Coulomb's law gives the force between two point charges

Electric Fields & Potential — AQA A-Level Physics

Key Definition

Coulomb's law — The electrostatic force between two point charges is directly proportional to the product of the charges and inversely proportional to the square of their separation.

Coulomb's lawThe electrostatic force between two point charges is directly proportional to the product of the charges and inversely proportional to the square of the distance between them. is analogous to Newton's law of gravitationThe gravitational force between two point masses is directly proportional to the product of their masses and inversely proportional to the square of the distance between them.. Both are inverse square laws.
Sometimes written as $F = kQ1Q2/r^2$ where $k = 1/(4piepsilon0) = 8.99 x 10^9 N m^2 C^-2$.
Unlike gravity, the electric force can be attractive or repulsive.
Same sign charges: Q1Q2 is positive, force is repulsive.
Opposite sign charges: Q1Q2 is negative, force is attractive.
Only valid when the charges are small compared to their separation (point chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C). approximation). The separation r is always measured from the centres.

Examiner Tips and Tricks

You do not need to memorise epsilon0 -- it is on the data sheet.
However, k is not.
Unless told otherwise, assume charges are in a vacuum, so use epsilon0.