Question 1

What is the S.I. unit of electric charge?

Accepted Answer

The S.I. unit of electric charge is Coulomb ($C$). It is defined as the amount of charge transported by a constant current of one ampere in one second ($1	ext{ C} = 1	ext{ A}\cdot	ext{s}$).

Question 2

The number of electrons present in $1	ext{ Coulomb}$ of negative charge is:

Accepted Answer

Using the quantization formula $q = n e$: $$n = \frac{q}{e} = \frac{1	ext{ C}}{1.6 	imes 10^{-19}	ext{ C}} = 6.25 	imes 10^{18}$$

Question 3

What is the dimensional formula of the permittivity of free space ($\ mepsilon0$)?

Accepted Answer

From Coulomb's Law, $F = \frac{1}{4\pi\epsilon0} \frac{q1 q2}{r^2} \implies \epsilon0 = \frac{q1 q2}{4\pi F r^2}$.
Dimensional formula: $$\epsilon0 = \frac{A T^2}{M L T^{-2}L^2} = M^{-1} L^{-3} T^4 A^2$$

Question 4

Two point charges separated by a distance $r$ exert a force $F$ on each other in vacuum. If they are placed in a medium of dielectric constant $K = 4$ at the same distance, the new electrostatic force

Accepted Answer

The electrostatic force in a medium of dielectric constant $K$ is given by $F' = \frac{F}{K}$.
Since $K = 4$, the new force is $F' = \frac{F}{4}$.

Question 5

What is the direction of the electric dipole moment vector?

Accepted Answer

By convention, the electric dipole moment ($\ mecb{p}$) is a vector quantity directed along the dipole axis from the negative charge ($-q$) to the positive charge ($+q$).

Electric Charges and Fields