatively bigger uncharged spherical conductor and a wire is connected between the two, then a charge will be completely transferred from the small spherical conductor to the bigger spherical conductor as the charge resides on die outer surface

Question 13. Why it is not safe to stand under a tree during lightning?

Electricity seeks the path of least resistance. Due to the moisture inside, a tree is a much better conductor than its surroundings. The result, a tree provides a preferred path for lightning to easily reach the ground. This is why, one should not stand under a tree during lightning.

Question 14. A spherical shell of charge +Q has an outer radius r2 and an Inner radius r1. If a charge +q is placed at the center of the shell then what are the values of surface charge density of inner and outer surfaces?

Here charge +Q resides on the outer surface. As the +q charge is placed at the center, a charge -q is induced on the inner surface and a charge +q is induced on the outer surface.

Hence, the total charge on the outer surface =Q+ q; and the surface charge density of the outer surface,

⇒ \(\sigma_2=\frac{Q+q}{4 \pi r_2^2}\)

The surface charge density of the inner surface,

⇒ \(\sigma_1=\frac{q}{4 \pi r_1^2}\)

Question 15. Charge of 3.2 X 10-7 C Is obtained by rubbing a piece

Since polythene has a negative charge, electrons are transferred from flannel to polythene.

Number of electrons transferred

= \(\frac{\text { total charge transferred }}{\text { charge of an electron }}\)

⇒ \(\frac{3.2 \times 10^{-7}}{1.6 \times 10^{-19}}\)

= \(2 \times 10^{12}\)

Here some mass is transferred from flannel to pohthene.

Mass transferred = number of electrons transferred x mass of an electron

= 2 x 10-12 X 9.1 x 10-31

= 1.82 x 10-18 kg.

Question 16. Why does a spherical conductor retain its charge for a longer time in comparison with conductors of any other shape?

Any conductor has a higher charge density at its sharp ‘ edge. Such an edge discharges faster than a spherical surface. For a spherical conductor, the distribution of charge over its surface is uniform.

As it has no sharp edge, it loses its charge very slowly due to the discharging action. So a spherical conductor can retain its charge for a longer time.

Question 17. Does a solid metallic sphere retain more charge than a hollow sphere of the same diameter?

A solid sphere does not retain more charge than a hollow sphere of the same diameter, because charge resides on the outer surface of a conductor, and the outer surface areas of the two spheres are equal.

Question 18. When a conductor is charged, its charge resides on its outer surface. What is the reason?

According to the property of a conductor, a charge may flow easily through it. Like charges repel each other and repulsion causes them to move as far apart as possible. An equilibrium is reached only when they move to the outer surface of the conductor.

Question 19. What is the minimum amount of charge acquired by a charged body? The charge of a body 5.6 X 10-14C justifies it.

The minimum charge acquired by a charged body is ±e = ±4.8 X 10-10 esu of charge = +1.6 X 10-19C. All charges present in nature are always an integral multiple of an electronic charge e.

The charge of a body is 5.6 X 10-14 C this statement means that it has a deficit of electrons, and hence is positively charged.

The number of electron deficit

⇒ \(\frac{5.6 \times 10^{-14}}{1.6 \times 10^{-19}}\)

= 350000

Question 20. Can charge reside on the inner surface of a hollow conductor? Explain.

A charged body is kept inside a hollow conductor in such a way that it does not touch the inside of the conductor. In this case, unlike charges are induced on the inner surface of the hollow conductor and like charges on its outer surface. It is clearly a special arrangement in which the inner surface of a hollow conductor is charged.

Question 21. An isolated metallic conductor is positively charged. Did its mass increase, decrease or remain the same? How will the mass of the conductor change if it is negatively charged?

Since the conductor is positively charged, electrons have been removed from it So the mass of the conductor has decreased. Electrons are to be added to the conductor to charge it negatively. So the mass of the conductor will increase.

Question 22. A sensitive instrument is influenced by a strong electric field. Suggest a possible way to prevent the influence.

The instrument should be enclosed in a cage made of metal wire. The cage would act as an electric screen; external electric fields would have no influence inside

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