Coulomb's law defines the force between a pair of charges (q₁ and q₂) separated by a vacuum by a distance, r as

F = k*(q₁q₂)/r²,

where k is a constant.

When q₁ and q₂ have the same charge, the force is positive. This corresponds to a repulsive force.

When one charge is positive and the other is negative, the sign of the force is negative. This corresponds to an attractive force.

Such an attractive force holds crystals of sodium chloride together. Here sodium is positively charged and chloride is negatively charged.