Electric current

What Is Coulomb's Law? Examples and Formula

What is Coulomb's law? Examples and formula

Coulomb's law is a law that defines the force exerted by an electric field on an electric charge. This is the force acting between electrically charged objects, and is operationally defined by the value of the interaction between two stationary point electric charges in vacuum.

Coulomb's law states: "The electrical force of attraction or repulsion between two charges is inversely proportional to the square of the distance that separates them."

This force is called the Coulomb force and was described by In physics, the Coulomb force, described by Charles-Augustin de Coulomb.

Charles-Augustin de Coulomb invented the torsion balance to measure magnetic and electrical attraction. This instrument is capable of measuring the torque resulting from the application of one or more forces to your arms.

Coulomb's Law Formula

The Coulomb force can be calculated using the following equation:

Coulomb's law formula


  • F is the Coulomb force expressed in Newtons (N).

  • q 1 is the first point charge expressed in Coulombs (C).

  • q 2 is the second point charge (C).

  • r is the distance between two point charges expressed in meters (m).

  • k C  is an electrostatic constant whose value is approximately 8.988 × 10 9 N · m 2 · C -2

Drawing of the force between two electric charges

All the variables described in this definition correspond to the international system of units.

If the sign of the force is positive it means that the force is repulsive. If, on the contrary, it is negative, the force is attractive.

From the formula we can draw several conclusions that the force is directly proportional to the product of the electric charges q 1 and q 2

Examples of Coulomb's Law

Coulomb's law applies whenever electric charges and electric fields are involved. Some examples in everyday life are:

  • An electromagnet: The force that an electromagnet generates when a current flows through it.

  • Electric motors: the force of electric motors comes from the interaction of electrical charges.

  • Atoms: atomic nuclei are made up of positive particles that exert a repulsive force. Nuclear bonds have to compensate for these forces.

  • Balloon charged with electrostatic electricity: if we rub an air balloon with a woolen sweater, some electrons from the sweater pass into the balloon. If we place the balloon on the roof it does not fall, because the excess electrons (negatively charged) exert an attractive force on the protons (positively charged) of the atoms on the roof.

Exercise on the Application of Coulomb's Law

Determine the electric force between two charged spheres of 1 μC each, placed at a distance of 1.0 cm.

Carrying out the exercise

Two point electric charges are placed at a distance of 1 cm from each other.

We want to determine the electrical force that is established between the two electrical charges, knowing that each charge has an intensity equal to 1 μC.

We apply Coulomb's law directly:

In which:

Q1 = Q2 = Q = 1 μC = 10 -6 C.

r = 1.0 cm = 10 -2 m

By replacing the formula we have:

F = 8.988 · 10 9 · (10 -6 · 10 -6 ) / 10 -2 = 89.88 N


Published: September 17, 2021
Last review: September 17, 2021