Wikipedia
Electrostatic discharge (ESD) is the sudden flow of electricity between two electrically charged objects caused by contact, an electrical short or dielectric breakdown A buildup of static electricity can be caused by tribocharging or by electrostatic induction The ESD occurs when differently-charged objects are brought close together or when the dielectric between them breaks down, often creating a visible spark ESD can create spectacular electric sparks (lightning, with the accompanying sound of thunder, is a large-scale ESD event), but also less dramatic forms which may be neither seen nor heard, yet still be large enough to cause damage to sensitive electronic devices
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