Calculate the electrostatic force between two point charges with Coulomb’s law, F = k q₁q₂ / r², or solve for a charge or the distance. The results show whether the charges attract or repel, the electric field each produces and the electric potential energy.
Electrostatics formulas
Worked example
Charges of +3 µC and −5 µC sit 20 cm apart. F = 8.988 × 10⁹ × (3 × 10⁻⁶)(−5 × 10⁻⁶) ÷ 0.2² = −3.37 N. The negative sign means attraction. Halve the distance and the force becomes four times larger, the same inverse-square law as gravity, but about 10³⁶ times stronger between two protons.
Frequently asked questions
What is Coulomb’s law?
The electric force between two point charges is proportional to the product of the charges and inversely proportional to the square of their separation: F = kq₁q₂/r².
What does a negative force mean?
Unlike charges (one positive, one negative) attract, which gives a negative value in this convention. Like charges repel and give a positive force.
What is Coulomb’s constant?
k = 8.988 × 10⁹ N·m²/C² in a vacuum (1/4πε₀). In other materials, divide by the relative permittivity.
Can I enter microcoulombs?
Yes. Choose µC or nC from the unit menu; the calculator converts to coulombs.
Related tools
Free educational tool by HawkInc. Results are calculated in your browser and rounded for display; check critical work by hand or with a second method.