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Inside the copper of a current-carrying conductor the Poynting vector points

  • A.circumferentially, following the magnetic field
  • B.radially inward, accounting for the ohmic loss
  • C.along the conductor, in the direction of the current
  • D.radially outward, energy leaving the conductor

Answer: B

The small axial electric field within the copper crossed with the circumferential magnetic field gives a vector directed into the conductor, and its surface integral is exactly the \( I^2R \) loss. The energy that heats a wire arrives through its surface from the field.

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