Two straight long parallel conductors carry currents i1 and i2 in the same direction. Define one ampere after deducing the expression for the force per unit length between them.

a) F = (μ₀ i₁ i₂)/(2πr)
b) F = (μ₀ i₁ i₂)/(4πr)
c) F = (2πr)/(μ₀ i₁ i₂)
d) F = (4πr)/(μ₀ i₁ i₂)