The correct answer is:
Point charges must be in a vacuum.
In fact, the usual form for of the Coulomb's law is:
[tex]F= \frac{1}{4 \pi \epsilon_0} \frac{q_1 q_2}{r^2} [/tex]
where
[tex]\epsilon_0[/tex] 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:
[tex]F= \frac{1}{4 \pi \epsilon_0 \epsilon_r} \frac{q_1 q_2}{r^2}[/tex]
where [tex]\epsilon_r[/tex] is the relative permittivity, which takes into account the dielectric effects of the material.
