Coulomb's Law Calculator

How to Use This Calculator

1. Enter charge q₁ and q₂ in microcoulombs (μC). Use negative values for negative charges.
2. Enter separation distance r in meters.
3. Results show force magnitude, attractive/repulsive type, and electric potential energy. Use Extended tabs for three charges or dielectric medium.

Formula

Example

Frequently Asked Questions

• What is Coulomb's law? ▼
  Coulomb's law states that the electrostatic force between two point charges is F = k × q₁ × q₂ / r², where k = 8.99 × 10⁹ N·m²/C², q₁ and q₂ are charges in coulombs, and r is the separation in meters.
• When is the force attractive vs repulsive? ▼
  When the charges have opposite signs (one positive, one negative), the product q₁q₂ is negative and the force is attractive. When both charges have the same sign, the force is repulsive. The magnitude |F| = k|q₁q₂|/r² is always positive.
• What is Coulomb's constant k? ▼
  Coulomb's constant k = 1/(4πε₀) = 8.9875 × 10⁹ N·m²/C². It is related to the permittivity of free space ε₀ = 8.854 × 10⁻¹² F/m. The NIST CODATA 2022 recommended value is k = 8.9875517923 × 10⁹ N·m²/C².
• How does a dielectric medium affect the force? ▼
  In a medium with relative permittivity εᵣ, the force is reduced: F_medium = F_vacuum / εᵣ. Water (εᵣ ≈ 80) reduces Coulomb force to just 1.25% of its vacuum value, which is why ionic compounds dissolve easily in water.
• How does Coulomb force compare to gravity? ▼
  The ratio of electrostatic to gravitational force between an electron and proton at hydrogen bond distance is approximately 10³⁹. Coulomb force is vastly stronger than gravity at the atomic scale, which is why electromagnetism governs chemistry and materials.

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