# 13: Physical Constants

- Page ID
- 19454

**The speed of light in free space** (\(c\)), which is the phase velocity of any electromagnetic radiation in free space, is \(\cong 2.9979 \times 10^8\) m/s. This is commonly rounded up to \(3 \times 10^8\) m/s. This rounding incurs error of \(\cong 0.07\)%, which is usually much less than other errors present in electrical engineering calculations.

**The charge of an electron** is \(\cong -1.602 \times 10^{-19}\) C. The constant \(e\triangleq +1.602176634 \times 10^{-19}\) C is known as the “elementary charge,” so the charge of the electron is said to be \(-e\).

**The permittivity of free space** (\(\epsilon_0\)) is \(\cong 8.854 \times 10^{-12}\) F/m.

**The permeability of free space** (\(\mu_0\)) is \(4\pi \times 10^{-7}\) H/m.

**The wave impedance of free space** (\(\eta_0\)) is the ratio of the magnitude of the electric field intensity to that of the magnetic field intensity in free space and is \(\sqrt{\mu_0/\epsilon_0}\cong 376.7~\Omega\). This is also sometimes referred to as the *intrinsic impedance of free space*.

**Boltzmann’s constant** is \(\cong 1.381 \times 10^{-23}\) J/K, the amount of energy associated with a change of one degree of temperature. This is typically assigned the symbol \(k\) (unfortunately, the same symbol often used to represent *wavenumber*).