Equation for Coulomb's law
Charges are...
not equal
Charge 1 (q₁)
nC
Charge 2 (q₂)
nC
Distance (r)
mm
Force (F)
lbf

Coulomb's Law Calculator

By Bogna Szyk
Last updated: May 21, 2021

This electric force calculator will enable you to determine the repulsive or attractive force between two static charged particles. Continue reading to get a better understanding of Coulomb's law, the conditions of its validity, and the physical interpretation of the obtained result.

How to use Coulomb's law

Coulomb's law, otherwise known as Coulomb's inverse-square law, describes the electrostatic force acting between two charges. The force acts along the shortest line that joins the charges. It is repulsive if both charges have the same sign and attractive if they have opposite signs.

Coulomb's law is formulated as follows:

F = keq₁q₂/r²

where:

  • F is the electrostatic force between charges (in Newtons),
  • q₁ is the magnitude of the first charge (in Coulombs),
  • q₂ is the magnitude of the second charge (in Coulombs),
  • r is the shortest distance between the charges (in m),
  • ke is the Coulomb's constant. It is equal to 8.98755 × 10⁹ N·m²/C². This value is already embedded in the calculator - you don't have to remember it :)

Simply input any three values into our electric force calculator to obtain the fourth as a result.

Electric charge units

The unit of electric charge is a Coulomb (symbol: C). It is defined as the charge that is transported by a constant current of 1 ampere during 1 second. Hence, 1 C = 1 A * 1 s expressed in SI units.

If you don't remember what an ampere is, head to our Ohm's law calculator.

Conditions for validity

Three main conditions must be fulfilled for the electrostatic force calculator to return valid values:

  1. Charges must be stationary - they cannot move with respect to each other.
  2. Point charges are assumed. This assumption also holds for any charges that are spherical and symmetric. For example, a charged metal sphere fulfills this condition, but a charged metal box doesn't.
  3. Charges cannot overlap - they must be distinct and have at least a minimal distance between them.

Interpretation of results

Force obtained with the help of our Coulomb's law calculator can be either positive or negative. Positive force implies a repulsive interaction between the charges. Negative force means that the interaction is attractive.

Did you notice that the default unit for charge in our Coulomb's law calculator is a nanoCoulomb (nC)? It is because the typical order of magnitude of an electric charge is 10-6 C or even 10-9 C.

Interested in electricity? Check out our electricity cost calculator, too!

Bogna Szyk
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