# 10.7: Physical Constants

- Page ID
- 9418

**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*.

## Contributors and Attributions

Ellingson, Steven W. (2018) Electromagnetics, Vol. 1. Blacksburg, VA: VT Publishing. https://doi.org/10.21061/electromagnetics-vol-1 Licensed with CC BY-SA 4.0 https://creativecommons.org/licenses/by-sa/4.0. Report adoption of this book here. If you are a professor reviewing, adopting, or adapting this textbook please help us understand a little more about your use by filling out this form.