EMC Question of the Week: February 20, 2023

Illustration of surface currents flowing around slots and apertures on a metal surface

At high frequencies, which field quantity just above a metal surface has a magnitude approximately equal to the magnitude of the surface current density?

  1. magnetic field
  2. tangential electric field
  3. normal electric field
  4. total electric field.


The best answer is “a.” At the boundary of a good conductor, the tangential magnetic field is perpendicular to the surface current and has the same magnitude. The normal component of the magnetic field is zero. The tangential electric field is also zero. 

The normal component of the electric field is related to the surface current. The magnitude of |εE| equals the charge density, and the magnitude of the current is the charge density times the frequency. However, the field strength does not equal the current density. In fact, one way to eliminate options (b), (c) and (d) is to recognize that the units of electric field strength (V/m) are not the same as the units of surface current density (A/m).

Note: A discontinuity in metal surface that reroutes a surface current creates a high current density at the discontinuity. Magnetic field probes can be used to detect spikes in the current density revealing seams or gaps that compromise an enclosure's shielding integrity.

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