Coulomb’s Law

The coulomb’s law states that the electrostatic force between two point charges is proportional to the product of their charges and inversely proportional to the square of the distance between them.

$F = \frac{Q_{1} Q_{2}}{4 \pi \epsilon_{o} r^{2}}$

, where

  • Q is charge,
  • r is the distance between the two charge,
  • $\epsilon_{o}$ is the permittivity of free space

Like charges, repulsion occurs. F is positive.
Unlike charges, attraction occurs. F is negative.

Electric Field Strength

The electric field strength at a point in an electric field is the electrostatic force per unit charge experienced by a small positive test charge placed at that point.

  • Electric field strength is a vector and it is in the direction of the force experienced by the positive test charge.

$E = \frac{Q_{1}}{4 \pi \epsilon_{o} r^{2}}$, Only valid for spherical objects or point charges


  • Q is charge,
  • r is distance from the charge

Electrostatic Force

It is useful to calculate the force on a charge particle in an electric field – this force is known as electrostatic force. To calculate the electrostatic force, rearrange the equation of the electric field strength above.

$F = Eq_{o}$


  • F is electrostatic force,
  • E is electric field strength,
  • q is charge of positive test charge

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