For a plane electromagnetic wave in vacuum, which relation between the instantaneous electric and magnetic energy densities is correct?
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Question

For a plane electromagnetic wave in vacuum, which relation between the instantaneous electric and magnetic energy densities is correct?

G1 Physics · Accepted Answer

Let uₑ denote electric energy density and uₘ denote magnetic energy density. Electric energy density: uₑ = ½ε₀E². Magnetic energy density: uₘ = B²/(2μ₀). For a plane electromagnetic wave in vacuum, E = cB and c² = 1/(μ₀ε₀). Therefore, uₘ = B²/(2μ₀) = ½ε₀c²B² = ½ε₀E² = uₑ. So the electric and magnetic energy densities are equal at every point and every instant. Correct option: C.