In physics, the electrostatic force is the force arising between static (that is, non-moving) electric charges. This force is proportional to the product of the electric charges, and inversely proportional to the distance between the charges. The magnitude of the force takes the form:

where F is the magnitude of the force (in Newtons),

ε₀ is the permittivity of free space,

q₁ and q₂ are the charge magnitudes (in coulombs),

and r is the distance between the two charges in metres. The direction of the force vector is along the axis joining the two charges, which can be expressed in the following vector equation:

where is the electrostatic force vector,

and is the vector between the two charges, such that

where r₁ is the charge on which the force acts,

and r₂ is the other charge. Note that when q₁ and q₂ are the same sign the force vector acts in the same direction as r - Like charges repel. When they are of opposite signs the force vector acts in the opposite direction to r - unlike charges attract.

In the cgs system of measurement, the force coefficient is included in the unit definitions, giving the simpler equation:

where is the force vector (in dynes),

q₁ and q₂ are the charges (in statcoulombs),

and and r are the distance vector and magnitude, measured in centimetres. The above equation can also be interpreted in terms of atomic units with the force expressed in Hartrees per Bohr radius, the charge in terms of the elementary charge, and the distances in terms of the Bohr radius.