EMC Question of the Week: March 29, 2021

Stack of metal plates

At high frequencies, the magnitude of the magnetic field tangential to a metal surface, such as the copper plane in a circuit board, is approximately equal to 

  1. zero
  2. the surface current density on the plane
  3. the electric field magnitude divided by 377 Ω
  4. all of the above


The best answer is “b.” The boundary condition for magnetic fields at a conducting surface is that the magnetic field is equal in magnitude and perpendicular to the surface current density. The tangential electric field is approximately zero.

On the other hand, at high frequencies, the normal (perpendicular) component of the magnetic field is zero. The magnitude of the normal component of the electric flux density is equal to the surface charge density.

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