What is the intrinsic impedance of free space Z0, and its approximate value in ohms?

What is the intrinsic impedance of free space Z0, and its approximate value in ohms?

• A. 120 Ω
• B. 377 Ω
• C. 600 Ω
• D. 1 Ω

Answer: (B)

The key idea is the ratio between the electric and magnetic field amplitudes in a plane wave traveling through free space. In vacuum this ratio is a fixed constant called the intrinsic impedance Z0, and it has a value about 377 ohms. For a plane wave, E and H are related by E = Z0 H, and Z0 equals sqrt(μ0/ε0). Using μ0 = 4π×10^-7 H/m and ε0 ≈ 8.854×10^-12 F/m gives Z0 ≈ 376.7 Ω, commonly rounded to 377 Ω. This value connects to the speed of light via c = 1/√(μ0 ε0), and the energy flow is described by the Poynting vector S = E×H. The other numbers are not the vacuum impedance; they appear as characteristic impedances in certain transmission lines or contexts but do not represent the intrinsic impedance of free space.

The key idea is the ratio between the electric and magnetic field amplitudes in a plane wave traveling through free space. In vacuum this ratio is a fixed constant called the intrinsic impedance Z0, and it has a value about 377 ohms.

For a plane wave, E and H are related by E = Z0 H, and Z0 equals sqrt(μ0/ε0). Using μ0 = 4π×10^-7 H/m and ε0 ≈ 8.854×10^-12 F/m gives Z0 ≈ 376.7 Ω, commonly rounded to 377 Ω. This value connects to the speed of light via c = 1/√(μ0 ε0), and the energy flow is described by the Poynting vector S = E×H.

The other numbers are not the vacuum impedance; they appear as characteristic impedances in certain transmission lines or contexts but do not represent the intrinsic impedance of free space.