Which formula represents Coulomb's law for the electric force between two point charges?

Which formula represents Coulomb's law for the electric force between two point charges?

• A. F = k q1 q2 / r
• B. F = k q1 q2 / r^3
• C. F = k q1 q2 / r^2
• D. F = k r^2 / (q1 q2)

Answer: (C)

This question tests how the electric force between two point charges depends on charge size and separation distance. The force is proportional to the product of the charges and inversely proportional to the square of the distance between them, which is the inverse-square law. The proper formula for the magnitude is F = k |q1 q2| / r^2, where k is Coulomb’s constant (about 8.99 × 10^9 N·m^2/C^2) and r is the distance between the charges. This means stronger charges push or pull harder, and doubling the distance reduces the force by a factor of four. The force acts along the line between the charges and is attractive if the charges have opposite signs and repulsive if they have the same sign. The other forms fail because they have the wrong distance dependence (1/r, 1/r^3), or place the distance or charges in the wrong positions, which would not match the observed inverse-square behavior.

This question tests how the electric force between two point charges depends on charge size and separation distance. The force is proportional to the product of the charges and inversely proportional to the square of the distance between them, which is the inverse-square law. The proper formula for the magnitude is F = k |q1 q2| / r^2, where k is Coulomb’s constant (about 8.99 × 10^9 N·m^2/C^2) and r is the distance between the charges. This means stronger charges push or pull harder, and doubling the distance reduces the force by a factor of four. The force acts along the line between the charges and is attractive if the charges have opposite signs and repulsive if they have the same sign. The other forms fail because they have the wrong distance dependence (1/r, 1/r^3), or place the distance or charges in the wrong positions, which would not match the observed inverse-square behavior.