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A dielectric with permanent molecular dipoles, such as water, has a relative permittivity that

  • A.falls as the frequency is raised into the microwave region
  • B.rises steadily with frequency throughout the radio spectrum
  • C.is independent of frequency at all frequencies
  • D.becomes negative above a certain frequency

Answer: A

The molecules cannot turn fast enough to follow a rapidly reversing field, so their contribution drops out and the permittivity falls from about eighty at low frequency toward the value set by electronic polarisation alone. The energy absorbed during that relaxation is what heats food in a microwave oven.

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