Coulomb's law #1

Consider two charge particles. One is test particle another one is source particle.

Test particle: $$Q_t$$
Source particle: $$Q_s$$

Now assuming both of the charges are static, we can write according to coulomb's law:

$$F = \dfrac{k . Q_t . Q_s}{{r_{st}}^2}$$

Where ' k ' is the proportionality constant. In SI units value of  ' k ' is

$$8.9875517873681764 * 10^{-9} N . M^2 . C^{-2}$$

It is represented in SI units as

$$k = \dfrac{1}{4\pi{\epsilon_0}}$$

Where,

$${\epsilon_0}$$

is the permittivity of free space.