Chapter 23: Coulomb’s Law

 Coulomb’s Law
For any point outside a spherical conductor, the charge on the sphere may be considered to act as a point charge at the center of the sphere.

Coulomb’s law: Any two point charges exert an electrical force on each other that is proportional to the product of their charge and inversely proportional to the square of the distance between them.

F α q1q2/r2

                        F = k q1q2/r2

Where k is constant of proportionality, the value of which depends on the insulating medium around the charge and the system of units.

K= 1/4πɛ0


F = q1q2/4πɛ0r2

ɛ0 is called the permittivity of free space.

ɛ0 = 8.85 × 10-12C2N-1m-2


Electric field strength

It is the force per unit positive charge at a point. E= F/Q

We know that the force between two points is given by;

F = Q1Q2/4πɛ0r2

Then electric field strength is;

E = F/Q

E = Q1Q2/4πɛ0r2Q2

E = Q/4πɛ0r2

The E is not constant as it decreases as distance r increases.


Electric Potential

It is the energy per unit charge due to charged body’s position in an electric field.

Similar to electric field strength, electric potential is defined as the potential energy per unit positive charge.


The electric field strength is equal to the negative of potential difference at a point.

The potential V is;

V = kQ/r

V= Q/4πɛ0r

Comparing Electric fields and Gravitational fields

  • Force is inversely proportional to square of distance.
  • Potential is inversely potential to distance.
  • Gravitational field strength g = F/m and Electric field strength E=F/Q.
  • Gravitational force is always attractive and electric force may be attractive or repulsive.