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For an electromagnetic wave propagating in free space, the Poynting vector (energy flux per unit area per unit time) is represented by:

1. B cross E / mu0
2. E cross B / (2*mu0)
3. B cross E / (2*mu0)
4. E cross B / mu0

Correct answer: E cross B / mu0

Solution

The Poynting vector is defined as S = (1/mu0) * (E cross B). This gives both the direction and magnitude of energy flow per unit area per unit time (power per unit area, or intensity) in an electromagnetic wave. The factor of 1/mu0 (not 1/(2*mu0)) appears in the instantaneous Poynting vector; the 1/2 factor appears when computing the time-averaged intensity. Note: B cross E = -(E cross B), so B cross E / mu0 would point in the opposite direction (wrong). The correct expression is E cross B / mu0.

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