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Asked in GSEB Board March 2020 · Dielectric slab
Dielectric strength is the largest electric field a dielectric can withstand before it breaks down and conducts.
For dry air at ordinary pressure this is about 3×10⁶ V m⁻¹.
In everyday terms: about 3 kV per millimetre, or 30 kV across a 1 cm gap.
Beyond this field, stray ions in the air are accelerated enough to ionise the molecules they hit, and a spark jumps.
So the dielectric strength of air is 3×10⁶ V m⁻¹.
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