Question

Which condition is required for Coulomb's law to hold true?

Answer 1

The correct answer is:
Point charges must be in a vacuum.

In fact, the usual form for of the Coulomb's law is:
$F= \frac{1}{4 \pi \epsilon_0} \frac{q_1 q_2}{r^2}$
where
$\epsilon_0$ is the permittivity of free space
q1 and q2 are the two charges
q is the separation between the two charges

However, this formula is valid only if the charges are in vacuum. If they are in a material medium, the law is modified as follows:
$F= \frac{1}{4 \pi \epsilon_0 \epsilon_r} \frac{q_1 q_2}{r^2}$
where $\epsilon_r$ is the relative permittivity, which takes into account the dielectric effects of the material.