For a moving charge is km = μ0/2pi or is km = μ0/4pi? examples: Biot-Savart (magnetic field dB at a point P due to a length element ds that carrues a steady current I is: dB = (μ0/4pi) (I ds x r) / r^2 While for an long straight wire, (≥10m?) B = (μ0 I)/(2pi r) What is the different use 4pi when length is less than 10meters? wikipedia says that km is "In another system, the "rationalized-metre-kilogram-second (rmks) system" (or alternatively the "metre-kilogram-second-ampere (mksa) system"), km is written as μ0/2π, where μ0 is a measurement-system constant called the "magnetic constant". The value of μ0 was chosen such that the rmks unit of current is equal in size to the ampere in the emu system: μ0 is defined to be 4π × 10−7 N A−2." https://en.wikipedia.org/wiki/Magnetic_constant However in my notes from class I have Km = 1E-7 as well as Km = 2E-7. Which do i use for a charge particle. And why the difference in the stated formulas? is it the distance? Km is suppose to be a constant not a variable.