Two parallel conductors of infinite length sit 1 m apart in empty space and each carries 1 A. What force per metre does each feel?

1. Use F/l = mu0 I1 I2 / (2 pi r), with both currents equal to 1 A and r equal to 1 m. 2. Substitute: F/l = (4 pi x 10^-7 T m/A)(1 A)(1 A) / (2 pi x 1 m). 3. The factor pi cancels, leaving 4 x 10^-7 divided by 2. 4. So F/l = 2 x 10^-7 N/m. 5. Because mu0 is exactly 4 pi x 10^-7 T m/A by definition, this result is exact rather than approximate. 6. This exact force per metre is the operational definition of the ampere. 7. An answer of 4 x 10^-7 N/m drops the factor of 2 in the denominator. 8. An answer quoted in newtons alone forgets that the quantity is a force per unit length. _Source: OpenStax College Physics (CC BY 4.0), Ch 22 "Magnetism", section 22.10 Magnetic Force between Two Parallel Conductors_