EMC Question of the Week: September 14, 2020

Magnetic field lines associated with current-carrying coil

The SI-derived unit for magnetic field intensity is

  1. gauss
  2. tesla
  3. weber/m2
  4. ampere/m


The best answer is "d". The first three choices are all units of magnetic flux density. In the SI system, magnetic flux density, B, is related to magnetic field intensity, H, by the permeability of the medium. In free space and in non-magnetic materials, B = μ0H.

The earth's magnetic flux density at its surface ranges from 25 to 65 microteslas (0.25 to 0.65 gauss). That's a magnetic field intensity of about 0.2 to 0.5 A/m in free space. By contrast, the magnetic field 1 cm away from a wire carrying 1 ampere of current is approximately 16 A/m.

Note: If you did an internet search to find the answer to this question, you probably encountered sites that describe the Tesla as the SI-derived unit for magnetic field intensity. This is because scientists and engineers in many fields don't find it necessary to distinguish between magnetic flux density and magnetic field intensity, so the terms are sometimes used interchangeably. Nevertheless, in the field of electromagnetic compatibility, where magnetic materials are widely used, the distinction is important.

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