Two long parallel wires carry currents I₁ and I₂ in the SAME direction, separated by distance d. The force per unit length between them is:

1. Field from wire 1 at wire 2: B = μ₀ I₁ / (2π d). 2. Force per unit length on wire 2: F/L = I₂ B = μ₀ I₁ I₂ / (2π d). 3. Same-direction currents ATTRACT; opposite currents repel. 4. This was the historic definition of the ampere. _Source: NCERT Class 12 Physics Ch 4 §4.7 Force between Parallel Currents_