Answer: <u> = ε_0 E0²/2 = ε_0 Erms².
- A <u> = ε_0 E0²/2 = ε_0 Erms²
- B <u> = ε_0 E0² in most textbook accounts
- C <u> = ε_0 E0²/4 during normal conditions
- D <u> = 2ε_0 E0² as generally observed
Correct answer: A. <u> = ε_0 E0²/2 = ε_0 Erms²
Explanation: Instantaneous u = ε_0 E² (total, including both E and B contributions). Time-averaged: <u> = ε_0 Erms² = ε_0 E0²/2. Intensity = <u> × c = ε_0 c E0²/2.
In an electromagnetic wave, the oscillating electric field (E) and magnetic field (B) are perpendicular to each other and to the direction the wave travels - a purely transverse wave that needs no medium, unlike sound.
Concept context
Maxwell's equations, EM spectrum, properties, and applications of each band.