Question

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

Point charges must be in a vacuum.

Point charges must be at infinite distances apart.

One charge must be much greater in magnitude than the other.

Both charges must carry the same sign, either positive or negative.

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= (1)/(4 \pi \epsilon_0) (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= (1)/(4 \pi \epsilon_0 \epsilon_r) (q_1 q_2)/(r^2)`
where
`\epsilon_r` is the relative permittivity, which takes into account the dielectric effects of the material.