vacuum

# Vacuum

 Stan Zurek, Vacuum, Encyclopedia Magnetica, https://E-Magnetica.pl/vacuum

Vacuum - in electromagnetism, the “vacuum” is a hypothetical medium used to denote lack of any matter, without molecules, atoms or even sub-atomic particles such as electrons.1)

Illustration of magnetic field strength H, flux density B, magnetisation M, and polarisation J in vacuum

The lack of matter means lack of any magnetic dipole moments which can respond to the applied magnetic field by aligning to it.

Therefore, in vacuum always magnetisation M = 0 and also the equivalent magnetic polarisation J = 0, regardless the level of excitation.

The ratio between magnetic flux density B and magnetic field strength H is the permeability of vacuum which is also known as the magnetic constant $μ_0$:2)

 $$μ_0 = \frac{B_{\text{vacuum}}}{H_{\text{vacuum}}} = 4 · π · 10^{-7}$$ (H/m) ≡ (T·m/A)

Magnetic response of any matter can be quantified by absolute magnetic permeability $μ$, but a more useful figure of merit is the relative magnetic permeability $μ_r$, which is calculated as the ratio of the absolute permeability to that of vacuum so that:

 $$μ_r = \frac{μ}{μ_0}$$ (unitless) ≡ (H/m)/(H/m)

And by definition for vacuum $μ_r ≡ 1$.

For non-magnetic materials $μ_r \approx 1$, and for ferromagnets $μ_r \gg 1$.