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Dielectric breakdown
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Insulation fails above a critical field.
At a critical field (air 3 MV/m) electrons are torn from atoms and a spark forms. Materials with high breakdown strength are used in capacitors.
Lightning occurs over kilometres.
Breakdown fields vary with humidity and shape.
In more detail
Dielectric breakdown happens when the electric field is strong enough to ionise the insulator, allowing a sudden current to flow. Air breaks down at about 3 MV/m.
Real-world uses
- Lightning is dielectric breakdown of air.
- Insulator design sets maximum voltage ratings to avoid breakdown.
- Engineers designing high-voltage cables and transformers check that the insulation will not break down under the expected field.
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Frequently asked questions
Why does breakdown happen?
High fields accelerate free electrons enough to ionise other atoms in an avalanche.
Does breakdown always destroy the insulator?
Not always, but it often leaves a conducting path that damages it.
What causes dielectric breakdown?
When the field is strong enough to tear electrons from atoms, the insulator conducts and may be permanently damaged.
Related topics in Electrostatics
Energy density of field · Dielectrics · Polarisation · Spherical capacitor · Cylindrical capacitor · Van de Graaff generator