What is colombus law?

Coulomb's Law.

"the force of attraction
or
repulsion
between two 'point' charges
is
directly proportional to the product of their charges
and
inversely proportional to the squared of the distance between them."

consider two very very very tiny particles. both have a positive charge.
now we know that opposite charges attract each other. so, if we bring them close they will try to stick to each other.

lets call charge one q1 and charge two q2.
the force they experience(according to coulomb's law) will be

f ɑ q1 x q2

ɑ=alpha, sign of proportionality
f=force

the force will be inverse of the distance between them(visualize this scenario)

f 1/ɑ r^2

r is the distance

the final equation becomes.

f=K (q1q2)/r^2

and k is the coulombs constant. it has a value which is 8.9x10^9 Nm2C^-2

that's all ..