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The instantaneous Poynting vector of a plane wave in a lossless medium

  • A.is constant in time
  • B.reverses direction twice in each cycle
  • C.is zero at every instant
  • D.pulsates at twice the wave frequency and never reverses

Answer: D

With the two fields in phase their product is proportional to the square of a sinusoid, which pulsates between zero and twice its mean at double the frequency but never goes negative. Its mean is the power the wave carries.

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