ELECTROSTATICS
Important short answer for HSEB
Electrostatics
1. What is electrostatic?
The branch of physics that deals with the electric charge at rest is called electrostatics.
1. What is electrostatic?
The branch of physics that deals with the electric charge at rest is called electrostatics.
2. What do you mean by quantization of charge? (HSEB 2068)
The fact that electric charge occur in discrete amounts instead of continuous amounts is called quantization of charge. Thus according, according to the quantization of charge the total charge (q) on a body is always equal to an integral multiple of electronic charge (the charge on an electron) i.e., q=±e, ±2e, ±3e……or q=ne , where n is an integer.
3. Why is meant by relative permittivity? What is its minimum value ? (HSEB 2063 )
The ratio of permittivity of a medium to the permittivity of vacuum is called relative permittivity. It may also be defined as the ratio of the force acting between two charges placed at some distances in the given medium. It is also called dielectric constant of the medium. It is also called dielectric constant of the medium. Its minimum value is 1 and it is for vacuum.
4. Give some points of dissimilarity and similarity between electrostatic and gravitational forces.
Dissimilarity:
I. Electrostatic forces act between two charges while gravitational forces act between two m asses.
II. Electrostatic forces may be attractive or repulsive , but gravitational forces are always attractive.
III. Electrostatics forces are stronger while gravitational forces are weaker.
IV. Electrostatic forces operates over distance which are not large but gravitational forces operate over large distance.
Similarity
I. Both the forces obey the inverse square law.
II. Both are central forces.
III. Both are conservative forces.
5. A comb run through one’s dry hair attracts small bits of paper. Why?
What the comb runs through hair, it gets charged by friction and so it attracts small bits of paper. With wet hair, the friction is reduced , the comb is not charged. On a rainy day, the charges of the comb goes into the moist air as the latter is good conductor of electricity. Thus, if the hair is wet or if it is a rainy day , the comb will not attract paper bits.
6. Because of higher moisture content, air is a better conductor of electricity in the summer than in the summer than in the winter. Would you expect the shocks from static electricity to be more sever in summer or winter? Explain.
because of lower moisture contains during winter , the air act as better insulator and allow larger static charges to build up on objects during winter than during summer. As a result, shocks from static electricity are more sever in winter months than in summer months.
7. Vehicles carrying inflammable materials usually have chains that hang down and drag on the ground. Why? (HSEB 2053)
When a vehicle is in motion, its tiers rub against the road and get charged due to friction. Further, due to friction of air, the body of the vehicle also gets charged . if the accumulated charge becomes excessive ,sparking may occur and the inflammable material may catch fire. Since the chain ropes are touching the ground , the charge leaks to the earth. Hence , the danger of fire is avoided.
8. A bird perches on a bare high power line , and nothing happens to the bird. A man standing on the ground touches the same line and gets a fatal shock. Why?
When a bird perches on single bare high power line, nothing happens to it as no current flows through is body because the circuit is not complete. In case of person standing on a ground , the circuit is completed through ground. Current flows through his body resulting in a fatal shock.
9. Why does a charged glass rod attract a piece of paper?
paper is a dielectric . when a positively charged glass rod is brought near it , atoms of paper get polarized, with centre of negative charge of atoms coming closer to glass rod. Therefore , force of attraction between glass rod and piece of paper becomes greater than the force of repulsion between the glass rod and the piece of paper. This results in attraction of the piece of paper towards the glass rod.
10. Why are sharp edges or points avoided in electrical machines? (HSEB 2058,2063)
Due to the action of point the charges accumulate at sharp edges or points which get discharged. If this discharge occurs in the electrical machines , the machines may get damaged. That is why, sharp edges or points are avoided in electrical machines.
11. Why more charge be placed on metal if its highly polished than when its surface is rough? (HSEB 2061)
The rough surface has many tiny problems which are pointed parts from where charges leak due to the action of points but polished surfaces has very few such projections so leakage of charge is very low. Obviously , more charge can be placed on a highly polished surface than when its surface is rough.
12. You have a negatively charged object. How can you use it to place a net negative charge on an insulated metal sphere? To place a net positive charge on the sphere?
method). In this method same nature of charges are transferred to a neutral body from the charged body.
To place a net positive charge on the sphere , we have to go through electrostatic induction method in which opposite nature of charges is induced on the neutral body.
13. Two equal point charges exert equal forces on each other. But if one charge is twice the other , do they still exert equal forces on each other or does one exert twice the force than the other ?
yes , still they exert equal forces on each other. From coulomb’s law , force exerted on (2q) by the charge q is given by F 1= k q x 2q / r2
similarly ,the force exerted on a q by the charge 2q
F1= k q x 2q/r2
obviously , F1= F2 . According to Newton’s third law of motion , one is action and another is reaction having opposite directions.
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