Final answer:
The electric force between two negative charges increases by a factor of four when the distance between them is halved, according to Coulomb's law.
Step-by-step explanation:
When the distance between two negative charges decreases by a factor of two, the electric force between the charges increases by a factor of four. This relationship is described by Coulomb's law, which states that the electrostatic force between two point charges is inversely proportional to the square of the distance separating them. Therefore, if the distance is halved, the force becomes the square of two (2^2), which is four times stronger.