Coulombs law formulas and interpretation
Coulomb’s law describes the electric force between stationary point charges.
The calculator preserves charge-unit conversion and attraction-versus-repulsion interpretation.
How to use the Coulombs law calculator
- Choose a model: Select the relationship matching the problem.
- Choose the unknown: Select the quantity to calculate.
- Enter values: Enter all known values with matching units and signs.
- Calculate: Review the result, formula, units, and direction.
Formula and variables
Force magnitude is proportional to both charges and inversely proportional to squared separation.
F = |q₁q₂|/(4πεr²)- F — Electric force
- Electrostatic force magnitude (N)
- q₁,q₂ — Charges
- Point-charge quantities (C)
- r — Distance
- Charge separation (m)
- ε — Permittivity
- Medium permittivity (F/m)
Point-charge example
Two 1 μC charges are separated by 0.1 m in vacuum.
- Charges
- 1 μC each
- Distance
- 0.1 m
- F = kq₁q₂/r²
- F ≈ 0.899 N
Result: Force magnitude is about 0.899 N.
Like charges repel; opposite charges attract.
Understanding your results
Interpreting the result
The point-charge model is best when object dimensions are small compared with separation.
Assumptions
- The selected equation represents the physical system.
- Inputs use a consistent reference direction.
- Values are converted through coherent SI units.
Limitations
- Vector components must be resolved along a common axis.
- External forces or energy losses are not added automatically.
- Results depend on the accuracy of entered measurements.
Common mistakes
- Mixing incompatible units.
- Dropping negative signs that represent direction.
- Using weight where mass is required.
- Entering a zero divisor.
Practical use cases
Physics problems
Check classroom, laboratory, and mechanics calculations.
Practical estimates
Estimate motion, forces, and energy for real systems.
Frequently asked questions
Can a result be negative?
Yes. For directional quantities, the sign indicates direction relative to the chosen positive axis.
Should I use SI units?
The interface can convert supported units, while the formulas are evaluated through coherent SI units.
Sources and review
- SI Brochure, 9th edition — BIPM. Accessed 2026-07-11.
- Special Publication 811 — NIST. Accessed 2026-07-11.
Reviewed 2026-07-11.