EMC Question of the Week: January 18, 2021

field lines encircling a trace above a plane

In the figure on the right representing a cross-sectional view of a 3.3-V trace over a ground plane, the lines encircling the trace represent

  1. electric field lines
  2. magnetic field lines
  3. equipotential lines
  4. antenna patterns


The best answer is “c.” Electric field lines would start on one conductor and terminate on the other conductor. At DC and low frequencies, the equipotential lines are perpendicular to the electric field lines.

Note that magnetic field lines wrapping a trace due to a DC or low-frequency current flowing in the trace would not change direction at the dielectric interface. The electric fields are influenced by the dielectric, but not the magnetic fields.

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