If this quantity is negative, the force is attractive. If it is positive, the force is repulsive.
Examples of static, motionless, interacting currents are the [[Static forces and virtual-particle exchange#The Yukawa potential: The force between two nucleons in an atomic nucleus|Yukawa Potentialpotential]], the [[Static forces and virtual-particle exchange#The Coulomb potential in a vacuum|The Coulomb potential in a vacuum]], and the [[Static forces and virtual-particle exchange#Coulomb potential in a simple plasma or electron gas|Coulomb potential in a simple plasma or electron gas]].
The expression for the interaction energy can be generalized to the situation in which the point particles are moving, but the motion is slow compared with the speed of light. Examples are the Darwin interaction [[Static forces and virtual-particle exchange#Darwin interaction in a vacuum|Darwin interaction in a vacuum]] and [[Static forces and virtual-particle exchange#Darwin interaction in a plasma|Darwin interaction in a plasma]].
Finally, the expression for the interaction energy can be generalized to situations in which the disturbances are not point particles, but are possibly line charges, tubes of charges, or current vortices. Examples areinclude: [[Static forces and virtual-particle exchange#Two line charges embedded in a plasma or electron gas|Twotwo line charges embedded in a plasma or electron gas]], [[Static forces and virtual-particle exchange#Coulomb potential between two current loops embedded in a magnetic field|Coulomb potential between two current loops embedded in a magnetic field]], and the [[Static forces and virtual-particle exchange#Magnetic interaction between current loops in a simple plasma or electron gas|Magneticmagnetic interaction between current loops in a simple plasma or electron gas]]. As seen from the Coulomb interaction between tubes of charge example, shown below, these more complicated geometries can lead to such exotic phenomena as [[Fractional quantum Hall effect|fractional quantum numbers]].
