20.4.3—The origin of the forces between current-carrying conductors and determine

Each current creates a magnetic field that acts on the other conductor, producing attraction for currents in the same direction and repulsion for opposite directions.

Use the field of one wire and F=BIL on the other; the force per length falls as separation increases.

Two long parallel wires carrying equal currents in the same direction pull toward each other.

The force is not an action of one current on itself; it is an interaction between the fields and the other conductor’s current.