Coulomb's Law Calculator
Coulomb’s law calculator: electrostatic force between two point charges, or the charge or distance needed for a given force.
F = ke · q1 · q2r²
How to use this calculator
- Choose the quantity you want under “Solve for”.
- Type the known values and pick their units: any combination works.
- The result updates as you type. Open “Show the steps” to see the substitution.
Coulomb’s law gives the force between two point charges, F = ke·q₁·q₂/r², with ke ≈ 8.988 × 10⁹ N·m²/C².
A positive result means the charges repel (same sign); a negative one means they attract. The formula is for vacuum; in a medium divide by its relative permittivity.
Formula
F = ke · q1 · q2r²
| Symbol | Quantity | Units |
|---|---|---|
| q1 | Charge 1 | C, mC, µC, nC, e, mAh |
| q2 | Charge 2 | C, mC, µC, nC, e, mAh |
| r | Distance | m, km, cm, mm, µm, nm, pm, Å, in, ft, yd, mi, nmi, au, ly |
| F | Force (+ repulsive, − attractive) | N, kN, mN, dyn, lbf, kgf |
Constants used: kₑ = 8.9875517923 · 10⁹ N·m²·C⁻²
Worked example
Given: q1 = 1 µC · q2 = 2 µC · r = 10 cm
F = keq1·q2r²
Result: F = 1.79751 N
Frequently asked questions
What force acts between 1 µC and 2 µC 10 cm apart?
F = 8.988 × 10⁹ × 1×10⁻⁶ × 2×10⁻⁶ / 0.1² = 1.80 N, repulsive.
How does force change with distance?
It follows an inverse-square law: doubling the distance cuts the force to a quarter.