# 13: Physical Constants

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).