The physics
A long straight wire's field goes round it in circles. Its size falls as 1/r.
B = μ0I / 2πr
Use the right-hand rule. Point your thumb along the current. Your fingers curl the way the field goes.
Two wires each make their own field. At any point, add the two fields as arrows.
A current in the other wire's field feels a force. Currents the same way pull together. Opposite currents push apart.
F / L = μ0I₁I₂ / 2πd
Here μ0/2π = 2 × 10⁻⁷ T m/A.