The Poynting Flow

Plate 19 speaks for itself: “The energy never travels along the wire.”

The Physics

In a circuit, energy flows through the fields around the wires. The Poynting vector points from the battery through the space outside the wires and into the resistor from the side.

S = (1/μ₀) E × B

• S: Poynting vector (energy flux), W m⁻²
• μ₀: vacuum permeability, N A⁻²
• E: electric field, V m⁻¹
• B: magnetic flux density, T

It enters the resistor from the side. Poynting, 1884.

Honest caveat: A little energy does enter a real wire: the heat lost to its resistance flows in radially through its surface, never along it.

Sources

• J. H. Poynting, Phil. Trans. R. Soc. 175, 343 (1884).
• D. J. Griffiths, Introduction to Electrodynamics, 4th ed., §8.1.

On the poster

Plate 019, “The Poynting Flow”: the pixel-art plate with its slogan “The energy never travels along the wire.”, the key equation and the facts panel shown above. Pixelated Physics. Physics. Personified.