EMC Question of the Week: January 29, 2018

Spark between electric guitar and connector

At standard temperature and pressure, air breaks down becoming an electrical conductor at field strengths of approximately

  1. 10,000 volts
  2. 30,000 volts/cm
  3. 3 MW/m2
  4. 300 MW/m2

Answer

The correct answer is "b". The dielectric breakdown of air occurs at electric field strengths of about 30,000 volts/cm. This is why a spark will form between a person (or electrostatic discharge tester) charged to around 15,000 volts and a grounded object several millimeters away. [The length of the spark will depend on several factors including the geometry of the conductors.]

The first answer, 10,000 volts, is not an electric field strength. The last two answers have units of power density, not electric field strength. If we were to assume that these were average power densities for electromagnetic plane wave propagation, they would represent peak electric field strengths of 480 V/cm and 4.8 kV/cm, respectively. These would be extremely intense electromagnetic waves, but not strong enough to break down air.  

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