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Engineering LibreTexts

13: Physical Constants

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  • 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).