Q.The force acting between two parallel current carrying wires is F. If the current through each wire is doubled and separation becomes three times, then the acting force between them will be

Two long, straight, parallel wires, separated by distance d and carrying currents $I_1$ and $I_2$, exert a magnetic force on each other: the field of wire 1, at the location of wire 2, is $B=\mu_0I_1/(2\pi d)$ (from the long-straight-wire Biot-Savart result), and wire 2, carrying current $I_2$ in this field, experiences a force per unit length of $F/L=\mu_0I_1I_2/(2\pi d)$. This force is ATTRACTIVE when the two currents flow in the SAME direction (parallel currents) and REPULSIVE when they flow in OPPOSITE directions (antiparallel currents) -- the precise, quantitative version of the simple two-wire demonstration that opens the chapter. …