为什么自由空间磁导率的值有圆周率Pi？
2πrB = μI from Ampère's circuital law where r is the separation between wires.
εμ = 1/c² from the EM wave equation where c = speed of light ≈ 3 x10⁸.
The Ampère which is defined as the amount of current that generates a force of 2 dynes per centimeter of length between two wires one centimeter apart.
Combine the first two equations to get μ = 4 π x 10⁻⁷.
Combine that with the third equation to get ε = 1/(μc²) ≈ 10⁻⁹/(9 x 4π).
Reference:
Where does 1/4(pi) in vacuum permittivity come from?
Roy McCammon
https://www.quora.com/Where-does-1-4-pi-in-vacuum-permittivity-come-from
https://e-magnetica.pl/magnetic_permeability_of_vacuum
