EMC Question of the Week: February 12, 2024
A 15-kV potential difference causes the air between two points to break down forming a 1-cm arc. The arc extinguishes when the
- voltage drops below the threshold
- current drops below the threshold
- temperature exceeds the threshold
- air is burned up
Answer
The best answer is “b.” Once the breakdown occurs, the current flowing in the arc superheats the air forming a plasma. In this super-heated state, large voltages are no longer required to sustain the arc. The arc will extinguish when there is no longer enough current to maintain the high temperature that sustains the plasma.
No specific voltage is required, although there must be enough voltage to drive a current through the arc resistance. The temperature is important, but higher temperatures will not extinguish the arc.
Finally, the air isn't burned up. In its plasma state, it has a relatively low density due to the high temperature. Once the arc is extinguished, the air returns to its original state (though perhaps with a higher density of ionized molecules).
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