18. Given two point charges $q_1$ and $q_2$ such that $q_1q_2<0$. What is the nature of the force between them?

Ans. One charge is positive and the other is negative. Unlike charges attract each other.

19. What is the absolute permittivity of a medium whose dielectric constant is one?

Ans. $\epsilon=\epsilon_0 \epsilon _r= \epsilon_0 \times 1= 8.85\times 10^{-12}C^2N^{-1}m^{-2}$

20. Force between two point charges kept in a distance $d$ apart in air is $F$. If these charges are kept at the same distance in water, how does the force between them change? $\epsilon_r$ of water$=80$ ?

Ans. The force will decrease by 8 factor of 80

21. What ts the dielectric constant of a metal?

Ans. Infinity.

22. Give the names of two conservative forces in nature.

Ans. (a) Electrostatic force and (b) Gravitational force

23. What is the cause of charging?

Ans. Transfer of electrons from one body to another.

24. Does the coulomb force that one charge exert on another charge change, if ether charges are brought nearby?

Ans. No, in accordance with the principle of superposition.

25.How many electrons make up one coulomb of negative charge?

Ans. $n=\frac{q}{e}=\frac{1}{1.6\times10^{-19}}=6.25\times 10^{18}$ electrons.

26. State limitations of Coulumb’s law.

Ans. Coulomb's law holds good only for stationary charges and for distances $r$ greater than $10^{-15}m \quad (r>10^{-15})m$