This calculator finds the electrostatic force between two point charges using Coulomb's law. Enter the two charges and their separation; Coulomb's constant is filled in automatically.
What the symbols mean
- F — electric force (newtons); positive means repulsion, negative attraction.
- k — Coulomb's constant, 8.99×10⁹ N·m²·C⁻² (auto-filled).
- q₁, q₂ — the two charges (coulombs).
- r — distance between the charges (metres).
Worked example
Two 1-microcoulomb charges one metre apart repel with a force of about 9×10⁻³ N.
Want the full story behind this formula? Read our guide: Coulomb's Law.
How the formula works
Coulomb's law gives the electric force between two charges. It has exactly the same inverse-square shape as Newton's law of gravitation, but it is enormously stronger and it can both attract and repel: like charges push apart, opposite charges pull together. Coulomb's constant sets the strength of the interaction.
Common mistakes to avoid
- Ignoring the sign. Two positive or two negative charges repel; opposite charges attract.
- Forgetting to square the distance. As with gravity, r is squared.
- Charge units. Enter charges in coulombs; a microcoulomb is 10⁻⁶ C.
Where it is used
Coulomb's law governs the forces inside atoms and molecules, the behaviour of capacitors, static electricity, and the fundamentals of chemistry and electronics.