1. Explain the difference between electrostatic force and gravitational force. 

Ans: (a) Gravitational force ia always attractive. Electrostatic force can be attractive or repulsive, depending on the nature of the charges, 

(b) The gravitational constant G (s independent of the nature of the medium. But the electrical constant depends on the nature of the medium (i.e relative permittivity). 

(c) The electrostatic force is always large compared to the gravitational force

2. Vehicles carrying inflammable materials usually have metallic ropes touching the ground during motion. Why ? 

Ans: When vehicles move through air, they get charged due to friction. Charge will be developed on tyres also. If the change is sufficient, they will produce spark. The vapour which escapes from the inflammable materials carried by the vehicle may catch fire. To prevent this metallic ropes touching the ground are suspended. Through this conducting metal the charge flows to the earth. 

3. When a polythene piece is rubbed with wool, it acquires negative charge ? Is there a transfer of mass from wool to polythene ? 

Ans: During rubbing, electrons are transferred from wool to polythene. Electrons have mass. So mass is transferred from wool to polythene. 

4. A comb run through one’s dry hair attracts small bits of paper. Why ? What happens if the hair is wet or if it is a rainy day ? {Remember a paper does not conduct electricity}. 

Ans: The comb gets electrified due to friction : so the comb attracts bits of paper. Wet hair is a conductor. So the charge goes out of comb and hence it will not attract paper bits. 

5. Ordinary rubber is an insulator. But the special rubber tyres of aircrafts are made slightly conducing. Why is this necessary ?

Ans: Tyres of aircrafts as well as its body get charged due to friction. If the tyres are non-conducting, it causes spark. Since the tyres are conducting the charge flows to the earth. 

6. A bird perches on a bare high-power line, and nothing happens to the bird. A man standing oa the ground touches the same line and gets a fatal shock. Why ? 

Ans: No current passes through the bird because the circuit is not completed and its body is at the same voltage as the live wire. 

When a man touches the same line the circuit is completed. There is a large potential difference between his hand and feet. A large current flows through his body and he gets a fatal shock. [But if the man wears shoes with insulating soles he will not get a shock.] 

7. A glass rod held in hand can be charged by rubbing with silk. Why cannot a metal rod be charged like this ? 

Ans: Metal and our body are conductors. So charge gets transferred to the earth. 

8. Can two like charges attract each other ?

Ans. Yes. If one charge is very large and is placed very close to each other then the larger charge will induce an opposite and equal charge on the other body. The induced charge is greater than the charge present * on the body. So there will be attraction.

9. How can you transfer the whole charge of a conductor to another isolated conductor ? 

Ans. Place the charged conductor inside the hollow insulated conductor. Now connect the two conductor using a metallic wire. Then the whole charge will flow from the charged conductor to the outside conductor.

10. Can a body have a charge of $0.8 \times 10^{-19}C$ ?

Ans. No. The given charge is $q = e/2$ and is not allowed, as charge is quantised.

11.State Coulomb’s law in electrostatics.

According to Coulomb's law, the force of attraction or repulsion between two charged bodies is directly proportional to the product of their charges and inversely proportional to the square of the distance between them.

12. What does $q_1+q_2=0 $ signify in electrostatics?

Ans. $q_1=-q_2 $. The two charges are equal and opposite. Unlike charge attract.

13. In Conlomb’s law, on what factors the value of electrostatic force constant $\epsilon _ r(\kappa)$ depend?

Ans. It depends on the nature of the medium in which the charges are placed and the system of units used.

14.Does Coulomb’s law obey Newton’s third law of motion?

Ans. Yes. The coulombian force of interaction is equal and opposite and they act on different bodies.

15. Does the motion of a body affect its charge?

Ans. No. The charge does not depend on the speed of the body.

16. A body is charged negatively. Will its mass increase, decrease or remain the same?

Ans. Electrons must be added to charge a body negatively. So its mass will increase .

17. Given twe point charges $q_1$ and $q_2$ such that $q_1q_2>0 $. What is the nature of force between them? 

Ans. $q_1$ and $q_2$ are either both negatrve or both positive. So force is repulsive.