Electromagnetism
In classical electromagnetism, the vacuum of free space, or sometimes just free space or perfect vacuum, is a standard reference medium for electromagnetic effects. Some authors refer to this reference medium as classical vacuum, a terminology intended to separate this concept from QED vacuum or QCD vacuum, where vacuum fluctuations can produce transient virtual particle densities and a relative permittivity and relative permeability that are not identically unity.
In the theory of classical electromagnetism, free space has the following properties:
- Electromagnetic radiation travels where unobstructed at the speed of light, the defined value 299,792,458 m/s in SI units.
- The superposition principle is always exactly true. For example, the electric potential generated by two charges is the simple addition of the potentials generated by each charge in isolation. The value of the electric field at any point around these two charges is found by calculating the vector sum of the two electric fields from each of the charges acting alone.
- The permittivity and permeability are exactly the electric constant ε0 and magnetic constant μ0, respectively (in SI units), or exactly 1 (in Gaussian units).
- The characteristic impedance (η) equals the impedance of free space Z0 ≈ 376.73 Ω.
The vacuum of classical electromagnetism can be viewed as an idealized electromagnetic medium with the constitutive relations in SI units:
relating the electric displacement field D to the electric field E and the magnetic field or H-field H to the magnetic induction or B-field B. Here r is a spatial location and t is time.
Read more about this topic: Vacuum